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If the exhaust of the small mold is not done well, the product can be messed up!
Author:HAMSNA Mold

If the exhaust of the small mold is not done well, the product can be messed up!

In a sense, the injection mold is also a displacement device, that is, the plastic melt enters the cavity and replaces the air in the cavity at the same time. In fact, the air in the mold is not limited to the cavity, especially for the three-plate injection mold, the air in the runner cannot be ignored. In addition, plastic melts produce minute amounts of decomposition gases. These gases must be discharged in time.

If the exhaust performance of the mold is poor, it is easy to produce bubbles, silver streaks, clouds, unsatisfactory filling, surface scorching marks, intermittent injection and other defects. Therefore, the mold must be equipped with a reasonable exhaust structure to avoid defective products caused by poor exhaust.


1. Several common gas methods

1) Exhaust slot exhaust:

For molds for forming large and medium-sized plastic parts, a large amount of gas needs to be removed, and exhaust grooves should usually be provided on the side of the die on the parting surface. The location of the exhaust groove is preferably at the end of the melt flow, and the size of the exhaust groove is based on the principle that the gas can be discharged smoothly without overflow. The width of the exhaust groove is generally about 3-5mm, the depth of the front end of the exhaust groove is less than 0.05mm, and the length is generally 0.7-1.0mm. The depth of common exhaust grooves can be checked in <Plastic Mold Technical Manual>.

2) Parting surface exhaust:
For small molds, the parting surface gap can be used to vent, but the parting surface must be located at the end of the melt flow.
3) Exhaust through gaps in mosaic inserts:
For combined dies or cavities, the combined gaps can be used to vent.
4) Push rod clearance exhaust:
Use the gap between the push rod and the template or core to exhaust, or deliberately increase the gap between the push rod and the template.
5) Exhaust of powdered unsintered gold nuggets:
Powder unsintered alloy is a material sintered with spherical particle alloy, which has poor strength, but loose texture, which allows gas to pass through. Placing a piece of such an alloy at the part to be vented can meet the venting requirements, but the diameter of the vent hole at the bottom should not be too large to prevent it from being squeezed and deformed by the pressure of the cavity.
6) exhaust well exhaust:
On the outside of the joint of the plastic melts, a cavity is set to allow the gas to be discharged into it, and a good exhaust effect can also be obtained.
7) Mandatory exhaust:
Install an exhaust rod at the part where the gas is closed. This method has a good exhaust effect but will leave traces of the rod on the plastic part, so the exhaust rod should be installed in a hidden place of the plastic part.
2. Matters needing attention in exhaust design:
1. When the exhaust is extremely difficult, the mosaic structure is used. If the dead corner of some molds is not easy to open the exhaust groove, first of all, the mold should be properly changed to mosaic processing without affecting the appearance and precision of the product. This is not only conducive to processing Exhaust can also improve the original processing difficulty and facilitate maintenance.
2. At the closed shape of plastic products, thimbles must be added for exhausting and preventing burns and fusion marks.
3. For product molds with complex geometric shapes, it is best to determine the opening of the exhaust groove after several mold trials.
4. The thinner the product, the farther away from the gate, the opening of the vent groove is particularly important.
5. For small parts or precision parts, attention should also be paid to the opening of the exhaust groove, because it can eliminate various defects of the product in addition to avoiding product surface burns and insufficient injection volume.
3. Summary:
Appropriately opening the exhaust groove can greatly reduce the injection pressure, injection time, holding time and clamping pressure, making the molding of plastic parts easy from difficult, thereby improving production efficiency, reducing production costs, and reducing energy consumption of the machine.
Details determine success or failure, and the details of the mold must be carefully handled!

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