Injection Mold Venting Problem

Venting can be one of the most important parts of an injection molded product. Many engineering materials like PBT (Polybutylene terephthalate) have a tendency of out-gassing. The best practices that I have to date are:

• Add 10mm wide vents spaced 30mm from each other all the way around parting line.
• Always follow the recommendations of the material company for vent depth.
• If the product has a 50mm or larger core out the injection mold must be vented through the ejector plate, which means that you must drill through the core side (without hitting any water lines etc.) I usually use a 5mm or 1/4" drill bit to do so.
• Always vent to atmosphere.
• Polish the vents and vent channels. This makes the vent self-cleaning. It also allows the gases to escape much better.

A really old trick to ascertain whether the injection mold has an inherent venting problem or not, and this trick is somewhat dependent upon injection mold construction so must be evaluated as applicable on a case by case basis (more about this at the end) is to use shipping tape, or masking tape, depending upon the gap desired laid across the injection mold major parting line to artificially "spread" the mold and create a vent gap. Evaluate the fill in the taped v non taped shots without making changes to the injection molding process settings.

The reason why this must be evaluated case by case is that given the potential actions built into an injection mold, such as lifts, slides, or core pulls, creating this artificial parting line may cause binding, or inability of moving features to "lock" properly leading to potential damage to the injection mold. Additionally forcing mold clamp tonnage to spots not intended to carry all that clamp load could cause cracking or breakage of hardened steel. SO USE THIS TROUBLE SHOOT METHOD WITH CAUTION and make sure to consult with injection mold design engineering first.

Also some injection mold designs/builds have pre-load built into the cavity and core blocks meaning that the actual contact surface area of the parting line is greatly reduced to ensure a parting line seal off. In this case taping the mold base parting line will have legible effect unless you tape directly between these blocks which may in fact seal off any vents that are built in at the parting line.