This is Part Two: how to Injection Cast Urethane low volume high quality plastic parts in a Silicone mold. Sometimes also referred to as Reaction Injection Molding (RIM), the process allows you to meet your low volume, injection molded plastic part needs with out aluminum or steel tooling. In my case I just need a few high quality parts, but they need to be as strong or stronger than an injection molded part.
You will need the following for this project.
-Two part resin, I recommend a Quality Urethane from Innovative Polymers I will be using IE-3076
-Digital cooking scale for accurate mixing of the resin
-Protective gloves to keep your body from getting sticky and absorbing harmful materials
-Colorant, a pigment of some sort to make your parts a specific color
-Wax paper or Aluminum foil to keep resin off your work surface
-Kitchen Timer so you know how long you have to work your resin
-An good place to work
Optional: Vacuum Tank/Chamber and a pressure tank for bubble free parts
Step 1: Mixing Your Urethane Resin
There are many types of resin out there with all kinds of hardness (shore) properties. This is great if you need a flexible part for something like a mobile phone case, or if you need a super strong part like I am making.
I set out my digital kitchen scale set to grams (metric is way more accurate don't waste your time with oz.)
I tear the scale with the dollar store cup on it to zero and add the Resin (yellow tinted), about 30g. Make sure you mix according to your manufactures specifications. At this point you would add a colorant to tint your resin. There are many kinds out there as well, most resin suppliers will sell tints as well. As long as it's not water based you should be good (water moisture will cause the resin to bubble and you will have lots of air bubbles in your part)
Next mix the Hardener according to the specified weight ratio for your product and start your timer. I know that this resin has about a 14 minute working (pot) time. It allows me a lot of time to mix and De-gas the resin and get into the mold.
Mix your resin really well, scrape the sides of the cup numerous times and get down into the bottom edges of your cup. Notice how I am using a squared off mixing stick and not a rounded one like a tongue depressor, see video for more details
Step 2: De-Gas Your Resin in a Vacumm Chamber If You Have One
Once the resin is thoroughly mixed we quickly place it in the vacuum tank, see video
I use a three gallon top load tank with a 4CFM vacuum pump. If you don't have a Vacuum tank you can still make the parts, but they my have air bubbles in them. Some resins are also a bit less susceptible to air bubbles from mixing. I would suggest one of those if you don't have a tank to De-gas your material.
You can see the bubbles being pulled out of the resin in the video. Once you have a full 30hg of vacuum and the bubble stop then you know you have gotten all the air out of the resin and you can use it to cast your parts. With this resin I usually have about 5-6 minutes still left in my 14 minute working time to actually get the resin into the silicone mold.
Next: getting the resin into the mold!
Step 3: Getting the Resin Into Your Silicone Mold
Injection time: Now we need to get the resin into two part Silicone mold. In the video I begin by pouring the resin into the mold. I show this because it does not work well. You will need to inject the resin into the mold with a syringe. You will need a syringe that has the capacity of the cavity of your mold. You may need a bigger or smaller syringe than I have, depending on the size of your part you are making.
Plunge the stopper of the syringe to the bottom, and place it in your De-gassed cup of resin and draw the syrup into the syringe making sure to always keep the tip in the fluid so that you don't introduce any air into the resin.
Once you have enough resin in the syringe, inject it into your mold slowly as you wait for it to level, and come out of the vent hole.
Once it's filled with resin you are done. If you have a pressure tank then you can go to the next step.
Step 4: Add Some Pressure and Some Time to Get Good Parts
If you don't have a Pressure tank you can still make parts. The tank however will allow you to cast parts that are bubble free. I use the one in the video for most of the casting work that I do. Theoretically the parts should be bubble free since we already evacuated all the air out of the resin, but this is the real world and our system is not closed so there does exist the possibility of air having entered the system. (plus urethane react with the moisture in the air to expand to create air bubbles) So just in case we did not remove all the air in the de-gasing process we will attempt to crush whatever air is in the resin down to a microscopic level by placing the mold with the resin into the tank under about 60 PSI of pressure.
I am using a converted paint pressure tank tank as a pressure pot for my casting. They are readily available on eBay.
Let the resin set up under pressure. Once it's set you are finished and ready to de-mold your part form the silicone mold.
Step 5: Demold and Clean Up, You Are Done!
Let the air slowly out of the tank and remove the silicone mold and the part. I am very gentle in the video as the part has cast threads and I don't want to rip the silicone and ruin the tool! Take your time. You should get a great part if you followed all the steps. If may take you a few times to get a perfect part. It's totally achievable with some practice and patience. There should be very little clean up, if you made a good silicone mold. Just trim the Sprue and Runner at the gate and you should be done.
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Leave your comments below and let me know how your project turns out.
About Me: I am an Industrial Designer living in Southfield MI USA. I have a home-based Product Design Studio called “Botzen Design”and have been designing consumer products for 25+ years ranging from sunglasses for Bauch & Lomb, Traps eyewear, entry level luxury vehicles for Ford, wireless charging PowerMat for Homedics, to magnetic toys for Guidecraft. I specializes in tabletop and handheld products, ranging from WiFi routers to cosmetic products to Bluetooth devices and everything in between, I also teach Industrial Design at Wayne State University and CCS (College for Creative Studies) Follow me on Twitter @botzendesign and Subscribe to my Youtube channel Here.
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