Most Common Injection Moulding Defects and How to Fix Them

September 22, 2017
Most Common Injection Moulding Defects and How to Fix Them

When an injection moulding production run is optimised, manufactured products are perfectly formed and ready for market. The process continues in this manner for as long as the feeder hopper contains plastic pellets, meaning productivity is an assured factor. Unfortunately, this optimised manufacturing scenario is susceptible to speed bumps, defects that corrupt the structure of a solidifying plastic object. We’re switching to troubleshooting mode for this article so that these issues can be properly addressed and solved. Let’s begin with structure-weakening bubbles in the plastic.

Bubbles and Blisters

Moisture and gaseous byproducts become trapped in the melting plastic. The ejected product has blistered surfaces and small bubbles are spotted inside the hardened material. The blisters spoil the surface finish while the bubbles undermine mechanical strength. In addressing this injection moulding defect, eliminate moisture and troubleshoot causes for gaseous byproducts. If necessary, adjust the process by reducing melt temperature.

Fixing Short Shot Problems

If the cavity isn’t fully filled, the product can’t assume its final form. Short shot is the technical term that describes this phenomenon. There may be a blockage or bottleneck in the injection assembly, but gas venting issues can also cause this error, with back pressure issues stopping the plasticized material from filling the cavity. Resolve the problem by inspecting input and output stages. Is the injected material flowing freely? Are gases leaving the air traps? Apply Occam’s Razor, a principle that looks for obvious solutions. Injection temperature should be evaluated alongside material flowability checks.

Eliminating Flash

This defect sounds like it should be assigned to a luminance problem in a light fixture, but it’s a common injection moulding term, one that relates to excessive material flow. The plastic fills the cavity, which is what we want, but it then continues to flow and overfill mould channels. The defect is often seen in finished production lines as a lip surrounding the ejection point, or there’s a thin seam penetrating the mould plates. This “lip” can permanently damage the mould when the plasticized material is injected. This event causes pressure to “blow” the clamped plates open, reducing the process to a hot mess. Reduce pressure and troubleshoot the problem. Contaminants could be preventing the metal parts from sealing properly. Additionally, the poorly mated moulding plates may need to be retooled and replaced if they’re worn.

The list doesn’t end here, but we can resolve these and other issues by implementing an intelligent troubleshooting program, one that effectively divides defects into tooling, material, and temperature-related puzzles. And, once properly identified, all problems quickly submit to a sound repair or replacement strategy.