I received this general venting question the other day…
Many of our molds have venting issues as evidenced by either burns or filling problems. I realize that a “flow analysis” done previous to cutting the steel would answer most of these questions. The gases have to get out… What do you recommend regarding placement, size and depth?
With I am asked how many vents should be added, I always say more. Each material has design guidelines for vent depth which are typically very helpful.
Ultimately, the more vents you can have on the part, the better. Obviously… strategic venting, such as a porous steel in a boss, can be very important; but getting as much gas out of the mold prior to that boss will minimize the amount of localized venting that is required. Also consider venting the runner, sprue, and cold slug wells.
Since the polymer only flashes as a result of thickness, the width of the vent has virtually no effect. As a result, you can make the vents relatively wide. I have seen large vents taking up high percentages of the parting line with great success.
One commonly overlooked aspect to venting is the vent drop. Behind any well designed vent is a substantive vent drop. These drops are deep grooves, which channel the gas from behind the vent to the outside the mold. Think of the vent as a gate designed to transfer the gas from the mold to the vent drop.
Lastly, high clamp pressures may reduce the effectiveness of your vents… and may close them completely.
Many mold designers put extensive thought into the getting the polymer into the mold… but pay little attention to how the gas will get out of the way. Most of the molds I see with vent clogging problems often have thin vents, too few vents, no vent drop, or combinations of all three. Give the gas trapped within the mold many wide vents with a big vent drop to help it escape to the atmosphere.
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You wrote: Each material has design guidelines for vent depth which are typically very helpful- where I could find these guidlines?
Although generic materials will often come with minimal support, most major suppliers such as Du-Pont, Eastman, etc. will have design guidelines for general grades of their materials. If your supplier does not have access to this information, try a one of the major suppliers, or a materials resource such as IDES.
An example of an Eastman design guide can be found here:
Internet searches will also generate some good references such as: http://www.plastictroubleshooter.com/ThePlasticTroubleshooter/mold_venting.htm