Why Do Plastic Parts Easily Deform After CNC Machining?

Compared to metals, plastics are more prone to deformation during machining due to their low rigidity, low thermal conductivity, and high thermal expansion coefficient. The main causes include:

Release of Residual Internal Stress

Many plastic materials, especially extruded or injection-moulded sheets/rods, develop residual stresses during the moulding process. When CNC machining removes part of the material, it damages the original stress balance, causing uneven redistribution of the remaining stresses. This uneven release leads to warping, bending, or deformation of the part.

Heat-Induced Deformation from Machining

Plastics have low thermal conductivity and low softening points. If the heat generated during CNC machining is not dissipated quickly, it can accumulate in the machining zone and on the part surface. So it will cause local overheating, thermal expansion, or even melting, resulting in dimensional changes or surface defects.

Clamping Deformation

Plastic materials have poor rigidity and are prone to deformation under clamping force. This is especially true for thin-walled structures, which may deform under clamping pressure. But then spring back when the force is released, causing shape changes and dimensional deviations.

Material Hygroscopicity and Batch Variability

Plastics such as nylon and PEEK absorb moisture. During and after machining, exposure to environmental humidity can change their dimensions. Additionally, different batches of plastic materials may have varying mechanical properties and stress distributions, leading to inconsistent processing results.

How to Prevent or Reduce Deformation of Plastic Parts After Processing?

To effectively address post-processing deformation issues, optimization should be done in several areas, including material handling, process parameters, clamping methods, and processing path strategies.

Stress Relief Annealing Before Processing

Annealing the material before processing can effectively release residual internal stresses. For example, annealing PC material at 120°C for 2 hours can significantly reduce warping deformation after processing. Especially for parts with high structural and aesthetic requirements, such as transparent optical components.

Use Sharp Tools and Control Heat Accumulation

Select high-sharpness, high-back-angle tungsten carbide tools, combined with appropriate spindle speed and feed rate, to reduce cutting heat. Avoid high-speed machining that exacerbates thermal expansion. For cooling, use air blowing or minimal lubrication to prevent water cooling from causing plastic moisture absorption and expansion.

Reduce Clamping Force and Use Flexible Fixtures

Use vacuum fixtures or fixtures with soft pads to avoid concentrated clamping that causes plastic compression deformation. For thin-walled parts, reduce single-cut force through staged semi-finishing to minimize deformation risk.

Control Material Storage and Pre-Treatment

Moisture-absorbing materials such as nylon should be stored in a low-humidity environment. Dry them thoroughly before machining (e.g., 6 hours at 80°C) to prevent size changes caused by moisture.

Adopt a Symmetrical Machining Strategy

Optimize toolpath and process sequence, such as alternating roughing operations on opposite sides in the roughing stage to balance stress release. Avoid large-area cutting on a single side, which can cause stress concentration and result in part warping.

Case Study: Deformation Control of a Thin-Wall POM Gearbox Housing

Thin-walled plastic parts are especially vulnerable to deformation during CNC machining. This case examines a POM gearbox housing with demanding dimensional and structural requirements.

Part Overview

This gearbox housing, made from black POM, was designed for a micro-actuator. It measured approximately 90 mm × 60 mm × 26 mm and featured:

• Four sides with 1.8mm-thick thin-walled structures;
• Multiple precision mounting holes on two perpendicular surfaces (e.g., M4 threaded holes and H7 tolerance positioning holes);
• A high-precision bearing mounting position at the centre (tolerance requirement: 0.02mm);
• An open box-shaped structure with limited internal reinforcing ribs.

Issue Description

After the initial machining process, the following issues were identified during inspection:

• Side walls showed outward warping, with maximum deformation reaching 1.5mm.
• Installation hole positions were offset by 0.2mm, exceeding design specifications.
• Bearing holes were slightly elliptical, preventing proper press-fit accuracy.
• The workpiece exhibited elastic springback deformation after release from the fixture, indicating residual stress release.

Therefore, the part could not be used for assembly verification and functional testing and required rework.

Problem Analysis

Inappropriate Clamping Strategy

Initial machining used full-perimeter clamping, applying excessive clamping force to the thin-walled areas, causing elastic deformation. After releasing the clamps, the material released stress, leading to outward warping of the side walls.

Unreasonable Machining Sequence

Internal features (bearing seats, reinforcing ribs) were completed before rough machining of the outer contours, prematurely removing structural support. This caused the part to undergo micro-displacement during subsequent outer contour machining due to a lack of support, resulting in cumulative errors.

Material Thermal Response Characteristics

POM has a certain thermal expansion coefficient and is prone to thermal melting and tool chip adhesion during machining. The tools used in the initial machining were dull, and the feed rate was too low, causing localised heating and exacerbating stress concentration and warping risks.

Optimization Methods

Fixture Adjustment

Switched to a vacuum suction fixture with custom support blocks and limit pins. This provided gentle support for thin-walled areas and avoided forced deformation.

Toolpath and Sequence Changes

Moved the finishing of the outer contour to the last step. This kept the inner cavity and thin walls supported until the end, reducing deformation.

Cutting Parameter Optimisation

Used an 8 mm, three-flute flat-end tool for dynamic roughing with a 3 mm allowance.

• Spindle speed: 3,500 rpm
•  Feed rate: 2,000 mm/min
• Cutting depth: 20 mm
• Side allowance: 1.6 mm

Dynamic roughing reduced heat build-up compared to step-down roughing and improved chip removal.

Intermediate Annealing

Added annealing between roughing and finishing (60°C for 1 hour, then air cooling) to release stresses and improve stability.

Final Results

• Part warpage was controlled within 0.3 mm, with stable appearance and dimensions;
• Installation hole position accuracy was restored to within ±0.05 mm.
• Bearing hole accuracy met H7 tolerance, and assembly was completed smoothly.
• No significant springback or deformation occurred after releasing the fixture.

Engineering Practice Insights

• Plastic processing cannot be approached using metal processing experience; specialized strategies are required to address issues such as heat, stress, and humidity.
• The core of residual stress control is ‘prevention’;
• Details such as tool sharpness, cooling methods, and fixture design determine success or failure.
• Processing solutions must be dynamically adjusted: processing methods should be optimized based on differences in part material, structure, and precision requirements.

Conclusion

With the growing demand for high-precision plastic structural components, gaining a deep understanding of their processing characteristics and deformation mechanisms has become a key challenge in the field of CNC machining. For CNC engineers, mastering these key details will effectively improve the dimensional consistency of plastic parts and the overall product pass rate.