Technical Requirements for Mold Venting
In the manufacturing of molds, proper mold venting is a crucial aspect.
Mold venting can represent the overall quality of the mold structure to a great extent.
Even if a mold is excellent, without effective venting, it is unlikely to produce consistently high-quality products. A good mold should have excellent sealing properties while ensuring the free release of gases, rather than compressing them significantly. So, what are the standards for mold venting?
Let’s explore the reasonable technical requirements for venting:
Technical Requirements for Mold Venting
1. Full Circumference Venting
Efforts should be made to create a full circumference venting structure, with a depth within the plastic overflow value. Full circumference venting is optimal, but it’s essential to leave 1/2 of the steel material on the parting line to compensate for pressure.
If irregularities on the PL surface prevent full circumference venting, a good alternative is to use a 12mm-wide vent groove towards the female mold, ensuring that the gas is eventually vented into the atmosphere. Utilize milling cutters with flat bottoms and chamfered edges for vent groove machining whenever possible.
If a ball-end mill must be used, it is crucial to calculate the machining depth to align with the depth produced by the flat-bottom cutter.
The length of the vent sealing is a factor often overlooked by many but is closely related to proper venting issues.
2. Specification of Dimensions and Venting Depth
There are various statements regarding dimension specifications on this issue, with material supplier specifications ranging from 3 to 5mm.
Through validation, it has been proven that if the length exceeds 3mm, it will no longer be effective.
For safety reasons, the standard specification is set at 3mm.
The venting depth depends on the molding material, and if there are doubts, consulting the processing specifications is advisable.
3. Understanding the Relationship Between Flashing and Venting
A common misconception is that too much venting causes flashing. In reality, the opposite is true.
Excessive venting requires excessive pressure, and when gas is allowed to escape, the material follows suit.
The only cause of excessive venting leading to flashing is when the vent groove depth is too deep.
4. Ejector Pin Venting and Runner Venting
It is generally advisable to use all ejector pin venting, especially crucial for large plastic components, which, of course, depends on the diameter of the ejector pin.
If this is not possible, flat venting should be applied with an appropriate length and depth, along with a reduction in the clearance between ejector pins.
For runners, especially large or long runners, venting must be implemented.
5. Treatment of Non-Perimeter Venting Components
Another common issue arises when components cannot have perimeter venting.
In such cases, the final fill position and the second-to-last fill position must be considered, and both positions require venting.
Deep core positions, column positions, small pin injection positions, or thin injection positions need special consideration for venting. This can be achieved using flat-topped pins, semi-needle structures, venting steels, or even vacuum venting devices.
Additional Considerations for Mold Venting
It’s crucial not to over-ventilate a part, even though it’s achievable along the parting line. This can lead to the following issues related to venting:
There may not be enough steel on the parting line to withstand pressure, causing venting on the mold surfaces to be compressed to close.
Even the best venting can eventually diminish. After a certain material cycle, if there isn’t enough steel on the parting line to compensate for pressure, the mold may bite together, resulting in wear on the venting grooves. It’s advisable to avoid reducing the parting line in mold sections, which is a significant challenge molds encounter.
For molds with front and rear mold inserts, the front and rear mold cores should not come together before the mold parting surfaces meet. The correct approach is to mold simultaneously using pressure-resistant blocks between the mold frame AB plates.