Injection Molding Defects And The Moulding Process

The injection moulding process is so complex that it is often quite difficult to pinpoint exactly what is causing the molding defects. Also, because deadlines are always short and most companies operate with a JIT philosophy, there is a tendency to look for simple answers to complex problems.

On the other hand, the trouble shooting process is generally geared toward beginning with simple causes and proceeding to more the complicated. Many times the obvious is overlooked in the attempt to find something more involved or technical.

From a mold makers viewpoint and experience, a lack of proper venting is the most common cause for injection molding defects. The short shots, unfilled corners, burned areas, sink marks, flow marks, jetting, weld lines, flash, and warping all have distinct characteristics and unique causes, but often it is venting that is lacking.

Most mold makers are very shy about venting, for fear of flash. Also, many designers fail to properly determine the correct vent location and depth, leaving it up to the mold maker. He may or may not have access to the material being used in the plastic part, so he probably guesses at the location and depth. Plus, because of the fear of flash, he will always stay on the conservative side and make it shallow.

Runners are also commonly overlooked as a venting area. When you think about it, the gas buildup and air found in the runners has to go someplace, and it ends up in the molding area. Properly vented runners alleviate many problems that seem to be difficult to solve.

Improper gate placement and size is also a huge factor in molding defects. Without proper sizing and location, the part is subject to all kinds of problems that simply will not go away until the remedy is applied. Engineered plastics are very finicky in their molding demands and the designer must find out the proper characteristics in order to produce a quality product. Guessing is very much a thing of the past and is very costly in time, labor and material.

Adding adequate radii to runners, gates and sprues will also minimize defects. The turbulence caused by sharp corners will result in a defective part. This is easily remedied by simply taking the time to add a radius where needed. It is surprising how many molds are made with these corners dead sharp.

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A tunnel gate, for example, needs a radius at the parting line in order to avoid flaking when the gate is ejected. You often see molds with massive amounts of plastic flaking on the parting lines, causing more damage to the molding shut-offs and surface finish.

This is so easy to fix, yet the mold just keeps running because the order must be filled, now! Such problems lead to other problems and soon there is significant damage to the tool steel. A plastic injection mold is a precision piece of tooling, made by very skilled toolmakers, CNC machinists, EDM operators, designers, polishers, WEDM operators, surface grinders and a host of suppliers for the various mold components.

Overlooking these simple details is foolish in the long run and taking care of the easy things is much less expensive than putting it off until later.