This two-piece, injection-molded automotive intake manifold   made of nylon 6 has its two halves vibration welded together.

This two-piece, injection-molded automotive intake manifold made of nylon 6 has its two halves vibration welded together.


Fibers at the knit line oriented 90° to applied   loads don't add strength. In fact, knit lines have about the strength   of unfilled resin.

Fibers at the knit line oriented 90° to applied loads don't add strength. In fact, knit lines have about the strength of unfilled resin.


Moldflow software from Moldflow Corp., Wayland, Mass.,   simulates liquid plastic flowing into an injection mold. This helps designers   predict where knit and meld lines are likely to form. Areas colored red   take the least time to fill, and blue ones, the longest.

Moldflow software from Moldflow Corp., Wayland, Mass., simulates liquid plastic flowing into an injection mold. This helps designers predict where knit and meld lines are likely to form. Areas colored red take the least time to fill, and blue ones, the longest.


Knit lines form when two opposing flows meet. Meld lines   form at the interface between two parallel flows.

Knit lines form when two opposing flows meet. Meld lines form at the interface between two parallel flows.


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.

More designs use nylon-based plastics
Less weight and lower production costs are two reasons why the use of nylon for automotive under-the-hood components has grown from 87,500 tons in 1999 to 165,000 tons today, and is expected to hit 230,000 tons by 2005 (North America and Europe). Fiberglass and mineral fiber-reinforced nylons allow automakers to build welded fluid reservoirs, resonators, covers, and chassis components that can weigh 40 to 55% less than stamped steel or cast equivalents.

Similar weight reductions are possible for various power tools and lawn and garden gear. Fiberglass-reinforced and filled nylons also help reduce waste because they mostly retain their mechanical properties, even after several remolding/regrind cycles. More than a dozen classes of nylon resins (polyamides) are available today.


Influence of fillers on knit and weld line strength
GLASS FIBER, wt. %
MINERAL FIBER, wt. %
IMPACT MODIFIER, wt. %
PLASTIC TENSILE STRENGTH, MPa
KNIT LINE TENSILE STRENGTH, MPa
WELD LINE TENSILE STRENGTH, MPa
0
0
0
82.0
85.5
81.0
0
40
0
90.0
77.0
81.5
0
0
4
54.0
51.6
6
0
0
85.0
83.1
14
0
0
125.0
89.1
90.7
15
25
0
126.0
90.0
84.8
25
0
0
160.0
90.2
33
0
0
185.0
89.2
85.6
33
0
5
152.0
62.0
45
0
0
208.0
82.1
50
0
0
220.0
83.3
80.5
63
0
0
229.0
83.8
79.2
At 23°C, dry as molded nylon 6-based plastics and optimized processing conditions. Tests performed at Honeywell International, Engineered Applications & Solutions, Morristown, N.J.

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