Plastic Machining Guidelines

Introduction

Plastic machining is a process of cutting, drilling, milling, and turning of plastic materials, using equipment such as CNC machine tools to manufacture high-precision parts. 

Applications

1) Small quantity/prototype development

● Advantages: No need for mold, direct machining, low cost, and short cycle time

● Typical applications:

– Product function test prototypes (such as gears, housings)

– Customized medical parts (surgical guides, prosthetic parts)

2) High-precision/complex structure parts

● Advantages: CNC can achieve ±0.05mm or even higher accuracy, suitable for strict tolerance requirements

● Typical applications:

– Optical lens brackets (PMMA, PC)

– Precision gears (POM, PA+glass fiber)

– Electronic insulation parts (PTFE, PEEK)

3) Large size/thick wall parts

● Advantages: Injection molding is limited by molds and machine tonnage; large parts are difficult to be injection molded, while machining can be made in sections

● Typical applications:

– Large equipment lining (UHMW)

– Chemical tank lining (PP, PVC)

4) Special materials or high-performance plastics

● Advantages: Some engineering plastics (such as PEEK, PTFE) are difficult to injection mold, so machining is the first choice

● Typical applications:

– Aerospace high-temperature resistant parts (PEEK)

– Ultra-low friction bearings (PTFE)

5) Modify or repair existing parts

● Advantages: Existing plastic parts can be directly machined(such as hole expansion and trimming)

● Typical applications:

– Modify mechanical parts (nylon slider trimming)

– Repair worn plastic guide rails (UHMW welding and machining)

Plastic Machining VS. Other Processes

How to Choose the Right Plastic for Your Product 

Plastics suitable for machining need to have good machinability, dimensional stability, and low moisture absorption.

The following are common plastics suitable for machining and their properties for reference:

1) General-purpose plastics (easy to machine, low cost)

● ABS (Acrylonitrile Butadiene Styrene)

– Property: Moderate strength, impact resistance, easy to machine, surface can be polished

– Applications: prototype parts, housings, gears, etc

– Note: Avoid overheating when cutting (may soften)

● POM (Polyoxymethylene, Race Steel/Delrin)

– Property: High rigidity, low friction, good dimensional stability, but prone to internal stress

– Applications: precision gears, bearings, slides

– Note: Requires annealing to minimize deformation

● HDPE/LDPE (High/Low Density Polyethylene)

– Property: chemical resistance, good toughness, but poor rigidity

– Applications: containers, bushings, and other low-load parts

2) Engineering plastics (high performance, temperature, and abrasion resistant)

● PC (polycarbonate)

– Property: high transparency, impact resistance, temperature resistance (~120°C), but prone to stress cracking

– Applications: protective covers, optical parts, insulating parts

● PA (nylon, including glass fiber reinforcement)

– Property: wear-resistant, high-strength (rigidity increased after glass fiber reinforcement), but prone to deformation due to moisture absorption

– Applications: gears, bearings, structural parts

– Note: Drying is required before machining, and moisture-proofing is required after machining

● PEEK (polyether ether ketone)

– Property: high temperature resistance (260°C), high strength, chemically inert, but high cost

– Applications: aerospace, medical implants, high-load parts

● PTFE (Polytetrafluoroethylene, Teflon)

– Property: very low coefficient of friction, corrosion resistant, but soft and easily deformed

– Applications: seals, bushings

– Note: Requires ultra-sharp tools + low speed machining

3) Filled/reinforced plastics (high rigidity, low deformation)

● Glass fiber reinforced plastics (e.g., PA-GF, PC-GF)

– Property: high rigidity, good thermal stability, but fast wear on tools

– Applications: structural support parts, high-strength housings

● Carbon fiber reinforced plastics (e.g., PEEK-CF)

– Property: Ultra-high strength/weight ratio, but harmful machining dust

– Applications: high-end industrial parts, racing car parts

● Mineral-filled plastics (e.g., PP + talc)

– Property: dimensionally stable, low cost, but brittle

4) Transparent/Special Function Plastics

● PMMA (Acrylic)

– Property: High transparency, easy to polish, but brittle

– Applications: lampshades, signs, decorative parts

– Note: Low speed cutting + cooling to avoid melt marks

● PVC (Polyvinyl Chloride, rigid model)

– Property: chemically resistant, but may release harmful gases when cutting

Key Factors for Material Selection

● Rigidity Requirements: PEEK, glass fiber reinforced PA for high load; HDPE for flexible parts

● Temperature Resistance: PEEK, PI (polyimide) for high-temperature environments

● Dimensional stability: POM, glass fiber reinforced material is better than pure plastic

● Cost: ABS, POM are suitable for regular use; PEEK is used in high-end fields

Plastics not recommended for machining

● Soft TPE/TPU (elastomers): easy to stick to the cutter, molding process is recommended

● Unreinforced PP/PE: too soft, easy to deform by machining

● Foamed material: loose structure, easy to break at the processing edge

Keeping Parts Straight and Preventing Warping

The following are some ways to maintain dimensional stability:

1) Material selection and pre-treatment

● Choose plastics with high stability:

– Glass fiber reinforced plastics (such as PA-GF), PEEK, ABS, PC and other engineering plastics are preferred, as their rigidity and thermal stability are better than ordinary plastics (such as PP, PE)

– Avoid materials with strong hygroscopicity (such as nylon), or dry them before machining (such as baking at 80~120℃ for 2~4 hours)

● Material stress release:

– Anneal the blanks of injection molding or extrusion molding (such as keeping warm and cooling slowly at 10~20℃ below the heat deformation temperature) to eliminate internal residual stress

2) Process optimization

● Reduce cutting heat:

– Use sharp tools (diamond coating or carbide tools) to reduce cutting resistance

– Use parameters with high speed, small cutting depth, and large feed (for example: speed 5000~15000 rpm, cutting depth 0.1~0.5mm)

– Use compressed air or coolant (be careful as some plastics will crack when cold)

● Layered machining and symmetrical cutting:

– Machine to the final size multiple times to avoid excessive single cutting volume, causing local heating and deformation

– Symmetrical machining (such as alternating cutting on both sides) to balance stress

3) Clamping and support design

● Flexible clamping:

– Use soft claw clamps, vacuum suction cups, or low-pressure clamps to avoid deformation caused by excessive extrusion

– Add support structures (such as process ribs or auxiliary support points that need to be removed after machining)

● Avoid overhang:

– When machining thin-walled parts, use soluble fillers (such as wax) or low-melting-point alloys to support them from below

4) Tool and path planning

● Tool selection:

– Use large rake angle tools (such as 20°~30°) to reduce cutting forces

– Avoid heat generated by blunt tool friction

● Path strategy:

– Use spiral feed or ramp cutting to avoid vertical impact.

– Use equal height contour processing for final finishing to keep the cutting force uniform

5) Post-treatment and environmental control

● Stress release:

– Anneal again after machining (especially for thick-walled or complex parts)

● Environmental stability:

– Store and machine in a constant temperature and humidity environment to avoid moisture absorption or temperature difference deformation (such as nylon swelling after absorbing water)

● Natural aging:

– For parts with high precision requirements, place them for 24~48 hours after machining, and then measure or secondary machining

6) Design optimization

● Structural improvement:

– Avoid designing large planes or thin-walled structures, adding reinforcing ribs or rounded corner transitions

 – Reserve processing allowance (such as 0.1~0.3mm) for fine adjustment

By comprehensively controlling materials, processes and external factors, the warping can be significantly reduced.

The key is to verify the parameters through small batch trial cutting and adjust the production according to the specific material properties.

Getting the Perfect Finish on Your Parts

There are some measures to improve the surface finish after machining:

● Mechanical polishing:

– Rough polishing: 600# sandpaper + ethanol lubrication

– Fine polishing: diamond grinding paste (W1-W0.5)

● Chemical polishing:

– PC/PMMA can be polished with dichloromethane vapor (control time <30 seconds)

Conclusion

In conclusion, plastic machining is a fast, efficient, and cost-effective way to validate your prototype, a perfect method for your small order quantity.

Contact us to get started with your next projects.