Engineering Plastics: A Selection Reference

Short answer

• UHMW-PE suits abrasive conveyor wear strips, POM suits gears, guides and bushings, PTFE offers the lowest friction and chemical inertness, PEEK handles heat and aggressive chemistry together, and polycarbonate suits impact-prone guards.
• Nylon absorbs moisture and changes dimension with humidity, so tight-tolerance parts should use POM or PEEK, and a short trial under real load and temperature should confirm the choice.
• UHMW-PE is a common choice for conveyor wear strips and chute liners because of its outstanding abrasion resistance, a coefficient of friction of 0.15 to 0.20 against steel and excellent impact resistance.

Which temperature and chemical limits rule out a plastic?

When a part will see hot oil, aggressive cleaning solvents or cryogenic cycles, the polymer’s temperature envelope and chemical compatibility are the first things to rule out. The common engineering plastics each have a well‑defined sweet spot.

• UHMW‑PE (Ultra‑High‑Molecular‑Weight Polyethylene) stays usable from about –40 °C to +80 °C. It tolerates most acids, bases and salts, but it softens quickly above 80 °C and will melt near 135 °C. Its resistance to hydrocarbons and chlorinated solvents is good, yet strong oxidizers can attack it.
• Acetal (POM – Polyoxymethylene) offers a usable range of –40 °C to +120 °C. It resists many alcohols, fuels and light oils, but it is vulnerable to strong acids (especially mineral acids) and halogenated solvents. In a food‑processing line you would need a food‑grade grade such as POM‑FG.
• Nylon (PA – Polyamide) works from –30 °C up to +120 °C, but its chemical resistance is limited. It tolerates mineral oils and many hydrocarbons, yet it swells in strong acids, phenols and most chlorinated solvents. The hygroscopic nature means dimensional change with humidity – a factor often missed in design.
• PTFE (Polytetrafluoroethylene) covers –200 °C to +260 °C and is essentially inert to acids, bases and solvents. The trade‑off is low load‑bearing capability and a tendency to creep under sustained stress.
• PEEK (Polyether‑ether‑ketone) handles –60 °C to +260 °C and survives most aggressive chemicals, including strong acids, aromatics and chlorinated solvents. It retains strength at temperature, making it the go‑to when both heat and chemistry matter.
• Polycarbonate (PC) is rated from –40 °C to +125 °C. It resists many alcohols and oils but is attacked by strong alkalis, ketones and some refrigerant oils. Its impact strength is excellent, but prolonged exposure to certain coolants can cause stress cracking.
• Acrylic (PMMA) works from –40 °C to +80 °C. It offers superb optical clarity and good resistance to many aqueous solutions, yet it cracks in the presence of hydrocarbons, ketones and strong bases.

For applications that must meet food‑contact regulations, look for grades certified to FDA 21 CFR 177.2600 (e.g., food‑grade POM, PTFE, or certain UHMW‑PE). When static dissipation is required – for example on guard panels that can accumulate charge – choose a conductive‑filled grade of POM or polycarbonate, often marketed as “static‑dissipative”.

Which plastics resist wear and carry load?

Wear pads, guides and sliding surfaces are where the polymer’s tribological properties dominate. The key metrics are coefficient of friction (CoF), specific wear rate and the ability to carry load without excessive deformation.

• UHMW‑PE has a CoF of 0.15‑0.20 against steel and excellent impact resistance. Its high molecular weight gives it outstanding abrasion resistance, making it a common choice for conveyor components such as chute liners and wear strips. The downside is low stiffness; under high point loads it can deform.
• POM offers a CoF of 0.20‑0.25 and a wear rate roughly one‑third that of UHMW‑PE. Its higher modulus lets it carry point loads better, which is why it is frequently machined into gear teeth, linear guides and plain bearings and bushings.
• PA (Nylon) provides a CoF of 0.20‑0.30 but its wear resistance drops sharply when moisture is absorbed, because the swollen matrix reduces hardness. In dry conditions it can be a good bearing material, but designers must account for the ~2 % dimensional increase at 90 % relative humidity.
• PTFE is the lowest‑friction polymer (CoF 0.05‑0.10) but its wear rate is high; it will scar under repeated sliding. It is therefore used where a non‑sticking surface is essential – for example, non‑metallic seals or low‑load sliding guides.
• PEEK combines a moderate CoF (0.20‑0.25) with a wear rate comparable to metal‑on‑metal in many lubricated applications. Its high compressive strength (≈100 MPa) lets it replace metal bushings in high‑temperature pumps.
• Polycarbonate has a CoF around 0.30 and a wear rate higher than POM but lower than unfilled acrylic. Its impact toughness makes it suitable for protective guards that may be struck, though it is not the first choice for high‑load sliding surfaces.
• Acrylic wears quickly under sliding contact; its CoF can be 0.35‑0.40 and it is brittle, so it is limited to static guards, light‑weight covers or clear windows where wear is not a factor.

When a design calls for a sliding element that also serves as a seal, consider pairing a low‑friction PTFE face with a tougher backing material such as POM or PEEK. The backing carries the load while the PTFE provides the chemical inertness and slip.

How do machinability and stability differ between plastics?

Most machine shops evaluate a polymer by how it behaves in CNC milling, turning or drilling. The same properties that give a polymer its chemical resilience also affect tool wear and final tolerance.

• UHMW‑PE is extremely easy to cut – it produces long, continuous chips and generates little heat. However, it is prone to spring‑back; final dimensions can be 0.2‑0.5 mm larger than the programmed cut unless a finishing operation is added.
• POM is regarded as the most machinable engineering plastic. It chips cleanly, holds tight tolerances (±0.02 mm typical) and can be tapped without special inserts. Its low moisture absorption means dimensions remain stable after machining.
• PA machines well when dry, but when the material has absorbed moisture it can cause tool chatter and result in surface tearing. Pre‑drying at 80 °C for several hours restores machinability and dimensional control.
• PTFE is soft and tends to smear rather than chip. It is best machined at low spindle speeds with a sharp carbide tool; otherwise the surface can become gummy and dimensional accuracy suffers.
• PEEK requires high‑speed steel or carbide tooling and generous coolant. It can be machined to tight tolerances (±0.01 mm) but tool wear is significant; expect frequent tool changes in production runs.
• Polycarbonate machines cleanly but is sensitive to heat buildup. Excessive spindle speed can cause melt‑back and lead to dimensional creep after the part cools.
• Acrylic cuts easily with a fine‑tooth blade, yet it cracks if the feed is too aggressive. It is best used for visual guards where a polished surface is desired.

Dimensional stability over time is a function of both thermal expansion and moisture uptake. Typical linear expansion coefficients (α) are:

• UHMW‑PE: ~200 µm/m·°C
• POM: ~110 µm/m·°C
• PA (dry): ~80 µm/m·°C; wet: up to 150 µm/m·°C
• PTFE: ~130 µm/m·°C
• PEEK: ~50 µm/m·°C
• Polycarbonate: ~68 µm/m·°C
• Acrylic: ~70 µm/m·°C

Designers should incorporate these values into tolerance stacks, especially for sliding guides that must maintain clearance over a temperature swing of 50 °C or more.

When are filled or special-grade plastics needed?

Beyond the baseline polymers, manufacturers offer filled or blended grades that address specific shortcomings.

• Glass‑filled POM raises stiffness and wear resistance by up to 30 % while retaining most of the base material’s chemical resistance.
• Carbon‑filled PTFE improves load‑bearing capacity and reduces creep, making it suitable for high‑pressure seals.
• Fluorinated ethylene propylene (FEP) over‑molded on PTFE gives a smooth surface that can be welded to metal frames, useful for guarding panels and doors that must be both transparent and chemically resistant.
• Conductive‑filled polycarbonate or POM contains carbon black or metal particles to provide a surface resistivity of 10⁶‑10⁸ Ω·sq, meeting static‑dissipative specifications for explosion‑prone environments.
• Food‑grade grades of POM, PTFE and UHMW‑PE are formulated without heavy metal stabilizers and meet FDA or EU food‑contact regulations. They are the only safe choice for parts that touch consumables.
• Low‑permeability PEEK blends are used in sealing applications where aggressive solvents could otherwise permeate a standard polymer. Pair such a blend with a gasket or packing material that matches the chemical environment.

When a part must serve as a barrier – for example a gasket separating a hydraulic fluid from a metal housing – the polymer’s permeability to the fluid is as important as its chemical resistance. PTFE and PEEK have the lowest permeation rates; acrylic and polycarbonate are much higher.

How do you select a plastic for a machine part?

Below is a step‑by‑step checklist that engineers can apply to any new component, whether it is a wear pad for a conveyor, a guide for a linear actuator, or a protective guard on a motor.

1. Define the service environment. List the maximum and minimum operating temperatures, the chemicals it will contact, and whether the part will be exposed to moisture or steam.
2. Identify the load profile. Is the part a static shield, a sliding guide, or a load‑bearing bushing? Note the maximum contact pressure and whether the load is steady or cyclic.
3. Rank the required properties. Typical order: temperature stability → chemical resistance → wear resistance → dimensional stability → machinability.
   • If temperature is the dominant factor, narrow the field to PTFE, PEEK, or UHMW‑PE.
   • If chemical resistance to strong acids is needed, eliminate POM and PA.
   • If low friction is critical and load is light, PTFE or UHMW‑PE become candidates.
4. Check hygroscopic behavior. For parts that must stay within tight tolerances, avoid nylon unless you can guarantee a controlled humidity environment or select a moisture‑stable grade.
   • For example, a linear guide that must stay within ±0.05 mm over a year should not be nylon; POM or PEEK are safer choices.
5. Consider machining and finishing. If the part will be CNC‑machined in‑house, choose a material that does not require exotic tooling. POM and UHMW‑PE are the most forgiving; PEEK will need carbide inserts and abundant coolant.
   • If you need a transparent window on a guard, acrylic is easy to machine but must be sealed against solvents.
6. Match the material to ancillary components. A wear pad of UHMW‑PE on a conveyor may be paired with a steel backing plate; the interface can be secured with standard plain bearings and bushings if alignment is critical.
   • When a seal is required, select a PTFE face with a PEEK backing, and use a compatible gasket or packing material.
7. Validate against standards. For safety‑critical guards, verify that the chosen polymer meets the required NEMA or IP rating for the enclosure. For explosion‑proof zones, confirm static‑dissipative grades.
   • Guarding panels and doors often need an IP65 rating; polycarbonate with a conductive filler can meet that while providing impact resistance.
8. Prototype and test. Even with a thorough data sheet review, run a short‑term trial under real load and temperature. Observe wear patterns, dimensional change, and any chemical staining.

Following this workflow reduces the chance of a “surprise” – such as a nylon guide that swells and jams after a humid summer – and ensures the selected polymer aligns with the functional demands of the equipment.

What do typical plastic part selections look like?

To illustrate how the comparison translates into real parts, here are three common scenarios.

• Conveyor wear strip: The strip must resist abrasion from bulk material, operate at 30‑80 °C, and be replaceable without tools. UHMW‑PE extruded into a 10 mm thick strip, bolted to the frame, provides low friction and high wear life. Pair it with a steel backing plate and use the conveyor components catalog for fasteners.
• Linear guide bushing: The bushing experiences 5 MPa contact pressure, runs at 120 °C, and sees occasional exposure to hydraulic oil. A glass‑filled POM bushing offers the needed stiffness and oil resistance, while still being machinable to a 0.02 mm tolerance. Reference the plain bearings and bushings section for standard dimensions.
• Protective guard for a motor: The guard must stop impact from tools, be visible for inspection, and survive occasional coolant spray. Polycarbonate with a conductive filler gives impact strength, visual clarity, and static‑dissipative properties. Mount it to the frame using the guarding panels and doors hardware.

Each case demonstrates how the material’s core attributes – temperature, chemistry, wear, and machinability – drive the final selection.

If you need help cross‑referencing a specific part geometry with the appropriate polymer grade, feel free to reach out for sourcing assistance.

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