CNC Machining Polyethylene
Polyethylene is a family, not a single material. Low-density PE is film and squeeze bottles and is almost never machined. High-density PE (PE 300) is the everyday machining grade. PE 500 and PE 1000 — the latter better known as UHMW — are the wear materials: chain guides, liners, star wheels and sliding pads that outlast steel in abrasive service. Picking the right one is most of the job.
Which polyethylene
Molecular weight goes up the table; so do wear resistance, impact strength and machining difficulty.
| Form / grade | Best for | Notes |
|---|---|---|
| LDPE | Flexible parts, seals, prototypes | Very soft, hard to hold and to get a clean edge on; rarely worth machining |
| HDPE / PE 300 | General machined parts, tank fittings, food-contact | Best all-round machinability; see the HDPE page |
| PE 500 (HMW-PE) | Wear strips, guides with moderate abrasion | Noticeably better wear than PE 300, still cuts cleanly |
| PE 1000 (UHMW-PE) | Chain guides, wear pads, liners, star wheels, impact parts | Outstanding abrasion and impact resistance, self-lubricating; gummy under the cutter, long stringy chips, will not hold a sharp edge on thin sections |
| UHMW anti-static / glass / oil-filled | Conveyor and packaging machinery | Modified grades for static dissipation, stiffness or lower friction; name the grade on the drawing |
Sharp tools, light clamping, patience with UHMW
All polyethylenes machine with the same rules: very sharp, high-rake tooling, high spindle speed, a definite feed so the cutter cuts rather than rubs, and air or a light mist to carry heat away. Heat is the enemy — the material softens around 80 °C, and a rubbing tool melts the surface into a smeared edge.
UHMW-PE adds its own habits. The chips come off as long, tough ribbons that wrap the tool, so chip breaking and clearing matter. Drilled holes close up slightly after the drill leaves, so we drill and then ream or bore to size. Sawn or milled edges carry a stubborn burr that is trimmed by hand.
None of the polyethylenes like clamping pressure. Plates are held on vacuum or with many light clamps, machined, released, and skimmed again to remove the distortion the first pass introduced. Thermal expansion is high, so we state the temperature at which tight dimensions were measured.
| At a glance | |
|---|---|
| Service temperature | LDPE around 60 °C; HDPE and UHMW around 80 °C continuous |
| Chemical resistance | Excellent across the family; attacked only by strong oxidisers |
| Moisture absorption | None |
| Weakness | Soft, high thermal expansion, creep under load, poor UV resistance unless black, cannot be glued or painted |
| Typical parts | Chain and belt guides, wear strips and pads, liners, star wheels, spacers, cutting boards, food-contact parts |
Three rules for a polyethylene part that stays flat
Match the grade to the job
PE 300 for anything static or chemical, PE 1000 for anything that slides or is hit. Specifying UHMW for a simple bracket costs more and machines worse for no gain.
Design flat parts with room to move
Long thin plates in polyethylene warp as stress releases and as temperature changes. Slotted holes, generous clearances and a stiffer backing plate save arguments later.
Use inserts and shoulders, not adhesive
Nothing bonds to polyethylene. Threaded inserts, through-bolts with washers and dovetail keys are the reliable joints.
Finishes that work on polyethylene
Polyethylene parts are delivered machined and deburred; small parts can be tumbled. Coatings and paints do not adhere, so the sheet colour is the finish.
Send the polyethylene drawing
Send the drawing and the grade, or the drawing and a sentence about what the part has to survive. You get a price, a lead time, and an engineer’s note on anything that will fight the tolerances.