Gas vent is space that air can go out of mold. When molten plastics fills up the mold. The air or molten plastics left in the mold will generate gas. If they can not go out of mold, appearance of samples will not good: have gas, fire, …
There are several ways to place a gas vent to the mold, but it can be classified broadly into the following three ways.
・ Parting line:from the divided face of the mold.
・ From the cavity and core part, ejector system.
・ Gas venting by other special way.
1. Gas vent from the divided face of the mold (parting line).
Vent depth should be 0.005mm to 0.02mm, and generally placed on the following location.
+ Preferably far from the gate.
+ Place where weld lines tend to occur.
+ End part of the runner or sub runner.
2. Gas vent from the cavity and core part.
2.1 Using ejector pin.
Ejector pin and its hole clearance is used in this method. The clearance of pin and its hole should be 0.02mm to 0.03mm if the pin diameter is about 5mm to 10mm. If it is thinner than this, 0.01mm to 0.02mm is general for the clearance.
2.2 Using core pin.
If there is tall boss or rib on the part of the product, vent by making clearance around the core pin.
2.3 Layered nesting block.
As a method of gas venting of tall rib, use the clearance made by layered nesting composed of thin blocks. Also, insert a flat part of the cavity as layered nesting, and vent from the clearance of that thin plates.
3. Gas vent by other special way.
This is a method that ventilate a gas instantaneously by making high-vacuum state inside the cavity, using the vacuum pump. This method is an ideal as a gas venting method, and also effective
for transfer accuracy upgrading against the mold cavity, but as disadvantage, the cost of equipment will be expensive, and mold construction will be complicated.