Originally Published MDDI May 2001PLASTICS WELDINGInfrared Welding of Polymers

May 1, 2001

15 Min Read
Infrared Welding of Polymers

Originally Published MDDI May 2001

PLASTICS WELDING

Infrared Welding of Polymers

Using a transmitting substrate as a waveguide enables
the creation of difficult-to-fabricate polymer joints.

Robert A. Grimm

Through-transmission infrared (TTIR) welding of plastics is a process in which light of suitable wavelength is transmitted through a transparent substrate that is in contact with an absorbing one. Energy absorption at the interface heats and melts both the absorbing substrate and, through conduction, the transmitting one that is in contact with it.1 The light that is used is mainly in the near-IR range, from 750 to 3000 nm. Common sources of such IR radiation include Nd:YAG lasers (1064 nm), diode lasers (808 nm and other frequencies), and xenon and quartz halogen lamps (from 300 to 4000 nm).

The IR energy can be delivered to the weld zone using a fiber-optic bundle. The fibers, commonly made from quartz or optically clear polymer, can carry the light energy around curves and for substantial distances. These properties are derived from the phenomenon of internal reflection. Light rays entering a substrate continue to be reflected by the walls of the substrate if the incident angle is smaller than the critical angle. This value, the critical angle, follows the relationship:

 

Substrate refractive index (ams) = 1/sin C

 

where a is air, s is the substrate (such as polymer, quartz, or liquid), and C is the critical angle for that substrate.

  0105d158a1.jpg

0105d158a2.jpg

Figure 1. Curved strips with black acrylic on the ends being held in position with binder clips. The photo on the left shows a curved strip viewed through an IR viewer as a defocused Nd:YAG laser beam is aimed at one end.

This article describes a number of less-common joint types that are made based on the concept of internal reflection. In most cases, the process involves transmission of light from an Nd:YAG or diode laser through various substrates, using the substrate as the waveguide for the light energy. The concept is detailed in a soon-to-be-published report on studies involving quartz-halogen sources and quartz light pipes.2 Results of the study suggest that it may be possible to weld around corners, or at least around curves and in remote locations that could be hard to reach with other joining processes. So long as the angle of bending did not exceed a critical angle (theoretically 44° for quartz), light was transmitted efficiently through the rod. A study of extinction coefficients shows that optically clear polymers and some semicrystalline ones can transmit light through a considerable thickness at the near-IR wavelengths.3

These findings suggest that even translucent substrates might be used as waveguides for the IR radiation welding of polymers. This would enable the creation of welds at locations that are significantly remote from the point where the light first enters the transparent substrate. One of the intentions of the study reported on below was to spark some new applications and to generate additional ideas on the potential of this joining method.

 

SUBSTRATES AS WAVEGUIDES

 

Optically clear polymers, such as acrylic, polycarbonate, and polystyrene, transmit light energy of suitable wavelengths with very low loss levels. In this study, clear acrylic strips about 200 mm long were heated, softened, and manually bent into a curved shape. Figure 1 shows one of the curved strips viewed through an IR viewer as a defocused Nd:YAG laser beam is aimed at one end. Information gained through welding demonstrated that the laser beam is not widely scattered in optically clear materials and, to make a weld along the whole end of the strip, the beam had to be tracked along the other end of the curved strip. The figure also shows that to accomplish this, a sheet of black acrylic was placed against the end of the curved strip and held in position with binder clips. Passage of a laser beam along the other end of the curve created a weld.

0105d158b.jpg

Figure 2. Polyvinyl chloride sheets joined in a T-joint using black vinyl plastisol and an Nd:YAG laser.

Welding of plastics normally requires application of force to press the two melted parts together, resulting in weld flash. In this case, as is common with many of the through-transmission processes, the weld force is created by thermal expansion of the polymers, allowing clean joints to be obtained.4

As shown in Figure 2, polyvinyl chloride sheets (PVC, 0.25 mm thick) were joined in a T-joint by the welding process. A line of black vinyl plastisol ink was preapplied onto one side and stainless-steel bars were clipped onto the other sheet near the edge to be welded. The steel bars and film were then placed upright on a line of plastisol ink. The beam from an Nd:YAG laser was aimed at the free (upper) edge of the clipped sheet and moved in a linear manner along that edge. A weld was created at the interface on the other end of the sheet. Because the stainless-steel supports prevented the laser beam from reaching the weld, energy had to be delivered through the vertical film. Weld speed depended on applied energy and the beam diameter (energy density).

0105d158c.jpg

 

Figure 3. Acrylic rod joined to a black acrylic sheet.

This basic approach was used to join an acrylic rod (0.6 m long and 6.4 mm diam) to a sheet of black acrylic (see Figure 3). One end of the rod was placed against the sheet of black acrylic, while an unfocused beam from an 808-nm diode laser (45-W capability) was aimed into the other end. A weld was created at the rod-sheet interface in approximately 5 seconds. The weld time could have been greatly reduced had special care been taken to focus the beam on the end of the rod. Welding should be possible at much greater distances. Clear tubes were welded to sheets of black acrylic that had a hole the size of the tube ID (approximately 6 mm) in the same manner. A weld time of 10 seconds at a power level of 15 W was used.

It was more difficult, but still possible, to weld a Teflon PFA tube to a sheet of black Teflon PFA. The reference on extinction coefficients showed that a number of semicrystalline polymers had surprisingly high levels of light transmission. Low-density polyethylene, polypropylene, and Teflon PFA were among these. Table I identifies a few of the light-transmission equations to provide some perspective.

 

Polymer

Nd:YAG
(1064 nm)

Diode
(810 nm)

Quartz-Halogen, Unfiltered (300 to 4000 nm)

Acrylic(crystal clear)

Y = <0.009x

Y = <0.009x

Y = 0.025x + 0.02

Teflon PFA

Y = 0.036x + 0.13

Not measured

Not measured

LDPE

Y = 0.052x + 0.05

Y = 0.048x + 0.25

Y = 0.08x + 0.32

Polyoxymethylene

Y = 0.14x + 0.42

Y = 0.1x + 0.53

Y = 0.14x + 0.73

Where Y = Ln(incident light/transmitted light), x is sample thickness (mm), coefficient of x is the extinction coefficient (mm-1), and the constant is a scattering, reflection, or thin-layer absorption level.

 

Table I. Light-transmission equations for selected polymers.

0105d158d.jpg

 

 

Figure 4. A Teflon PFA tube observed through an IR viewer during the welding process.

0105d158e.jpg

 

Figure 5. Examples of welded Teflon PFA tubing constructions.

In several test runs, a tube composed of natural Teflon PFA (6.4 mm OD and 3.2 mm ID) was pressed into a 6.4-mm hole in an aluminum sheet. The aluminum was used both as a fixture and to shield the black sheet from stray radiation. An Nd:YAG laser beam defocused to a diameter of approximately 6.4 mm was then aimed at the end of the tube in the aluminum. When the tube length was 50 mm, 30 W of laser power produced a sound weld in a few seconds. With a tube length of 115 mm, 50 seconds was needed. No effort was made to optimize this process. A portion of the beam might have been reflected along the inside of the tube. If that was the case, however, the tube should have welded in a shorter time. Figure 4 shows a Teflon PFA tube during welding as observed through an IR viewer, and additonal welded samples are shown in Figure 5.

WELDING TUBES THROUGH A BULKHEAD

 

In other test runs, short lengths of high-density polyethylene (HDPE) tubing (9.5 mm OD) were cut from both natural and black stock. These were inserted into predrilled black or natural HDPE sheets in a press fit. The pieces were then mounted in a chuck and the joint area rotated past an Nd:YAG laser beam at a low rate (approximately 60 rpm). Figure 6 shows the three configurations examined.

0105d158f.jpg

 

 

Figure 6. The three HDPE tubing configurations that were examined during the tests with bulkhead interfaces.

In the first configuration, a natural HDPE tube was inserted into a 3.2-mm sheet of black HDPE. Because a laser beam will not pass through the black bulkhead material, it was passed through the tube wall on the side opposite of active welding. Although the method was successful, some reflection was found to occur from the surface, and a ring of roughened surface caused by melting encircled the tube and extended some distance from it (see Figure 7).

 

0105d158g.jpg

 

Figure 7. Melting caused the formation of a roughened surface around the tubing.

In the second configuration, the colors were reversed, and a black tube was pressed into a hole in a bulkhead made from natural HDPE. As in the first case, the tube and bulkhead were rotated past a defocused laser beam that was aimed to span the desired weld interface. In both cases, successful welds were made in approximately 10 seconds with 50 W of power. Figure 8 shows sections of some of these joints.

0105d158h.jpg

 

Figure 8. Black tubing welded to a clear bulkhead materials.

In the third weld configuration, a tube made from natural HDPE was pressed through a black bulkhead until it projected about one tube diameter past the end of the bulkhead. A metal washer was then pressed over the projecting end of the tube and seated against the black bulkhead. This arrangement was then rotated past a laser beam that was defocused to a spot the size of the tube wall and that was aimed straight onto the end of the natural tube (see Figure 10).

The tight fit of the washer should have prevented the beam from directly contacting the interface between the two parts being welded. Surprisingly, a weld was formed that was approximately equivalent to those used for the other two cases. Because direct welding with the laser beam seems unlikely, the most probable explanation is that light scattering as it travels along the tube is actually involved in melting the black bulkhead, creating a weld.

 

FUTURE POSSIBILITIES

 

0105d158i.jpg

 

Figure 10. The third weld configuration entailed pressing a metal washer over the projecting end of the tube, seating it against the bulkhead, and rotating the tubing past a laser beam.

These findings suggest that there are a significant number of potential directions and applications for internal reflection welding technology, many of which could be pertinent to medical manufacturing. Possible areas for future study include:

 

  • Welding at great distances and through curved substrates.

  • Creation of delicate, thin-walled structures by welding sheets against other sheets.

  • Welding of microtubes into manifolds.

  • Fabricating optical conduits using welding processes that operate at a wavelength that is outside the planned-use wavelength. For example, pigmented layers that are weldable at one wavelength but transmit at another could be developed. In addition, clear rods could be joined end to end with 810-nm light using transparent green films as the weld layer (green acrylic films absorb 810-nm light but are much less sensitive at 1064 nm, so the joint would be nearly transparent to 1060-nm light).

  • Development of weld layers that fade or bleach on welding with, for example, ultraviolet light sources of suitable wavelengths.

  • Using other waveguide shapes and mirrors to expand versatility. In some experiments with acrylic tubes, aiming the laser beam so it spiraled down the tube wall produced a more-uniform weld at the other end.

  • Further exploration of welding by scattered light. The finding that tube-to-bulkhead welding is possible using light that is traveling inside the tube wall was surprising. If this mechanism is attributable to scattering, it could have wider applicability to indirect welding.   No doubt many more applications will be found for this technology following additional research and development efforts.   ACKNOWLEDGEMENT   The author would like to acknowledge the contributions of Kevin Hartke, Michael Fallara, and John Robinson.   REFERENCES   1.RA Grimm, "Through-Transmission ," in Proceedings of the Society of Plastics Engineers Annual Technical Conference (Indianapolis, IN: SPE, 1996), 1238. 2.RA Grimm, TTIR--Use of Light Pipes and Development of TTIR Welding Tools, EWI Cooperative Research Report, in press. 3.RA Grimm, Laser Welding of Plastics-- Extinction Coefficients, EWI Cooperative Research Report (Columbus, OH; Edison Welding Institute, 2001). 4.RA Grimm, "Methods for Making Nearly Invisible Welded Joints in Clear Polymers," in Proceedings of the Society of Plastics Engineers Annual Technical Conference (Orlando, FL: SPE, 2000), 1153.  

    Robert A. Grimm, PhD, is a lead research engineer in the microjoining and plastics section of the Edison Welding Institute Technology Department (Columbus, OH).

     

    Copyright ©2001 Medical Device & Diagnostic Industry

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