11 Tips on how to design injection mold gate

Gate is defined as an opening on the plastic wall through which the molten plastic is injected into the final part. Gate location can make or break your part design.  Therefore, it is very important to do a pre-analysis to decide the location, size, and shape of the gate as all these can greatly affect everything from the structural integrity to the visual appearance of a finished piece.

Yet most CAD designers give little thought as to where to and how to put the gate, or injection point, on the part they design. They simply leave that detail to the mold maker.This is a big mistake. Along with choosing the wrong gate size and type is the root cause of many molding difficulties such as welding line, shrinkage, warp and more.

How to design gate on injection mold then?

Firstly, decide what kind of gating systems for your part ,if necessary run mold analysis

Secondly, below are some guidelines for designing the proper gate to avoid problems from your gate location.

  1. Gate should be placed on the thickest cross section to allow for part packing and minimize voids & sink.
  2. Gate should minimize flow path length toreduce loss of injection pressure and avoid cosmetic flow marks
  3. If filling problems occur with thin walled parts, add flow channels or make wall thickness adjustments to correct the flow.
  4. Be sure that stress from the gate is in an area that will not affect part function or aesthetics. If you are using a plastic with a high shrink grade, the part may shrink near the gate causing “gate pucker” if there is high molded-in stress at the gate
  5. Avoid opening gate at the insert core of slender shape, otherwise the plastic flow will have direct impact on the core,  resulting in deformation,dislocation or bending
  6. For large or flat products, it is recommended to use multi-gating system as it can prevent product warpand missing plastic.
  7. Open gateat the edge or bottom of the product as possible so that it wont affect the appearance and function of the product.
  8. Gate size is determined by partsize, geometry, structure and plastic material.  Design small size at start and then modify it according to mold test result.
  9. For multiple cavitiesfamily injection mold, if all parts are the same, use symmetrical gate feeding system, whereas the parts are different in size, place the largest part close to the main channel.
  10. Design sprue puller as possible around cold slug near the gate to facilitate the demoulding of runner.

In general, gates vary in size and shape depending upon the type of plastic being molded and the size of the part. Large parts will require larger gates to provide a bigger flow of resin to shorten the mold time. Small gates have a better appearance but take longer time to mold or may need to have higher pressure to fill correctly.

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