Guys,
I have been really busy at work and this thread started off slowly but I am amazed and grateful at the EXCELLENT info provided here! Thanks everyone, especially Chris and Jim. Hey if anyone is near the greater Boston area and wants a free worker for a week, I'd be happy to work for nothing to learn a little more about this
I feel I have a pretty good understanding of the whole process. I have been reading about mold design for a long time.. the thing that always got me was it was described as a "black art" and very specialized, but it didn't seem like there were really THAT many rules to follow, just a few basics and go from there (i.e. drafts, part thicknesses, gate locations, etc). Maybe the intricasies of mold design are being unveiled quicker due to groups like this!
So based on the excellent posts above, I had a few followup questions. I am assuming (could be wrong) that on the two halves of the mold, one will be fixed and the other will move. Generally the fixed piece will contain the sprue bushing so that you align it with the screw nozzle when you insert the mold then you don't need to worry about its alignment from then on (am I right on that?). Now it would seem to me that for most molds, the part will tend to stay in one side or the other but it will generally always stick in the same side and that's where I would want my ejector pins. Now how are ejector pins actuated? Do they get activated by a seperate mechanism, or does the moving platen coming backwards somehow actuate the ejector pins? And if the moving platen actuates them, that would mean the moving half of the mold would be the one where I want my part to "stick" upon mold opening so that the ejectors can remove the part, and given I want my sprue bushing to be on the fixed part, doesn't that limit my options of where to inject the plastic? Or am I way off here?
I was just looking online and I think I am wrong... I read about "ejector mechanisms" that seem to be standalone mechanical mechanisms not related to the opening/closing of the mold. How does an ejector mechanism work - air powered? electric? hydraulic? On ejector systems, when I look online, I see pics like this:
http://www.royalpins.com/products/ProdRhxHardCore.jpg
Now for many molds, there may not be a flat surface where you can place the ejector pin to push against... so how does that flat ejector pin fit into the puzzle? Are ejector pins sold as blanks and meant to me machined (or ground?) to the profile you need? I picture the ejector pin being totally flush inside the mold with it's other end sticking out the back of the mold. When you push the pin, a small piece of the mold surface will protrude, knocking the part out. Or do I have this all wrong? Once the pin is actuated to remove the part, what retracts the pin? Is it spring loaded or affixed to a retraction mechanism?
I am looking at specs of machines... I use Pro/E to design parts so it can tell me the size and volume of the part so I can determine what size machine I need. I've heard you want to be between 60-80% of the machines "shot size" per cycle, including runners/gates and the part(s). Does that sound reasonable?
And finally (I know, I know, I ramble on.. sorry) can anyone give some pointers about buying a used molding machine i.e. on eBay? There seems to be some great deals on there. Lets assume I determine the size machine I want, is there any common brands to look for? Or does it not matter? Better to buy a newer machine and see it under power before buying? Any pointers or advice would be very helpful in this area.
You guys have helped out so much and I can't thank y'all enough. Its such a great resource to be able to converse with experts on this subject.