The weak link in reinforced nylons

Nylon thermoplastics serve in a variety of demanding applications from automotive air-intake manifolds and bumpers to appliances and power tools. Injection-molded nylons tend not to warp when welded, making them ideal for assemblies with complex geometry. Nylon also remains rigid at near-molten temperatures and has predictable shrinkage rates so parts tend to eject easily from tooling and can be built to close tolerances. A relatively low viscosity lets it flow easily into complicated mold shapes including those with thinner sections. And thinner-walled parts lower cooling and cycle times and cut product weight.

But simply having the ability to fill every mold nook and cranny with plastic or weld two molded parts together isn't enough. Melt flows must also be uniformly distributed and oriented, especially when using fiber-reinforced plastics. In this regard, injection molding and welding share something in common: knit lines. Although the mechanisms responsible for their formation are different, the results are similar.

Knit lines in molded parts
Liquid plastic (melt) flowing around obstacles in a mold tool, such as inserts, ribs, cores, etc., produces what are called knit and meld lines. Knit lines (planes) form wherever flow fronts meet from opposite directions, and meld lines, from the same direction.

Knit line count is determined by:

N = G + Co - 1

where N = the number of weld lines, G is gate count, and Co = the number of shutoff cores or pins. Knit lines are generally of greater concern because they're mechanically weaker than meld lines and can be significantly more so than the bulk material. Knit-line tensile strength for nonfilled nylon is roughly equal to or about 17% less than that of the bulk material. The same resin filled with 30% (by weight) glass fiber, in contrast, may lose 50% or more of its strength at knit lines. And mechanical strength doesn't improve with additional or stronger fibers.

Such a precipitous drop in strength around knit lines comes from stress being concentrated at sharp, V-shaped notches. Notches form when fibers orient orthogonal to the principal melt flow. This, in turn, promotes incomplete molecular entanglement or diffusion and even micro voids. At gates (injection sites), fibers arrange randomly then align with the principal flow. When flow fronts meet, fibers that turn 90° to the principal flow don't add strength. In fact, test specimens (33 wt. % glass fiber nylon 6) show 50 to 60% less strength in the transverse direction.

Also promoting these fissures are impact modifiers. Impact-modified plastics when injected into a mold — especially at excessively high melt temperatures — exhibit what is called "fountain flow." Here, the additive doesn't reach the frozen wall layer directly and instead flows first down the center of the mold cavity to the melt front. This can alter flow direction and orientation of polymer molecules and fibers to favor V-notch formation.

Boosting mold and melt temperatures — below levels that degrade the polymer — promote slower cooling and in most cases improve strength. Although melt temperature effects dominate, excessively cool mold walls can too rapidly solidify liquid plastic, producing skins with lower crystallinity than at the slower-cooling core. Raising mold temperature, filling molds faster, eliminating release agents, and packing charges to higher pressures all can strengthen knit lines.

Still, parts built from filled thermoplastics (fiber-glass reinforcements, fillers, impact modifiers, etc.) should have their allowable working stress derated accordingly. Moreover, surfaces bearing higher loads should not contain knit lines. The same holds true for welded assemblies.

Weld line formation
Weld lines closely resemble knit lines in that they form when two melt flows meet. The melting, in this case, is confined to the component interface or weld line. Heat for the process comes from friction (linear or orbital vibration, spin, or ultrasonic), contact with a hot plate, or laser light. Infrared-laser transmission welding is relatively new but expected to grow rapidly, whereas other techniques such as linear vibration welding are already widely used.

As with injection molding, linear vibration welding has a number of adjustable parameters, all of which may affect weld integrity. These include amplitude, clamp-and-hold pressure and duration, and meltdown. Raising weld amplitude and lowering pressure boosts weld-line tensile strength. Increasing the melT-down or inter-phase thickness improves tensile strength as well. Similar improvements come with higher melt temperatures. However, oscillation shape and direction have no measurable affect on weld mechanical properties.

Many thanks to Dr. Val Kagan, Honeywell International, Engineered Applications & Solutions, Morristown, N.J. for his help with this article.