High-Density Polyethylene (HDPE) is the default engineering plastic when the design requirements are chemical resistance, low moisture absorption, and impact toughness — without the cost or weight of metals or the stiffness demands of Nylon or Delrin. With a tensile strength of 26–33 MPa, near-zero moisture uptake (<0.01%), and broad compatibility with acids, alkalis, and cleaning agents, it covers the majority of CNC-machined part requirements in food processing, chemical handling, marine, and general industrial environments.
Two factors govern whether a machined HDPE part performs correctly in service: grade selection (standard HDPE, HMW-HDPE, and UHMW-PE have meaningfully different property profiles) and thermal management (HDPE's high coefficient of thermal expansion — 100–200 × 10⁻⁶/°C — requires controlled machining strategy for tight-tolerance features). Both are covered in full below.
At a Glance
| Available grades | Standard HDPE, HMW-HDPE, UHMW-PE |
| Standard tolerance | ±0.25 mm |
| Precision tolerance | ±0.10 mm (staged roughing and finishing required) |
| Min wall thickness | 0.75 mm (Standard / HMW-HDPE) · 1.0 mm (UHMW-PE) |
| Max part size | 1,200 × 500 × 150 mm (milling) · 394 × 152 mm dia. (turning) |
| Surface finish | As-machined Ra 1.6–3.2 µm · Precision pass Ra 0.8–1.6 µm |
| Lead time | 1–5 Days |
| Certifications(vetted network) | ISO 9001:2015 · AS9100D · ISO 13485:2016 |
| Ready to machine HDPE parts?
Upload your CAD file and get DFM-reviewed pricing, with lead times from 1–5 days across Standard HDPE, HMW-HDPE and UHMW-PE grades. |
HDPE Grades for CNC Machining
HDPE is not a single material — it is a family of grades differentiated primarily by molecular weight. The grade determines impact resistance, wear behaviour, and how the material responds to the cutter. Specifying the wrong grade is the most common source of functional failure in machined polyethylene parts.
Standard HDPE
Molecular weight range: 200,000–500,000 g/mol.
The default grade for the majority of CNC-machined HDPE applications. It offers a balanced combination of stiffness, chemical resistance, and machinability at the lowest cost in the family. Forms clean chips, holds acceptable surface finishes with standard carbide tooling, and machines efficiently on both milling and turning equipment.
Specify Standard HDPE when:
the part requires chemical resistance, low moisture uptake, or dimensional stability in wet environments, and tolerances are ±0.25 mm or wider.
High Molecular Weight HDPE (HMW-HDPE)
Molecular weight range: 500,000–1,500,000 g/mol.
The longer polymer chains in HMW-HDPE produce measurably higher impact resistance and better wear performance than standard grade, without the machining difficulties of UHMW-PE. Surface finish is slightly more demanding to achieve — feed rates typically need to be reduced 10–15% compared to standard HDPE to avoid surface tearing on finishing passes.
Specify HMW-HDPE when:
the part is subject to repetitive impact, sliding wear, or abrasive contact and standard HDPE has shown inadequate service life. Typical applications include conveyor wear strips, impact pads, dock bumpers, and sliding liners.
Ultra High Molecular Weight PE (UHMW-PE)
Molecular weight: ≥ 3,500,000 g/mol.
UHMW-PE offers the best abrasion and impact resistance in the polyethylene family — it outperforms standard HDPE significantly on both measures. However, its machining behaviour is different enough that it is treated as a distinct material in process planning. Chips are fibrous rather than discrete, heat management is more critical, and dimensional control is harder to achieve and maintain. Tolerances tighter than ±0.25 mm require careful staging and are not always achievable consistently.
Specify UHMW-PE when:
abrasion resistance and impact toughness are the primary design drivers and tolerances are not tighter than ±0.25 mm. Typical applications are bearing liners, star wheels, chain guides, and bulk material handling surfaces. Where tighter tolerances are required, standard HDPE is the more reliable choice.
Grade Comparison at a Glance
| Property | Standard HDPE | HMW-HDPE | UHMW-PE |
|---|---|---|---|
| Molecular Weight (g/mol) | 200,000–500,000 | 500,000–1,500,000 | ≥ 3,500,000 |
| Tensile Strength (MPa) | 26–33 | 25–30 | 20–28 |
| Impact Resistance | Good | Very Good | Excellent |
| Abrasion Resistance | Moderate | Good | Excellent |
| Machinability | Excellent | Good | Moderate |
| Achievable Tolerance | ±0.10 mm | ±0.10 mm (staged machining required) | ±0.25 mm |
| Relative Cost | Low | Low–Medium | Medium |
| Best For | General parts, housings, fittings, food contact | Wear pads, conveyor components, impact surfaces | Liners, star wheels, chain guides, bulk handling |
| Not sure which HDPE grade fits your part?
Our engineers will review your drawing and recommend Standard, HMW, or UHMW-PE based on your tolerance, load, and environment requirements. |
HDPE Material Properties
Values below are indicative reference ranges for engineering-grade HDPE stock used in CNC machining. Always confirm against the specific supplier datasheet before finalising structural calculations or tolerance stacks.
| Property | Value |
|---|---|
| Density (g/cm³) | 0.94–0.97 |
| Tensile Strength (MPa) | 26–33 |
| Yield Strength (MPa) | 15–30 |
| Elongation at Break (%) | 500–800 |
| Flexural Modulus (MPa) | 800–1,400 |
| Hardness (Shore D) | 60–70 |
| Glass Transition Temperature (°C) | –110 |
| Melting Point (°C) | 125–135 |
| Heat Deflection Temp @ 0.45 MPa (°C) | 45–60 |
| CTE (×10⁻⁶/°C) | 100–200 |
| Moisture Absorption (% by weight) | < 0.01 |
| Flammability Rating | UL 94 HB |
| Dielectric Strength (kV/mm) | 18–20 |
Dimensional Stability and Thermal Behaviour
HDPE's moisture absorption of less than 0.01% by weight means parts are dimensionally stable in humid, washdown, and submerged environments. No moisture allowance is needed in the tolerance budget — unlike nylon, where equilibrium moisture uptake of 2.5–3.5% produces measurable dimensional growth after machining.
The dimensional risk is thermal. With a CTE of 100–200 × 10⁻⁶/°C — roughly 5–10× higher than steel — heat generated during longer cuts causes real dimensional shift. Parts measured immediately off the machine can read within tolerance and shift out after cooling.
Rule of thumb: For ±0.25 mm or wider, standard machining practice applies. For ±0.10 mm or tighter, use a staged roughing and finishing strategy with a stabilisation period between passes, and measure after the part has returned to ambient temperature.
Tolerances and Design Limits for CNC-Machined HDPE
Achievable tolerance in HDPE depends on grade, feature size, and whether the controlled machining sequence described above is used. The values below reflect what is consistently achievable under controlled conditions, measured after full thermal equilibration.
Achievable Tolerances by Grade
| Tolerance Class | Value | Standard HDPE | HMW-HDPE | UHMW-PE | Typical Application |
|---|---|---|---|---|---|
| Standard | ±0.25 mm | ✓ | ✓ | ✓ | General geometry, housings, clearance fits |
| Precision | ±0.10 mm | ✓ with staged machining | ✓ with staged machining -more process-sensitive than Standard HDPE | Not recommended | Bores, sealing interfaces, alignment features |
| Fine | ±0.05 mm | Consult at enquiry | Consult at enquiry | Not recommended | Critical fits — requires controlled conditions |
Measure at ambient temperature after full thermal equilibration — not immediately off the machine.
Wall Thickness, Holes and Feature Design
| Feature | Standard HDPE / HMW-HDPE | UHMW-PE | Notes |
|---|---|---|---|
| Max part size (milling) | 1,200 × 500 × 150 mm | 1,200 × 500 × 150 mm | Larger parts available on request — contact team for custom stock sizes |
| Max part size (turning) | 394 mm dia. × 152 mm length | 394 mm dia. × 152 mm length | Confirm with engineering for oversized bar stock |
| Minimum wall thickness | 0.75 mm | 1.0 mm | Thinner walls deflect under clamping; dimension shifts on release |
| Minimum end mill diameter | 0.8 mm | 1.0 mm | Below minimum, deflection and chatter risk increases |
| Minimum drill diameter | 0.5 mm | 0.8 mm | Standard geometry; peck drill above 5:1 depth-to-diameter |
| Max hole depth-to-diameter ratio | 10:1 (peck above 5:1) | 8:1 | Chip evacuation critical at high L/D — fibrous UHMW-PE chips pack |
| Tapped holes (direct) | M3 minimum | M3 minimum | Minimum 2× diameter engagement; low-cycle use only |
| Threaded inserts | M2 minimum | M2 minimum | Preferred for any repeat-assembly feature |
| Internal corner radii | ≥ 1/3 pocket depth | ≥ 1/3 pocket depth | Sharp internal corners are stress concentrators under impact load |
| Sliding clearance (bore/shaft) | 0.15–0.25 mm per side | 0.20–0.30 mm per side | Allow for thermal expansion in service across temperature range |
| Need ±0.10 mm or tighter on an HDPE part?
Upload your CAD file and we'll confirm achievable tolerances, flag any design risks, and return a DFM-reviewed quote — typically within 2 hours. |
Joining and Assembly of CNC-Machined HDPE Parts
HDPE's non-polar surface chemistry is the key constraint when designing assemblies — standard structural adhesives do not bond reliably to untreated HDPE. Design for one of the two reliable joining methods from the outset:
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Mechanical fastening— screws, bolts, heat-set or press-fit threaded inserts, and snap-fits. For tapped holes directly in HDPE, specify a minimum thread engagement of 2× the nominal diameter and limit to low-cycle-count applications. For any repeat-assembly feature, specify a heat-set threaded insert (M2 minimum) — direct tapped threads in HDPE will strip with repeated use.
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Plastic welding— hot-gas welding, extrusion welding, and butt fusion welding all produce strong, reliable joints in HDPE. Correctly executed welds approach the base material strength and are the correct solution for sealed housings, tanks, and assemblies that cannot use fasteners.
Adhesive bondingis only viable as a joining method after flame treatment or plasma surface activation of the HDPE surfaces. This increases surface energy enough to support primer-based adhesive systems, but adds process steps, cost, and risk. Unless the application specifically requires adhesive joining, mechanical fastening or welding is the practical default.
Surface Finish Options for Machined HDPE
As-machined HDPE is the correct finish for most functional applications. HDPE's non-polar surface chemistry limits the finishing options available — paint and coating adhesion require surface activation, and polishing yields less visual improvement than on denser engineering plastics. The options below are the practically useful ones for CNC-machined HDPE parts.
| Finish | Ra (µm) | Dimensional Impact | Notes |
|---|---|---|---|
| As-machined (standard) | 1.6–3.2 | None | Default for all functional applications. Retains HDPE's natural low-friction surface. |
| As-machined (precision pass) | 0.8–1.6 | None | Achievable on Standard and HMW-HDPE with sharp tooling and light finishing pass. |
| Bead blasted | 1.6–3.2 (uniform) | None | Removes directional tool marks. Use fine glass bead only — coarse media can damage thin features. |
| Tumbling / vibratory | 0.8–1.6 | Negligible | Smooths sharp edges and light tool marks; suitable for batch processing. |
| PTFE / Teflon coated | < 0.5 (applied layer) | +5–25 µm per side | Further reduces friction and improves chemical barrier performance. Confirm coating thickness against toleranced features before specifying. |
| Powder coated | Depends on application | +50–100 µm per side | Requires plasma or flame surface activation prior to application. Less common on HDPE than on metals — confirm adhesion requirements with engineering. |
| Painted | Depends on primer | Negligible | Requires flame treatment or plasma activation before priming. Standard solvent and water-based primers do not bond reliably to untreated HDPE. |
Note on colour and coating on HDPE:
HDPE cannot be dyed. For colour-coded parts, use black UV-stabilised stock for outdoor and UV-exposed applications, or natural (white/off-white) stock for indoor and food-contact use. Both painting and powder coating require surface activation prior to application — confirm with engineering before specifying on toleranced features.
HDPE Properties — Advantages and Limitations for CNC Parts
HDPE's Shore D hardness of 60–70 means low cutting forces, minimal tool wear, and clean chip formation across all three grades — standard carbide tooling is sufficient, with no specialist process controls required.
| Advantages | Limitations |
|---|---|
| Near-zero moisture absorption (<0.01%) — dimensions stable in wet, submerged, and washdown environments; no tolerance allowance for moisture uptake required, unlike nylon | Lower stiffness and tensile strength than Delrin, Nylon 6/6, and metals — not suitable for high-load structural applications |
| Excellent chemical resistance — resistant to acids, alkalis, salts, solvents, and industrial cleaning agents at room temperature; suitable for direct contact with process fluids without degradation | High CTE (100–200 × 10⁻⁶/°C) — tight tolerances require a controlled staged machining sequence; see Tolerances section |
| Good impact toughness — resists brittle fracture under shock and handling loads; suitable for guards, bumpers, and components exposed to incidental impact in service | Poor adhesive bondability — HDPE's non-polar surface does not bond reliably with standard structural adhesives; mechanical fastening or plastic welding required |
| Excellent electrical insulator — dielectric strength 18–20 kV/mm, stable in wet and chemically aggressive environments; suitable for enclosures, terminal housings, cable guides, and separator plates | Prone to creep under sustained compressive load — not suitable for long-term press-fit or heavily loaded bearing applications |
| Low coefficient of friction (0.2–0.4 dynamic against steel) — naturally self-lubricating surface reduces wear in sliding guides, wear strips, seal housings, and plug bodies without added lubricant
|
Continuous service temperature limited to ~80°C — not suitable for elevated-temperature environments; check against application operating range
|
| FDA-compliant grades available — suitable for direct food-contact applications; confirm compliance status with material supplier before releasing to production
|
UV degradation on natural (white/off-white) grades — specify black UV-stabilised stock for outdoor or prolonged UV-exposed applications
|
| Lowest material cost in the engineering plastics group — practical default for medium-duty parts where stiffness and tight tolerances are not primary design requirements
|
| HDPE not quite right for your application?
Compare it against Delrin, Nylon, and PEEK — or let our team suggest the best-fit material for your load, temperature, and budget requirements. |
HDPE Compared with Other Engineering Plastics
| Property | HDPE | Nylon PA6/6 | Delrin (POM) | UHMW-PE | Polypropylene (PP) |
|---|---|---|---|---|---|
| Tensile Strength (MPa) | 26–33 | 75–90 | 60–75 | 20–28 | 25–40 |
| Impact Resistance | Good | Moderate | Moderate | Excellent | Good |
| Chemical Resistance | Excellent | Good | Good | Excellent | Excellent |
| Water Absorption | <0.01% | 2.0–3.5% | 0.2–0.9% | <0.01% | <0.01% |
| Dimensional Stability (humid) | Excellent | Moderate | High | Excellent | Excellent |
| Heat Deflection Temp @ 0.45 MPa (°C) | 45–60 | 75–90 | 100–110 | 40–55 | 55–65 |
| Machinability | Excellent | Good | Excellent | Moderate | Good |
| Relative Material Cost | Low | Medium | Medium–High | Medium | Low |
| Typical CNC Applications | Chemical handling, food contact, marine, guards | Gears, bearings, wear pads | Precision gears, bushings, close-tolerance parts | Liners, chain guides, bulk handling | Chemical tanks, low-load fittings |
When to choose HDPE over the alternatives:
- Over Nylon— when moisture-driven dimensional stability is the priority, or when the part operates in a humid, submerged, or washdown environment. Nylon absorbs 2.0–3.5% moisture at equilibrium; HDPE absorbs essentially none.
- Over Delrin (POM)— when chemical resistance to strong acids and alkalis is required, or when FDA food-contact compliance is needed. Delrin wins on stiffness, strength, and dimensional precision under load.
- Over UHMW-PE— when tolerances tighter than ±0.25 mm are required, or when machining efficiency and cost matter more than maximum wear performance.
- Over Polypropylene— when impact toughness or service in low-temperature environments is a factor. HDPE is tougher than PP at temperatures below 0°C and better suited to applications with handling loads.
Where HDPE Is Used — Industries and Applications
| Industry | Typical CNC-Machined Components | Key Property Driver |
|---|---|---|
| Food Processing & Packaging | Cutting surfaces, product guides, chute liners, conveyor wear strips, auger bushings, washdown spacers | Near-zero moisture absorption; FDA-compliant grades available |
| Marine & Outdoor Hardware | Dock fenders, pile guides, cleats, bearing pads, structural spacers | Black UV-stabilised grade required; natural white not suitable for outdoor UV exposure |
| Fluid & Chemical Handling | Manifolds, valve bodies, pump housings, pipe fittings, chemical-contact liners | Resistant to acids, alkalis, salts, and industrial cleaning agents; handles continuous immersion |
| Electrical & Electronic | Terminal housings, cable guides, separator plates, plug bodies, sensor housings in washdown environments | Dielectric strength 18–20 kV/mm; stable in wet environments unlike nylon |
| General Industrial & Mechanical | Guards, bumpers, impact pads, low-stress brackets, protective covers | Impact toughness, low cost, easy machinability; default when stiffness is not a design driver |
| Ready to machine HDPE parts?
From food-grade guides to marine-grade UHMW liners — get a quote in 2 hours. Lead times from 1 day. ISO 9001:2015 · AS9100D · ISO 13485 certified network. |
Frequently Asked Questions
What tolerances can be held when CNC machining HDPE?
Standard tolerances of ±0.25 mm are achievable on all three grades. Precision tolerances of ±0.10 mm are achievable on Standard HDPE and HMW-HDPE using a staged roughing-and-finishing strategy, with measurement taken after full thermal equilibration at ambient temperature. UHMW-PE is not recommended for tolerances tighter than ±0.25 mm due to its fibrous chip behaviour and greater dimensional variability under cutting loads.
What is the difference between HDPE and Delrin (POM) for CNC machined parts?
Choose HDPE when chemical resistance to strong acids and alkalis is required, when moisture-driven dimensional stability is the priority, or when food-contact compliance is needed. Choose Delrin when higher stiffness, tensile strength (60–75 MPa vs 26–33 MPa for HDPE), and tight dimensional stability under sustained mechanical load are the design drivers. Delrin machines to tighter tolerances more reliably and creeps less under compressive load — it is the better choice for precision gears, close-fitting bushings, and load-bearing sliding parts.
Can HDPE be welded?
Yes — hot-gas, extrusion, and butt fusion welding all produce reliable joints in HDPE, with correctly executed welds approaching base material strength. This makes welding the preferred solution for sealed housings, tanks, and assemblies where mechanical fasteners are impractical. For full guidance on joining method selection, thread engagement rules, and adhesive bonding requirements, see the Joining and Assembly section above.
What is the difference between HDPE and UHMW-PE?
Both are polyethylene grades differentiated by molecular weight — standard HDPE at 200,000–500,000 g/mol versus UHMW-PE at ≥ 3,500,000 g/mol. UHMW-PE offers significantly better abrasion and impact resistance, making it the right choice for bearing liners, chain guides, and high-wear surfaces. Standard HDPE is preferred when tighter tolerances, machining efficiency, or lower cost are the priority — UHMW-PE is harder to machine to tight dimensions and generates fibrous chips that require careful evacuation.