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Open Source Laser Polymer Welding System: Design and Characterization of Linear Low-Density Polyethylene Multilayer Welds John J. Laureto 1 , Serguei V. Dessiatoun 2 , Michael M. Ohadi 2 and Joshua M. Pearce 1,3, * 1 2 3

*

Department of Materials Science and Engineering, Michigan Technological University, 601 M&M Building, 1400 Townsend Drive, Houghton, MI 49931-1295, USA; [email protected] Smart and Small Thermal Systems Laboratory, Department of Mechanical Engineering, University of Maryland, College Park, MD 20742, USA; [email protected] (S.V.D.); [email protected] (M.M.O.) Department of Electrical & Computer Engineering, Michigan Technological University, Houghton, MI 49931, USA Correspondence: [email protected]; Tel.: +1-906-487-1466

Academic Editor: Xiaoliang Jin Received: 26 May 2016; Accepted: 22 June 2016; Published: 1 July 2016

Abstract: The use of lasers to weld polymer sheets provides a means of highly-adaptive and custom additive manufacturing for a wide array of industrial, medical, and end user/consumer applications. This paper provides an open source design for a laser polymer welding system, which can be fabricated with low-cost fused filament fabrication and off-the-shelf mechanical and electrical parts. The system is controlled with free and open source software and firmware. The operation of the machine is validated and the performance of the system is quantified for the mechanical properties (peak load) and weld width of linear low density polyethylene (LLDPE) lap welds manufactured with the system as a function of linear energy density. The results provide incident laser power and machine parameters that enable both dual (two layers) and multilayer (three layers while welding only two sheets) polymer welded systems. The application of these parameter sets provides users of the open source laser polymer welder with the fundamental requirements to produce mechanically stable LLDPE multi-layer welded products, such as heat exchangers. Keywords: polymer welding; laser welding; polymer laser welding; additive manufacturing; open hardware; linear low density polyethylene; LLDPE; heat exchangers

1. Introduction Focused laser radiation absorbed into a polymer interface produces an elevated temperature, which can be used for inter-layer bonding. A contact free manufacturing method, such as laser welding, provides increased flexibility and further application than its conventional joint bonding processes [1]. Advancement in the field of polymer welding has expanded applications to microfluid polymer packages [2], aseptic packaging [3], hermetic sealing of an electronic car key [4], microfluidic channels [5], and additively manufactured and complex microchannel heat exchangers [6,7]. Characterization of polymer welds and in-process monitoring techniques have been explored with acoustic, optical, thermal, ultrasonic, and emission techniques [8,9]. Thus, the application of polymer sheet material(s) for lap-joint laser welding applications is not uncommon. Ghorbel et al. characterized the thermal and mechanical behavior of some thermoplastic polymers [10]. They successfully welded polypropylene sheets by diode-laser transmission welding [11] and selected soundness variables for the diode laser welding of polypropylene thermoplastic polymers by experimental and numerical analysis [12]. Also, Torrisi et al. characterized the adhesion susceptibility of polyethylene sheet materials [13,14]. The work described indicates that efficient welding of polymer materials is the result Machines 2016, 4, 14; doi:10.3390/machines4030014

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of not only thermally induced melting effects, but also the development of ions near the laser-polymer interface. Subsequently, pulsed laser radiation allows for adequate polymer weld adhesion, through photo-chemical and ion implantation effects, while not elevating the polymer beyond its melting temperature. All work described suggests that the resultant weld seam quality correlates to diode laser process parameters (laser power (W) and cross-head speed (mm/s)) and the optical/absorption properties of the incident polymer [11]. Dowding et al. successfully demonstrated the production of viable adhesive bonds between LLDPE (linear low density polyethylene) on PP (polypropylene) at an appropriate laser line energy (J/m), similar to a linear energy density (Coulombs/mm). In this study, maximum peel force was used for quantification [15]. The response behavior of the material system is constant in regard to incident laser line energy. Specifically, the linear energy density delivered to the polymer system requires, at a minimum, a critical value to induce bonding. This paper provides open source designs for a laser polymer welding system and then explores the mechanical properties and weld width appearance of LLDPE lap welds manufactured with the system. Specifically, apparent peak load (lbf) and linear energy density (coulombs/mm), corresponding to weld width (mm), are quantified. The designed, open-source, system is meant to provide a reliable manufacturing tool to be readily adapted to a multitude of polymer welding applications. Available source code and the provided component build files allow a multitude of users the ability to utilize the technology as they see appropriate. 2. Materials and Methods 2.1. Laser Welder An open-source computer numeric control (CNC) laser welder [16] was modified for this experiment. The apparatus is a gantry device with NEMA17 motors driving 20 tooth GT2 pulleys, one set for the x-axis and one for y. The frame is constructed with 20-20 extruded aluminum with accommodating fittings and fixtures. Utilized bearings and guide rods are readily available standard equipment for purchase. Printed members (Table 1) were redesigned in OpenSCAD [17], an open source parametric scripting computer aided design (CAD) program, and printed on a standard RepRap [18–21] using polylactic acid (PLA). Parts were designed so as to maximize rigidity while minimizing plastic consumption to minimize printing time, embodied energy, environmental impact, and economic cost. All SCAD files are available for free [22] under the GNU GPLv3 [23] along with operational instructions [24]. Boxed idler ends were designed to maximize rigidity and to assure proper belt tracking under tension. 624-ZZ roller bearings on 4 mm shafts were used as idlers. Belt tension was applied and maintained through the use of large nylon wire ties stretched between belt terminators previously designed for the MOST delta RepRap [25]. The x-carriage can adjust the position of the laser in the z-direction to assist in focusing. A pair of printed thumbscrews clamp the position of a threaded rod upon which they ride to the x-carriage. The laser mount is fixed to one end of the threaded rod and additionally constrained in the x and y-directions by a 6 mm smooth rod that is press-fit into the mount and passes through the x-carriage. The assembled x-carriage and z-adjustment system are shown in Figure 1. Mechanical snap-action switches to eliminate the need for a 5 V power supply and to simplify the design. A Melzi controller [26] was mounted to the frame with three dimensional (3-D) printed components and this is driven by a Raspberry Pi [27] with custom Franklin firmware [28,29], Arroyo Instruments 4320 20 A LaserSource, and 5305 5A/12V TECSource. Gcode, for the laser profile scans, was user-generated and imported into Franklin. As designed, the 4320 20 A LaserSource and X/Y laser-head movement is controlled by commands the user prescribed, while the 5305 TECSource is a standalone unit. The 4320 20 A LaserSource provides the incident laser source while the 5305 TECSource is the cooling system for the laser apparatus.

Table 1. Three-dimensional printed parts. Table 1. Three-dimensional (3-D) printed(3-D) parts. Part Part Part Part Count Table Rendered Image Count Rendered Image Count Rendered Image Count parts.Rendered Image (3-D) printed parts. 1. Three-dimensional (3-D) printed Name/DescriptionTable 1. Three-dimensional Name/Description Name/Description Name/Description Table 1. Three-dimensional (3-D) printed(3-D) parts. Table 1. Three-dimensional printed parts. Part Part Part Part Count Table Rendered Image Count parts.Rendered Image Count Rendered Image Count Rendered Image 1. Three-dimensional (3-D) printed parts. Table 1. Three-dimensional (3-D) printed Name/Description Name/Description Name/Description Part Name/Description Part Part Part Count Rendered Count Rendered Image Count Image Rendered Image Count Rendered Image Name/Description Name/Description Name/Description Name/Description (3-D) printed parts. Table 1. Three-dimensional (3-D) printed Part PartstandoffTable 1. Three-dimensional Part Controller Laser carriage for parts. Laser carriage for Machines 2016,Controller 4, 14 Partstandoff 3 of 14 Count Rendered Count Rendered Image Count Image Rendered Image Count Rendered Image Name/Description Name/Description Name/Description Name/Description for attaching1 1 Three-dimensional mounting laser for attaching mounting laser to 1 toparts. 1 Table 1. (3-D) printed parts. Table 1. Three-dimensional (3-D) printed Part Part Part Part Count Count Rendered Image Count Image Renderedholder Imageapparatus Count Rendered Image controller to frame Rendered holder apparatus controller to frame Controller standoff Laser carriage for Controller standoff Laser carriage for parts. Name/Description Name/Description Name/Description Name/Description Table 1. Three-dimensional (3-D) printed parts. Table 1. Three-dimensional (3-D) printed Part Part Part Part for attaching 1 mounting laser to 1 for attaching 1 mounting laser to 1 Controller standoff Laser carriage for Count Rendered Image Count Rendered Image Controller standoff Laser carriage for Count Rendered Image Count Rendered Image Table 1. Three-dimensional (3-D) printed parts. Name/Description Name/Description Name/Description Name/Description Part Part Part Part controller to frame holder apparatus controller to holder apparatus for attaching 1frame Rendered mounting laser to carriage 1 for for attaching 1 mounting laser to 1 Controller standoff carriage for Count Image Count Rendered Image Controller standoff Laser Count RenderedLaser Image Count Rendered Image Name/Description Name/Description Name/Description Name/Description controller tofor frame holder apparatus controller to 1frame holder for attaching mounting laser to apparatus 1 to Limit switch attaching 1 mounting laser 1 Limit switch Part Name/Description Count Part Name/Description Rendered Image Controller standoff Laser carriage for carriage for Count Controller standoff Rendered Image Laser controller frame for to frame holder apparatus mount M8 thumbscrew holder apparatus mount for tocontroller M8 thumbscrew for attachingfor attaching1 mounting laser to 1 to 1 mounting laser 1 Controller standoff Laser carriage Controller standoff Laser carriage for mechanical forfor adjusting mechanical for adjusting Limit switch Limit controller tocontroller frameswitch holder apparatus to 2frame holder apparatus 2 2 2 to for attaching 1 mounting laser to carriage for attaching 1 mounting laser 1 switches to z-position of 1 for switches z-position of Controller Laser carriage for Controller standoff Laser Controller standoff for Laser carriage for mountswitch fortostandoff M8 thumbscrew mount for M8 thumbscrew Limit Limit switch controller to frame holder apparatus controller to frame holder apparatus appropriate guide laser carriage appropriate guide laser carriage for attaching 1 mounting laser to 1 for attaching 1 mounting laser to attaching controller 1 mounting laser to 1 1 mechanical for adjusting mechanical for mount for Limit M8 thumbscrew mount for M8 adjusting thumbscrew Limit switch switch 2 2 2 rods rods to frame holder apparatus controller tocontroller frame toto 2frame holder apparatus holder apparatus switches z-position ofM8 switches z-position of mechanical for adjusting mechanical for adjusting mount forto mount M8 thumbscrew for 2 thumbscrew 2 2 2 Limit switch Limit switch appropriate guide to guide laser carriage appropriate laser carriage switches to mechanical z-position offor switches z-position of mechanical for adjusting adjusting mount for mount M8 thumbscrew for guide M8 thumbscrew 2 2 2 2 X-carriage for X-carriage for rods rods appropriate guide laser carriage appropriate laser carriage Limit switch Limit switch switches to switches z-position ofz-position of tocable X-carriage X-carriage mechanicalcable for adjusting mechanical for adjusting connecting x-axis connecting x-axis rods rods mount for M8 thumbscrew mount for M8 thumbscrew 2 2 2 2 appropriate guide switch laser carriage appropriate laser carriage Limit Limit mount forto guide for switches to switches z-position ofz-position of to Limit switch mount mountswitch for M8 thumbscrew for linear bearings linear bearings to mechanical for adjusting mechanical for adjusting X-carriage for X-carriage for1 rods rods mount for M8 thumbscrew mount for M8 thumbscrew attaching a cable 1 attaching a cable 1 2 2 2 mechanical switches to cable adjusting z-position of belt, 2 21 appropriate guide 2 cable laser carriage X-carriage X-carriage appropriate guide laser x-axiscarriage drive x-axis driveof belt, switches to mechanical z-position switches to z-position of connecting x-axis connecting x-axis X-carriage for X-carriage for mechanical for adjusting for adjusting carrier to carrier to appropriate guide laser carriage rods rods mount forthe mount forthe rods X-carriage cable X-carriage cable 2 2and 2 2 laser carriage, laser carriage, and appropriate guide to guide laser appropriate laser linearcarriage bearings to carriage linear bearings to connecting x-axis connecting x-axis switches z-position of switches z-position of x-carriage x-carriage X-carriage for X-carriage for attaching acable cable 1 1 attaching cable 1 1 mount forto mount foracable cable carrier cable carrier X-carriage X-carriage rods rods x-axis drive belt, x-axis drive belt, linear bearings to linear bearings to appropriate guide laser carriage appropriate guide laser carriage connecting x-axis connecting x-axis carrier to carrier to attaching a cable 1 1 attaching a cable 1 1 X-carriage for X-carriage for mount forthe mount forthe laser carriage, and laser carriage, and x-axis drive belt, x-axis drive belt, rods rods X-carriage cable X-carriage cable linear bearings to bearings to linear X-carriage for x-carriage x-carriage carrier to the carrier to the connecting x-axis connecting x-axis attaching a cable 1 1 attaching a cable 1 1 X-clamp X-clamp for mount cable carrier cable carrier laser carriage, andcarriage, laser and X-carriage X-carriage mount for forfor x-axis drivefor belt, x-axis drivefor belt, connecting linear X-carriage cable mount x-carriage x-carriage X-carriage cable X-carriage cable linear bearings to carrier linear bearings to carrier to the carrier tox-axis the X fixed cable securing X fixed cable cable carrier cable connecting x-axis connecting x-axis attaching a cable 1 1and attaching 1 1 1 bearings tofor x-axis drive for attaching securing a cable x-axis 1 a cable laser carriage, and laser carriage, X-carriage X-carriage for mount for cable mount for cable x-axis drive belt, x-axis drive belt, x-carriage x-carriage X-carriage guide rods to carrier mount for rods toX-carriage carrier mount for linear bearings to belt, laser carriage, and carrier to theguide x-carriage linear bearings to carrier tofor the X-clamp X-clamp carrier tofor the cable carrier cable carrier connecting x-axis connecting x-axis 2 2 1and attaching a cable 1 1 attaching a cable 1 11 laser carriage, and laser carriage, mount forfor mount attaching cable y-bearings and for attaching cable y-bearings and for forfor carrier x-axis drive belt, x-axis drive belt, x-carriage Xcable fixedbearings cable securing x-axis X fixed cable securing x-axis x-carriage X-clamp X-clamp linear to carrier linear bearings to carrier tox-axis the carrier tox-axis the cable carrier cable attaching a cable 1 1 attaching a cable 1 1 holding carrier to y-bearing holding carrier to y-bearing laser carriage, andcarriage, laser and guide rods toX-clamp carrier mount for guide rods to carrier mount for X fixed cablex-axis securing x-axis X fixed cablebelt, securing x-axis x-axis drive belt, drive X-clamp for for x-carriage x-carriage 2 1 2 1 carrier to the carrier to the idler idler cable carrier cable carrier attaching cable y-bearings and for rods attaching cablefor y-bearings and guide rods tosecuring carrier mount for guide to for carrier mount laser carriage, and laser carriage, and X fixed cable securing x-axis X fixed cable x-axis X-clamp for securing X fixed cable carrier 2 1 2 1 x-carriage x-carriage X-clamp for X-clamp for holding x-axis carriercarrier to y-bearing holding x-axis carrier to y-bearing attaching cable y-bearings and for rods attaching cablefor y-bearings and cable cable guide toguide carrier mount for carrier to for carrier mount x-axis guide rods torods for attaching Xmount fixedto cable securing x-axis X fixedto cable securing x-axis 2 1 2 1 2 1 idler idler holding x-axis carrier y-bearing holding x-axis carrier y-bearing X-clamp forand X-clamp y-bearings and for cable carrier attaching cable y-bearings for forand for attaching cable y-bearings guide rods toguide carrier mount for mount for rods to carrier idler idler X cable securing x-axis X fixed cable securing x-axis holding x-axis idler cap tofixed y-bearing 2 1 for 2 1 holding x-axis carrier to y-bearing X-idler cap for X-motor mount holding x-axis carrier y-bearing X-idler for X-motor mount for to X-clamp for X-clamp for attaching cable y-bearings and for attaching cable y-bearings and for guide rods tosecuring carrier mount for guide to1 idler carrier mount for idler boxing x-axis 1 mounting x-motor 1 idler boxing x-axis idler rods mounting x-motor 1 X fixed cable securing x-axis X fixed cable x-axis 2 1 2 1 holding x-axis carrier to y-bearing holding x-axis carrier to y-bearing attaching cable y-bearings for rods attaching cablefor y-bearings for bearing andand shaft to for y-bearing bearing andand shaft to y-bearing guide to carrier mount guide to carrier mount X-idler cap for X-motor mount for X-idler cap for X-motor mount for idler rods idler 2 1 2 1 holding x-axis carrier to y-bearing holding x-axis carrier to y-bearing attaching cable y-bearings for x-axis attaching cable y-bearings for boxing x-axis idler 1 idler mounting x-motor 1 for boxing 1 mounting x-motor 1 X-idler cap and for X-motor for X-idler cap and for X-motor X-idler cap for boxing X-motormount mount for mount idler idler holding x-axis carrier to y-bearing holding x-axis carrier to y-bearing bearing and shaft to y-bearing bearing and shaft to y-bearing boxing x-axis idler 1 mounting x-motor 1 boxing x-axis idler 1 mounting x-motor 1 x-axis idler bearing 1 mounting x-motor 1 X-motor saddle X-motor saddle X-idler cap for X-motor mount for mount for X-idler cap for X-motor idler carrier idler and shaft toy-bearing y-bearing bearing and boxing shaft to bearing and 1shaft to y-bearing cable cable boxing x-axis idlercarrier mounting x-motor 1 x-axis idler 1 mounting x-motor 1 X-idler for cap for X-motor mount for mount X-idler cap for X-motor mountfor for Y cable mount Y cable mount for mount bearing and shaft for to y-bearing bearing and shaft to y-bearing X-motor saddle X-motor saddle boxing x-axis idler x-axis 1 idler mounting fixed x-motor 1 boxing 1 mounting x-motor 1 mounting fixed mounting a cable mounting mounting a cable X-idler cap for X-motor mount for X-idler cap for X-motor mount for cable carrier cable carrier X-motor saddle X-motor saddle 1 1 1 1shaft X-motor saddle cable bearing and shaft to y-bearing bearing and to y-bearing end of y-axis carrier for the end of y-axis cable carrier for the boxing x-axis idlercarrier 1 idler mounting x-motor 1cable boxing x-axis 1 mounting x-motor 1 Y cable mount for mount for Y cable mount for mount for cable cable X-idler cap for X-motor mount for X-idler cap for X-motor mount for carrier mountX-motor for carrier saddle X-motor saddle Y y-bearing cable mount for carrier y-axis and carrier y-axis and bearing and shaft to bearing and to y-bearing fixed mounting afor cable fixed mounting afor cable Y cable mount for mount mount for Y cable mount for mounting a cable boxing x-axis idlercarrier 1shaft mounting x-motor 1 for boxing x-axis idler 1 mounting x-motor 1 cable carrier cable 1 of 1 1saddle mounting fixed end of 1 1 1 added X-motor saddle added of rigidity X-motor carrier for themount y-axisrigidity end of y-axis cable carrier for the end of y-axis cable carrier for the mounting fixed mounting a cable mounting fixed mounting a cable bearing and shaft to y-bearing bearing and shaft to y-bearing Y cable mount for for mount for 1 Y cable mount for y-axis cable carrier 1 1 1 the x-motor mount cable carrier the x-motor mount cable carrier and for added rigidity carrier y-axis and for carrier y-axis and for end of y-axis cable carrier forsaddle end of y-axis cable carrier forsaddle X-motor X-motor mounting fixed mounting athe cable mounting fixed mounting athe cable Y cable mount for mount1 for mount for for Y cable mount for for of the x-motor mount 1 of 1 1 added rigidity of added rigidity carrier y-axis and carrier y-axis and cable carrier cable carrier end of y-axisend cable carrier forsaddle the of y-axis cable carrier forsaddle the X-motor X-motor mounting fixed mounting a cable mounting fixed mounting a cable the x-motor mount the mount added rigidity ofx-motor added rigidity Y cable mount for mount1 for mount for for Y cable mount for for 1 of 1 1 carrier y-axis and carrier y-axis and cable carrier cable carrier end of y-axis cable carrier for the end of y-axis cable carrier for the Y-carriage for Y-carriage for the x-motor mount the mount mounting fixed mounting a cable mounting fixedfor mounting a cable rigidity ofx-motor added rigidity of Y-carriage foradded Y cable mount for mount for Y cable mount mount for 1 1 1 1 carrier y-axis and for carrier y-axis and for connecting Y-idler for holding connecting Y-idler for Y-idler forholding holding end of y-axis cable carrier for end of y-axis 2cable 2 2 carrier for the x-motor mount 1 the mount 1 connecting Machines y-bearing toathe 1 athe 2016, 4, 14x-motor 2 of 2 mounting fixed mounting cable mounting mounting cable added rigidity of rigidity added y-axisidler idler bearingidlerfixed y-bearing to y-axis bearing y-bearing to y-axis bearing 1 1 of 1 1 Y-carriage Y-carriage for carrier and for carrier y-axis and y-drive belt y-axis end of y-axis cable carrier for the end of y-axis cable carrier for the Machines 2016, 4, 14 2 of 2 Machines 2016, 4, 14 2 of 2 the x-motor mount the x-motor mount y-drive beltfor y-drive beltfor connecting Y-idler for holding connecting Y-idler for holding Y-carriage Y-carriage added rigidity of and added rigidity of Fixed belt carrier y-axis and for carrier y-axis for 1 2 1 2 y-bearing to y-axis idler bearing y-bearing to y-axis bearing connecting Y-idler for holding connecting Y-idleridler for holding the x-motor mount theofx-motor mount terminator Y-carriage for Y-carriage for added rigidity added rigidity 1 of 2 1 2 Fixed belt Fixed belt for Y-motorbelt mount for belt y-drive y-drive y-bearing to connecting y-axis idler bearing y-bearing to mount y-axis idler bearing Fixed belt terminator attaching drive belt connecting Y-idler for holding Y-idler for holding the x-motor mount the x-motor terminator for2 terminator for Y-carriage for Y-carriage for attaching y-motor 1 1 2 y-drive y-drive mount for Y-motor mount for belt Y-motor mount for belt forx attaching driveidler belt bearing to and y-carriages y-bearing to Y-motor y-axis idler bearing y-bearing to y-axis attaching belt attaching drive belt fordrive connecting Y-idler for holding connecting Y-idler holding to frame y-motor attaching 11 22 attaching 11 14; doi:10.3390/machines4030014 22 2 www.mdpi.com/journal/machines attaching y-motor 1 y-motor to xtensioning and y-carriages Y-carriage Y-carriage for Machines 2016, 4, Machines 2016, 4, 14; doi:10.3390/machines4030014 www.mdpi.com/journal/machines and ofand and y-drive beltfor y-drive belt to x y-carriages to x and y-carriages y-bearing to y-axis idler bearing y-bearing to y-axis idler bearing to frame tensioning of open to frame to frame connecting Y-idler for holding connecting Y-idler for holding open ended belting Y-carriage Y-carriage andoftensioning and tensioning 1 2 of 1 2 y-drive beltfor y-drive beltfor ended belting y-bearing to4, y-axis idler bearing y-bearing to y-axis idler bearing connecting for holding connecting Y-idler for holding Machines 2016,Machines 14; doi:10.3390/machines4030014 www.mdpi.com/journal/machines 2016, 4, 14; doi:10.3390/machines4030014Y-idler www.mdpi.com/journal/machines open ended belting open ended belting 1 2 1 2 y-drive belt y-drive belt y-bearing to4, y-axis idler bearing y-bearing to y-axis idler bearing Machines 2016, 14; doi:10.3390/machines4030014 www.mdpi.com/journal/machines Machines 2016, 4, 14; doi:10.3390/machines4030014

Free beltbelt y-drive belt y-drive Machines 2016, 4, 14; doi:10.3390/machines4030014 2016, 4, 14; doi:10.3390/machines4030014 Free belt terminator for Machines terminator for Free belt2 2 Free -tensioning of openbelt Machines 2016, 4, 14; doi:10.3390/machines4030014 Machines 4, 14; doi:10.3390/machines4030014 tensioning ofterminator open 2016,for ended beltingterminator for 2 2 ended belting tensioning tensioning of4, open Machines 2016,Machines 14; doi:10.3390/machines4030014 2016,of4, open 14; doi:10.3390/machines4030014 ended belting ended belting Machines 2016,Machines 4, 14; doi:10.3390/machines4030014 2016, 4, 14; doi:10.3390/machines4030014

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Cable carriers are used to support the laser fiber and were mounted such that the fiber is nearly continuously protected. The laser source is positioned under the frame. The entire apparatus (Figure 2) is placed in a shielded aluminum box for the safety of operators. 2.2. Materials Liner low-density polyethylene (LLDPE), which is typically utilized as an underground encasement of ductile iron pipes per ANSI/AWWA C105/A21.5, is analyzed. Large industrial LLDPE rolls are readily available [30]. Material was obtained in a continuous length measuring

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16 in (406.4 mm) ˘0.5 in (12.7 mm) in width and manufactured to a minimum thickness of 0.008 in (0.203 mm). The supplier’s technical data sheets indicate a density of 0.910 to 0.935 g/cm3 and a carbon Machines 2016, 4, 14 4 of 14 black additive of no less than 2% [31].

Figure 1. Closeup of the x-carriage z-axis adjustment assembly.

Cable carriers are used to support the laser fiber and were mounted such that the fiber is nearly continuously protected. The laser source is positioned the assembly. frame. The entire apparatus Figure 1. Closeup of the x-carriage z-axis under adjustment (Figure 2) is placed in a shielded aluminum box for the safety of operators. Cable carriers are used to support the laser fiber and were mounted such that the fiber is nearly continuously protected. The laser source is positioned under the frame. The entire apparatus (Figure 2) is placed in a shielded aluminum box for the safety of operators.

Figure 2. Completed laser welder welder apparatus. apparatus. Figure 2. Completed open-source open-source laser

2.2. Materials 2.3. Fabrication 2. Completed open-source laser apparatus. Liner low-density Figure polyethylene (LLDPE), which is welder typically utilized as an underground The LLDPE sheeting was sectioned into dimensions 2.25 ˆ 4.5 in (57.15 ˆ 114.3 mm) ˘0.5 in encasement of ductile iron pipes per ANSI/AWWA C105/A21.5, is analyzed. Large industrial (12.7 mm). The specified dimensions allow for sufficient bonding area to be analyzed while fitting into 2.2. Materials LLDPE rolls are readily available [30]. Material was obtained in a continuous length measuring 16 in the tensile testing grips used for analysis. Prior to all welding operations, foreign particulate (e.g., dust (406.4Liner mm)low-density ±0.5 in (12.7polyethylene mm) in width and manufactured to a minimum of 0.008 in (LLDPE), which is typically utilized thickness as an underground 3 and a carbon (0.203 mm). The supplier’s technical data sheets indicate a density of 0.910 to 0.935 g/cm encasement of ductile iron pipes per ANSI/AWWA C105/A21.5, is analyzed. Large industrial black additive no lessavailable than 2% [31]. LLDPE rolls areofreadily [30]. Material was obtained in a continuous length measuring 16 in (406.4 mm) ±0.5 in (12.7 mm) in width and manufactured to a minimum thickness of 0.008 in (0.203 mm). The supplier’s technical data sheets indicate a density of 0.910 to 0.935 g/cm3 and a carbon black additive of no less than 2% [31].

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2.3. Fabrication

The LLDPE sheeting was sectioned into dimensions 2.25 × 4.5 in (57.15 × 114.3 mm) ±0.5 in and debris) from the surface with a wet cloth then allowed to dry. while Contaminates, (12.7 mm). was The removed specified dimensions allow for sufficient bonding area to be analyzed fitting as into described, may depreciate the validity of the analysis. the tensile testing grips used for analysis. Prior to all welding operations, foreign particulate (e.g., dust and debris) was removed from the surface withto a wet then Contaminates, A single sample component is comprised of two threecloth layers of allowed LLDPE, to to dry. dimensions specified as described, may depreciate validity ofThree the analysis. prior, depending upon testingthe conditions. individual samples are placed inside the polymer component is samples comprised two to three layers of LLDPE, dimensions welder A at single a time,sample thus providing three peroftesting condition. Multiple testingtoconditions were specified prior, depending upon testing conditions. Three individual samples are placed inside the analyzed beyond variable layer count. Incident current (I) and cross-head speed were intentionally polymer welder at time, thus providing the three samples per testing condition. Multiple varied throughout theaanalysis. Specifically, incident current was incremented 0.5 A pertesting analysis conditions were analyzed beyond variable layer count. Incident current (I) and cross-head speed within the range of 5 A–20 A, and all collected data was done in two scenarios: one using a 10 mm/s were intentionally varied analysis. the incident was incremented cross-head speed, and the throughout other usingthe a 20 mm/s Specifically, cross-head speed. Lasercurrent scan patterns proceeded 0.5 A per analysis within the range of 5 A–20 A, and all collected data was done in two scenarios: one linearly across the sample component, parallel to the rolled direction, near mid length ~2.25 in using a 10 mm/s cross-head speed, and the other using a 20 mm/s cross-head speed. Laser scan (57.15 mm). Table 2 describes the test parameters in further detail. patterns proceeded linearly across the sample component, parallel to the rolled direction, near mid length ~2.25 in (57.15 mm). Table 2 describes the test parameters in further detail. Table 2. LaserSource 20A 4320 Set Up Values. Table 2. LaserSource 20A 4320 Set Up Values. Variable Value Units Variable Value Units Mode Io (ACC) Mode1 Io5.5–20 (ACC) - (A) Amps Io Limit Limit 1 5.5–20 Amps (A) (µm) ImIo Limit 20,400 Microamps I m Limit 20,400 Microamps (μm) Vf Limit 5.1 Volts (V) f Limit 5.1 Volts (V) VfVSense Internal Vf Sense Internal - (Ω) Cable R 0.0 Ohms Cable RIo 0.0 Ohms (Ω)(mA) Tolerance 100 Milliamps On Delay Io 0.0 Milliseconds (ms) Tolerance 100 Milliamps (mA) 1 Variable in On Delay 0.0experimentation. Milliseconds (ms) 1

Variable in experimentation.

Low-iron glass plates, 0.60.6 cm thick, were Low-iron glass plates, cm thick, wereutilized utilizedtotoensure ensuresample samplestability stabilityand andflatness flatnessduring duringthe welding operation. The experimental setup involved layering three samples adjacent to one the welding operation. The experimental setup involved layering three samples adjacent toanother, one another, their 4.5 mm length,byfollowed another secondary low-iron plate on along their along 4.5 mm length, followed anotherby secondary low-iron glass plateglass placed on placed top. Second, Second, laser head,with modifiable with awas set screw, placedto adjacent the top glass surface. thetop. laser head,the modifiable a set screw, placedwas adjacent the toptoglass surface. Figure 3 Figure 3the describes set-up during involved all experimentation. describes set-up the involved allduring experimentation.

Figure 3. Sample dimensions and two/three layer experimental set up. Linear low density Figure 3. Sample dimensions and two/three layer experimental set up. Linear low density polyethylene polyethylene (LLDPE), linear low density polyethylene. (LLDPE), linear low density polyethylene.

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2.4. Characterization Characterization 2.4. 2.4.1. Peak Peak Load Load Determination Determination 2.4.1. Procured materials materials for for this this analysis analysis are are assumed assumed to to not not be be anisotropic. anisotropic. Specifically, Specifically, all all tensile tensile Procured tests performed performed induce induce force force normal normal to to the the rolled rolleddirection directionofofthe themanufactured manufacturedLLPDE LLPDEand/or and/or tests normal to to the the weld weld line. line. Baseline Baseline analysis analysis of of “virgin” “virgin” LLDPE LLDPE samples samples (e.g., (e.g., no no weld weld line line specimens) specimens) normal can be be directly directly compared compared to to their theirwelded weldedcounterparts. counterparts. An An Instron Instron 4206 4206 tensile tensile tester tester with with testing testing can proceduresmodeled modeledafter afterASTM ASTMD2990-01 D2990-01and andD638-02a D638-02aallowed allowedfor for determination determination of of peak peak load load (lbf) (lbf) procedures for all sample conditions [32,33]. Specimens comprised of two and three layers were subjected to for all sample conditions [32,33]. Specimens comprised of two and three layers were subjectedthis to analysis. All All twotwo layered components exhibiting were deemed deemed this analysis. layered components exhibitingadequate adequatelayer-to-layer layer-to-layer adhesion adhesion were adequate. If If visual visual analysis analysis post-welding post-welding determined determined any any delamination delamination and/or and/or lack lack of of weld weld cross cross adequate. section, the sample was omitted from the analysis. Similar inspection criteria were employed on the section, the sample was omitted from the analysis. Similar inspection criteria were employed on the three layer samples. Ideally, the bottom layer (third layer—Figure 3) will not bond to the three layer samples. Ideally, the bottom layer (third layer—Figure 3) will not bond to the near-adjacent near-adjacent first and second enables 3-Dtogeometries to be fabricated with first and second layers, which layers, enableswhich complex 3-D complex geometries be fabricated with this system this system (e.g., heat exchangers). Thus, the near-adjacent layers can be welded independently of (e.g., heat exchangers). Thus, the near-adjacent layers can be welded independently of the previous the previous layers of LLDPE.methods, Fabrication described prior,toare aimed to ensure bottom layersbottom of LLDPE. Fabrication as methods, describedas prior, are aimed ensure this. Thus, this. Thus, tensile testing on three layered specimens was performed pending the observation that tensile testing on three layered specimens was performed pending the observation that the first and the first and second layers are adequately bonded while the third has not. second layers are adequately bonded while the third has not. 2.4.2. 2.4.2. Weld WeldWidth Width(mm) (mm)and andResultant ResultantEnergy EnergyDensity Density(Coulombs/mm) (Coulombs/mm) The The application application of of imaging imaging software software ImageJ ImageJ 1.49 1.49 [34] [34] allowed allowed for for the the quantitative quantitative analysis analysis of of each each respective respective weld weld width. width. Images Images selected selected for for analysis analysis were were captured captured utilizing utilizing aa standard standard digital digital camera. The image frame (i.e., contained contained in the the image(s)) image(s)) were were aa representative representative top-down top-down view view of of camera. each weld line. Each with 0.50.5 andand 1.0 1.0 mmmm resolution/gradations. The each Eachimage imageframe framecontained containeda ruler a ruler with resolution/gradations. rulerruler provided the the ability to to utilize ImageJ is The provided ability utilize ImageJ1.49 1.49totoproperly properlyscale scale the the captured images. This is accomplishedbybythethe software measurement correlation to themeasurements “real” measurements using a accomplished software measurement correlation to the “real” using a “pixels/in” “pixels/in” determination. of three profile length measurements ensured determination. An averageAn of average three distinct linedistinct profileline length measurements ensured statistical statistical confidence operator measurement(s). Figurea4representative displays a representative weld width confidence in operatorinmeasurement(s). Figure 4 displays weld width photograph photograph used for width determination. used for width determination.

Figure 4. 4. Representative Representative weld weld width width analysis analysisphotograph. photograph. Figure

Correlating laser cross-head speed to incident laser current derives an expression for linear Correlating laser cross-head speed to incident laser current derives an expression for linear energy density (Coulombs (A·s)/mm). Thus, linear energy density, weld width, and peak load can energy density (Coulombs (A¨ s)/mm). Thus, linear energy density, weld width, and peak load can be characterized. be characterized.

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3. 3. Results Results 3. Results 3.1. Weld Width at Various Linear Energy Densities 3.1. Weld Width at Various Linear Energy Densities weld width vs. vs. linear energy density data data showshow that weld Linear regression regressionanalysis analysisofofmeasured measured weld width linear energy density that Linear regression analysis of measured weld width vs. linear energy density data show that width increases with increased linear energy density.density. Figures Figures 5–8 describe the correlation. Directly weld width increases with increased linear energy 5–8 describe the correlation. weld width increases with increased linear energy density. Figures 5–8 describe the correlation. comparing the regression analysis of Figuresof5 Figures and 7 (two layered systems) thatshows the slopes are Directly comparing the regression analysis 5 and 7 (two layeredshows systems) that the Directly comparing the regression analysis of Figures 5 and 7 (two layered systems) shows that the near equivalent and greaterand thangreater one. Conversely, Figures 6 and 8 (three layered systems) alsosystems) display slopes are near equivalent than one. Conversely, Figures 6 and 8 (three layered slopes are near equivalent and greater than one. Conversely, Figures 6 and 8 (three layered systems) a similar slope, although at although a different of ~0.5. Weld width datawidth was recorded linear also display a similar slope, atmagnitude a different magnitude of ~0.5. Weld data was for recorded also display a similar slope, although at a different magnitude of ~0.5. Weld width data was recorded welds with, at a minimum, incident laser appearance. Specifically,Specifically, solid linearsolid weldslinear to observable for linear welds with, at a minimum, incident laser appearance. welds to for linear welds with, at a minimum, incident laser appearance. Specifically, solid linear welds to faint heat lines spread,data in reference toreference the trendto line, apparent observable faintwere heatrecorded. lines wereSignificant recorded. data Significant spread, in theistrend line, in is observable faint heat lines were recorded. Significant data spread, in reference to the trend line, is Figure 5 atinaFigure range of 0.5–1.3 (Coulombs/mm). At relativelyAt low linear energy densities the densities resultant apparent 5 at a range of 0.5–1.3 (Coulombs/mm). relatively low linear energy apparent in Figure 5 at a range of 0.5–1.3 (Coulombs/mm). At relatively low linear energy densities weld width isweld a gradient a thin linear gradually at distances to the resultant width is(e.g., a gradient (e.g., aindication thin linearthat indication thatfades gradually fades atnormal distances the resultant weld width is a gradient (e.g., a thin linear indication that gradually fades at distances the weld Conversely, relativelyrelatively high linear energy welds develop weld seams normal todirection). the weld direction). Conversely, high linear density energy density welds develop weld normal to the weld direction). Conversely, relatively high linear energy density welds develop weld with measurement with ImageJ, identification of the apparent seamsa visible with a finite visiblewidth. finite Thus, width.upon Thus, upon measurement with the ImageJ, the identification of the seams with a visible finite width. Thus, upon measurement with ImageJ, the identification of the weld is subjective as some zone within the within gradient selected as the edge.asThe operator apparent weld is subjective as some zone theis gradient is selected thedeviation edge. Theindeviation apparent weld is subjective as some zone within the gradient is selected as the edge. The deviation measurement, which is identified the edgeasofthe theedge weld, thecauses spreadthe shown inshown the Figure 5 in operator measurement, which isas identified of causes the weld, spread in the in operator measurement, which is identified as the edge of the weld, causes the spread shown in the data set. Figure 5 data set. Figure 5 data set.

Figure 5. Weld width as a function of linear energy density for 10 mm/s on two layers of LLDPE. Figure5.5.Weld Weldwidth widthasasaafunction functionofoflinear linearenergy energydensity densityfor for10 10mm/s mm/sonontwo twolayers layersofofLLDPE. LLDPE. Figure

Figure 6. Weld width as a function of linear energy density for 10 mm/s on three layers of LLDPE. Figure 6. Weld width as a function of linear energy density for 10 mm/s on three layers of LLDPE. Figure 6. Weld width as a function of linear energy density for 10 mm/s on three layers of LLDPE.

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Figure 7. Weld width as a function of linear energy density for 20 mm/s on two layers of LLDPE.

Figure 7. Weld width as aasfunction ofoflinear for2020mm/s mm/s layers of LLDPE. Figure 7. Weld width a function linearenergy energy density density for on on twotwo layers of LLDPE. Figure 7. Weld width as a function of linear energy density for 20 mm/s on two layers of LLDPE.

Figure 8. Weld width as a function of linear energy density for 20 mm/s on three layers of LLDPE. Figure 8. Weld width as a function of linear energy density for 20 mm/s on three layers of LLDPE.

Figure 8. Weld width as a function of linear energy density for 20 mm/s on three layers of LLDPE. Typical weld cross sections are as shown in Figures 9 and 10. Figure 9 demonstrates a quality Figure 8. Weld width as a function of linear energy density for 20 mm/s on three layers of LLDPE. Typical weld cross sections are aasdelaminated shown in Figures 9 and 10. Figure 9 demonstrates a quality weld, while Figure 10 demonstrates failed weld. weld, while Figure 10 demonstrates a delaminated failed weld. Typical weld cross sections are as shown in Figures 9 and 10. Figure 9 demonstrates a quality Typical weld cross sections are as shown in Figures 9 and 10. Figure 9 demonstrates a quality weld,weld, whilewhile Figure 10 demonstrates a adelaminated weld. Figure 10 demonstrates delaminated failed failed weld.

Figure 9. Representative photograph of a quality two layer LLDPE polymer laser weld. Similar Figure Representative photograph quality two layerweld LLDPE polymer laser weld. Similar surface9. topology, as shown, is apparentofinathree layer LLDPE systems. surface topology, as shown, is apparent in three layer LLDPE weld systems. Figure 9. Representative photograph of a quality two layer LLDPE polymer laser weld. Similar Figure 9. Representative photograph of a quality two layer LLDPE polymer laser weld. Similar surface surface topology, as shown, is apparent in three layer LLDPE weld systems.

topology, as shown, is apparent in three layer LLDPE weld systems.

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Figure 10. Representative photograph of a degraded two layer LLDPE polymer laser weld. Similar

Figure 10.line Representative of alayer degraded twosystems. layer LLDPE polymer laser weld. Similar width decrease isphotograph apparent in three LLDPE weld line width decrease is apparent in three layer LLDPE weld systems.

3.2.

Figure 10. Representative of a Layered degraded two Systems layer LLDPE polymer laser weld. Similar 3.2. Polymer Weld Adhesionphotograph of Two and Three LLDPE Available line width decrease is apparent in threeoflayer LLDPE weld systems. Figure 10. Representative photograph a degraded two layer LLDPE polymer laser qualitatively. weld. Similar Adhesion susceptibility due to an increase in LLDPE linear energy density was analyzed Polymer Weld Adhesion of Two and Three Layered Systems Available line width decrease is apparent in three layer LLDPE weld systems. Post welding operations/attempts, operators would analyze generated welds and exert a small pull

3.2. Polymer Weld Adhesion of Two and Layered Systems Available Adhesion susceptibility due anThree increase in LLDPE linear qualitatively. force (by hand) in attempts toto shear the weld zone. Welds energy requiringdensity minimal was effort analyzed (e.g., tackiness) 3.2. Polymer Weld Adhesion of Two and Three Layered LLDPE Systems Available were deemed unacceptable for further analysis. Welds exhibiting greater adhesion (i.e., greater than Post welding operations/attempts, operators would analyze generated welds and exert a small pull Adhesion susceptibility due to an increase in linear energy density was analyzed qualitatively. minimal force) were subjected to further mechanical testing. Laser welds requiring further welding analyze generated andeffort exertqualitatively. a(e.g., smalltackiness) pull forcePost (by Adhesion hand) inoperations/attempts, attempts to shear the weldwould zone. Welds requiring minimal susceptibility due tooperators an increase in linear energy densitywelds was analyzed mechanical testing and those sheared are shown in Figures 10 and 11, respectively. The linear line force (by hand) in attempts tofurther shear the weldwould zone. Welds requiring minimal effort (e.g., tackiness) welding operations/attempts, operators analyze generated welds and exert a small pull werePost deemed unacceptable for analysis. Welds exhibiting greater adhesion (i.e., greater than indication in Figures 11 and 12 represent solid weld regions. A proper weld contains a solid line were deemed unacceptable for further analysis. Welds exhibiting greater adhesion (i.e., greater than force (by hand) in attempts to shear the weld zone. Welds requiring minimal effort (e.g., tackiness) minimal force) were subjected atopoor further testing. Laserindications welds requiring mechanical (Figure 11). Conversely, weld mechanical (Figure 12) will have dashed displayingfurther improper minimal force) subjected to in further mechanical testing. welds requiring were deemed unacceptable further analysis. Welds exhibiting greater (i.e.,line. greater than adhesion. It iswere to be concluded that the ideal weld appearance will be aLaser solidadhesion uninterrupted testing and those sheared arefor shown Figures 10 and 11, respectively. The linear linefurther indication mechanical testing andsubjected those sheared are shown in Figures 10 andLaser 11, respectively. The linear line minimal force) were to further mechanical testing. welds requiring further in Figures 11 and 12 represent solid weld regions. A proper weld contains a solid line (Figure 11). indication intesting Figures 11those and 12 represent solid weld regions. proper weld contains solid line mechanical and sheared are shown in Figures 10 A and 11, respectively. Thealinear Conversely, a poor weld (Figure 12) will have dashed indications displaying improper adhesion. It is (Figure 11).inConversely, poor (Figure 12)weld will have dashed indications improper indication Figures 11 aand 12 weld represent solid regions. A proper weld displaying contains a solid line to be(Figure concluded that theconcluded ideal weld appearance will behave a solid uninterrupted line. adhesion. It Conversely, is to be the(Figure ideal weld appearance will be a solid uninterrupted line. 11). a poorthat weld 12) will dashed indications displaying improper adhesion. It is to be concluded that the ideal weld appearance will be a solid uninterrupted line.

Figure 11. Laser weld subjected to further mechanical testing. Linear indications signify proper adhesion at the weld interface of the LLDPE sheeting.

Figure 11. Laser weld subjected to further mechanical testing. Linear indications signify proper

Figure 11. Laser weld subjected to further mechanical testing. Linear indications signify proper adhesion the weld interface of the Figure 11.atLaser weld subjected to LLDPE further sheeting. mechanical testing. Linear indications signify proper adhesion at the weld interface of the LLDPE sheeting. adhesion at the weld interface of the LLDPE sheeting.

Figure 12. Laser weld not subjected to further mechanical testing. Broken/dashed linear indication represented a degraded weld seam between the LLDPE sheeting.

Figure 12. Laser weld not subjected to further mechanical testing. Broken/dashed linear indication represented a degraded weld seam between themechanical LLDPE sheeting. Figure 12. Laser weld not subjected to further testing. Broken/dashed linear indication Figure 12. Laser weld not subjected to further mechanical testing. Broken/dashed linear indication represented a degraded weld seam between the LLDPE sheeting.

represented a degraded weld seam between the LLDPE sheeting.

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each testing scenario and their shear point(s) where(e.g., mechanical Figure 13 13describes describes each testing scenario andrespective their respective shear(e.g., point(s) where testing is not required for quantification). mechanical testing is not required for quantification).

Figure 13. Shear and Bond Zone comparisons of 10 and 20 mm/s cross-head speeds at variable Figure 13. Shear and Bond Zone comparisons of 10 and 20 mm/s cross-head speeds at variable incident incident laser current (A). laser current (A).

In application of three layered based systems a delaminated (i.e., un-bonded) third layer is ideal. In application of three described layered based systems a delaminated (i.e., un-bonded) third layer is Specifically, the information in Figure 13 indicates that multilayered systems are applicable ideal. Specifically, the information described in Figure 13 indicates that multilayered systems are to this technology. By proper control of the linear energy density (vector speed x incident current applicable to this technology. proper control of the energy density speed incident (i.e., laser power)) the overall By depth of penetration canlinear be controlled. Thus,(vector providing an xadequate current (i.e., laser power)) the overall depth of penetration can be controlled. Thus, providing an system to develop multichannel and multi-layered laser welded LLDPE polymer systems. adequate system to develop multichannel and multi-layered laser welded LLDPE polymer systems. Specifically, in the developed system for three layered manufacturing processes at 10 and 20 mm/s Specifically, in8.5 theand developed system for three layered manufacturing at 10 20 mm/s are to be set at 10.5 A, respectively. At these specified zones, theprocesses laser system hasand successfully are to be set at 8.5 and 10.5 A, respectively. At these specified zones, the laser system has successfully welded two layers of the three layered systems. Amperage settings greater than those recommended welded two layers of the threethree layered systems. AmperageConversely, settings greater than those recommended will yield completely welded layered components. amperages settings below the will yield completely three layered components. Conversely, amperages settings below the recommendations maywelded fail to allow the top two layers to bond recommendations may fail to allow the top two layers to bond. 3.3. Mechanical Testing—Peak Load (lbf) 3.3. Mechanical Testing—Peak Load (lbf) Mechanical testing was performed on all sample components abiding similar criteria, to the Mechanical testing was performed on all sample components abiding similar criteria, to the energy density determination, were met. Typically, recorded mechanical data is resultant of an energy density determination, were met. Typically, recorded mechanical data is resultant of an average of three different peak load determinations. Specifically, all mechanically tested samples average of three different peak load determinations. Specifically, all mechanically tested samples resemble those described in Figure 11. Raw (i.e., non-welded LLDPE) samples set the baseline for the resemble those described in Figure 11. Raw (i.e., non-welded LLDPE) samples set the baseline for analysis. Maximum sustained peak loads for each experimental condition (10 mm/s—two layers, the analysis. Maximum sustained peak loads for each experimental condition (10 mm/s—two layers, 10 mm/s—three layers, 20 mm/s—two layers, and 20 mm/s—three layers) are displayed in Table 3. 10 mm/s—three layers, 20 mm/s—two layers, and 20 mm/s—three layers) are displayed in Table 3. Representative values shown indicate maximum peak load at the experimental setting just after the Representative values shown indicate maximum peak load at the experimental setting just after the shear zone (no-bond region). Thus, any incident current greater than the critical shear zone limit shear zone (no-bond region). Thus, any incident current greater than the critical shear zone limit amperage will provide, at a minimum, this corresponding peak load. Furthermore, for comparative amperage will provide, at a minimum, this corresponding peak load. Furthermore, for comparative purposes, typical load-extension curves are displayed in Figure 14. Samples were subjected to a purposes, typical load-extension curves are displayed in Figure 14. Samples were subjected to a cross-head displacement rate of 1 in/min with the maximum allowable extension set at 1 in. The test cross-head displacement rate of 1 in/min with the maximum allowable extension set at 1 in. The test was completed if a break/rupture was measured and/or the maximum cross-head displacement was completed if a break/rupture was measured and/or the maximum cross-head displacement was reached. was reached.

Sample Material LLDPE LLDEP LLDPE LLDPE Machines 2016, 4, 14 LLDPE

Condition/Speed (mm/s) RAW 10 10 20 20

LLDPE Layers 2 3 2 3

Peak Load (±σ) (lbf) 26.6 (2.1) 19.6 (3.8) 25.3 (3.4) 25.7 (1.4) 25.4 (2.8)

Incident Current Setting (A) 5.5 8.5 9.5 11 of 14 10.5

Figure 14. 14. Typical Typical load-elongation load-elongation curves curves for for sample sample conditions conditions described described in Table Table 3. 3. Samples Samples were were Figure subjected to a cross-head displacement of 1 in/min. subjected to a cross-head displacement of 1 in/min.

4. Discussion Table 3. Maximum sustained peak load above shear point of LLDPE weld(s). The proposed welding system was shown to adhere multi-layered systems. Sustained peak load Condition/Speed (mm/s) LLDPE Layers Peak Load (˘σ) (lbf) Incident Current Setting (A) measurements of the resultant weld width(s) are equivalent to a virgin/raw LLDPE sample sheet. The LLDPE RAW 26.6 (2.1) experimental shear zones2 of the particular weld systems (e.g., 10 LLDEP trials have identified 10 19.6 (3.8) 5.5and 20 mm/s LLDPEspeeds coupled with 10 3 beam current). 25.3 (3.4) cross-head variable incident Quantification of the8.5 rigidity of two LLDPE 20 2 25.7 (1.4) 9.5 layered LLDPE systems, specifically the shear zone, allows for confirmation of a quality lap weld LLDPE 20 3 25.4 (2.8) 10.5 seam. Furthermore, shear zone identification in three layered systems determines the appropriate linear energy density for a given multi-layered system. 4. Discussion Mechanical property results describe a system in which a welded interface will perform The proposed welding system was shown to adhereComparison multi-layered Sustained peak similarly to that of its not welded raw/virgin counterpart. of systems. the representative data in load measurements of the resultant weld width(s) are equivalent to a virgin/raw LLDPE sample Table 3 shows, at a maximum, the overall degradation in sustain peak load (lbf) is 26.32% (10 mm/s sheet. The experimental have mechanical identified shear zones load of the particular weld systems (e.g.,just 10 and two layers of LLDPE).trials Collected data (peak (lbf)) is representative of a weld and 20 mm/s cross-head speeds coupled with variable incident beam current). Quantification beyond the potential shear zone. These data points described are theoretical operating minimums of of the of two layered LLDPEThus, systems, the shear zone, confirmation of a the rigidity polymer welding system. an specifically adequate safety factor is allows to befor applied to further quality lap weldoperations seam. Furthermore, shear identification in three systems determines manufacturing to ensure, at azone minimum, the peak load layered of the theoretical minimumthe is appropriate linear energy density for a given multi-layered system. achieved. For example, in a three layer 20 mm/s condition the incident current setting should be Mechanical describe a system in which a welded willofperform similarly 10%–20% largerproperty than theresults recommended minimums of 9.5 A. Theinterface clustering the mechanical to that of its not welded counterpart. of the representative datamechanical in Table 3 property results suggestraw/virgin that a weld interface Comparison does not significantly impact the shows, at a maximum, the overall degradation in sustain peak load (lbf) is 26.32% (10 mm/s and performance of the polymer in this test scenario. Various energy densities have been shown to two layers of LLDPE). data (peak load (lbf)) weld just produce quality welds.Collected Refer to mechanical Table 3, a LLDPE polymer weld is at representative 10 mm/s with of 8.5a A current beyond the potential shear zone. aThese pointsload described operating minimums of (0.425 Coulombs/mm) produces peak data sustained of 25.3are lbf.theoretical Comparatively, a LLDPE polymer the polymer welding system. Thus, an adequate safety factor is to be applied to further manufacturing weld at 20 mm/s with 10.5 A current (0.525 Coulombs/mm) produced a peak load of 25.4 lbf. Therefore, operations to ensure, at a minimum, peak load the theoretical minimum achieved. example, a linear energy variance of 21.95%the produces a of LLDPE weld seam where isthe averageFor mechanical in a three layer 20 mm/s condition the incident current setting should be 10%–20% larger than the property variance is relatively small at 0.39%. recommended minimums of 9.5 A. The clustering of the mechanical property results suggest that Larger scaled application(s) are possible with large X-Y build platforms. Increased productivity a(i.e., weld interface does not significantly impactby theimplementation mechanical performance of laser the polymer in this speed of manufacturing) is achievable of multiple head systems. test scenario. Various energy densities have been shown to produce quality welds. Refer to Table 3, Situations and models described in these experiments utilize a single laser source head, whereas amultiple LLDPE systems polymerwould weld at 10 mm/s 8.5part A current (0.425 Coulombs/mm) peak allow for a with similar (laser paths) to be replicated produces during thea same sustained load of 25.3 lbf. Comparatively, a LLDPE polymer weld at 20 mm/s with 10.5 A current (0.525 Coulombs/mm) produced a peak load of 25.4 lbf. Therefore, a linear energy variance of 21.95% produces a LLDPE weld seam where the average mechanical property variance is relatively small at 0.39%. Larger scaled application(s) are possible with large X-Y build platforms. Increased productivity (i.e., speed of manufacturing) is achievable by implementation of multiple laser head systems. Sample Material

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Situations and models described in these experiments utilize a single laser source head, whereas multiple systems would allow for a similar part (laser paths) to be replicated during the same manufacturing cycle. Similarly, increased laser power allows for increased manufacturing speed [35]. High power laser systems have been shown to be valuable in the current scope in laser welding applications [4]. Thus, quick-high power systems are achievable. In addition, large-scale mass manufacturing is possible with this process using roll-to-roll technology [7]. Furthermore, numerous direct applications are available for implementation of the proposed system. For example, the system can be used for additive manufacturing of vehicle heat recovery ventilators for the automotive industry [36], industrial heat exchangers [6,37], heat exchangers for solar water pasteurization [38], hermetic thermoplastic medical device encapsulation [39], bio-microfluidic channels in transparent polymer materials [40], and consumer goods packaging [41]. The polymer laser welder described is ideal for rapid prototyping. For example, the new floating photovoltaics (FPV) can be combined with aquaponics to makeaquavoltaics (AV), which use thin film flexible substrate based solar photovoltaic (PV) modules to float on water, yet designs have largely been untested [42,43]. The low mass allows a significantly diminished supporting structure and the flexible nature of the system allows for designed yield to oncoming waves while maintaining electrical performance [44]. This enables FV to take advantage of the superior net energy production of thin film PV materials like amorphous silicon [45,46]. To maintain the flexibility and long term structural integrity of the module, thin-films should be encapsulated by a polymer with high transparency, low rigidity, and be waterproof [42], and during the encapsulation process air pockets or voids can be purposefully introduced to increase buoyancy without increasing mass [44]. The system described in this article can be used to test various thin-film FPV designs by prototyping them at minimal costs. 5. Conclusions Modification of a standard RepRap system has allowed for the development of a novel laser welding system and weld protocol. Previously custom developed Franklin firmware has provided an intuitive graphical user interface in which to control the welding system. Mechanical property analysis and weld width characterization of representative LLDPE polymer welds have shown applicability to multiple industrial, medical, and end user/consumer systems. Results have shown success in both dual (two layer) and multilayer (three layer) systems. Proper incident laser power and machine parameters (i.e., linear energy density) have been determined. Application of these parameter sets will provide user(s) with the fundamental LLDPE requirements to produce adequate mechanical polymer welds. Incident laser current (A) has been shown to display a positive linear relationship with relative weld width data. Thus, weld width increases as incident laser current increases. However, increased laser current did not show any increase and/or degradation to the LLDPE weld mechanical properties. Acknowledgments: Financial support of this work by U.S. Department of Energy ARPA-e, Award # DE-AR0000507 is greatly acknowledged. The authors would also like to acknowledge technical assistance from Gerald Anzalone and Paul Fraley and helpful discussions with David Denkenberger, James Klausner, and Geoffery Short. Author Contributions: J.L. performed all polymer welding experimentation, including data analysis and interpretation. S.V.D. provided guidance and materials selection. M.M.O. led the project and overall collaborative effort of all entities. J.M.P. conceived of the application and development of the designs and the experiments, and helped analyze the data. All authors were responsible for writing and editing the final manuscript. Conflicts of Interest: The authors declare no conflict of interest.

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