Need Help!- Injection Mold Venting

[airfidget]

Newbie to CNC Zone and injection molding. Need some help figuring out what a typical vent size is. I'm using Morgan-Press injector rated for 12 tons, and I'll be using low density polyethylene. It's a very small mold compared to some I've seen, it's got about 18 square inches of surface area on the inside including the gates and the sprue, and it makes 4 caps at a time. Any help anyone can give me would be awesome.

[JCMills]

Im not the king of vents but try parting line vent the entire cavity if it is flat. It depends on how flashy your material is. If its very flashy then .0006 might be necessary--if its low density something then it may not flash hardly at all and you might go deeper such as .0010. then just do a groove with a ball end mill maybe .020 deep that goes into that undercut area and run the groove to the outside..with the groove coming up maybe .100 from the edge.. Also when molding and having vent problems dont forget the tape and shim trick between the platens to space them apart.. I am a big believer in venting the entire periphery of the cavity even though it gives alot of people the willies. A good rule of thumb is that when venting that you probably need MUCH more.. keep in mind that a true full periphery parting line vent gives not less then TWENTY TIMES the venting....at the same depth. If .001 will not flash--then there is no logical reason to be afraid of venting the entire cavity. This proves out in actual practice and not just theory..

[anchange]

I just made my first injection mold and shot it this weekend. Used PP and some other plastics. My vent lines were .0015 deep with a 1/8" cutter and there was no flashing at all. I faced the piece with the same cutter so that I wouldn't lose tolerance with a tool change and they'd be exactly .0015

[JCMills]

Originally Posted by airfidget Newbie to CNC Zone and injection molding. Need some help figuring out what a typical vent size is. I'm using Morgan-Press injector rated for 12 tons, and I'll be using low density polyethylene. It's a very small mold compared to some I've seen, it's got about 18 square inches of surface area on the inside including the gates and the sprue, and it makes 4 caps at a time. Any help anyone can give me would be awesome.

and of course the smaller a mold is--the tighter it will shutoff so venting can become more important. A large mold with ej pins and unusual parting line--they automatically are usually venting all over the place. Even down the pin holes especially--and remember--ej pins are never greased--they are always installed bone dry.