Alexei Volkov
Plastic is a versatile material that can be moulded into various shapes, and there are several methods to achieve this. One common method involves melting plastic, such as styrofoam or plastic bags, and reshaping it into the desired form. This can be done through vacuum forming, rotational molding, or by simply heating and reshaping individual pieces. Additionally, polymers can be created at home by mixing ingredients like borax, glue, and food colouring to form a slimy, mouldable substance. On an industrial level, there are eight different plastic-forming methods, each serving a unique purpose, and they can be combined to create complex shapes. The chosen technique depends on the desired shape, precision, thickness, and intended use of the final product.
Characteristics and Values of Changing the Shape of Plastic
| Characteristics | Values |
|---|---|
| Raw Material | Elastomer or Polyurethane |
| Plastic Type | Polymers, PETE/PET or #1, LDPE, HDPE |
| Heating Method | Oven, Double Boiler, Oil |
| Shaping Method | Rotating Mold, Vacuum Forming, Compression Molding, Hand Molding |
| Use Case | Industrial, DIY, Medical Devices, Aerospace |
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What You'll Learn
Rotational molding: heat and rotate plastic powder in a mold
Rotational molding, also known as rotomolding, is a process that uses heat and rotation to transform plastic powder into a desired shape. It is a manufacturing process that uses heat and biaxial rotation to form hollow, one-piece parts from plastic powder.
The process involves placing plastic powder into a mold, heating the mold, and rotating it simultaneously to form the desired shape. Polyethylene is the most common material used in rotational molding, but polypropylene, polycarbonate, nylon, acetal, and polyesters are also used. Polyethylene is a particularly good material for rotational molding because it has good heat stability and can easily form a solid part. When working with polyethylene, a common practice is to use a thickness of 0.125 to 0.25 inches.
The mold is heated and rotated simultaneously, melting the plastic powder, which sticks to the inside of the mold. This process is known as fusion. Once the plastic powder has melted and adhered to the inside of the mold, the mold is cooled and the plastic part is removed. The mold rotates at all times during the heating and cooling phases to avoid sagging or deformation. The rotating trick helps the plastic stick to the mold evenly.
The process of rotational molding is quite slow compared to other high-pressure processes, with only one or two cycles per hour. However, it is a relatively inexpensive process and very little material is wasted. Excess material can often be reused, making it a cost-effective and environmentally viable manufacturing process.
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Vacuum forming: heat and stretch plastic sheet over a die
Vacuum forming is a technique for shaping sheet plastics into 3D shapes. It is a simple, cheap, and fun method that can be done at home. The process involves clamping a sheet of plastic to a frame, such as an aluminium window screen frame. The plastic is then heated in an oven until it becomes soft, rubbery, and stretchable. At this stage, it is important to work quickly as you stretch the plastic over a mould of the desired shape. The mould should be placed on a board with a hole, allowing air to flow from around the object and under it to the hole.
Once the plastic is stretched over the mould, a vacuum system is used to suck the plastic inward, forcing it to adopt the shape of the mould as it cools. After cooling, the plastic can be removed from the mould and any excess material can be trimmed off, leaving a copy of the desired shape. This technique can be used to create an infinite variety of shapes and is a great way to make moulds for casting shapes in other materials such as concrete.
When designing your mould, it is important to consider the draw ratio, which is the height of the part to be formed compared to its footprint. Taller, narrower shapes will require thinner plastic to cover the whole part effectively. It is also recommended to avoid sharp corners and instead use larger radii to strengthen the corners and reduce the chance of webbing, which is the wrinkling of the plastic caused by excessive stretching.
Additionally, the choice of moulding tool material is crucial. Plastic shrinks onto the tool after forming, so the forces on ejection can be significant. Aluminium tools, for example, require a medium to coarse shot blast to avoid air entrapment on the mould surface. Wood tools, on the other hand, are not recommended for moulding materials as the plastic can get stuck in the grain of the wood.
Vacuum forming is a versatile and accessible technique for shaping plastic at home. With the right tools and considerations, you can create a wide range of shapes and moulds with ease.
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Compression molding: heat and pressure-cure plastic in a die
Compression molding is a process used to manufacture composite parts, typically larger, flat or moderately curved parts. It is one of the oldest manufacturing techniques, initially developed for metal replacement applications. The process is ideal for producing automotive parts such as hoods, fenders, scoops, and spoilers, as well as smaller, more intricate parts. It is also used for creating laminated plastics, which are a special form of polymer-matrix composite.
The compression molding process involves placing the molding material, which is often preheated, into an open, heated mold cavity. The mold is then closed with a top force or plug member, and pressure is applied to force the material into all areas of the mold. This pressure ensures that the material conforms to the mold, and it is maintained until the molding material has cured. The typical mold shape is like a clam shell, with the bottom being the mold cavity. A 300-400 ton clamp pressure is commonly used.
The molding material is usually in the form of pellets or sheets, or it may be loaded from a plasticating extruder. For thermoplastic compounds, the whole package must be cooled under pressure before the article can be ejected from the mold. In the case of thermosets, rapid cycle times are necessary to prevent defects and warping. Once the curing is complete, the pressure is released, and the part is removed from the mold.
Compression molding is a high-volume, high-pressure process suitable for molding complex, high-strength parts using either thermoset or thermoplastic resins. It offers geometric flexibility and faster cycle times compared to other methods. The process parameters include molding time, temperature, and pressure.
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Make plastic at home: melt styrofoam in paint thinner
Making Plastic at Home by Melting Styrofoam in Paint Thinner
Making industrial plastic requires a degree in chemistry and access to heavy machinery. However, there are some easy do-it-at-home projects that allow you to make something very similar to plastic with common household items.
To make plastic at home by melting styrofoam in paint thinner, you will need the following:
- Styrofoam
- Paint thinner
- A jar
- Safety glasses
- Gloves
First, cut the styrofoam into small pieces and push them to the bottom of the jar. Next, pour paint thinner into the jar. You should start to see a reaction relatively quickly, so it is important to wear safety glasses and gloves. Put the cap on the jar and let it sit for several minutes. The paint thinner will break down the styrofoam, and the more you wait, the more solid the melted product will become. Generally, it will be at a good moldable consistency between 3 and 5 minutes.
Once the styrofoam has dissolved, remove the plastic from the jar. You will have a moldable plastic substance that you can shape and form. Keep your gloves on and mold it into your desired shape, then let it dry. It should be fully formed and solid within 12-24 hours.
You can also add pigments to create colored plastic. For darker paints like black or metallic blue, use colored styrofoam. For lighter paints like white or yellow, use white styrofoam to avoid tinting the final color.
This method of making plastic at home by melting styrofoam in paint thinner is a fun and creative way to experiment with plastic-making and molding.
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Make polymers: mix glue, borax, water, and food colouring
Making industrial plastic requires a degree in chemistry and specialised equipment. However, there are some simple do-it-at-home projects that allow you to make something very similar to plastic with household ingredients. One such method involves making polymers from glue, borax, water, and food colouring.
Ingredients and Tools
You will need the following ingredients and tools:
- Borax
- Water
- Food colouring (optional)
- Glue
- Measuring cups
- Measuring spoons
- Two containers
- A popsicle stick or a similar tool for stirring
- Safety goggles
Dissolve Borax in Water
In one of the containers, add a small amount of borax and enough water to dissolve it. Stir the mixture until the borax is completely dissolved. If you want to colour your polymer, add a few drops of food colouring to this mixture and stir until combined. Set this mixture aside.
Prepare the Glue
In the other container, add a small amount of glue. Thin the glue out by adding a few spoonfuls of water and stirring until combined. If you did not add food colouring to your borax mixture, you can add a few drops of food colouring to the glue mixture and stir to create your desired colour.
Combine the Mixtures
Slowly pour the borax mixture into the glue mixture and stir as you pour. As you stir, you will notice that the mixture starts to become slimy and may stick to your stirring utensil. This is because the borax cross-links with the polyvinyl acetate molecules in the glue, creating a network of linked molecules that cannot move and flow as easily.
Play with the Polymer
Once the mixture becomes thick and slimy, you can remove it from the container and move it back and forth between your hands. The more you play with it, the less sticky it will become. You can mould it into different shapes and stretch it.
Store the Polymer
To store your polymer, keep it in a sealed plastic bag. If you let the polymer sit out in the air, the water will evaporate, and it will eventually become a solid piece of plastic.
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