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Compression Moulding

03/13/2018 2:10 PM

Why can't we use molten or pre-heated rubber in compression moulding instead of using direct pellets? If that's possible then the curing time, pressure required and total labor may be minimized or does it seem inappropriate ?

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#1

Re: Compression Moulding

03/13/2018 4:16 PM

Pellets are stable and can be handled. Thermo-set resins polymerize rapidly when melted, and many have a room temperature shelf life.

Thermo plastic can be used this way, in some cases, if the melt viscosity is low enough and if they do not decompose thermally in the melted state.

The above is simple, there are hundreds of variations that are special cases

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#2

Re: Compression Moulding

03/13/2018 7:17 PM

In a word, inappropriate.

You don't say what you are molding, but I assume it is a thermo-set/vulcanizing rubber since you referenced curing time. I do not believe "the curing time, pressure required and total labor may be minimized" but preheating the shot is common place.

Pellets are usually associated with thermo-plastic molding, so with the tiny amount of information you provide, no one can say.

You do know what compression molding is, and how it works, don't you?

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#3

Re: Compression Moulding

03/14/2018 7:11 AM

Because it would cure before you could inject it.

Bazzer

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#4

Re: Compression Moulding

03/14/2018 11:29 AM

If the rubber is a vulcanizing type, melting it won't work as it will vulcanize and burn, rather than stay liquid.

If its not vulcanizing, then melting would work fine, except for the burn hazard from the molten liquid. Handling pellets turns out to be easier and less messy and has fewer safety concerns form burns and fumes than pouring liquid. Compression molding equipment is also cheaper to purchase and maintain than injection molding machines and tooling.

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#5

Re: Compression Moulding

03/14/2018 2:35 PM

My first "real" employment was an apprentice Mold Maker in a "rubber" molding company. I built hundreds of molds of various types. Compression, Transfer compression, vulcanizing splice molds, and many more.

There is a lot of information WE need to assist you. First, what type of material are you molding and what are its properties, specifications, Durometer, curing time and temperature?

This is my definition of vulcanization.

Vulcanization or is a chemical process for converting natural rubber or related polymers into more durable materials by mixing and heating them with sulfur or other equivalent curatives or accelerators. These additives modify the polymer by forming cross-links (bridges) between individual polymer chains.

Natural rubber or gum rubber?

Buna? Buna-N?

Neoprene?

Nitrile?

Silicone?

Butyl?

The above is just a sample of what would be called vulcanizing or thermo-curing materials.

We actually dealt with this type of issue back in 1989 when the owner (boss who loves used machinery auctions) purchased 2 Husky plastic injection molding machines thinking it would be simple to convert then to rubber molding. OK, everyone please stop chuckling.

Below are the trials and tribulations we experienced.

  1. Build a mold that runs HORIZONTALLY and suspends stripper or center plates requires a lot more design and materials such as longer and larger diameter locating pins and possibly springs.
  2. Original barrel, screw and nozzle are completely incompatible with a vulcanizing material. YES, we ended up having to clean that mess up several times by heating the barrel to where the material burned and crystallized.
  3. Even after changing the nozzle to and open ended design, just the action of the screw rotating was enough to heat the material to vulcanizing temperature and even when the screw was at rest, the material would drool out of the nozzle during the curing period and the pile of drool would be larger than the actual molded part and would cure because of the heat from the screw rotation. Talk about waste! Though we had tons of gag gifts that looked like piles of black dog poo!
  4. The process of rubber injection molding and plastic injection molding are essentially reversed. Plastic requires heat at the barrel, screw and nozzle and cooling at the mold while rubber needs cooling at the barrel, screw and nozzle and heating at the mold. I'm not even going to try to explain the problems with runners, sprue plates or parting lines.
  5. Next comes removal of the molded part from the mold and machine. UGH! You simply cannot revise a standard VERTICAL mold to run HORIZONTALLY, it does not work. The part must be designed to be compatible with the horizontal process, easily removed from the mold and the flashing has to be minimal or you end up having to scrub it off with a rag or wire brush.
  6. These Husky injection machines were designed for speed and at times it takes MINUTES not seconds to remove the rubber part and start a new cycle because on most rubber parts you cannot use ejector pins due to the marks being left in the part and the rubber being fluid enough to flash into the space between the ejector pin and its bore thus fouling the pin and requiring a FULL mold tear-down and cleaning.

After many, MANY months and LOTS of money being thrown at those injection machines I ended up leaving that company about 9 months later. At that time the machines had basically been mothballed until we (now they) had free time to work on them. I stayed in touch with them over the years and they finally did get the machines to run in production. How did they accomplish this you say?

  1. They spoke with a consultant with experience in that process.
  2. They hired said consultant.
  3. They switched to TPR (Thermo-Plastic-Rubber).
  4. They listened to the consultant and are now very proficient in that process!
  5. Oh, and it only took 4 years to get the machines to run production.

So, please enlighten us with the information we need and we may be able to assist you!

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#6
In reply to #5

Re: Compression Moulding

03/15/2018 7:50 AM

ZZZZZZZZZZZzzzzzzzzz

Bazzer

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#7

Re: Compression Moulding

06/09/2018 5:38 PM

Rubber injection molding is the way to go.

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