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Mold Failure and Why - Plastic Injection Tests
🕑 Added 2025-05-01 22:34:22 +0000 UTCSo I started using my plastic injection machine and oh boy it has taken a bit to get it to work just right. Still in the "tinkering" stage as this is all new to me and very little information on things so I am learning as I go. The mold up top was one of my first test molds and once printed I saw all the glaring issues with it, the biggest one was the overhangs.
This is my first mold and it printed fine but I was a bit too rough taking it off my resin printer and clipped the bottom of it. You can't really see it from this angle but it was a large chunk. No matter since the inside was fine. So you can resin print molds without an issue, just make sure you use high temp resin for your resin 3d printer. I am using a high temp Siraya translucent resin which I highly recommend if you are wanting molds that can take a beating. The metal topper I bought separately from the same company that created my plastic injection machine Buster Beagle 3D. They sell metal parts and molds for you to use on your own molds. Making metal molds is very costly and this way you can skip all that and make molds in a few hours at home.
The two parts connected well, this resin has little shrinkage so I was overall pretty happy with how easy it was to print. The metal puck clips in like a lego piece making it very easy to use on a few molds.
I made a little bump out for the muzzle so it was overall easier to make this mold fit with the current metal puck I had. Making the mold itself isn't too complicated since you are working with simple shapes like boxes. I like using the translucent resin (it does come in black and white) so I can see when the mold is full of plastic.
So I mentioned above the dreaded undercuts, oh boy did I learn the hard way. So I was being extra ambitions with my first mold and blanked on how many undercuts this test had which was my mistake. So let's go over why it failed. The neck went in, you can see it in the above photo showing the render of the mold, that locked the part into place. The ears are thinner and it needs more pressure to go into those areas to really fill it but they are also in a weird area to pull out making them easy to get stuck in the mold. As you can see I did not have enough pressure or material to fully fill this mold and I did not get a muzzle. The small pot holes I see on skin also show lack of pressure so overall the idea was partially there but not quite. So what do I now? Create a new mold where the ears are their own mold and cast separate, maybe remove the hair as it is small and more intricate. I have limitations with my machine as I can only go up to a pressure of 90 PSI for my air compressor. Most of the molds I was testing with a pressure of 60 PSI which was too low so I have cranked it to 80 now and going to see how it compares to fill the molds more evenly. The other issue was heat, I was heating the plastic too low of a temp.
This is the material of choice, Polypropylene is an excellent easy material to cast. You can make action figures out of this. The melting temp is lower than other plastic and usually more predictable. My machine caps at 300 C which is more than enough heat to handle this material. I was melting the plastic too low before which made the material too goopy and not runny enough to fill the mold correctly, so I am working on new molds to re test this with the right heat and pressure. I got these plastic pellets from a company online called LNS but these can also be found in bulk on eBay since a lot of plastic companies just want to get rid of their not so perfect plastic pellets so you can find this in a lot of colors. Right now I am testing so any color is fine until I fine tune what I am doing.
So I tested a few things, did test my metal mold that I use now as my "main test" which is shaped like a little heart which gives me an idea of how my pressure is too much. But I made a resin mold out of regular resin and the results were gross. So the nose shape, yes it has a seam in the middle as the mold was split there just to create a quick test came out odd. Aside from the lack of proper heat, the lack of good pressure regular resin is not strong enough for heat and pressure combined. The mold itself develops cracks as you use it and the walls of the mold tend to have tiny pits after one use which was expected. I had some old resin I wanted to use in my machine so it was just a test. I plan to re do the nose test again and try it with the high temp resin and see what works best when it comes to pressure. So the first tests have been a learning process but I am slowly getting there. I knew when I was getting into this it would take some testing until I got it just right to be able to replicate parts easier. I want to be able to make my toys using this process but it will take some time to get it tweaked just the way I need it.
Will continue posting my findings until I start to get predictable results and document my process as I go.
