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Plastic machines are rated by clamping pressure. The "rule" is 2 tons of pressure per cubic inch of surface area of your part, including the runners. I treat this as merely a guideline but that is what the pros say. This is another limiting factor, not just shot size.
My hard learned lesson is to start with simple things and work your way up. If you look at the troll I made on the blog it took some time because it is organic and had complex parting lines. I was trying to run before I could walk, so to speak. He still stands on my shelf and I like him. Someday I will make more things like it. The star trek engines were also complex, but not quite as bad. A simple flat mold is much more likely to work out. And smaller molds are in some ways better. I worked on a 32 cavity mold for a client that was full of torsos, heads, etc. Out of the 32 cavities about 3 were crap. So the whole thing was a waste of time. Breaking the sprues up in to smaller bites like a lot of the Wargames factory stuff means that you have to run more cycles to make your product, but if the mold is crap it is less to replace.

 poda_t wrote:
how much work would it be to turn the sprues into something useful?

From my understanding of how the injection process works it would be fairly difficult to add proper details to the sprue. The reason that they are always smooth is because the plastic that enters first into the mold cools much more quickly than the liquid plastic behind it which forms a plug of material that needs to travel down the sprue past the intake point of the parts. The plug is too stiff to make it around tight corners or press properly into details.

Think of it like a cork being pushed down a length of pipe. Once the plug moves past the intake the hotter liquid flows down into the detail area.

If you have a bunch of ribbed surfaces for instance on the sprue then the plug would likely get snagged up on the irregular surfaces and cause it to foul up, in best case scenario you'd get some detail on the sprue but it'd be "soft " as the harder plug material wouldn't take proper shape as easily.



The manual machines can use epoxy molds so it's possible to make casts directly from a model green and not have to resort to having to machine every part. They use an epoxy that has metal embedded in the forumla so it's very strong material. From what I remember it runs about $50 ish to make a 2x3 mold die (and you also need a metal form to create the mold which runs about $150 but it can be reused over and over). The epoxy molds wear lots quicker than a metal die but if you are using it simply to run small batches or test parts before investing in a milled die it saves a ton of money.


It's been quite a while since I was digging through services but here's a couple links I found that might be useful

http://www.injectionmolder.net/

http://www.protomold.com/

http://www.makeyourownstuff.org/