By Val A. Kagan Robert Bray
Engineered Applications & Solutions
Automotive parts made from thermoplastics often weigh and cost less than equivalent metal castings and stampings. Typically, these assemblies are welded together by heat from friction, hot plates, or radiant heaters. Although these methods produce welds of adequate strength, each has some drawbacks. For instance, vibration during frictional welding can damage installed electronic devices or the plastic pieces themselves. Moreover, part size and positioning accuracy are limited because one part must move relative to the other during the procedure. Melt residue deposited on hot-plate tooling requires frequent cleaning. And the relatively high temperatures required for noncontact, radiant heating and some laser techniques, can degrade materials at the weld joints or overheat sensitive electronics.
But a new experimental laser-transmission welding (LTW) technique eliminates many of these shortcomings yet produces welds of near or equal strength. Here, laser light transmits through an optically transparent (at a particular wavelength) thermoplastic part and is absorbed by a nontransmitting, adjoining part to be welded. Heat from the beam melts and fuses the two materials at the interface or weld-plane zone. The amount of energy deposited and therefore weld temperature is controllable with laser power, optics, and sweep rates. Often of greater influence, however, is the thermo-plastic material itself.
The Beer-Lambert law gives the transmission of light, T, through a material:
T = 10 -A
where A is the absorbance defined by:
A = a l
Here, a is the material's absorption coefficient (mm 2 /gm), is the density (gm/mm 3 ), and l is the thickness (mm). When absorbance is relatively low, as is the case for polycarbonates in the near-infrared (NIR) region, transmission can be approximated by:
T = 1 - A - R
The additional term, R, is reflectance from the material surfaces. Glasslike polymers such as polycarbonates, only specularly reflect at the two surfaces about 7% of the incident beam intensity. But many thermoplastics including nylon 6 are not glasslike (homogeneous), either because of additives or inherent crystallinity. In these cases, surface specular reflections still happen but are minor compared to diffuse scat-tering about the polymer matrix. In other words, a substantial portion of transmitted light may deflect away from the incident beam direction reducing the energy density at the weld interface. Furthermore, the effective pathlength may be greater than the actual part thickness which makes the sample absorb more and decreases transmission as well.
TRANSMISSION AND ABSORPTION
The amount of diffuse scattering depends on additive composition and concentration, particle size, and its refractive index relative to the base polymer. In general, laser transmission drops with rising levels of reinforcements such as mineral and glass fiber, fillers, colorants, and impact modifiers.
For example, the short fiberglass reinforcements and mineral fillers commonly used in nylon thermoplastics to boost strength also increase effective pathlength and promote light scattering. Of the two additives, however, mineral fillers more greatly blocks transmission. For example, the effective pathlength of nylon 6 with 40% (by weight) mineral filler is four times that of the same material with 45% fiberglass reinforcement. This is because the mineral filler's relatively smaller particle size leads to more light scattering centers. Scattering reduces the available energy at the weld interface, spreading it out over a wider area. So, highly filled materials require substantially more laser power for LTW than natural nylons.
Impact modifiers and fire retardants, depending on type, can block transmission even more than fillers and reinforcements for a given amount. The small inhomogeneities introduced by impact modifiers scatter light more effectively than glass fibers resulting in transmission losses of about 50% relative to natural nylon 6. Flame retardants can reduce transmission by 60 to 70%, although the mechanism is not well understood.
Nylon 6 thermoplastic blended with about 1% by weight of blue or yellow pigment transmits 75 to 85% less near-infrared (NIR, 0.8 to 1.1 m) light than the uncolored polymer. However, similar concentrations of red colorants and pigments do little to NIR transmission. On the other hand, the same amount of carbon black blocks transmission entirely, desirable for light-absorbing components.
The optical properties of the thermoplastic and additives not only impact transmission, but also dictate which laser type is appropriate for the job. The three most commonly used lasers for welding plastics are CO2 (10.6 m), Nd:YAG (1.06 m), and diode (808±10, 830±10 or 940±10 nm).
Because most plastics, including nylon 6, strongly absorb IR wavelengths (10.6 m), the use of CO2 lasers is limited to samples on the order of 20 m thick. In contrast, NIR absorbance is several orders of magnitude lower, letting millimeter-thick parts be welded with either Nd:YAG or diode lasers. However, material thickness is an issue regardless of wavelength. NIR transmission for uncolored or red nylon 6, for example, drops monotonically from 85 to 42% as thickness grows from 0.8 to 6.25 mm. In contrast, transmission through yellow, green, and white nylon 6 drops precipitously from about 50 to 2% over the same thickness range.
Weld strength is a function of four parameters: the material to be welded, the laser, weld joint design, and clamping pressure. Obviously, welds made on polymers with relatively higher tensile strengths will be stronger than welds on weaker ones. And weld quality is determined largely by the laser power level, scan rate (welding speed), beam focus, and beam diameter. Parts should be designed such that welds get enough laser energy. Here, material properties and part geometry are key metrics. Equally important is that the materials to be welded firmly contact one another during the procedure. Some research suggests a clamping pressure of about 2 MPa.
Assuming welds are of good quality, weld tensile strength is expressed as a percentage of the material's bulk strength by what are called weld factors. One factor, wm, is based on the strength of the unfilled material, and the other wpl, of the filled material. This is an important distinction because the addition of glass fiber or other filler can dramatically boost plastic strength.
Although this study focuses on nylon 6, other researchers have successfully joined nonreinforced, semicrystalline plastics including polypropylene and high-density poly-ethylene with light from a Nd:YAG solid-state laser. Here, tensile strength of a butt joint is equal to that of the base polymer.
Experimental LTW rig
This laser-transmission welding system consists of a robotically controlled high-power diode laser and a pneumatic clamping mechanism for holding workpieces at pressures to about 2 MPa. Parts are clamped firmly together during the operation to aid heat transfer. Both the thickness and composition of the transmitting part influence the laser-beam width and energy density at the joint interface or weld bead. In general, thicker parts and plastics with more additives reduce energy density for a given beam diameter and power. For optimal results, the laser beam must be properly aligned/oriented with respect to the weld-bead axis and weld-joint plane during the welding process.
LTW joins two thermoplastic materials one optically transparent at the laser wavelength, and the other optically dense. The laser beams through the transparent part with minimal losses via an optical fiber and gets totally absorbed within the surface layer of the dense part. The subsequent heating melts and fuses the materials at the interface.
LASER SYSTEM SPECS
SOME ADVANTAGES OF LASER-TRANSMISSION WELDING
- Accurate, noncontact, heat transfer at the weld interface and better control of welding temperatures.
- Parts can be welded in the same orientation as the assem-bled product.
- Minimal limitations on component geometry.
- Rapid welding speeds.
- No vibration permits welding of sensitive electronic and medical components.
- More localized heating reduces melt flash at weld joints.
|Tensile strength of T-shaped, butt-joint welds in nylon 6 thermoplastics|
WEIGHT % GLASS FIBER
MATERIAL TENSILE STRENGTH (MPa)
WELD TENSILE STRENGTH (MPa), VIBRATION WELDING
WELD TENSILE STRENGTH (MPa), LTW
N / A
|The welds were tensile tested until failure at 23°C according to ISO 527 procedures. The weld flash bead was not removed.|
|Comparison of weld strength by technology|
WELD FACTOR ƒWM
WELD FACTOR, ƒWPL
|Hot plate|| |
|Linear vibration|| |
|Orbital vibration|| |
|For 33% by weight fiberglass-reinforced nylon 6 at 23°C, dry as molded.|