I’m pretty sure the cast into silicone is under extremely low pressure, in fact a vacuum. Pushing hot melted plastic under pressure into a silicone mold would be asking for distortion.
We tried casting into silicone with the machine too, but still had distortion. But we didn’t think of using a vacuum. Not sure if that would have solved the issue. The molten plastic itself still has weight.
Injecting wax into silicone moulds is often done with a combination of Vacuum and pressure. Typically a short vacuum to evacuate as much air as practical without collapsing the mould which can cause distortion in itself and then wax injection at around 0.5 bar up to a maximum for filigree or really difficult moulds of 2 bar.
Seems fine for jewelry - but I’m still skeptical it could be done for mechanical parts like gears.
I can understand you about the materials… it took me quite a lot of time and money to find the right materials, properties and techniques before I reached perfect castings. My company is making 99% engineering parts, most of them are complex and with tight tolerances.
There are a lot of great urethanes with properties very, very close to those of the thermoplastics infact. After trying a lot of brands, I can say we are now settled using Axson and BJB - I can hardly tell the difference between them and thermoplastics. We are using urethanes close to ABS, PP, Nylon, high temp, rubber and fire retardant, there are rigid urethanes which are fine with temperatures >150C. I am also making overmolding - steel inserts, pins, rubber, etc.
Of course it took me a lot of time to get perfect results - most of the magic is when you prepare the part for molding and especially creating the parting line and vents. This is not something I can describe in a few words, it just takes a lot of thinking, reading & practice.
Here are some parts that I have vacuum cast:
https://drive.google.com/drive/folders/0B3BqL-smlR6RbDl0VFMzMTN1RDQ?usp=sharing
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Right now I’m using a couple of pressure chambers I have made, 2 air compressors and one small vacuum pump for degassing silicone. I don’t degass the materials, I’m doing mostly pressure castings, e.g. I’m casting the urethane in the mold by hand and then put it in the pressure chamber. For degassing silicone I’m using a small vacuum pump and the pressure chambers to put the material in.
I’m thinking about making a small vacuum casting machine (which is working in a different way than pressure casting), with almost no electronics and a few levers. It will work the same way as the ones on the market, but way cheaper then them. The cheapest vacuum casting machine I know starts from 20.000EU…
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I had useable parts injecting into ABS plastic, just not silicone that was mentioned.
I was interested in using something flexible like silicone because it is forgiving for undercuts.
My engineering company does production medical devices (mostly CNC), but also a little bit of prototyping, too. We have a Morgan injection molding machine which is great for prototype plastic parts.
We often use silicone RTV for the mold. It works fine and is accurate, too, within reason. We have used styrene and polycarbonate. We have also made plastic patterns for investment casting.
Think of RTV as a non-compressible fluid. Do not give it anywhere to go under pressure! If it has no place to go, it cannot distort much. Make an aluminum enclosure that fits the pattern closely. The sprue opening deserves special care because it is the one place where the RTV might bulge out. We have used several hundred psi without a problem if the RTV is constrained properly. I cannot go into any more detail as the exact procedure is a trade secret.
This technique works great for wax, too. (Really, really well!)
Bill Box
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Yeah, we would always use an enclosure for the silicone molds. Still never got a gear to come out good enough to use.
I agree in that scenario it can’t distort much. Much being the key word.
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I don’t understand this mentality since even without giving it anywhere to go, it is still compressible.
The other issue we struggled with was with just about any attempts under pressure blew plastic out the parting line which caused distortion to the mold. We tried up to shore 85 - anything more rigid defeats the usefulness of a flexible mold.
I think we are talking about different techniques here. I haven’t seen anyone doing injection molding in silicone molds. It is very different than vacuum casting polyurethane in silicone molds and I am not sure you can get the same accuracy and quality doing it this way.
Can you show us some photos of the results?
For parts that allowed it, we also tried casting molten PLA versus injecting it (one example is the gears shown below).
Sure, here are some examples. All parts shown are PLA plastic.
This silicone mold was made from the steel bolt shown in the center (actually it was a slightly longer bolt):
The Buddha in the foreground has kevlar and carbon fibers in the body. It and the other statues where created from a silicone mold of the light blue Buddha on the left.
These parts were injected into a printed ABS mold shown on the right. The first one injected we had coated the mold with some carbon powder which is the black on in the rear. Each following one had less carbon because it wasn’t added for the subsequent parts. These parts came out with high precision and ultimately went into service on a large format 3D printer.
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One example of a non-compressible fluid is brake fluid. The brake fluid does not compress when you use your brakes in the car. Be glad that it does or we would all have problems stopping our cars. All hydraulic systems act this way. Water acts the same, too. Most fluids act this way.
Generally speaking, solids and fluids are not compressible. When pressurized they may distort, but they do not change in volume to any significant degree to cause issues here.
The key is not to allow the RTV to distort and therefore not giving it somewhere to go when pressure is applied. If you had the aluminum cavity completely constraining the RTV mold improperly designed allowing the plastic to run out, you allowed the RTV somewhere to go. There was a space in your aluminum constraint design allowing the RTV to distort and it allowed the plastic to run out.
Machine two aluminum blocks with a cavity with about a 3 mm void around the printed pattern. Put the pattern into the aluminum blocks and fill that 3 mm space with RTV. Hardness of the RTV (or urethane for that matter) does not matter. After curing remove the RTV and split it to remove the pattern. Re-assemble the cured RTV into the blocks and inject plastic into the void left by the pattern in the usual way. Remove plastic piece and repeat.
I cannot legally show photos here because of the confidential nature of our work. I have given all the information I can. If designed properly, this really works. If you understand how fluids and solids act under pressure, you can build the mold properly. I can try to answer questions, but I cannot design the system for anyone. Do not be put off by the science. Just make it so the RTV is properly constrained by the aluminum parts.
Bill Box
marktwo.com
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I did think the same way, but I guess it is because we were using 3D printed plastic containers that must have flexed.
In any event, this is not something we are working on anymore, I was just sharing my experience with silicone since it came up.
The best use for this material is hobby molds that are fairly basic. The hobby machines don’t have either the closing tonnage or the injection pressures/speeds of an industrial machine.
The industrial use isn’t necessarily for running several different details in a single die. It should be useful for early prototypes that will see production. The customer pays for what is essentially a production die minus the cavity details. Printed details are relatively fast to produce and can have surface finishes at the prototype phase. This eliminates cutting details only to find that they have changed/don’t fit.
I received my high temperature resin Wednesday. I should have run the first tests by the end of the week… but this will be in a ‘real’ machine. 50-ton.
Out of curiosity how does your machine stack up to mine (pic in the original post). Please excuse my ignorance as I know absolutely nothing about injection molding machines…i just run our printer and am learning Solid Works!
We just got our resin over the holiday weekend and will also be doing some testing with it by early next week. I will be eager to see your results.
This is very exciting! I can’t wait to see how well this works.
kevinduhe,
Please give me the exact model of your press… I can’t quite get it from the pic… I will let you know the press model of what we use (as well as the MUD fixtures), once we have completed a successful run.
I am teaming up with a molding house of mine (a favorite vendor) for this experiment/exercise. They have tried many rapid tooling methods over the years, but nothing has been utilitarian enough… it either takes too long (vs, rapid machining) or too costly.
We’ve already done the math for cost and time… and so far so good… with the Form 2.
I’ll be sure to include the press, parameters, material, and MUD set provider.