I received this commentary form one of our frequent blog contributors…
I expect you to give a very strong message on tool venting in your Plastic Training and Technology Blog. Basically, many of the Moulders are not aware of venting. What should be the best way to make them understand?? Even though they agree, they still have a strong concern on venting which may cause flash on the product.
The best rule of thumb is ‘You can never over-vent your tool’.
Since the melted polymer will not flash if the vents are of appropriate depth, the number of vents has no affect on the likeliness of flash.
When I am involved in a tool design or revision, my motto is to vent everything… runners, cavities, parting lines, ejector pins, slides, and lifters. Drawing a vacuum on the mold or incorporating porous materials such as Porcerex are very helpful options.
Poor venting often contributes to defects such as burning, poor surface finish, short shot, and part hang-up. This also restricts your ability to increase injection velocity to help correct many defects including poor surface finish, short shot, sinks, runner flash, voids, large dimensions, and warpage.
Essentially, inadequate venting restricts your ability to develop and maintain a good Scientific, or Decoupled, molding process.