Plastic Injection Molding Gate Design

Injection Molding Gate Issues Caused by Improper Material, Size and/or Location

In plastic injection molding, gate design is critical to achieving molded part quality and productivity. Because gates are designed to allow melted plastic resins to move into the mold cavity, it’s imperative that gate material, size and location are optimized for structural, operational and aesthetic considerations.


Proper material selection is critical in plastic injection molding gate design. The last thing you want is the wrong material based on machining a part from a stock resin that’s been cast into bar stock or flat stock.

Castings are always stronger & less prone to stress fracturing. A part machined out of cast material will often give you a resulting stronger part. If machining the part includes sharp corners, this could become a trap into the material selection being completely wrong for the molding process.


Gate size will depend on the resin being used and the size of the part being molded.

Small gates provide a better appearance, but require longer mold time or higher pressure to fill properly. Higher cavity pressures cause more stress to be molded into the part. This higher stress can cause structural failure in a finished part down the road & inevitably shows up at the most inopportune time.

Large gates allow for greater resin flow and shortened mold time.

In plastic injection molding, if gate size is inadequate the flow rate of the molten resin will be restricted as it tries to move through. This restriction can cause the filling rate to slow down enough to cause an excessive pressure drop from point-of-gate to last-point-to-fill.

Inadequate gate size can often cause injection molding gate defects like mold warpage and/or black streaks in the molded part.


Gate location can affect a variety of critical factors, including:

  • Polymer molecule orientation
  • Overall strength & structure of the part
  • Part shrinkage
  • Mold warpage
  • Ability to hold a required roundness or flatness
  • Operational considerations, such as injection pressure
  • Aesthetics, such as injection molding gate marks, sink marks, voids and flow lines

In some cases, more than one gate may be necessary to achieve the desired outcome. When incorporating multiple gates it’s important to consider where the flows will meet as the meeting area can cause aesthetic issues and part weakness.


Whether it’s improper gate size or positioning, injection molding gate defects can become extremely expensive and time-consuming.

Injection molding gate issues can range from moving one gate location to re-configuring the entire mold!

If your plastic injection molder suggests a particular gate size or location that you do not feel is optimal it’s important to seek out a second opinion from another plastic injection molding expert.

For more information, check out our Plastic Injection Molding Defect Guide.

Reach out to Plastic Molded Concepts today if you’re concerned about injection molding gate issues with your part.

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