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This Christmas, one of the most popular items for kids will be bought and given thousands of times over. Whether it be a plane or car kit, model planes and model cars are still as popular with kids as they were decades ago. Adults now spend time putting together more complex kits of the same type.

Most of these kits are comprised of lots of plastic parts that have to be popped out or popped off plastic grids. Have you ever wondered how are these parts and grids made? Could you make spare parts at home? Let's take a closer look at how plastic parts and grids for these toy kits are made and whether or not you can make spare plastic parts at home.

Plastic Injection Molding

Plastic injection molding is the industrial process whereby thousands of things are made from plastic. You can always tell that something was created from injection molding at a place like Accurate Products Inc. because there are extra plastic pieces that have to be snapped or cut off of the parts you need. These extra bits of plastic are the direct result of creating a grid mold and/or a lead-in line to the mold for the plastic injection machine.

As the melted plastic is injected through an opening, it goes in a line down to the mold, fills the mold, and stops. When the plastic product is removed and cooled, you have the extra pieces of plastic leading from the outside of the mold injection machine to the mold inside. With model plane and model car kits, a mold grid is designed to connect all of the smaller molded parts together to prevent them from getting lost. It also helps with the molding process, as it would be too costly to design thousands of very tiny, separate molds for each and every piece of a kit.

New Molds for Every Model

Additionally, for every model kit, an engineer has to design new molds. Every model has a mold grid, consisting of a square or rectangular frame and several components attached to the inside of this frame. Every model has its own set of mold grids, which is why you typically cannot use parts from one model to replace missing parts in another model. Since the plastic is also dyed to fit with the model type, and dye lots in plastics are never the exact same shade from the first couple of thousand injections to the next, you could get two of the "exact" same kits and have them be just a very slightly different shade of army green or navy blue.

Making Your Own Replacement Parts Via Injection Molding

There is a way you could make replacement parts by recreating injection molding at home. It is very difficult to do and takes a great deal of skill, but you could try it. You would need a double boiler or slow cooker to melt the extra bits of plastic you have, a bar of soap, some soap carving tools, a drill with a small-boring drill bit, a plastic syringe, and something to cut the bar of soap in half lengthwise.

To start:

1. Use the drill to put a hole, dead center, in the end of a bar of soap. This is going to be your injection hole.
2. Cut the bar of soap through the center of the hole and lengthwise through the bar of soap to the very opposite end of the soap. Be careful not to break the bar of soap when you do this!
3. With the carving tools, mark and carve out the shape of the missing piece from your model kit. the duplicate piece (if applicable) should fit right into the carvings you made on both sides of the soap.
4. Melt the extra plastic and then draw it up inside the syringe. 
5. Hold both halves of the soap together as you quickly inject the melted plastic into the hole in the end of the soap. 
6. Wait about a half hour for the plastic to cool. Then chip the soap away and you will have your replacement part!

You may have to repeat this a couple of times to get the hang of it, but you will learn a new skill in the process.