Plastics can be injection moulded, cast, printed or machined. Forming is most commonly used for high-volume part production. Using CNC machining is the most popular choice when a high-quality finish and precision cut are required. Most applications are for high-quality, low-volume parts and prototyping. Warping, cracking and misshapen plastics mean that the relationship between raw material suppliers, knowing your machine and the end use are important factors when machining plastic parts on CNC machines. Industrial grade polymers have been introduced into mainstream plastics due to their lightweight, inexpensive and durable composition for a variety of applications.
The stability of the plastic is one of the first considerations for CNC machining. Unlike metals, the properties of plastics can vary by manufacturer and even by lot number. Thermal expansion, outgassing and material stress must be considered. Thermal expansion certainly affects machining processes that must use coolants to control tolerances, but raw material inventories also suffer from tolerance issues. Outgassing is a long-term problem where compounds continue to be released and can lead to part size reduction. Material stress is the least controllable, and this is where experienced machine shops take to prevent parts from deforming during machining.
Plastics can be divided into two categories: thermoplastics and thermosets. The raw material may arrive in the form of sheets, rods or tubes. Interestingly, the color and hardness of the same manufacturer and lot can vary by shape and size. Consistent color can be an issue with many plastics.
Thermoplastics – These plastics can be heated and even melted many times and retain their chemical properties. They respond well to additive compounds, such as glass or carbon fibers, that can alter their chemical characteristics. This is the most commonly used category. Common types you may be familiar with include PVC, ABS, polyethylene, nylon, and acrylic.
Nylon is preferred due to its strength and durability compared to its lower cost. It is also chemical and corrosion resistant.
Acrylic is also prized for its crack resistance, flexibility and high tensile strength. Acrylic is a special example where hardness varies depending on the shape of the raw material. Cast acrylic is a better choice for complex shapes and precision work. Extruded acrylic is less expensive and better suited for food storage and handling, display and packaging.
ABS is a special example of inconsistent colors within the same manufacturer or even within the same batch. The size and shape of the raw material can change the base color from pure white to a yellowed appearance.
Thermoset Plastics – Once cured, these plastics form stable bonds that change the physical properties of the plastic. They are suitable for fillers similar to thermoplastics. They may be crunchy, but won’t squirm once cured. Brands such as bakelite and melamine fall into this category. You’ll be familiar with melamine for durable, heat-resistant kitchenware, but its qualities also provide compounds for flame-retardant clothing and other applications. Bakelite, the first plastic to be invented, became less popular with the advent of newer plastics, but is still used in electrical, automotive and aerospace components due to its low electrical conductivity and chemical resistance.
cutting and clamping
Plastic parts are machined using the same tools as metals, but certain parameters must be considered to handle the properties of the plastic. Some plastics and skilled mechanics can achieve tolerances as low as ±0.0002 inches.
Chip evacuation is achieved through smaller chips, sharper cutting tools and lower angles. Chips must be removed efficiently because if they remain in the hot volatile cutting area, they may melt back into the plastic part.
Plastics are compounds of various chemicals. Additives provide anything from microbes to flame retardant properties. Chemical additives or carbide particles used to achieve the desired heat resistance, flexibility or rigidity will alter the machinability of pure plastics. This affects the cutting method as well as clamping.
Soft buffers are usually used in conjunction with clamps to avoid high pressure on the plastic material. Plastic can easily deform or even crack if you hold it too tightly. The cutting method is usually best if the cutting action is very close to clamping to avoid movement in the plastic.
Material selection plays a key role in successful CNC machining of plastics. Dimensional tolerances and finishing processes are often trial and error as each application can see differences in raw material specifications.
It is important to note that dyes will change color due to heat during processing. Friction is a particular problem since the feed and spindle speeds are 3 times faster than aluminum machining. This higher speed is used in conjunction with other parameters for fast chip evacuation.
If your end-use product requires some flexibility, freezing the plastic before processing may be an option to increase its rigidity for easy cutting. The flexibility of the raw material will allow movement resulting in unwanted rough cuts or breakage.
To achieve a precision finish, many plastic parts are tempered, polished or coated after machining.