Do inserts need to be preheated to tooling temperature in injection molding?

[sadhusong]

Hello all
as instructed injection with serts such as shaft, the shaft should be pre-heated as tooling temperature,i would like to get help for theory of this and is this mandatory?

and i found some supplier does not do this.

[Phil Fields]

My past experience in injection molding (15 yrs ago), we never pre-heated inserts.

Phil

[john]

I have been insert molding for over 30 years and the only time we pre heated inserts was on a small nylon over steel insert part that was machined afterward. The rational for this was to keep the bubbles / shrink voids out of the part so it could be machined smoothly with no pits.

[Randy]

What do your engineers say?

[Edward Nell]

Originally Posted by sadhusong

Hello all
as instructed injection with serts such as shaft, the shaft should be pre-heated as tooling temperature,i would like to get help for theory of this and is this mandatory?

and i found some supplier does not do this.

If you are doing rubber to metal/plastic injection moulding, then preheating will start the adhessive to start reacting before you have had time to inject the product. That would lead to failures

[bgwiehle]

Originally Posted by Edward Nell

If you are doing rubber to metal/plastic injection moulding, then preheating will start the adhesive to start reacting before you have had time to inject the product. That would lead to failures

True, but there are times when it is an advantage -- in our case, we pre-heat when the metals are particularly large or thick and when the rubber portion is especially thin. We control time and temperature of the pre-heating as well as the molding process, and test adhesion as well as dynamic properties. Also, the pre-heat temperatures are not as high as the molding temperature.

B.G. Wiehle

[Jim Wynne]

Originally Posted by sadhusong

Hello all
as instructed injection with serts such as shaft, the shaft should be pre-heated as tooling temperature,i would like to get help for theory of this and is this mandatory?

and i found some supplier does not do this.

I think the answer is, "it depends." For example, I found the attached PDF from DuPont that says this about some of their plastics:

Inserts should be preheated before molding (200 °F for Delrin® , 250°F for Zytel® , Minlon® and Rynite® ). This minimizes post mold shrinkage, pre-expands the insert and improves the weld line strength.

If your suppliers lack technical resources in their own compaies, it might be best to talk to an application expert from their resin supplier about your parts and the best way to proceed.

[Ajit Basrur]

Originally Posted by Jim Wynne

I think the answer is, "it depends." For example, I found the attached PDF from DuPont that says this about some of their plastics:

Jim, the info provided is very good but the attachment is not correct - it titles 1984 and is something else