07-18-2014 10:33 PM
Hello, Would someone have any idea why one or two parts are sticking in the mould every time it comes back from Toll Room after being serviced. We are running a 2-shot process: PP and TPE seal 8 cavity mould, mould temperatures: 20C and 25C
Why one or two parts are sticking in the mould?
07-19-2014 01:05 AM
Do the parts randomly stick throughout the molding process or for the first hour of startup or any other "set" period of time?
07-19-2014 03:58 AM
what happens is that one or two cavities get stuck on the fixed side after the mould service for several hours sometimes for a couple of days. polishing the cavities doesn't seem to make much difference. after that period the mould runs perfect, never any parts stick until the next service.
07-19-2014 06:07 AM
Sometimes when this kind of thinks happens is for little details. For example, the oil used for clean and protection of mould. There are some oil that stick plastic in metalics parts.
07-19-2014 08:30 AM
07-19-2014 11:21 AM
slight coating of oil and a high polished surface can make flexible materials stick. It is usually better to have a fine etched surface or fine glass bead blasted surface allows the air to filter in easier.
07-19-2014 02:08 PM
Please clarify if it is the same two cavities or if it can be different ones that stick. If the same two cavities have you done a fill balance study? Once the tool has been cleaned vents are open and it may be that the cavities that stick are filling first and significantly overpacking. The sticking may come and go as a result of the normal thermal variability of your molding machine.
07-19-2014 04:52 PM
Thanks. The cavities that stick are different every time. The surface of the mould is highly polished as it is a transparent part. I had an idea that cleaner surfaces allow better heat transfer thus affecting the shrinkage.
07-19-2014 07:13 PM
Oleg, In 2-Shot tools (shuttle or rotary) the Startup process may require a longer time period in order to stabilize the temperature of all of tooling components. Since you say that the parts are randomly sticking I would ask; 1) Is this Sticking due to marginal Draft ? or 2) Are the parts actually sticking to the cavity surface and possibly leaving residue on the steel surfaces ? Since the process eventually runs OK after a certain time I would suggest that you revise the Startup procedure and adjust the Temperatures and Pressures to be slightly lower at Startup in order to Saturate the entire tool before ramping up to the Production Cycle. This will also be effected by the type of cooling you have in your process. I would also ask if you have ample features designed into your tooling cores which hold the parts in place prior to final ejection.