One of the main attributes of plastics materials is their ability to be moulded into a finished component with no need for subsequent work to be carried out. Complicated shapes, holes and undercut features can be moulded into the component using tooling and moulding techniques. However all this comes at a cost of tooling expense.
Why Machine Plastics?
Moulding tools and forming equipment used in the various plastic moulding processes are invariably hand made one off creations. They can often take weeks and months to manufacture with a resultantly high cost. Where a plastics component is specified and the numbers to be used are not large, then machining the component becomes more economical. Not all plastics materials can be machined. The more rigid a plastic then the easier it is to be machined. The more flexible and the softer plastics are not suitable for machining.
What are the main points to note when machining plastics?
The cutting tools used in the machining of all materials rely on the rigidity of the component being cut.
In the case of cutting metals, the materials' natural rigidity is good. Therefore the component resists distortion when the cutter (saw, drill or machine bit) cuts the metal.
In the case of plastics, machining tends to lend itself better to rigid materials, such as fibre reinforced thermosetting plastics materials, glass reinforced nylons, acrylic or PEEK have good relative stiffness. Less rigid plastic tends to deform and bend away when the cutter attempts to cut the component, making the achievement of fine dimensional tolerances difficult.
Aavantages of Machining Plastics
No mould costs are needed
Ability to manufacture plastic components with short lead times
Ability to manufacture low volumes economically
Can trial a design before committing to tooling
Thicker wall sections can be accommodated
Components too large to be moulded can be machined from fabricated plastic
The forces required to machine plastics are low
Plastics normally machine dry
Swarf can be recycled back into the compounding process
Disadvantages of Machining Plastics Materials
Machining ability limited to the more rigid plastics materials
Relative high cost of block plastic material
High scrap (relative to other plastics forming processes) can result
High volume of swarf to be removed can present difficulties
High costs of CNC machine time
Volume production by machining will require robust jigs and fixtures
Plastics materials do not conduct away any heat generated in the machining process
Dust producing composite plastics require an effective dust collection system
Considerations When Machining Plastics Materials
Due to the softer nature of plastics materials, the holding jigs and fixtures have to be designed with jaws which protect the plastic being machined, this can be with other plastics materials shaped to the form of the block being machined. In addition the jigs require to be robust in order to support the material being cut.
Thermoplastic Plastics Materials being machined can be cooled with an air blast providing the resultant swarf is continuous and not in chipping form. Thermosetting plastics can be cooled using a liquid coolant, but care needs to be taken in terms of plastics prone to swelling in water to ensure that machined dimensions do not change.
Heat generated in the process can cause thermal expansion – this effect must be factored in as dimensions may alter on cooling.
Methods of Machining Plastics Materials:
CNC Machining If the component to be cut has a complex shape, its profile can be programmed into a computer. A CNC machining centre can be used to manufacture duplicate numbers of components. Multiple interchangeable cutters typically used on CNC machines enable complex and varied components to be machined.
Turning If the shape to be achieved is round, then a simple turning operation can be used. Specialist supplementary equipment attached to the lathe can extend the capabilities of the lathe's operation.
Milling This method of machining can vary from simple milling to profile and CNC milling. Again as with lathe work, either additions to the milling machine, or the use of a more complex milling machine can extend the milling machine's capability to make more complex shapes.
Sawing Invariably this method of machining is solely for parting off sections of plastic material from bar stock for subsequent working by other machining operations.
Die Cutting In certain cases the use of die cutting of plastics material can produce a simple component. The process is limited to sheet material. A male and female die are used to punch out a predetermined shape. The process can be either a manual process or automated using a special machine.
Hot Knife Cutting The softer less rigid types of plastic can be cut using a hot knife to slice through the plastic. An electrically heated wire or blade melts the plastic locally. This type of process is commonly used to cut blocks of foam and Expanded Polystyrene (eps).
Punching Certain shapes can be cut on metal type punching presses. Like a CNC machine, they are invariably computer controlled and are multi tool bit equipped. This process is limited to the thinner thermoplastic and thermoset sheet.
Water Jet Cutting This process is used to edge trim fibre reinforced thermosetting components, which would otherwise prove difficult to trim by other processes. The tough reinforcing layers in the material defy trimming by conventional knives and cutting equipment. The narrow cutting path and fast progess without dust or chippings are an advantage.
Separating Acrylic and laminated sheet can be separated by means of scoring using a sharp knife and breaking about the scored line.
Laser Cutting This process can be used for cutting and profile boring of certain types of acrylic and other plastics although not thermosetting. The process uses an industrial laser to melt the plastic often with computer controlled profile following.
Ultra-sonic Cutting Some of the softer thinner plastics can be cut using ultra-sonic equipment. The high frequency generated by ultra-sonics in the tool, have the effect of locally melting the plastic being cut. Again, integrated with computer profile control the process lends itself to high speed automated production lines.