Design for Transfer Molding

[ryandias]

I have been asked to do some concepts for a "transfer molded" package. I am not that familiar with the constraints on such a process.

Mainly can lead frames extend out vertically in the mold (into the mold cavity), or do they always have to be flat against the parting line?

Anyone have experience with this field? or recommend resources?

 

[Pud]

I have seen margarine tubs moulded at 3s cycle time for two cavity tooling with the insert around the side of the tub. This was at an exhibition, so I suspect cavity count was on the low side!

Took me about 5 minutes of careful observation to see what was going on the robot was so fast!

It was on the "Stork" (google will get you a video) stand.

I assume that's what you mean?...

H

http://www.tynevalleyplastics.co.uk

 

[patprimmer]

By transfer moulding are you talking about the process where melted or liquid or powdered polymer or resin is placed in an open mould, then the mould is closed to form the shape as the resin or polymer cures.

Regards
Pat
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[ryandias]

I am refering to the process Microchips & IC's are made.

 

[btrueblood]

Pat,

Transfer molds are used in molding rubber, and close to what you described.

To rubber part molders, what you described is compression molding.

Transfer molding is one step closer to injection molding - part is placed in a mold cavity, and mold cavity is closed. A second cavity atop/adjacent to the mold cavity holds a charge of uncured rubber/polymer, and a piston is placed into this second cavity. The whole shebang (stack) is then placed into heated press, press closes, rubber is squeezed from the transfer "pot", thru sprues, and into mold cavity. Stack/press is "bumped" (allowed to open slightly, then reclosed) several times to allow trapped air to work its way out (usually spill grooves/channels are placed just outside the parting line of mold cavity to help this process). Press is closed a final time, and part is allowed to cure under heat/pressure. Mold is then opened, and part is pried out of mold. Transfer pot must also be pried/jacked open, and residual rubber peeled/cleaned out of the pot, all mold surfaces must be cleaned and reprepped prior to loading next part.

Transfer molding is essentially a "one shot" injection molding process, used for polymers that cure under heat application (epoxies, rubbers), i.e. that would cure inside the injection/extruder ram of a more conventional plastic injection molding machine.

Although, there are injection molding machines for rubber parts as well - the cycle time must be short enough so that minimal rubber inside the sprue/gate/ram cures prior to the next injection cycle, and the extruder ram tough enough to expel as waste any product that has cured/partially cured prior to injecting the next part.

To the OP, yes you would want the lead wires of your part to be on the parting line, unless you can envision or design a way to eject the part from whatever is holding the leadwires tightly enough (.001 or less clearance?) to keep the rubber from shifting the position of the part within the cavity. Having a second axis of ejection makes it more time-consuming to load/unload parts...it's all a trade off, with time = $.

 

[patprimmer]

btrue

I normally heard it described as compression moulding if it's for powders like in Phenolic, Urea, Melamine etc and PTFE.

I normally herd it referred to as transfer if it was liquids or pre-melted like the moulding of vinyl records.

I have often seen the terms confused.

I have never been involved with thermosetting rubbers and was unaware of the exact details. Good to know.

Regards
Pat
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