The repeatability factor comes quickly to mind when compiling a list of the pros and cons of the injection-moulded fabrication process used in plastic manufacturing. This one discipline is responsible for an incalculable range of parts and products as used in the automotive industry, aeronautics, and any mass-production environment where plastic is the key material. A single mould is all it takes to replace the favourable habitat where machining is the standard fabrication solution. The plastic is relayed in a liquid form to the mould, exposed to pressure until it hardens, and then ejected to a finishing stage. The process is quick and efficient, able to adapt to new technologies, and scale to different industries. It should come as no surprise to hear that moulded plastic fabrication rules, but there are occasions when machining still wins out.
A machined plastic part skips over the problems caused by moulding plastics that create issues in a liquid or near-liquid form. While thermoplastics, thermoset plastics, and elastomers are all used in plastic moulding, some plastics exhibit complex polymerization properties that react unpredictably when heat and pressure are present. This can be a big issue, since both of these elements are key aspects in running an injection moulding operation. For example, ABS (Acrylonitrile Butadiene Styrene), a plastic that’s well known for its impact resistance properties, can blister and crack if there’s a mismatch in the temperature-to-pressure relationship during moulding. Therefore, we can still say plastic moulding is fast, efficient, and reliable, but the technology does carry complex parameters that have to be managed. A constant temperature and smooth flow of source material is required. Venting of the equipment and maintenance to the injection nozzles must be carried out at regular intervals. Changes to these variables can and will cause undesirable weaknesses in the moulded plastic, but a well-tuned operation can quickly eliminate these performance hiccups. Moulded plastic remains ever-popular due to the ability of plastic to conform to every feature of a complex mould with repeatable efficiency.
Regardless of the advantages of a speedy moulded plastics operation, machined plastic remains a simple way to build a prototype without having to setup complex heating and pressurization variables. There’s no need to build complex geometry on an expensive mould that will in all likelihood only be used once, and constant revisions can be machined into the part until the desired configuration is reached. Additionally, as precise a process as pressurized moulding has become, machining still delivers superior dimensional tolerances, and the part is free from weaknesses that could potentially be introduced by a poor injection cycle. We have to conclude that moulded plastics will always rule the mass-production environment, rightly so, but machined plastics have their place. They make excellent prototypes, and the components can be made from any material. Designed to recycle waste material and deliver high productivity, moulding is the industry choice for a winning profit margin, but every versatile factory should have a machining option for the ultimate in adaptability and high-volume production.