Machine Design #53: Plastic Parts — Design for Manufacturing, Assembly, and Long-Term Durability

Plastic can make an automation machine lighter, quieter, cleaner, and easier to prototype. It can also crack around a screw, creep under a constant load, swell with moisture, warp after machining, or lose a datum after repeated service.

The material name alone is not a design. A plastic part needs a controlled chain:

Function → material and process → geometry → load and environment → tolerance → manufacture → assembly → verification → maintenance.

1. Start from function and environment

Define load, support, motion, contact, cycle, temperature, humidity, chemicals, cleanliness, UV, ESD, and required life. Separate short shock from sustained load. A part that is strong for one minute may creep after a year.

2. Material choice is a system decision

Compare stiffness, strength, impact, fatigue, creep, wear, friction, moisture absorption, thermal expansion, chemical resistance, fire behavior, machinability, availability, and cost. Do not choose only by a tensile-strength number. The supplier grade, filler, color, orientation, and process affect behavior.

3. Manufacturing process changes the geometry

Machining, injection molding, 3D printing, cutting, and forming have different wall, radius, draft, shrinkage, grain, layer, and surface constraints. State the process before freezing tolerances. A prototype machined from a block may not represent a molded production part.

4. Avoid stress concentration

Use generous radii, smooth transitions, adequate section, and correct fiber or layer direction where relevant. Avoid sharp internal corners, abrupt thickness changes, unsupported bosses, and holes too close to an edge. Check both static load and repeated cycle.

5. Creep and preload

Constant clamp force, weight, or bending can relax over time. Define load duration, temperature, contact area, screw and washer, insert, support, and allowable deformation. If a datum matters, use a geometry that does not depend on plastic retaining preload alone.

6. Threads and fasteners

A plastic thread may strip, relax, or crack. Choose heat-set insert, captive nut, through-bolt, metal bushing, or a suitable self-tapping design based on torque, reuse, temperature, and access. Define pilot, boss, edge distance, torque, washer, and inspection.

7. Tolerance is a function of the assembly

Do not copy metal tolerances blindly. Define critical datum, hole pattern, clearance, interference, thermal condition, shrinkage, and adjustment. Use tolerance stack-up where alignment, sensor gap, or motion depends on several parts.

8. Surface, wear, and contamination

Contact surfaces may gall, collect dust, absorb oil, or scratch a product. Select texture, coating, insert, bushing, replaceable wear pad, and cleaning method. Define what wear is acceptable and how it is detected.

9. Assembly must not create cracks

Avoid forcing a warped part into alignment. Provide lead-ins, controlled clamp, correct driver access, torque sequence, support during fastening, and a visual crack check. Do not use a screw to pull a design error closed without checking residual stress.

10. Temperature and moisture

Plastic dimensions can change significantly with temperature and moisture. Define reference condition, operating range, storage, conditioning, and measurement method. Verify sensor gaps, seals, clearances, and moving interfaces at worst conditions.

11. Failure modes to review

Crack, creep, brittle fracture, wear, deformation, loosened insert, stripped thread, delamination, swelling, chemical attack, discoloration, contamination, short circuit, ESD damage, and assembly mix-up. For each, record cause, effect, detection, control, and evidence.

12. Design for machining and inspection

Provide tool access, datum, stock allowance, clamping surface, chip evacuation, edge break, measurement points, and inspection method. A small plastic part still needs a drawing that a supplier can manufacture and measure without guessing.

13. Design for assembly and service

Use orientation features, poka-yoke, labels, compatible fasteners, replaceable wear surfaces, and a safe removal route. Record torque, insert type, adjustment, calibration, and post-assembly verification.

14. Example: a plastic sensor bracket

Define sensor gap, target, vibration, temperature, material, cable load, fastener, datum, and adjustment. Compare machined and molded options. Add a metal insert if repeated service will damage the plastic thread. Test gap and signal at temperature, vibration, and after the expected cycle count.

15. Verification plan

Inspect dimensions, surface, material, insert, torque, and assembly. Run functional, load, cycle, environment, contamination, and service trials as justified. Record sample, condition, measurement, acceptance, and revision.

16. Plastic-part checklist

[ ] Function, load, cycle, environment, and life are defined.
[ ] Material grade and process are controlled.
[ ] Geometry avoids sharp stress concentration and unsupported bosses.
[ ] Creep, moisture, temperature, wear, and chemical effects are assessed.
[ ] Threads, inserts, fasteners, torque, and reuse are appropriate.
[ ] Datums, stack-up, clearance, shrinkage, and adjustment are explicit.
[ ] Machining or molding can be inspected with the drawing provided.
[ ] Assembly prevents cracks, distortion, and wrong-part installation.
[ ] Service, cleaning, wear, and replacement are designed.
[ ] Failure modes have controls and evidence.
[ ] Verification covers function, environment, life, and assembly.

Choose the plastic family from the operating condition, not from what is in stock

Most plastic-part failures start where the material was chosen by habit: the white rigid one is acetal, the tough flexible one is nylon. Four conditions decide the answer and have to be asked first: is the load static or dynamic, is there contact with chemicals or solvents, what is the working temperature, and does the part have to hold a tight tolerance.

Plastic family	Where it is strong	What to watch for
POM (polyacetal)	Dimensionally stable, self-lubricating, machines to a clean surface	Poor acid resistance; hard to bond and to paint; thermal expansion far above metals
PA6 / PA66 (nylon)	Tough, takes impact and wear, good for gears and rollers	Absorbs moisture, which changes both dimensions and mechanical properties — design for the conditioned state, not the as-machined state
UHMW-PE	Low friction, wear resistant, low cost, good for conveyor guide rails	Soft, creeps badly under a steady load, will not hold a tight tolerance
PTFE	Lowest friction of the group, wide chemical and temperature range	Very soft and creeps strongly; use it as a sliding or sealing face, not as a load-carrying part
PC (polycarbonate)	Transparent and impact resistant, the usual choice for viewing windows	Environmental stress cracking on contact with solvents or cleaning agents — the most common cause of cracked guard windows
PMMA (acrylic)	Clear, inexpensive, easy to cut and polish	Brittle; cracks propagate from drilled holes and from rough cut edges
PEEK	Highest temperature, chemical and load capability in the group	Substantially more expensive — it needs a specific technical reason

The case for PEEK and the threshold where it earns its cost is covered in Materials #07 — What is PEEK. The comparison between aluminium, steel and engineering plastic at panel and frame level is in Equipment Selection #23 — Frame and panel material.

Three failure mechanisms a steel part does not have

Creep. Loaded inside its elastic range, metal keeps its shape. Plastic deforms slowly and does not fully recover. A plastic part that passes inspection right after assembly can, after a few weeks under permanent load, settle, lose clamp force, or drift out of tolerance. For plastic parts a sustained static load is more dangerous than a short peak load — the opposite of the intuition trained on steel.

Thermal expansion and moisture uptake eat the clearance. The thermal expansion coefficient of engineering plastics is an order of magnitude above steel, so a plastic-to-steel pair sized at room temperature can seize once the machine warms up. The nylon family adds a second term through moisture uptake. The calculation method and worked examples are in Materials #08 — Thermal expansion coefficient.

Cracks starting at a stress raiser. Plastic is more notch sensitive than metal: a sharp internal corner, a pointed groove root, a hole edge left with burrs are all crack initiation sites. Add residual stress from machining and contact with a solvent and the crack appears weeks later with no abnormal load involved. Rounding every internal corner and deburring hole edges is the cheapest measure in this whole article.

Machining and joining: four decisions that belong on the drawing

Stock form. Extruded stock and cast plate carry different levels of residual

stress. Thin, long or flatness-critical parts should call out the stock form and a stress-relief step before or between machining passes, instead of leaving it to the shop.

Order of material removal. Removing material from one side only bows the part on

the machine. Removing it symmetrically from both sides and leaving stock for a final finishing pass holds flatness much better.

Threads under cyclic load. Tapping directly into plastic suits light loads and

few assembly cycles only. Where a joint is opened often or sees vibration, use a metal insert — selection and placement are covered in Machine Design #96 — Inserts in plastic parts.

Screw clearance holes. Use a larger clearance than on a steel part to leave room

for expansion, together with a wide washer to spread the clamping force. A screw head bearing directly on the plastic face will embed over time and lose the clamp load — which is creep seen from the joint.

Frequently asked questions

The part fitted at handover but seized a few weeks later. Why?

Three suspects, in order: thermal expansion once the machine runs warm, moisture uptake if it is a nylon grade, and creep if the part carries a permanent clamping load. Check by measuring in the hot state and after the part has stabilised in its working environment, not immediately after machining.

Should a plastic part carry the same tight tolerance as a steel one?

Not by default. Plastic parts move with temperature and humidity far more than steel, so a very tight tolerance may not hold even when the shop works correctly. Where a tight tolerance is genuinely required, state the measuring condition with it: temperature, humidity, and how long the part has been conditioned.

Can threads be tapped directly into plastic?

Yes for light loads and few assembly cycles. Under vibration or repeated disassembly the plastic thread wears and loses clamp load — use a metal insert. Thread engagement in plastic also has to be longer than in steel because the material is softer.

Why does a plastic part bow after milling even though the drawing says flat?

Because residual stress in the stock is released as material is removed. The remedy is to select the right stock form, add a stress-relief step, machine symmetrically from both sides and leave stock for a final pass. These are design decisions that belong on the drawing, not shop-floor improvisation.

A polycarbonate guard cracked around the screw holes — what is the usual cause?

A combination of three things: machining marks left at the hole edge, screws tightened directly against the panel, and a solvent-bearing cleaning agent used during washdown. Polycarbonate is prone to environmental stress cracking, so deburr the hole edges, use wide washers, leave clearance for expansion, and name the permitted cleaning agents in the operating documentation.

Conclusion

Plastic parts need engineering across material, process, geometry, load, environment, tolerance, assembly, and service. The goal is not merely a light part; it is a part that can be made, assembled, inspected, maintained, and trusted over its life.

17. Dimensional stability and datum strategy

Choose datums that remain meaningful after machining, assembly, temperature change, moisture, and service. If a sensor gap depends on a plastic face, define whether the face is a reference, a replaceable wear surface, or only a cover. Use metal inserts, hard stops, bushings, or adjustment where the tolerance stack requires stability.

18. Supplier drawing and first-article review

Send material grade, process, shrinkage assumption, surface, edge, insert, torque, inspection, and packing requirements with the drawing. Review a first article for dimensions, warpage, cracks, flash, insert position, texture, and function. A supplier’s “looks good” is not an acceptance criterion.

19. Chemical and cleaning compatibility

List cleaning agent, lubricant, coolant, adhesive, alcohol, oil, and process chemical exposure. Check stress cracking, swelling, discoloration, loss of friction, and residue. Test the combination at temperature and duration, not only the dry material coupon.

20. Lifecycle and replacement baseline

Record part revision, material, process, supplier, approved alternate, service limit, replacement method, inspection evidence, and configuration baseline. When a supplier changes grade or process, recheck critical dimensions, load, environment, and life rather than accepting the old drawing automatically.

21. Plastic-part design review

[ ] Material and process are compatible with load and environment.
[ ] Datums and tolerances remain stable after temperature, moisture, and service.
[ ] Stress, creep, thread, wear, chemical, and ESD risks have controls.
[ ] Supplier can manufacture and inspect the defined geometry.
[ ] First article and lifecycle evidence are stored in the baseline.

Record the accepted condition, measurement method, sample, and reviewer for every critical plastic feature.

References

ISO 178 — Plastics flexural properties: https://www.iso.org/standard/70513.html
ISO 527 — Plastics tensile properties: https://www.iso.org/standard/75824.html

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