Casting from plastic. Scheme, description

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Casting from plastic. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Ham Radio Technologies

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Small parts of models are usually made of plastic or metal by machining. However, when a lot of them are required, it is better to cast parts from plastic in plaster molds.

Plastic can be prepared from AKR-7 acrylate powder mixed with a solvent to a doughy state. Both necessary components are available in denture sets.

If the part is symmetrical and has a simple configuration, such as an anchor, a wheel or a lifebuoy, then it is more convenient to make the form collapsible, from two halves that can be used repeatedly.

When forming, metal or plastic casting models are pressed into the gypsum that has not yet hardened to the plane of symmetry (partition) and allowed to harden. The second half of the mold is poured, after wetting the lower part with a mild soapy solution to facilitate the separation of the halves. The upper part of the mold should have a through hole: a diameter of 3-4 mm for filling with plastic and exiting its excess during polymerization. Having taken out the casting model and folded both halves, they are pulled together with twine or rubber - and the mold is ready.

Plastic molding
Rice. 1. Collapsible form: 1 - form base; 2 - upper half; 3 - mounting rod (wood); 4 - plastic model; 5 - cardboard box.

For the manufacture of a part of a complex configuration, a mold consisting of three or more parts is required. Here it is better to use the method used in precision casting - according to the investment model.

Plastic molding
Rice. 2. Non-separable form: 1 - plaster casting; 2 - paraffin model; 3 - mounting rod (wood); 4 - cardboard box.

In this case, the form will be one-piece, that is, disposable. The model for its production is made of paraffin, poured with a solution of gypsum, providing in the uppermost part a cylindrical or conical hole with a diameter of 3-5 mm widening upwards - a sprue. After 30-40 minutes, the frozen gypsum mold is immersed in a vessel with cold water with the sprue up and boiled until the casting model is completely melted. Paraffin, being lighter, is displaced from the mold by water and floats to the surface. Then, without removing the form, the water is cooled and a layer of solidified paraffin is removed from its surface. To remove its residues from the mold, heating should be repeated.

Thanks to this method, we obtain a cavity in plaster that accurately reproduces the shape of the part. If fasteners are desired inside the future part, then they should be inserted into the paraffin casting model so that they do not move during melting. For example, if you want to have a metal nut in the part, then it is placed with a protruding bolt and everything is poured with plaster. Then the free end of the bolt will be in the thickness of the gypsum and, after melting the paraffin, will hold the nut in the right place.

With a non-separable form, plastic is prepared in glassware, bringing it to the consistency of liquid sour cream. After mixing the mass with a glass rod, pour it through the sprue hole into the mold. The preparation of the polymer and the filling of the mold are carried out without delay, since the mass thickens quickly. At the time of pouring, the mold must be moistened from the inside, otherwise the surface of the finished part will turn out to be rough and porous.

The filled form is kept at room temperature for 15-30 minutes until the rubber-like thickening of the mass. After that, it is compacted through the sprue hole with a glass or wooden stick. The compacted surface of the mass should not reach the upper edge of the mold by 3-5 mm, since acrylate expands during polymerization.

The sprue hole is covered with moistened cellophane and plywood trim and tightened with a clamp. Then the form together with the clamp is immersed in a pan with water at room temperature and brought to a boil by gradual heating for 30-40 minutes. It should not be violent and lasts at least 45 minutes. After that, the heating stops, and the form is kept in the pan for another 15-20 minutes. To obtain a better casting, a slow complete cooling of the mold to room temperature is necessary.

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