Defects in Plastic Molding

July 11, 2022

Injection moulding is an effective manufacturing process for making plastic products. A high pressure injector squeezes molten thermoset or thermoplastic resin into a pre-cast steel die, where the resin is allowed to set and form into a proper state. It is an efficient mode of production that can produce long and short runs to tight tolerances. Depending on the type of plastic or foam used, the material can range from very durable to soft and malleable.

However, as with any manufacturing process, problems can arise due to machine, material and human error. Troubleshooting common errors can alleviate the frequency of these situations, however, and being prepared can foster a smoother manufacturing run.

Common Defects
A blister is a raised area on the plastic surface, very similar to the medical condition of the same name. It is generally the product of too much heat on the tool or by inadequate cooling or venting. Depending on the type of tool, you can also find areas where full coverage is not working. For instance, if the injector has a flow pattern issue, it might not inject all the resin at once, allowing air bubbles to enter the molten resin. A hot runner tool might also suck air into the die because of area constriction, slowing the passage of the resin.

Burn Marks
Burn marks are generally caused by problems similar to blistering. They manifest as literal burn marks on the plastic, black discolorations that resemble scorches. The general cause of burn marks is improper ventilation for the resin. When this happens, the burns are usually located in the position farthest away from the ventilation gate. Another cause might be that the resin is getting trapped in the injector and heating too long. Cleaning the tool often solves this problem.

A common problem in many types of machining, burrs appear on injected plastic products when extra pieces or scraps are attached to the finished piece. A common, everyday occurrence of burrs can be found on green plastic Army men, who often have little extra pieces on their heads or thin sheets of plastic connecting their arms to their bodies. Burrs are usually a result of dull or inaccurately cut dies or molds. Sharpening or cleaning the dies are typically the most effective ways to remedy the situation.

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