Introduction
During injection molding, the vents should not allow much molten material to seep out because the molten material will cool and solidify there, blocking the passage. The vents should be positioned away from the operator to prevent accidental molten material from spraying out and injuring anyone. Alternatively, venting can be achieved by utilizing the clearance between the ejector pin and the ejector hole, or between the ejector block, stripper plate, and core.
1, The function of exhaust groove
The exhaust slot has two main functions:
Expel the air in the mold cavity when injecting molten material;
Remove various gases generated by the material during the heating process.
The thinner the wall of the product is and the farther away from the gate it is, the more important it is to have a venting groove.
In addition, for small or precision parts, attention should also be paid to the opening of venting grooves, because in addition to avoiding burns on the surface of the product and insufficient injection volume, it can also eliminate various defects of the product and reduce mold contamination.
So, how can we say that the venting of the mold cavity is sufficient? Generally speaking, if the melt is injected at the highest injection rate without leaving any burn marks on the product, the venting in the mold cavity can be considered sufficient.
2, Exhaust method
There are many ways to vent the mold cavity, but each method must ensure that: the exhaust groove is designed to be sized to prevent material from overflowing into the groove while venting; secondly, it must also prevent clogging.
Therefore, for venting grooves longer than 6-12mm, measured from the inner surface of the cavity toward the outer edge, the groove height should be increased by approximately 0.25-0.4mm. Furthermore, having too many venting grooves is harmful. This is because if the clamping pressure acting on the unvented portion of the cavity parting surface is too high, it can easily cause cold flow or cracking of the cavity material, which is very dangerous.
In addition to venting the mold cavity at the parting surface, venting can also be achieved by installing venting grooves at the end of the material flow in the gating system and leaving gaps around the ejector pin. However, if the depth, width, and location of the venting grooves are not appropriately selected, the resulting flash and burrs will affect the appearance and precision of the product. Therefore, the size of the above gaps should be limited to prevent flash around the ejector pin.
It should be noted that when exhausting parts such as gears, even the smallest flash may be undesirable. For such parts, it is best to exhaust them in the following ways:
Completely remove the gas in the flow channel;
The mating surface of the parting surface is shot peened with silicon carbide abrasive with a particle size of 200 #.
In addition, a venting groove is opened at the end of the material flow of the pouring system, which mainly refers to the venting groove at the end of the branch channel. Its width should be equal to the width of the branch channel, and the height varies depending on the material.
3, Design Method
For molds with complex geometric shapes, it is best to determine the location of venting grooves after several trial molds. The biggest disadvantage of the overall structure in mold structure design is poor venting.
For the overall mold cavity and core, there are several venting methods:
Use the slot or insert installation area of โโthe cavity;
Utilize the side panel seams;
Partially made into a spiral shape;
Install the slotted slat core and make process holes in the longitudinal position;
When exhaust is extremely difficult, an inlay structure is used. If some molds have dead corners that are not suitable for opening exhaust grooves, the mold should first be appropriately changed to an inlay process without affecting the appearance and precision of the product. This will not only facilitate the processing of exhaust grooves, but sometimes also improve the original processing difficulty and facilitate maintenance.
4, Conclusion
Properly opening vent grooves can greatly reduce injection pressure, injection time, holding time and clamping pressure, making plastic part molding easier, thereby improving production efficiency, reducing production costs and reducing machine energy consumption.
