In a sense, the injection mold is also a replacement device, that is, the plastic melt enters the mold cavity and simultaneously replaces the air in the mold cavity. In fact, the air in the mold is not limited to the cavity, especially the three-plate injection mold, and the air in the flow channel cannot be ignored. In addition, the plastic melt will produce a small amount of decomposition gas. These gases must be exhausted in time.

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What Is Mold Exhaust?
In a sense, the injection mold is also a replacement device, that is, the plastic melt enters the
mold cavity and simultaneously replaces the air in the mold cavity. In fact, the air in the mold is
not limited to the cavity, especially the three-plate injection mold, and the air in the flow channel
cannot be ignored. In addition, the plastic melt will produce a small amount of decomposition
gas. These gases must be exhausted in time.
01 Several common gas ways
1) Exhaust
For molds that form large and medium-sized plastic parts, the amount of gas to be removed is
large. Usually, an exhausted groove should be set up, usually on the side of the concave mold on
the parting surface. The position of the vent groove is preferably at the end of the melt flow. The
size of the vent groove is based on the principle that the gas can be smoothly discharged without
overflowing. The width of the exhaust slot is generally about 3-5mm, the depth of the front end
of the exhaust slot is less than 0.05mm, and the length is generally 0.7-1.0mm.

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2) Parting surface exhaust
For small molds, the gap between the parting surfaces can be used for exhaust, but the parting
surface must be located at the end of the melt flow.
3) Exhaust in the gap of the mosaic
For the combined concave mold or cavity, the split gap can be used to exhaust.
4) Exhaust gap clearance
Use the gap between the pushrod and the template or core to exhaust, or intentionally add the
gap between the pushrod and the template.
5) The exhaust of powder unsintered alloy block
Powder unsintered alloy is a material sintered with spherical particle alloy, which has poor
strength but the loose texture, allowing gas to pass through. A piece of such an alloy can be
placed on the part to be exhausted to meet the exhaust requirements, but the diameter of the
bottom vent hole should not be too large to prevent the cavity pressure from squeezing and
deforming it.
6) Exhaust well exhaust
On the outside of the confluence of plastic melts, a cavity is provided to allow gas to be
discharged into it, and a good exhaust effect can also be obtained.
7) Mandatory exhaust
In the place where the gas is closed, an exhaust rod is provided. This method has a good exhaust
effect but will leave a trace of the rod on the plastic part. Therefore, the exhaust rod should be
located in a hidden place of the plastic part.
02 Matters needing attention in exhaust design
1. When the exhaust is extremely difficult, the mosaic structure is adopted. If the dead corner of
some molds is not easy to open the exhaust groove, first of all, the mold should be changed to
mosaic processing without affecting the appearance and accuracy of the product. Exhaust can
also improve the original processing difficulty and facilitate maintenance.
2. At the closed shape of the plastic product, a thimble must be added for exhausting to prevent
burns and fusion marks.
3. For product molds with complex geometric shapes, the opening of the exhaust slot is best
determined after several trial molds.
4. The more thin-walled products, the farther away from the gate, the opening of the exhaust slot
is particularly important.
5. For small parts or precision parts, we must pay attention to the opening of the exhaust slot,
because it can avoid the surface burns and insufficient injection volume of the product, and can
also eliminate various defects of the product.
To sum up
Properly opening the exhaust groove can greatly reduce the injection pressure, injection time,
holding time and mold clamping pressure, making the plastic parts from difficult to easy, thereby
improving production efficiency, reducing production costs, and reducing the energy
consumption of the machine.