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Injection Moulding And How To Design Gates And Runners

mold designAny injection moulding mold must have the correct design of the gates and runners to produce a quality part. This is obvious, yet it is surprising just how often these two details are overlooked in the design process.

Maybe it is because people are so focused on the actual plastic part being molded, and the gates and runners become somewhat of an afterthought. In any case, here are some tried-and-true guidelines for the design of gates and runners in a plastic injection mould.

General guidelines for gates and runners

  • Always place the gates symmetrically in relation to the sprue if you are using multiple cavities. This usually requires a simple rotating of the part to achieve the symmetry.

  • Use a central gate when molding round parts, such as a gear. A 3 plate mold or diaphragm gate is advised, which will ensure the concentricity of the round parts.

  • Gas buildup and burning can be avoided with cup shaped parts, such as a capacitor, by gating near the base.

  • Tunnel or submarine gates are advised for cosmetic parts. This eliminates most of the gate vestige and makes for a cleaner looking part.

  • Always place the gate in the area with the greatest wall thickness.

     

Examples of correct and incorrect gate and runner design
 

 injection molding plastic gates and runners

 injection molding plastic gates and runners2

 

The gating on the left is correct, it is balanced. The gates on the right are incorrect, the result would be uneven fills and poor quality.


 gates and runners

 gates and runners


The gate on the left is correct, it is filled from the end. The gates on the right are incorrect and would result in poor fill and quality.





These gates are quite common and have many different names, such as smiley gate, banana gate, cashew gate and sub gate. They are all variations on the theme of getting the plastic into the cavity below the parting line.

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How to design tunnel gates or submarine gates

Before sinker EDM became so common in moldmaking, tunnel gates were cut with single lip cutters on the vertical milling machine. This was always a challenging task, as the cutters tended to break and were also not that easy to grind.

Another difficulty was that job of setting the cavity up on the correct angle. Many times the gate angle is close to 45 degreess and it can be very vexing to figure out if you are in the correct location. You usually cannot tell until the gate is cut, which could be too late if it was in the wrong location.

The machining process was even more challenging if the cavity block was already hardened. So, you would have to use carbide, which is brittle and easily chipped.

Things are easier now with CAD design software and CNC EDM machines. The sub gates are nearly always burned in place with the EDM and the entire process is simpler and more efficient as well.

  Submarine or tunnel gate

sub-gate

Summary

No matter which type of gate you use, it must have sharp details and edges to function properly. The gate size and location can dramatically affect the molding of any plastic part.

The gates and runners must be balanced, sized correctly and have a good surface finish to enable the flow of the material and the shearing of the plastic part. Injection moulding is a very versatile process, but unless the gates and runners are correct, you will be fighting the process in an attempt to get a quality plastic molded part.

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