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.
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
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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