When it comes to plastic forming techniques, there are several procedures that can be used to create parts. Some of these are more useful for certain types of plastics than others, depending on the properties needed for the part being created and therefore the qualities needed in the plastic.
How is plastic shaped?
Plastic can be shaped by cutting down large blocks of material, or it can be formed into the desired shape from the beginning without the need to cut things down.
Plastic can begin the manufacturing process in blocks, sheets, pellets, or powders. The starting form is important because it makes certain forming techniques easier, such as injection moulding with pellets and powder.
Blocks of plastic are best for techniques like CNC machining. This is because CNC machining is a subtractive process, and so the overall volume of plastic needs to be large enough to ‘contain’ the finished shape inside. Think of it like a block of marble being cut down into a statue.
However, a thick block of plastic isn’t suitable for melting down or thermoforming.
Sheets of plastic are best used for thermoforming, wherein the plastic sheet is heated until it’s soft and pliable. It’s then stretched over a mould and trimmed to make the desired shape.
With pellet and powders, this form of raw plastic can only be shaped by melting it first. This makes the plastic into a uniform whole which can be worked with. Until heated, raw plastics in a particle form are largely useless. On the other hand, thermoplastics are ideal in this form as it makes them much easier to measure into the right amounts and melt into a uniform liquid. Therefore, thermoplastics will often come supplied in this form.
Plastic forming techniques
Here are some common forming plastic fabrication techniques and the kinds of plastics that would be used with them:
CNC machining, as mentioned earlier, typically starts with a block of plastic which it then cuts down. The subtractive work itself depends on the tools being used. Some CNC machines will focus on cutting, whereas others will drill and mill to create other features.
CNC machining can be done with a lot of different plastics, including acrylic, PVC, and high-strength plastics like nylon.
Injection moulding melts thermoplastics and extrudes them into moulds. Once cooled, the liquid plastic has become solid and hard, adopting the shape according to the mould and needing only some post-processing to get the full and final shape.
This plastic forming technique uses plastic pellets made up from materials including polycarbonate, polypropylene, and acrylonitrile butadiene styrene (ABS).
Thermoforming and line bending
Using thermoplastics in sheets, these two forming techniques create different results, but both use heat to manipulate plastic without melting or breaking it.
Thermoforming heats the plastic until it’s soft enough to stretch out. This stretched plastic is formed over or into a mould, using a vacuum or pressure to aid the process. This method creates items like plastic cups easily.
Line bending uses a heating element to make a plastic sheet malleable in a specific area, allowing it to be bent to create curves in the material. Once left to cool, the material rehardens and holds its new shape. Line bending uses thermoplastics like polycarbonate and PVC.
Fastening, bonding, and welding
Sometimes pieces need to be bonded together to create a larger composite. There are several techniques that can be used to join pieces in a way that’s tough and long-lasting.
Mechanical fastening uses parts like screws and bolts to keep parts held together. This method creates strong results and can be easily repaired when the fasteners eventually need replacement, but it doesn’t physically join the plastic components together in the same way as solvents or welding; it just holds them together.
Adhesive bonding uses industrial-strength glue to hold parts together, with solvent-based adhesives actually softening the surface of the plastic so that the parts can chemically bond. Similarly, welding melts the top layer of plastic on each half, allowing the melted plastic to mix and join, creating a composite part when it cools and hardens.
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