Secrets of building a Plastic injection moulding machine

[Spence]

After someone on the forum posted about this book by Vincent R Gingery, I ordered two copies to be sent here to the UK. Just a warning that Lindsay Publications Inc. insisted on charging shipping on each item regarless of the fact that my two copies were being shipped together. That's daylight robbery. I was under the impression that that sort of skullduggery only happened in Rip off Britain.
Such a shame as the book is pretty good. I will not use Lindsay Publications Inc. again as long as I have a hole in my ass.

[DoctorX]

concerns about the injection molding book...

please see this web site:

http://www.benziemi.com/plasticinject/

very brief, but some cautions are mentioned about this machine.

[Spence]

Lindsay publications denied charging double. i can't be bothered chasing it up further. as regards the machine, the shortcoming of it are quite apparent when you read the book. I can live with this as I have evry intention of improving on it on the first build. i don't really have a lot of time to piss about making one so it's got to be right first time. One big issue I have with it is that most injection machines have some method for evacuating the air in the mould prioe to injection. The small amount of plastic that this machine will inject is plenty for pickup covers, knobs, trem and switch tips etc.
On balance this book is excellent value for a builder who compalins component parts being unavailable. I couldn't believe it when I ordered one set of Strat covers from AllParts only to find that all they had in stock was one pack of three where two covers were parchment and one was white. So I'll be making my own. i have already successfully moulded 'bakelite' '54 Strat covers and some P90 soapbar covers. It's a good buzz when you make your own.

[DoctorX]

injection molding machine from the UK...

http://www.crclarke.com/Products/Injection.htm

check this one out.

[Spence]

Excellent machine but you guys are gonna shit when you see the Rip-off Britain price.

£1,197.00 + VAT

VAT is at 17.5% which translates at $2,664. That's before you make any moulds and it will only handle 25 grammes or 1 oz of plastic the same as the Gingery machine would. Then you'd pay import duty and have to get a step-up transformer for use in the US.

Such a shame. Nothing's easy in the UK.
I could buy one of these right here and now but the mould making is the big issue.

[Spence]

Here's a one I found on Ebay.

Item 320050042019

[DoctorX]

UK injection molding machine and others

I guess the question is, how many bobbins can you make to pay for the unit. By the way, I did some basic calculations, and the shot size for one humbucker bobbin is about .213 ounces (6.03 grams), not counting the runner and the sprue, so a small machine with some over capacity should do the trick -- assuming the mold is made properly.

Realistically, if some of you are handy out there (and have the time and a small welding outfit (wire feed welder from Pep Boys for $179.00)), you could build one that would work as well as the one from the UK. You would have to be careful not to make it too flimsy. Use heavier gauge steel to make the structural and clamping components. There was an article some time ago in Model Engineer Workshop (a UK magazine) where a Gingery injection molder was built (and modified very slightly) and the builder managed to bend a 1/2" rod of tool steel very easily using hand pressure because of a misalignment. He rebuilt the unit (and fixed the misalignment problem) with a heavier ram. He made some model airplane components using a threaded insert, that looked pretty good to me.

Regardless, what is not being mentioned here is the need for a substantial shield between you and the injection molder while it is operating, particularly over the mold area itself. Molten plastic burns. A lot. And it sticks to you while it is molten and hot. Use at least 1/2 inch piece of Lexan or some other shatterproof material between the mold, the nozzle, and any other place where molten plastic can shoot out at you, and you. Wear goggles. Did I mention molten plastic at over 400 degrees?

Remember, even with a manual machine you are applying many thousands of pounds of pressure to force molten plastic through a small opening into a mold. Use inferior material for the mold itself and / or the machine, and accidents will happen.

The real kicker is control and as Spence mentioned, mold making. There is a great article by this guy who used some rapid prototyping material to make a quick test "mold" fitted to a Morgan Press to make some parts. Very revealing information about the Morgan Press, as well as molding houses like "Protomold". If you study a bobbin closely (separately, not in the article mentioned below - there are no bobbins mentioned in this article), you can see where the parting lines are, where the side cores go, where the ejector pins push on the plastic, and you will see the gate where the plastic enters the mold cavity. Don't forget, plastic shrinks as it cools. How much depends on the plastic material and the thickness of the walls of the plastic part itself. Which is why you see holes in areas of some plastic bobbins where nothing goes (between the screws or slugs, for instance). To thin out areas to allow even cooling with less distortion as it shrinks.

Here is the link:

http://www.cs.berkeley.edu/~sequin/P...o_MSthesis.pdf

You can use servo motors or stepper motors to closely control some of the functions of a small injection molder (clamp pressure, clamp time, stroke length, pack and hold time, etc), then repeatability becomes easier to attain and modify.

More later....

DoctorX

[NightWinder]

Great info.This is the next step!

[corduroyew]

Excellent machine but you guys are gonna shit when you see the Rip-off Britain price.

You never really know the whole rip off britain thing untill you are here. Before I moved I was holding off on buying lots of tools untill I got here. Now I'm planning on going to America just to get tools.