plastic injection molding

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plastic injection molding

Post by zotdoc » Thu May 25, 2006 2:16 pm

I'm thinking about making a home brew small scale platic injection molding machine to make small custom plastic parts. The schemes I've seen involve a heating element, copression ram and injector nozzle for the mold. Anyone have any good circuits to controll the temperature for a nichrome wire element heater capable of heating an aluminum mold and the the plastic reservoir of such a contraption, and anyone have any experience with the process?

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Post by Externet » Thu May 25, 2006 2:32 pm

A kitchen electric range thermostat can do the job, they are very reliable and available, good to over a kilowatt.
How large is the mold and the reservoir, rating of heating elements and size of parts to make?
As experience none, but to make few small parts I would use potting compound or two-part epoxy injected by hand. -no heating, no nothing-

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Post by jwax » Thu May 25, 2006 7:39 pm

Gingery Books has just the book for you zotdoc-

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Post by Chris Smith » Thu May 25, 2006 7:54 pm

You can use commercial heating elements and their thermostats to heat the plastic.

Your main problems is the hydraulics and the mold.

The mold in order to eject fairly perfect must be polished fairly well, or the plastic will become a permanent fixture in the mold.

The hydraulics needed are also fairly high in pressure.

Manufacturing the mold in commercial applications is the big dollar item, with commercial molds costing as high a $75k each for one cubic foot size molds.

You can make your own if you know how, have a CNC machine, or just by hand if its simple but "polish, polish, polish" the final surface! [sand blast also]

You can do it but its not as simple as it sounds.

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Post by Engineer1138 » Fri May 26, 2006 8:12 am

What kind of plastic are we talking about?
You can buy liquid acrylic molding compound at any art/craft store like Michael's

Polymer "clay" is another possibility that doesn't need heating and can be stamped in simple shapes.

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Post by peter-f » Wed May 31, 2006 3:33 pm

Engineer1138 wrote:What kind of plastic are we talking about?
You can buy liquid acrylic molding compound at any art/craft store like Michael's
Well- thats another kind of plastic! Thermoset vs. Thermoformed... one can be re-ground and re molded- the other NOT.

As for small Inj. Mold machines... look for bankruptcies and business closings-- (auctions) some can be had for a song.

The complexity of heating a Pot of plastic to a specific temp and keeping it Equally hot (!) is no small task, either.. just think of your disappointment if the part is scorched before you extract it from the mold.

If the resin is overheated or kept hot too long, the characteristics of the plastic deteriorate- and the colorants are the first to go. that can happen, too, if the heat is not equally distributed in the pot.

As mentioned.. clamping pressure (tons PSI) can be daunting for a DIY project. Are your hoses new (I hope!). And the Inj.Molders here in NJ find the electric rates too high-- they moved west years ago unless they have high-tech capabilities.

I worked in a shop that had a "sampling" molding machine-- run by "hand" - well - the clamp and injecting screw were triggered manually... Wish I had access to that one!

Good luck with your investigations!

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Post by pwillard » Mon Jun 05, 2006 11:10 am


Somewhere in my collection of schematics I have a full schematic (no EPROM source code) for a 80's era, MC6809 based, controller for a small scale production injection machine. I seriously doubt it's a good idea to start with a MC6809 now, but the concept on this commercial controller was simple.

Relays control RAM (with RAM stroke time adjustments) and Heater coils. Simple A/D chips used for Thermocouple with an op-amp controlling Thermocouple input gain.

Used a small keypad and LCD display. Could easily be done with a PIC or AVR microcontroller.

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Home Shop Injection Molding

Post by Shopdawg » Tue Jun 06, 2006 8:28 am

Hi zotdoc

Injection molding on a small scale isn't as difficult as it might seem. While you certainly can pick up a used injection molder on the surplus market for under $5000, some things to consider are :
1) It will probably need three phase power.
2) It will probably be designed to use standard molds made from commercial mold bases.
3) For even the smallest commercial machine you will probably need a forklift to put it in your shop.
4) You will also need "support equipment" such as hydraulic power if it isn't contained on the machine itself, chillers for the mold cooling water, dryers for the plastic molding material....etc.

The molding machines are measures in two ways, Shot size, and Tonnage. Shot size is the amount of plastic that can be shot into the mold with one cycle of the machine (it's usually measures in ounces). The Tonnage is the amount of force pressing the material into the mold cavity(usually measured in tons). More ounces per shot = higher tonnage.

Vince Gingery does have a very good book on the subject of a small bench top injection molding machine. It has a 1/2" diameter chamber that translated into about a 2 to 3 oz shot. That might sound small, but you can product a lot bigger part with 2 - 3 oz of plastic than you think.

The design is easy to build and uses easily obtainable materials and components. The temperature indicator is a modified oven thermometer and the temperature control is accomplished with a thermostat.
While the temp control methods discussed in Vince's book work, Digital temperature controllers are plentiful on Ebay and you can set the desired temp, and forget it. Ebay is also a great place for molding material, cartridge heaters, and other components.

The tooling (molds) for the machine can easily be made from a metal filled castable epoxy. these are used in industry for prototype tooling. they are called "Short Run" molds, but you have to keep in mind that short run in an industrial context is anything under 10,000 shots.
The material is a little on the pricy side at $25 to $30 per pound, but a pound will make several molds. In Vince's book, He also discusses using plaster of paris as a mold material for one or two shot tooling. I haven't tried that, so I can't say anything one way or the other.

As far as the force for injecting the plastic, a lever and a bit of elbow grease is all it takes. One point most people don't realize is that you not only have to inject the molten plastic into the mold, but you also have to hold it while the plastic cools and sets. If you pull the lever back while the plastic is still soft or molten, it will try to suck the material back out of the mold you just shot it into.

I didn't mean for this to become such a long post... Can't wait to see your machine and the parts it makes.
Smoke is an excellent indication that the electrical problem isn't fixed.

From the infamous "ShopDawg's Rules of Industrial Maintnenace"

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