In the fields of medical equipment, consumer electronics, and automotive manufacturing, plastic parts have become core components of various products due to their advantages of lightweight, low cost, and high design flexibility. However, CNC machining of plastic parts has always faced a tricky challenge — deformation. Issues such as warpage, bending, and dimensional deviation not only lead to a surge in part scrap rates but also may delay production cycles and increase cost inputs. As a professional CNC service provider, Elite Mold Tech has been deeply rooted in the industry for more than ten years. Aiming at the pain points of plastic machining deformation, we have developed a full-chain solution from material preprocessing to finished product delivery, providing global customers with high-precision and high-stability plastic part machining services.
Plastics differ significantly from metals in physical properties, making the causes of deformation during machining more complex. Only by accurately identifying the root causes can effective countermeasures be formulated.
Most plastic raw materials (such as sheets and rods) form residual stress due to uneven cooling rates and uneven force during extrusion or injection molding, similar to a compressed spring. During CNC machining, when the tool removes part of the material, the original stress balance is broken, and the residual stress drives the remaining material to deform:
- Large-area thin-walled parts: Such as mobile phone casings and instrument panels, are prone to "saddle-shaped" deformation with the middle protruding and edges warping after machining;
- Complex structural parts: Parts with ribs and holes may suffer local distortion due to uneven stress release, with hole position deviation reaching 0.2-0.5mm.
We can intuitively observe the stress distribution through stress detection equipment, providing accurate basis for subsequent annealing treatment.
The thermal conductivity of plastics is only 1/10-1/100 that of metals (e.g., the thermal conductivity of aluminum is 237W/(m·K), while that of ABS is only 0.25W/(m·K)), and they have low softening points (most plastics soften at 80-150°C). During machining, frictional heat between the tool and the material cannot be dissipated quickly, leading to a series of problems:
- Local melting: When the temperature in the cutting area exceeds the softening point, plastics will stick to the tool edge to form "built-up edges", resulting in excessive surface roughness (Ra value up to 3.2μm or more);
- Differential thermal expansion: Uneven heating in different areas of the part leads to a thermal expansion rate difference of 0.1%-0.5%, and inconsistent shrinkage after cooling causes permanent deformation.
For example, when machining POM materials, if the spindle speed is too high (exceeding 5000rpm), the temperature in the cutting area may rise to 120°C within just 10 seconds, resulting in a part dimensional deviation of more than 0.3mm.
The elastic modulus of plastics is much lower than that of metals. For instance, the elastic modulus of PC is 2.2GPa, only 1/20 that of steel. The rigid clamping methods traditionally used in metal machining can easily cause deformation:
- Single-point clamping: The concentrated pressure exerted by the fixture on thin-walled parts causes "bow-shaped" bending, with a rebound of 0.1-0.8mm after release;
- Unsupported clamping: When machining long strip plastic parts, if only the two ends are fixed, the middle part will sag due to cutting force, resulting in excessive straightness deviation after machining.
A customer once used a vice to clamp 1.5mm-thick PC boards, leading to a flatness deviation of 1.2mm after machining, and all 200 parts in the batch were scrapped.
Differences in properties of different plastics directly affect machining stability, among which moisture absorption and batch fluctuations are the two main incentives:
- Moisture absorption deformation: Materials such as nylon and PEEK have a water absorption rate of 1%-3%. After absorbing moisture, their volume expands, and they shrink after drying during machining, resulting in a dimensional deviation of 0.5%-1%. For example, nylon 66 parts will increase in size by 0.3mm when placed in an environment with 60% humidity for 24 hours;
- Batch differences: Even for plastics of the same grade, different manufacturers have different raw material purity and additive ratios, leading to fluctuations in mechanical properties (e.g., tensile strength difference up to 10%), and inconsistent deformation is likely to occur under the same machining parameters.
Relying on a professional technical team, advanced equipment configuration, and rich practical experience, we have built a full-chain deformation control system of "preprocessing - machining - inspection - post-processing", providing customized solutions for different plastic materials and structural parts.
We formulate personalized annealing processes based on material properties, allowing molecular chains to fully relax through slow heating, heat preservation, and cooling:
- PC material: Hold at 120°C for 2-3 hours, control the cooling rate at 5°C/hour, which can reduce residual stress by more than 80%;
- PMMA material: Hold at 80-90°C for 4 hours, effectively solving the problems of "cracking" and "warping" after machining;
- POM material: Hold at 60-70°C for 1-2 hours, avoiding "residual stress cracking" after machining.
We use programmable constant-temperature annealing furnaces with a temperature control accuracy of ±1°C to ensure stable annealing effects.
For hygroscopic plastics, we establish a closed-loop management of "storage - drying - machining":
- Storage environment: Constant temperature and humidity warehouse (temperature 20-25°C, humidity 30-40%), equipped with dehumidifiers and real-time temperature and humidity monitoring systems;
- Drying process: Use hot air circulation dryers, and adjust parameters according to materials:
- Nylon 6/66: Dry at 80-90°C for 6-8 hours, reduce moisture content to below 0.1%;
- PEEK: Dry at 120-130°C for 4-6 hours to ensure no bubbles or expansion during machining;
- ABS: Dry at 70-80°C for 4 hours to avoid "silver streaks" on the surface after machining.
After drying, materials are tested by a moisture meter and can only enter the machining process if qualified.
Comprehensive performance testing is conducted for each batch of incoming raw materials:
- Mechanical properties: Test tensile strength and flexural modulus through a universal testing machine to ensure they meet machining requirements;
- Thermal properties: Test glass transition temperature and melting temperature using a differential scanning calorimeter (DSC) to provide a basis for cutting parameter setting;
- Appearance and dimensions: Check for scratches and impurities on the raw material surface, and measure sheet thickness tolerance to ensure uniformity.
We abandon the "one-size-fits-all" parameter setting and customize solutions according to the hardness, wear resistance, and thermal sensitivity of plastics:
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