'Difficult' Plastic Machined To Close Tolerances

High-density polyethylene (HDPE) is a versatile polymer mainly used for injection moulded bottle tops, food chopping boards, and milk crates, but it is not normally machined.

High-density polyethylene (HDPE) is not normally machined as it becomes unstable when its temperature varies.

However, Regenesys is one company that needs to turn, mill and water jet profile this difficult material for the manufacture of energy storage equipment, and they approached contract machinist, Holifields, to machine the required parts.

The work was assigned following the success of an initial contract involving water jet cutting of 1,200 x 800 x 3.8 mm thick sheets of carbon-impregnated HDPE to close tolerance on the contract machinist's Flow CNC water jet profilers.

Said Ashley Morley, Production Buyer at Regenesys, "The results have been impressive, with dimensional tolerance of mill-turned HDPE components held to within +/- 0.05 mm.

They are supplied to us just-in-time by Holifields in kits of six components that form a sub-assembly for our new regenerative fuel cell modules.

"Moreover, there was a considerable reduction in cost compared to that charged by our previous subcontract supplier during the R and D phase of the project.

One of the reasons for this is that they were set up for prototype and small batches whereas Holifields have the plant and resources at its Abingdon premises for the longer runs we need now that we have entered the production phase." On the measures they take to machine the temperature-sensitive material, Roy Marshall for the contract machinist commented, "We cut a lot of different plastics but this one is most testing of all.

It is important to pre-machine it and then finish it in a second operation, keeping the cross-section as large as possible to reduce heat generation.

"We use carbide-tipped tools with as large a clearance and rake angle as possible, subject to maintaining the required strength of the cut edges.

Feed speed is kept low and depth of cut per revolution is under 0.5 mm.

In this way we minimise the cutting force in the direction of the cut.".

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