Sunday, February 26, 2023

Preheating Molds Often Isn’t Necessary, but sometimes…

Recently on an Internet group I saw an often repeated mocking of preheating soft plastic bait molds. I assume they mean aluminum molds. I make aluminum bait molds for soft plastics (and lead) at CNC Molds N Stuff, so I thought I might have some thoughts with which to weigh in.

This was my reply:

I’m not sure why this is a hill to die on. In my learning to make molds, dealing with customers who want molds for the “nearly” impossible, and making baits on a morning when its so cold in the shop the fluorescent light cans need a while to warm up I’ve seen times when preheating might be beneficial for hand injection.

“Oh, but if you were a competent mold maker you wouldn’t say that!” Maybe. I’ve made a lot of molds and often I can see why a design might not work or might have issues just from the sketches and description. Some I learned the hard way by making them. More recently I’ve documented the potential problems based on my experience and rough understanding of the thermodynamic process involved, made sure the customer had my opinion, and let them make the decision. I’m not always right, but often enough to be convincing at least to myself that I have a rough understanding of what’s going on.

“Well, shoot it two or three times and it will be hot enough!” Maybe. Not always, but sometimes I would agree that will work. The thing is that a beginner or corner of the garage hobbyist who had to convince the significant other to let them buy one (1) quart of plastic out of the bread and egg money may not want to remelt that (unknown brand to you and me) plastic two or three times potentially reducing the remelt life of their plastic.

“Well if they can’t afford the hobby they shouldn’t be making baits anyway!” Maybe. If it means their baby’s will starve certainly. If it’s the difference between new seasonal tea towels and some plastics then it’s really a personal matter that is none of my business. Neither choice is wrong.

Back to the topic. A “bad” mold may well require preheat. The mold may be “bad” because the mold maker is inexperienced, but it may also be “bad” because the buyer refused to listen to their mold maker. Maybe it can be forced to work by pushing the heat limit of the plastic and preheating the mold (for the first shot or two anyway). Maybe it can be forced to work by using a pressurized plastic injection system and cranking up the pressure to more than the user is capable of by leaning on their hand injector.

Some molds certainly do not need preheat. My frog molds (Curly Buzz Frog) and stick molds (Club-O) work fine from the first shot to the last with maybe shrinkage from poor technique and too hot plastic being a more likely issue than a cold mold. Others like the RT35 and RT45 like a “warm” mold and plastic shot slowly closer to the high end of the working range. On a cold day in the shop I can see the first shot or two having issues. The fine details need time to let the air float/level up to the vents at parting line. If the plastic cools to quickly because its cold in your garage and the molds are cold to the touch maybe its an issue.

“Well Bob, obviously you make some bad molds then.” I try not to, but I have. Some just to try things, and others because customers insisted I was wrong. I don’t think I generally make bad molds, but maybe. I have in the past while learning the basics. I have to recognize that your experience, knowledge, and understanding may be greater than my own. I just don’t think the extreme point of view is 100% correct in most things including the view that preheating deserves to be universally mocked.

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