Plastics machining – types and techniques

Plastics – in other words: a vast variety of polymers – are an inseparable part of our daily life.  Although they are usually connected with PET bottles, there are types of plastics that have high thermal and mechanical rigidity. These features can be used in many branches of industry and applications. 

Well-machined and manufactured can replace many parts made of metal alloys – especially in places where low mass and efficient cost of production are more important than mechanical resistance.

What is plastic machining?

Plastic machining is a process of shaping and manufacturing parts out of blocks of polymer, usually in lossy processing (the excess material is removed) and the final parts have a lower mass than the block it is made off. Plastics and polymers machining is usually made with

• machining tools (lathe, mills, also CNC plastic processing),
• handheld tools (drills, saws, chisels, files, etc.),
• hydraulic and mechanical presses (eg. to bend bigger elements),
• lasers, plotters, or water in high pressure (eg. Waterjet).

Contemporary plastic materials are usually easy to shape and machine; some of them have high mechanical rigidity and good chemical and thermal resistance while maintaining lower mass than the same parts made of metal alloys. 

That makes plastic components a good alternative for many metal elements in a vast variety of industries. The best example is the production of light gear wheels in transmissions with low torque (eg. for electrical power windows). It is worth mentioning the differences between machining plastics and 3D printing. Said technology, although young for many industries, has many applications and also is based on polymers (eg. PET), and recently – even allows printing parts from aluminum and titanium. 

Still: 3D printing is more "building” parts than "machining” – that is why we only mention that subject in that article. Also – machining is focused mostly on altering the physical form of a block of polymers. The production of said blocks is another subject that we only mention. That process focuses on melting plastic granules into a desired form (eg. injection molding or rotational molding) and is called "plastic processing”.

Types of plastics processing methods

There are many types and technologies to shape and manufacture plastic machined parts. The increasing popularity of 3D printers and the growing availability of simple plotters and laser cutters make small-quantity production and quick prototyping widely available for almost anyone. 

Said technologies allow the production of small amounts of elements, usually for the needs of small home businesses and so on. On the other hand – the mass production of plastic parts requires a much more efficient and advanced machine park. 

Currently, most of the industries that require plastic-made parts focus on their production in the well-known process of machining polymers. Said parts are manufactured in a vast variety of professional machining devices. 

Utilization of said machines allows the production of high precision and extremely low tolerances of machined parts. That precision translates to high repetitiveness and a decrease in quality control discards. 

The most popular methods of shaping and machining plastic parts are:

• plastics machining – with the use of traditional (automatic) mills and lathes. That technology allows a quick shaping of the material with cutters and tool bits. Traditional machining is perfect for the production of small quantities of parts and the utilization of templates can make it even faster. The availability of tools and ease of use make that type of manufacturing polymer parts popular in small shops. On the other hand – even with the use of templates, said the process is time and work-consuming and is not efficient for the needs of mass production,
• plastics bending – performed on special presses and other devices/tools to bend and give angles. That form of shaping plastics utilizes many types of templates and can be – to some extent – automatized. Bending is usually performed on bigger objects (eg. profiles, piles, or panels). The process itself is usually divided for:
  • cold plastics bending, a modification of part shape within the limitation of its natural flexibility. That method is simple and efficient. In addition – allows us to achieve a better and more repetitive bending curve. The most popular types of polymers for cold plastic bending are polycarbonates and PETS, sometimes called poly(ethylene terephthalate), commonly used in 3D printing,
  • heat plastics bending, also called thermoplastic bending – shapes plastic elements at high temperatures, according to the specification of the processed material. The heating part increases its plasticity and eases the process;
• CNC machining – with CNC mills, lathes, and sanders. It is the most popular way of mass-producing polymer parts for many industries. Said technology provides very high precision and repetitiveness of production while maintaining negligible differences between proceeding parts. Utilization of CNC plastic machining devices fastens the manufacturing process of complicated parts for many industries (eg. gear wheels or 3D parts),
• plastics laser cutting – is a relatively new technology for shaping and producing polymer parts, very popular in prototyping and production of small amounts of elements. The desired shape of the part is cut by a high-energy beam of light. That process can generate big amounts of heat – it requires an efficient head removing system, especially for the cutting surface, and well-dialed parameters of work, according to the processed material. Currently, professional laser cutters provide high precision of work and allow the production of high quantities of parts. On the other hand – said tools are many times more complex and expensive than plain home-grade CNC lasers.
  
  

Tools used for the plastics machining

The most popular types of tools used to machine plastic parts are:

• machining tools (eg. mills and lathes that remove the material with specialized cutting tools, also CNC controlled),
• brake presses and other bending devices/tool,
• laser cutters/plotters,
• water cutters (Waterjet), often numerically controlled, are sometimes called "CNC cutters”.

Each of them allows the production and shaping of polymer parts. Just like in metal machining, the selection of technology to produce desired parts is strongly related to the characteristics of the part, its desired shape, and the material characteristics. Selection of the perfect device to produce said part is usually on the side of the professional machining services shop, based on customer specifications. 

The production of flat parts (2D) out of polymer sheets is usually performed on CNC-driven cutting lasers. High precision and the possibility to cut even the most complex shapes make that technology the most efficient for said task. Similar precision can be achieved by the utilization of Waterjet. 

On the other hand – production of the cylindrical shape objects out of polymers (eg. pins and bushings) is the most efficient on lathes. The utilization of CNC-driven lathes allows to speed the process and decreases tolerances between manufactured parts. Production of more complex parts can be achieved thanks to milling machines, especially numeracy controlled. It allows the shaping of plastic objects in 2D and 3D and the production of complex parts (eg. gear wheel rims) and even spatial objects (eg. with the usage of 5-axis milling machines). 

Presses and other bending devices allow the shaping of a polymer part to its desired shape (eg. plastic housings). Plastic milling is a vast subject, that is why we only mention it. On the top of mentioned machines and devices, there is a vast variety of other tools that allow you to shape and form laments made of polymers. Saws, hand-held mills, or orbital sanders are perfect for casual plastics machining but have no match to industry-grade devices and professional machine plants.

Plastics and polymers most popular for machining

Currently, industry-grade plastics machining shops are using a selection of polymer types. Said materials can differ by mechanical and thermal properties – such variety makes finding the best-suited material for the production of desired parts much easier. Among the most popular polymers for industry-grade production, it is worth mentioning:

• polyoxymethylene (commonly called "POM”), with good abrasion and thermal resistance (can be used for a long time in temperatures up to 90°C / 194°F). That polymer is commonly used in many industries, from small home appliances to the automotive,
• polycarbonates (commonly called "PC"), a transparent polymer, similar to acrylic glass, but with better mechanical abilities. The most significant feature of that material is very good abrasion resistance (much better than the acrylic glass), ease of machining but notably higher price,
• the acrylic glass (PMMA, plexiglass), transparent material with good mechanical properties and UV resistance. That material is a perfect example of a "good enough” type of polymer: it is light and is better suited for most applications,
• polycaprolactam (polyamide-6, PA-6), is a polymer with poor mechanical properties but is easy to machine and very cost-efficient. Commonly used in the production of electronic parts due to its "good enough” properties and low cost,
• high-performance polymers, such as polyetherimide (PEI), with very good mechanical properties and high resistance to temperature, acids, and abrasion. Another example of said type of plastic is a polyamideimide (PAI) with very high resistance to fatigue corrosion. These polymers are expensive, yet provide low mass and very good mechanical properties and are used in hi-tech applications.

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