Omar Khalid
Casting plastic models can be done cheaply and efficiently with a few simple tools. The process involves creating a mould and using resin to create the desired shape. It is important to prepare the plastic model by cleaning it thoroughly and considering whether to use a mould release spray. The mould can be made with Lego, lined with plastic wrap, and filled with Plaster of Paris. The resin should be mixed thoroughly, with added dye if desired, and poured into the mould. The mould can then be left to set and dry before removing the plastic wrap and sanding the model.
| Characteristics | Values |
|---|---|
| Materials | Resin, silicone, plasticine, plaster of Paris, plastic wrap, modelling clay, glue, blue tack |
| Tools | Miniature paint mixer, electronic precision scale, nitrile gloves, bowl/cup, mixing sticks, coffee, dremmel/hacksaw, Lego |
| Techniques | Remove plastic part from assembly, clean thoroughly, spray with mould release, use two-part mould, weigh item, mix resin and colourant, pressure cast, sand and trim |
| Tips | Oil tools before cutting clay, use water to mix plaster of Paris, use toothpicks to cut mould, use UV rays to harden resin |
| Safety | Resin is carcinogenic, wear gloves to avoid constant exposure to chemicals |
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What You'll Learn
Use modelling clay to create a box for your mould
When creating a mould for plastic casting, you will need a "master object" (the original clay creation you are reproducing) and a "mould box" (the negative space in which you'll pour silicone or plaster to create a cast). The mould box doesn't have to be a literal box shape, but it does need to fit your master object.
For small, flat-bottomed clay pieces, you can use a plastic cup as a mould box. Round cups require less silicone than square containers. Make sure the cup is larger than your master object by about half an inch to three-quarters of an inch, so the mould is thick enough to support itself. Secure your master object to the cup with super glue or hot glue.
For larger pieces, you can assemble a custom box mould. Start by placing your master object on a foam core board and drawing a perimeter with half an inch to three-quarters of an inch between the object and the intended box edge. The mould box can be rectangular or any other straight-lined shape.
You can also use modelling clay to create the mould box itself. Clay is great for sealing mould boxes and can serve as a containment area for testing cure inhibition against the model material. For example, you can embed one portion of the model in clay up to the desired parting line, then pour rubber to make the first half of the mould. Once the rubber has cured, remove the clay and pour the second half of the mould.
Polymer clay is a good choice for creating moulds as it is cheap, easy to work with, and doesn't cure until baked, so you have plenty of time to work.
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Prepare your plaster by mixing it with water
Preparing plaster for plastic casting is a relatively clean and simple process. However, it can be challenging for beginners. Here is a step-by-step guide to help you prepare your plaster by mixing it with water:
Firstly, ensure you have the right materials. You will need plaster, which can be purchased at most art stores, a plastic bucket, latex or rubber gloves, and a paper or plastic cup for scooping the plaster. It is also important to work in a space that can be easily cleaned, as plaster can be messy.
Next, weigh out your materials. Calculate the amount of plaster and water you will need. You can determine the volume in cubic inches and then divide accordingly to get the desired amount in gallons or quarts. Don't forget to deduct 20% to account for the volume of plaster.
Now, it's time to add the plaster to the water. Slowly sift the plaster onto the surface of the water. Do not dump or toss it in. This step should not take more than 3 minutes. Allow the plaster to soak for 1-2 minutes. Soaking ensures that each plaster crystal is surrounded by water, removing air from the mix. The soaking time is crucial: too short, and it may cause pinholes; too long, and it will lead to early stiffening and a gritty surface.
Let the plaster sit for 2-5 minutes before stirring. This step ensures the plaster has fully absorbed the water, resulting in a harder mix. You can stir with your hand, wearing a glove, or use another instrument carefully. Stirring by hand has the benefit of releasing air bubbles. Pour the mix steadily and evenly to avoid creating air bubbles. After filling your vessel, gently shake your worktable to release any remaining bubbles.
Finally, wait for the plaster to set and cool down before proceeding with your project. Remember that plaster gives off heat during the setting process, so be cautious.
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Remove air bubbles by tapping the mould
Air bubbles can form in your casting for two main reasons. Firstly, the mixing process may have introduced air into the mixture. Secondly, the design of your mould may trap air during the pour. To remove air bubbles, you can try "burping" the mould by tilting it during and after the pour. This lets the trapped air escape.
If you are still having trouble with bubbles, you can try casting the part under pressure. However, if your mould was not made under pressure, you may need to try other methods to remove air bubbles. One method is to use a handheld vibrator to remove bubbles from the surface of the mould. You can also try "thumping" the container on the ground several times before letting it sit for a minute or two before pouring or casting. This will cause the bubbles to rise to the top, and you can watch them disappear.
Another way to remove air bubbles is to create air vents in the mould. To do this, you will need to punch holes into the mould with brass tubing and an exacto knife. The vents allow air to escape and minimise bubble entrapment in the finished casting. It is important to plan where you will place these holes, as they should be over the highest points in the top half of the mould.
Finally, you can try to minimise the number of air bubbles that form in the first place by being careful during the mixing process. Stir gently, moving in a figure-eight pattern, and avoid whipping air into the mixture. Be sure to scrape the sides and bottom of the container while mixing, and pour gently to avoid creating turbulence.
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Use nitrile gloves and an electronic scale for safety and precision
When casting plastic models, it is important to prioritise safety and precision. Nitrile gloves and an electronic scale are essential tools to achieve this.
Nitrile gloves are made from synthetic rubber, which offers several advantages over other glove materials. Firstly, nitrile provides superior chemical resistance, protecting your hands from exposure to harmful substances. This is particularly important if you are working with chemicals during the casting process. Additionally, nitrile gloves are less likely to cause skin allergies compared to latex gloves, as they do not contain the proteins found in latex rubber that can trigger allergic reactions.
Nitrile gloves also offer excellent tactile sensitivity. Their snug fit allows for greater precision and control when handling small model parts or electronic components. This can be crucial when assembling intricate plastic models, ensuring that you can work with the necessary dexterity and accuracy.
The durability of nitrile gloves is another key benefit. They are puncture-resistant and tear-resistant, making them ideal for repetitive tasks and providing long-lasting protection. This durability also means they can withstand the rigours of daily use, making them a cost-effective choice over time.
An electronic scale is another essential tool for safety and precision in casting plastic models. It allows you to measure and weigh your materials accurately, ensuring that you use the correct proportions and ratios in your casting process. This precision can make the difference between a successful cast and a failed attempt. Additionally, using an electronic scale helps in maintaining safety. By accurately measuring substances, you can avoid using incorrect ratios that may lead to hazardous situations.
In conclusion, when casting plastic models, using nitrile gloves and an electronic scale is a prudent decision. These tools offer safety, precision, and accuracy, helping you work with confidence and achieve successful results while protecting your hands and ensuring the proper handling of materials.
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Polish and sand your plastic model after demoulding
To polish and sand your plastic model kits after demoulding, you can follow these steps:
Firstly, apply a thin coat of grey primer—a fast-drying nitrocellulose primer is recommended as it dries within 15-20 minutes and shows any imperfections in the model. It is important to be cautious during this step, as applying too much primer or letting it flow onto the model can damage the plastic parts. Once the primer has dried, use 1000-grit sandpaper to sand the model, removing any imperfections and dust. If necessary, you can apply another thin coat of grey primer and repeat the process.
Next, apply a coat of white primer or black primer, depending on your desired base colour. After this, wet sand the model with 1500-grit sandpaper to remove any remaining dust. If there are still grey dots or large grey areas, repeat the previous step.
Now, apply two thin coats of lacquer and let them dry completely. Nitrocellulose lacquers are a good option as they can be polished easily and effectively. Once the lacquer is dry, it's time for another round of wet sanding. Use 2500-grit sandpaper and soapy water for this step, ensuring that you only sand in one direction (forward-backward or top-bottom) and never in a circular motion.
Finally, to achieve a polished finish, use a polishing cloth and polishing compound to polish the paint.
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