Some molds are a simple A side and B side,two halves just open up. There might be through holes,created by core pins that just touch off. A little prussian blue checks the shutoff.
Molds have ejector pins.You don't want those contacting molding surfaces on the opposite side.So the ejector pins are mounted in a return plate,driven back into place by return pins.Return pins need to be flush with the parting line.
What if we are molding a pipe tee or other side action part? We have slides that may travel at 90 deg to the parting line,moved by cam pins that may best at 15 deg,,but they won't hold 15,000 psi injection pressure,so a heel block set into the plates at 18 deg accelerates and locks the slide.The slide has to close and clamp at the same time all the cores and the parting line does.
Since flash in unacceptable in quality molded parts,and its hard on tooling..
And since many plastics flash at clearances over .0004 to maybe .0007 in,
A whole lot of steel plates,inserts,cores,pins and slides all have to be fitted and timed just right.
I'd mostly get it just right with measurements,but these molds represent weeks of work for one man,are worth between $20,000 and $50,000 apiece,and many of the tolerances are less than .001.
And there is a deadline of having product ready to stock on the retailers shelf.
The "B"side of a mold is typically3 or 4 plates laminated together.Most of mine clamped with 40 tons pressure.
So its a real good idea to do a mockup mold assembly using Prussian Blue to verify shutoffs,parting lines,etc.
I use prussian blue a lot building a 1911. You really know how the barrel lugs fit the slide,for example.
