Transfer Molding

[bikswahla]

Hello Guys

Hope there are some guys who know about troubleshooting rubber transfer molding process. I am running the mold on specific parameters for rubber molding and every time the parts comes small when measured. I tried to reduce the temp and increase the pressure but didnt help. Is any one you guys who know about rubber transfer molding process??
What are the otherways we can make the part big.

 

[ornerynorsk]

Are you putting enough material in the pot to begin with? I know, dumb question, but I've seen it happen. Secondly, is the pot clearances tight enough to inject the material instead of extruding it back out of the plunger. Third, again obvious, but you may not have calculated enough shrinkage factor into the cavity dimensions. Try another material with less shrink, or add some filler, if it's allowed, and see what happens.

 

[bikswahla]

Thanks Ornerynorsk for replying. I start with half pot and then make the pot full and then run the mold by placing a small lump of rubber in every heat. It is a 8 Cavities 24"X24" mold with a 19" POT. The pot clearances is tight. The shrinkage was calculated to 2.75% and was added to the cavities by out tool supplier. I am facing the problem with this mold only, the same set of other mold of same dimensions and same spruce size same process settings and part comes out big in that mold. Can you tell me is the pot Temp plays a role in that? or is there some thing else we can try.

 

[ornerynorsk]

What is your overall part dimensions? How much too small are your parts coming out? The pot temp should depend greatly on the mold temp, do you have any temp sensors or a laser thermometer to verify that the process conditions are really identical. Could the mold platens be of a different material that is expanding at a different ratio than the other molds? This sounds like a mystery !

 

[bikswahla]

the overall part dimensions are 2.197 Nom, with tol of 0.020". I want to achieve the parts dim to 2.212 ie 0.015" over so that when it shrinks it comes to Nominal. but currently my parts are coming -0.010" after shrinking. The mold heat is controlled by the builtin heaters with thermocouples. This is a mystery. I dont know what to do now my be increase the sprue size to increase the volume of material flow in the cavity. Got any more ideas???

 

[ornerynorsk]

Is there any indication that the cavity is not filling completely, IE the press ram is not building sufficient pressure to fully pack the cavity? What result do you get when the mold is run in a different press. Other than that, it's hard for me to come with anything else, not being able to see the operation. Are the mold thermocouples and heaters in the same relation to each other as in the molds that are working fine? Are the parts fully cured / over-cured? Have you played around with the settings, up and down? Is it the same material as the other parts you are molding. Your best bet is to compare your successful molds with the problematic one. Nit-pick and compare them and try to come up with anything that is different. Good luck!