Plastic materials for precision injection molding
Plastic materials used in precision injection molding must exhibit excellent dimensional stability, meaning minimal dimensional change (typically ≤0.1%) after molding despite changes in temperature, humidity, and time. This is a core requirement for ensuring the precision of precision plastic parts (such as electronic connectors and optical components). Dimensional stability is closely related to the material’s crystallinity, glass transition temperature (Tg), and water absorption. Crystalline plastics (such as POM and PA66) require crystallinity control (e.g., by adding nucleating agents) to reduce post-molding shrinkage. For example, adding 0.5% polyoxymethylene nucleating agent to POM reduces dimensional change from 0.3% to 0.08% within 24 hours after molding. Amorphous plastics (such as PC and ABS) require high Tg materials. PC has a Tg of 143°C, which is higher than ambient operating temperatures and offers superior dimensional stability to PS (Tg 100°C), which has a lower Tg. Hygroscopic materials (such as PA and PBT) require strict control of water absorption. PA66, after drying, has a moisture content of ≤0.05%. When used in an environment with 85% humidity, the dimensional change caused by water absorption over 24 hours can be controlled to within 0.05%. A precision gear uses glass fiber-reinforced POM (20% GF). Through optimized molding processes, the dimensional accuracy reaches IT5 (tolerance ±0.01mm), meeting transmission requirements.
Plastic materials used in precision injection molding require excellent melt flow and formability to ensure accurate replication of mold cavity microstructures (such as narrow gaps and tiny bosses less than 0.1mm) and minimize dimensional deviations caused by underfilling. The melt flow rate (MFR) is a key performance indicator. Precision injection molding materials typically have an MFR 20%-50% higher than standard materials. For example, the MFR of precision polycarbonate (PC) is 20-30 g/10 min (300°C/1.2 kg), higher than the 10-15 g/10 min of standard PC, enabling filling of thin-walled structures as thin as 0.08 mm. The material’s molecular weight distribution must be narrow (molecular weight distribution index < 2.5) to avoid melt viscosity fluctuations caused by molecular weight differences. For example, the molecular weight distribution index of precision polycarbonate (POM) must be controlled within 2.0, reducing pressure fluctuations during the injection molding process to ±3%. For thin-walled precision plastic parts, low-viscosity specialty materials, such as precision-grade PP (MFR 40g/10min), can be used. This allows for filling a 1mm-thick cavity at a relatively low injection pressure (80MPa), minimizing the impact of mold deformation on dimensional accuracy. A mobile phone camera bracket (minimum wall thickness 0.5mm) uses precision-grade ABS (MFR 25g/10min), achieving 100% fill and dimensional accuracy within ±0.005mm.
Plastic materials used in precision injection molding must possess excellent mechanical properties and environmental resistance to ensure that precision plastic parts do not deform, crack, or experience performance degradation during use. They are suitable for long-term, stable operation in applications such as medical devices and aerospace components. Tensile strength, flexural modulus, and impact strength must meet design requirements. For example, the flexural modulus of precision gear materials must exceed 2000MPa (e.g., 2800MPa for POM + 25% GF) to prevent deformation under stress. The impact strength of precision connector materials must exceed 20kJ/m² (e.g., 25kJ/m² for PC + 10% GF ) to prevent breakage during insertion and removal. Temperature resistance must be tailored to the operating environment. Precision plastic parts in automotive engine compartments must be made from materials with a temperature resistance of 150°C or higher (e.g., PPS, PEEK). Plastic parts used outdoors must be resistant to UV aging (e.g., PC with added UV stabilizers). A precision sensor housing made of PEEK exhibited only a 0.03% dimensional change after 1000 hours of operation at 180°C, with mechanical property retention exceeding 90%.
Plastic materials used in precision injection molding require excellent color stability and low volatile content to avoid cosmetic defects or performance degradation caused by pigment migration, precipitation, or volatile contamination. They are suitable for applications requiring high cleanliness, such as optics and electronics. Colorants should be heat-resistant and non-migrating, such as inorganic pigments (titanium dioxide, carbon black) or high-performance organic pigments (such as phthalocyanine blue). The addition level should be controlled at 0.5%-2% to ensure they do not decompose during the molding process. For example, titanium dioxide (R902) is used as the white pigment for precision optical lenses, ensuring uniform dispersion and no yellowing. The material must also have a low volatile organic compound (VOC) (VOC) content, with a weight loss of < 0.1% at 200°C as determined by thermogravimetric analysis (TGA). This prevents gas formation during injection molding, which can cause bubbles and mold contamination. For example, low-VOC PP (VOC content < 50 ppm) is used for precision plastic parts used in cleanrooms. A certain optical lens holder was contaminated by volatiles. Switching to low-VOC PC reduced the contamination rate from 5% to 0.1%.
The selection of plastic materials for precision injection molding requires a careful consideration of both the molding process and cost. While meeting performance requirements, it also balances processing feasibility and affordability. For ultra-precision plastic parts (tolerance ±0.001mm), high-performance engineering plastics (such as PEEK and LCP) should be selected, coupled with dedicated precision injection molding machines to ensure dimensional accuracy. For medium-precision parts (tolerance ±0.01mm), modified general-purpose plastics (such as ABS+GF and POM+PTFE) can be used, which are 30%-50% less expensive than engineering plastics. The material’s molding shrinkage should be stable, with shrinkage fluctuations within a batch ≤0.05%, facilitating process parameter adjustment. For example, precision connectors utilize PA66+30% GF (shrinkage 1.2%±0.03%), which maintains stable shrinkage and provides excellent dimensional consistency after molding. By synergistically optimizing materials, processes, and molds, the cost-effectiveness of precision injection molding can be maximized. For example, a smartwatch casing utilizes a precision-grade PC/ABS alloy, which meets dimensional accuracy (±0.02mm) while reducing costs by 15% compared to pure PC, making it suitable for mass production.
