I haven't made a mold yet, but I plan to get into it once I get my CNC machine (next 60 days). I have read a lot of stuff online about part design for molds and my background is mechanical engineering. I know about runners, drafts, radiused corners, gussets. etc. etc. There are a few gaps in my understanding of molding though that I hope someone can fill in for me.
First, what is the mold opening supposed to be like? (where the nozzle goes and injects the melt) Is it a taper? What are the specs of the taper? Does it vary by the machine? Is it just a taper or something more involved?
Second, its my understanding that the nozzle would move into the mold opening with each shot - am I wrong? As I understand, the mold closes, pressure is applied, the melt is injected, it holds while it cools, the nozzle retracts, then the mold opens and the part comes out. Or does the nozzle stay in the mold?
Third, is there any info on water cooled molds? Is it as simple as machining water channels in the back side of the mold, putting a plate over them with a gasket and running water lines? How is the mold cooled - would a leftover car radiator with a pump inline and an automotive 12 fan do? I know it's not "professional" but would that be in the ballpark or are we talking $50,000 chillers to accomplish cooling?
How do you determine the thermal aspects of the mold? I mean, if the mass of the mold was big and it radiated a lot of heat wouldn't it potentially not ever get to operating temp because it would cool faster than the shot of plastic heated it up? Or is this not a real concern?
How do you detemine how many tons machine you need to mold a part? Is there a rule of thumb? I want to make car tail light housings (eventually).
About surface finish... if my mold has a mirror finish, do I get a smooth-as-glass finish on the part (think a brake light lens)? If I make the mold in AL and then anodize it, in my experience it reduces the finish, so would this mean I'd need to use a steel mold (I hope not, I'm talking part counts of 500 pieces or 1000 pieces over the life of the mold at most).
I know the layout of a mold machine... 4 guide bars (from what I've seen) that hold the halves of the mold. Do I just mount my mold in these bars? How does the ram attach to the mold? Do I buy a "mold blank" from the molding machine manufacturer or do I incorporate the fixturing for the ram and retraction mechanism onto the back side of my mold?
How do you determine where the shot will enter the mold and where to put a parting line? On a brake light lens, I know whereI would like my parting line which would imply the shot injection point would be at the top. Are there other considerations other than cosmetic about where to part the mold or inject the shot? Such as the further the melt travels the more chance it has to cool and cause problems?
I think thats it for now
Sorry to ask so many ???'s but I understand the molding process except these parts. I have been looking at books on Amazon but they are so expensive (like >$300 for some of them). I want to lean what I can on my own before I invest that much. I'm also signing up for a moldmaking course at my local engineering school but it doesn't begin until August and I'd like to play around with some molds before then.
Thanks!!!!