Plastic Mold Design


Every plastic mold design has to have vents, and learning how to add vents is an essential part of the plastic mold design process. Adding vents is an absolute must for any injection, compression or thermosetting mold.

Vents are called vents because that is what they do: they allow the air and any gases trapped in the closed mold to escape. If these gases are not allowed to escape, the trapped air will burn together with gases from the molten plastic and cause any one of several unacceptable conditions in the part. The plastic mold design has to include vents, at least the general placement details.

Symptoms of poor venting

  • Short shots, or non-filled plastic molded parts. If the air is trapped and cannot be absorbed by the plastic part, the molten plastic will be physically unable to fill the cavity.
  • Burned areas are a typical symptom of inadequate venting. The volatile gases present from the plastic mix with the air and are a under tremendous amount of pressure, due to the molding process. They will ignite, causing the easily recognized blemish of a burn.
  • Voids, blisters, charred parts, and bubbles are all common symptoms of a poorly vented injection mold.

How deep should vents be?

As deep as possible without causing the part to flash is the easy answer. The complicated answer is that it depends on the process and the material being molded.

Typically, vents range from 5 in. to .002 in. for a plastic mold design. Most mold makers err on the side of too shallow, for fear of flash.

Ideally, the two halves of a mold fit together so well that it is virtually air-tight. In reality, however, this is not the case and this needs to be taken into consideration before adding vents that are too deep. Some areas of the mold will usually have unintentional gaps, due to the complexity of manufacturing a parting line. The parting line may even be “naturally” vented because of this gap. Adding additional vents will cause a deeper gap, and so you will have a mold that flashes.

The farther away from the gate, the cooler the material is, therefore some mold makers choose to make the vents a bit deeper as you move away from the gate area. On the other hand, the pressure is also greater in these same areas, and it may also be wise to start out shallow and increase the vent depth as needed.

Resin companies supply plastic injection molding companies with venting charts, such as this:


Resin typeSuggested vent depth (inches)



Cellulose Acetate

Cell. Ace. Butyrate


Nylon 6/6




Polyphenylene Oxide

Polyphenylene Sulfide



PVC Rigid

Pvc Flexible

















How many vents should you use?

The short answer: as many as possible without causing flash. Some plastic mold makers actually vent the entire parting line. With the high-end CNC milling machines, high-speed applications and hard-milling in use today, this is more and more common.

Because a contoured parting line is very difficult to vent conventionally, it is often done in the CNC milling machine very accurately. This can be noted in the plastic mold design layout.

A general rule-of-thumb is about 30% of the perimeter can be vented. If the plastic molding process allows enough time to sample prior to venting, the molder and mold maker can get a very accurate, real-life assessment of how to vent, where to vent and how deep to vent.


Every successful plastic mold design must include vents. Learning how to vent, where to vent and how deep to vent is a skill that requires time and experience. The resin supplier chart is a good starting point, but only a starting point.

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