How to make molded plastic parts at home

How to make molded plastic parts at home

Most people think that producing plastic parts is for big industry.

That certainly used to be the case, but not anymore! Modern materials have made it possible for anyone with a kitchen table to produce extremely high-quality parts, out of a wide variety of plastics.

This became possible with the advent of two different classes of materials. One is RTV Rubber, and the other is Polyurethane Casting Resin. RTV is a generic term that stands for “Room Temperature Vulcanizing”, which means that the rubber doesn’t need heat and pressure to solidify. Instead, it hardens up at room temperature. RTV rubbers typically come in a form that is the consistency of liquid honey.

Each one is different, but normally a catalyst is added to the rubber and mixed in thoroughly, and then the rubber is poured into a box with the original part glued inside. Typical RTVs take anywhere from an hour to 24 hours to set up, and when they do, you have a high-quality rubber mold. It is possible to mold all kinds of items, from doorknobs to dollhouse furniture, to you-name-it! There are two main types of RTV rubbers, Silicone-based and Urethane based.

I recommend a silicone rubber for beginners, although it is a little more expensive. Use one that has a low viscosity, and is intended for hobbyists, as these are not so sensitive to contaminants on the original. They also don’t require expensive equipment (such as a vacuum chamber) to get the air out once it’s mixed. When purchasing Silicone RTV, ask your supplier for a “Tin” catalyzed RTV, not a “Platinum” catalyzed type. Tin catalyzed RTV is much less sensitive to contaminants than Platinum types.

Urethane RTVs are cheaper but usually require a “release agent” to be applied to the mold when casting parts. A list of web sites for suppliers is at the bottom of this page. Some of the makers of RTV Rubbers also have short casting manuals that show you how to prepare a master (original) for casting, and the proper techniques for mixing and pouring the rubber into the mold box to get the best results. The RTV will reproduce everything, so make sure your original is exactly the way you want it. If your master has a porous surface, it is critical that the surface is sealed using paint, varnish, or a special sealing compound such as is made by Smooth-On Inc.

Once you have mold, now you need to use it to make a “casting”. The same supplier that sent you the RTV should have a selection of Polyurethane Casting Resins. These are typically two-part liquids that are mixed together and then poured into the rubber mold. The fast setting ones only take a couple of minutes to set up, and about 10 minutes to reach the state where they can be pulled from the mold.

That’s it! Start to finish in only a few minutes! It’s like magic to watch, actually. The resin in the center will change color suddenly, and you will see the change move from the center of the liquid towards the outside edges. The plastic parts which come out of the mold will be an exact copy of your original (minus a little shrinkage with some resins).

You’ll notice that your supplier has resins in a wide variety of hardnesses, setting times, colors, and prices. Let them know what you are making, so they can advise you as to which resin to use. Resins vary in quality, so don’t use a cheap resin for a part that may take abuse.

The biggest problem encountered with casting and molding is air bubbles. When initially pouring your RTV rubber mold, pour a little bit into the mold box first, and then push it into all the cracks and crevices of the original with a gloved finger or a disposable brush. Then pour the rest of the rubber in, and gently tap the mold box to shake loose any other bubbles. This step will help ensure that all of the detail of the original is captured in the mold.

When pouring casting resin into the rubber mold, the same thing applies! Don’t get the casting resin on your skin any more than you can help it, as some people can develop an allergy to the material from repeated skin contact. Be sure to work in a well-ventilated area, and wear latex or vinyl gloves, and safety glasses.

The safety glasses are particularly important, as mixed resin can cause blindness if it solidifies on your eye. If you see RTV rubber or Polyurethane Resin in your local hobby store, inquire as to the date it was stocked, as most RTVs and resins only have a shelf life of about a year from its manufacture date.

The safety glasses

There are a great many techniques that can be used to cast parts, but they are beyond the scope of this article. Be sure to ask the suppliers listed below for their guides to molding and casting, as they are a huge help to beginners. There are also web sites that have been produced by amateur casters. Use your favorite search engine to do a search on “resin casting””. Good luck casting and have fun!

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