GlossaryKind of term: Manufacturing termPlastic stock shapes
What are plastic stock shapes?
Plastic stock shapes are bought-in rod, plate, tube or block in plastic, extruded, compression moulded or cast, from which you machine the part.
Besides round rod, plate, tube and block there is hollow bar, in materials such as POM, PA, PE, PEEK or PTFE. They are extruded, compression moulded or cast, and the production method helps decide the sizes available, the internal stress and the properties by direction. As with metal, you machine the part out of them.
What does the quality of plastic stock depend on?
Extruded rod and plate can carry internal stress that is released during machining; stress-relieved stock usually stays flatter after milling. Thick rod and plate can differ in the core from the edge, with residual stress or porosity. Purchase tolerances on diameter, thickness and flatness are often looser for plastic than for metal, and stock is often supplied oversize on the nominal dimension. How much allowance you need depends on grade, size, production method and the part's requirements.
How do plastic stock shapes work in practice?
The choice of stock shapes the route: a plate that moves calls for roughing, resting and finishing, or for symmetrical machining. Saw loss, a minimum purchase per plate or bar, and the price per piece, per metre or per kilo all count in the material line. With expensive grades such as PEEK they carry more weight.
What are plastic stock shapes often confused with?
Stock shapes are not injection mouldings: in moulding the part takes its shape in a mould, with stock you machine the shape out. Nor is cast stock the same as extruded: the same grade can have different properties and sizes depending on how it was made.
How do you see plastic stock shapes in Blake?
In Blake metals and plastics sit in one materials catalogue. Your own purchase price for each material is recorded with its source and history, and the estimate's material line calculates with it; the result is labelled as an estimate. More on materials.
Frequently asked questions
Why does a plastic plate bow after milling?
Because internal stress is released when you remove a lot of material from one side. Stress-relieved stock, symmetrical machining or roughing with a rest before finishing limit it.
How much allowance do you take on plastic rod or plate?
Enough to remove the skin and the distortion; how much depends on grade, size, production method and supplier. Ask for the stock tolerances, because in plastic they are often looser than in metal.
Does saw loss count in the material line?
Yes. What you saw off, you bought. With an expensive grade such as PEEK it adds up, and a minimum purchase per plate or bar belongs in it too.