Selection of Materials for Injection Molding

In injection molding, material selection is the “first step” that determines the success or failure of a product. It directly impacts the mechanical properties, appearance, texture, service life, and even production efficiency and manufacturing costs of the product. According to industry statistics, approximately 30% of injection molding defects (such as cracking, deformation, and short shots) are related to improper material selection. A well-suited material can achieve a perfect balance among product performance, cost, and process. Whether producing daily plastic tableware or high-precision automotive parts and medical consumables, material selection requires a comprehensive assessment based on product requirements, processing conditions, and application scenarios. This article will dissect the logic and methods of material selection for injection molding from four dimensions: core material properties, product requirement matching, processing adaptability, and cost control, helping practitioners avoid pitfalls and make scientific decisions.

I. Core Performance Indicators of Injection Molding Materials

Different injection molding materials exhibit significant performance differences, and their suitability for product requirements must be evaluated through key indicators. Here are six core performance aspects that must be considered:

1. Mechanical Properties: Determining Product “Durability”

Mechanical properties refer to a material’s ability to resist external forces, directly affecting the product’s strength, toughness, and wear resistance. The core indicators include:

• Tensile Strength: The maximum tensile force a material can withstand before breaking (unit: MPa). For example, PA66 has a tensile strength of approximately 60-80 MPa, making it suitable for load-bearing components (such as automotive brackets). PP has a tensile strength of about 30-40 MPa and is more suitable for non-load-bearing daily items (such as plastic basins).
• Impact Strength: A material’s ability to resist impact damage (unit: kJ/m²). Materials with high impact strength (such as ABS, with a notched impact strength of 20-30 kJ/m²) are suitable for products prone to collisions (such as mobile phone cases). Materials with low impact strength (such as PS, with a notched impact strength of 2-3 kJ/m²) are suitable for static parts (such as picture frames).
• Hardness: A material’s ability to resist pressing or scratching on its surface (commonly expressed in Rockwell hardness HRR and Brinell hardness HB). POM (with a hardness of 85-95 HRR) is wear-resistant and scratch-resistant, making it suitable for gears and bearings. TPE (with a hardness of 30-80 Shore A) is soft and elastic, suitable for anti-slip grips and seals.

2. Thermal Properties: Adapting to “Operating Environment Temperature”

Thermal properties determine a material’s stability at different temperatures, with core indicators including:

• Heat Deflection Temperature (HDT): The temperature at which a material begins to deform under a certain pressure (unit: °C), directly reflecting its high-temperature resistance. For example, PC has an HDT of approximately 120-140°C and can be used to make coffee machine accessories. PP has an HDT of about 60-80°C and is not suitable for high-temperature environments (such as microwave heating components).
• Continuous Use Temperature: The temperature range in which a material can operate stably over a long period. PA66 + glass fiber has a continuous use temperature of approximately 150-180°C, making it suitable for automotive engine compartment components. PEEK can withstand a continuous use temperature of over 250°C and is suitable for aerospace high-temperature environments.
• Low-Temperature Toughness: A material’s ability to maintain toughness at low temperatures. TPU (with a low-temperature brittleness temperature of -40°C) is suitable for products used in outdoor low-temperature environments (such as snow boot soles). Ordinary ABS becomes brittle below -20°C and requires impact-resistant grade ABS (with toughening agents added) for such conditions.

3. Chemical Properties: Resisting “Environmental Corrosion”

Chemical properties determine a material’s resistance to chemicals in the operating environment (such as oils, water, solvents, acids, and alkalis), with core indicators including:

• Corrosion Resistance: A material’s ability to resist damage from chemicals. PP is resistant to acids, alkalis, and organic solvents, making it suitable for chemical storage tanks and pesticide bottles. PC is not resistant to strong alkalis and should not be used to hold detergents.
• Water Resistance/Moisture Resistance: The extent to which a material’s properties change after absorbing water. PA (nylon) has high water absorption (2-3% water absorption rate) and becomes softer after absorption, requiring drying treatment. POM has an extremely low water absorption rate (<0.1%) and is suitable for humid environments (such as bathroom accessories).
• Weather Resistance: A material’s ability to resist aging from natural environmental factors such as sunlight, wind, rain, and ozone. ASA has excellent weather resistance (no significant aging after 5 years of outdoor use) and is suitable for outdoor furniture and automotive exterior trim. Ordinary ABS fades and becomes brittle after long-term outdoor use and requires the addition of anti-UV agents.

4. Appearance Properties: Meeting “Visual and Tactile Requirements”

Appearance properties affect a product’s aesthetics and texture, with core indicators including:

• Transparency: A material’s light transmittance (unit: %). PC has a light transmittance of over 90% and is suitable for lampshades and eyeglass lenses. PMMA (acrylic) has a light transmittance of 92% but lower impact resistance than PC, making it suitable for decorative transparent parts.
• Colorability: A material’s ability to be uniformly dyed and maintain color stability. PP and ABS have good colorability and can be made in various bright colors. Specialty plastics such as PEEK and PI are difficult to color and are mostly available in their natural colors (brown, black).
• Surface Gloss: A material’s ability to reflect light on its surface (unit: GU). High-gloss materials (such as high-gloss ABS, with a gloss of 80-90 GU) are suitable for home appliance housings. Matte materials (such as frosted PP, with a gloss of 10-20 GU) are suitable for products requiring anti-slip and anti-fingerprint properties (such as remote control housings).

5. Processing Properties: Adapting to “Injection Molding Process Conditions”

Processing properties determine whether a material can be smoothly molded with minimal defects, with core indicators including:

• Melt Flow Rate (MFR): A key indicator for measuring a material’s fluidity (unit: g/10min; higher values indicate better fluidity). High MFR materials (such as PP, with an MFR of 10-20 g/10min) are suitable for thin-walled parts (such as mobile phone charger housings with a wall thickness of 0.8-1.2 mm). Low MFR materials (such as PC, with an MFR of 2-5 g/10min) are suitable for thick-walled parts (such as home appliance panels with a wall thickness of 2-3 mm).
• Shrinkage Rate: The proportional shrinkage of a material’s dimensions after cooling and solidification (unit: %). High shrinkage rate materials (such as PE, with a shrinkage rate of 1.5-3%) require larger shrinkage allowances in mold design. Low shrinkage rate materials (such as glass fiber-reinforced PA, with a shrinkage rate of 0.3-0.8%) are suitable for high-precision parts (such as connectors with a tolerance of ±0.02 mm).
• Thermal Stability: A material’s resistance to degradation during high-temperature processing. PVC has poor thermal stability (decomposes to release hydrogen chloride above 180°C) and requires the addition of stabilizers. PPS has excellent thermal stability (no degradation at processing temperatures of 300-350°C) and is suitable for high-temperature molding.

6. Special Properties: Meeting “Functional Requirements”

For specific scenarios, materials need to possess special properties, including:

• Conductivity/Insulation: Conductive plastics (such as carbon black-filled PP) are used for antistatic trays. Insulating plastics (such as FR-PP, flame-retardant polypropylene) are used for electronic component housings.
• Biocompatibility: Medical-grade materials (such as PP, PE, and PC) must pass ISO 10993 certification, be non-cytotoxic, and be suitable for making syringes and surgical instruments.
• Flame Retardancy: Flame-retardant plastics (such as UL94 V0-rated ABS and PC) are used in home appliances and automotive interiors to prevent fire spread.

II. Material Selection Logic Based on Product Requirements

The core of material selection is “matching based on requirements”—first clarify the product’s application scenario, functional requirements, and regulatory standards, and then screen for materials that meet these conditions. The following are selection strategies for different dimensional requirements:

1. Selection by “Application Field”: Matching Industry Characteristics

Different industries have significantly different material requirements, and industry-specific standards must be prioritized:

• Daily Necessities/Packaging Field: The core requirements are “low cost, easy processing, and safety.” Priority is given to general-purpose plastics:
  • PP (the most cost-effective, shatter-resistant, and acid/alkali-resistant, used for plastic cups, food containers, and express delivery bags);
  • PE (soft and low-temperature resistant, used for cling film, plastic bags, and children’s toys);
  • PS (transparent, used for disposable lunch boxes and foam cushioning materials, but brittle with poor impact resistance).
• Electronics and Electrical Appliances Field: The core requirements are “insulation, flame retardancy, and dimensional stability.” Priority is given to engineering plastics:
  • ABS (rigid and easy to color, used for TV housings, keyboards, and routers);
  • PC (transparent and impact-resistant, used for computer monitor housings and LED lampshades);
  • POM (wear-resistant and self-lubricating, used for printer gears and mouse rollers).
• Automotive Field: The core requirements are “lightweight, temperature resistance, and weather resistance,” with materials selected by part:
  • Interior parts (instrument panels, door panels): ABS + PC alloy (balancing rigidity and toughness, low-temperature resistant);
  • Exterior parts (bumpers, rearview mirror housings): PP + glass fiber (lightweight, impact-resistant, weather-resistant);
  • Engine compartment components (oil pipes, sensor housings): PA66 + glass fiber (temperature-resistant above 150°C, oil-resistant).
• Medical Field: The core requirements are “biocompatibility, sterility, and disinfection resistance.” Priority is given to medical-grade materials:
  • PP (medical-grade, steam sterilization-resistant, used for syringes and infusion sets);
  • PC (gamma-ray sterilization-resistant, used for blood dialysis machine housings);
  • TPE (soft and skin-friendly, used for medical catheters and mask nose clips).
• High-End Industrial Field (Aerospace, Precision Instruments): The core requirements are “high-temperature resistance, high strength, and corrosion resistance.” Specialty plastics are selected:
  • PEEK (temperature-resistant up to 260°C, with strength close to metal, used for aircraft components and bearings);
  • PPS (acid/alkali-resistant and radiation-resistant, used for chemical equipment and semiconductor fixtures);
  • PI (polyimide, temperature-resistant above 300°C, used for spacecraft thermal insulation materials).

2. Selection by “Operating Environment”: Resisting External Influences

A product’s operating environment (temperature, humidity, chemical exposure) directly determines the material’s tolerance:

• High-Temperature Environment (such as kitchens, engine compartments): Select materials with an HDT > 150°C, such as PA66 + glass fiber, PPS, and PEEK;
• Low-Temperature Environment (such as cold storage, outdoor severe cold regions): Select materials with a low-temperature brittleness temperature < -30°C, such as TPU, impact-resistant ABS, and LDPE;
• Humid/Water Environment (such as bathrooms, underwater equipment): Select materials with a water absorption rate < 0.5%, such as POM, PVC, and PP;
• Chemical Exposure Environment (such as chemical plants, laboratories): Select materials resistant to acids, alkalis, and solvents, such as PP and PTFE (polytetrafluoroethylene, “the king of plastics,” resistant to almost all chemicals).

3. Selection by “Functional Requirements”: Focusing on Core Properties

Based on the product’s core functions (load-bearing, transparency, wear resistance, etc.), lock in key performance indicators:

• Load-Bearing/Structural Parts (such as brackets, frames): Prioritize tensile strength (>50 MPa) and flexural modulus (>2000 MPa), selecting PA66 + glass fiber, PC, and ABS;
• Transparent Parts (such as lampshades, lenses): Prioritize light transmittance (>85%) and impact resistance, selecting PC, PMMA, and transparent ABS;
• Wear-Resistant Parts (such as gears, bearings): Prioritize friction coefficient (<0.3) and wear rate, selecting POM, PA66 + molybdenum disulfide, and PTFE;
• Flexible Parts (such as seals, grips): Prioritize Shore hardness (30-80 Shore A) and elastic recovery rate, selecting TPE, TPR, and silicone.

4. Selection by “Regulatory Standards”: Ensuring Compliance

To enter specific markets or industries, materials must meet mandatory standards, including:

• Food Contact: Must comply with FDA (US) and EU 10/2011 certifications, selecting food-grade PP, PE, PC, and PMMA;
• Electronics and Electrical Appliances: Must comply with RoHS (restriction of hazardous substances) and UL (flame retardancy rating) certifications, selecting lead-free, flame-retardant grade ABS, PC, and PBT;
• Medical Devices: Must comply with ISO 10993 (biocompatibility) and FDA medical-grade certifications, selecting medical-grade PP, PE, PC, and PEEK.

III. Processing Adaptability

Even if a material’s properties meet product requirements, production failure may still occur if it is incompatible with injection molding equipment or molds. Three adaptability aspects must be considered during selection:

1. Adaptability to Injection Molding Equipment

• Clamping Force: High-viscosity materials (such as PC, PEEK) or large-sized products require larger clamping force equipment (such as 1000 tons or more). Small-sized, low-viscosity materials (such as PP, PS) can be processed on equipment with a clamping force of 500 tons or less.
• Barrel Temperature: High-temperature materials (such as PEEK with a processing temperature of 360-400°C) require a barrel capable of withstanding temperatures above 400°C. Low-temperature materials (such as PP with a processing temperature of 180-220°C) can use ordinary barrels, avoiding excessive temperatures that may cause material degradation.
• Screw Configuration: Materials containing glass fibers (such as PA66 + 30% glass fiber) require wear-resistant screws (alloy materials) to prevent wear on ordinary screws. High-viscosity materials require deep-groove screws to improve plasticization efficiency.

2. Adaptability to Molds

• Mold Material: Materials containing glass fibers or mineral fillers (such as PP + 40% glass fiber) will wear molds and require wear-resistant mold steel (such as H13, S136). Ordinary materials (such as PP, ABS) can use pre-hardened steel (such as 718H) to reduce mold costs.
• Mold Structure: High shrinkage rate materials (such as PE, PP) require larger shrinkage allowances in mold design (such as a product size of 100 mm requiring a mold size of 102 mm). Low shrinkage rate materials (such as glass fiber-reinforced PC) can have a reduced allowance of 0.5-1%.
• Venting and Cooling: High-fluidity materials (such as PS) are prone to air entrapment, requiring enhanced venting in the mold. Thick-walled parts (such as PC lampshades) require optimized cooling water channels to avoid uneven cooling and shrinkage.

3. Processing Window

Different materials have different processing parameter ranges (material temperature, mold temperature, injection pressure, speed), and the equipment must be able to cover their “processing window”:

• For example, PC has a narrow processing window (material temperature 280-320°C, mold temperature 80-120°C). Too low a temperature may cause short shots, while too high a temperature may cause degradation.
• PP has a wide processing window (material temperature 180-240°C, mold temperature 20-60°C), with a high error tolerance, making it suitable for beginners.

IV. Cost Optimization

Material selection must balance performance and cost, avoiding “over-engineering” (using high-end materials for ordinary products) or “cost traps” (focusing only on unit price while ignoring hidden costs). The following are cost optimization strategies:

1. Material Unit Price: Prioritize “Cost-Effective Options”

• General-purpose plastics (PP, PE, ABS): Unit price of 10-20 yuan/kg, suitable for large-volume, low-value-added products (such as daily necessities);
• Engineering plastics (PC, PA, POM): Unit price of 20-50 yuan/kg, suitable for mid-to-high-end products (such as electronics and automotive parts);
• Specialty plastics (PEEK, PI): Unit price of 100-500 yuan/kg, used only for high-end scenarios (such as aerospace and medical), avoiding abuse.

2. Processing Cost: Focus on “Molding Efficiency” and “Scrap Rate”

• High-fluidity materials (such as PP, PS) have short molding cycles (10-20 seconds/piece) and low scrap rates (<1%), resulting in low total processing costs.
• Low-fluidity materials (such as PC, PEEK) have long molding cycles (30-60 seconds/piece) and are prone to short shots and deformation, with high scrap rates (5-10%), leading to high processing costs.

3. Subsequent Costs: Reduce “Secondary Processing”

• Select materials that are easy to color and have good surface gloss (such as ABS, PC) to eliminate painting or electroplating processes.
• Select materials with built-in flame retardancy or antistatic properties (such as FR-ABS, conductive PP) to avoid adding additives or coatings later.

4. Rational Use of Recycled Materials

• For products with low performance requirements (such as turnover boxes, cushioning parts), 20-30% recycled materials (crushed defective products, runner materials) can be added to reduce raw material costs.
• For high-precision, high-requirement products (such as electronic parts, medical items), the proportion of recycled materials should be <5% to avoid affecting performance.

V. “Pitfalls to Avoid” and “Practical Process” for Material Selection

1. Common Pitfalls and Avoidance Methods

• Pitfall 1: Focusing only on unit price and ignoring performance matching
  • For example, using PP to make automotive engine compartment components (PP has insufficient temperature resistance) leads to high-temperature deformation. PA66 + glass fiber should be selected instead, despite its higher unit price, to meet temperature resistance requirements.
• Pitfall 2: Ignoring processing properties, leading to production failure
  • For example, using low MFR PC to make thin-walled connectors (PC has poor fluidity) causes short shots. High-flow grade PC (MFR 8-10 g/10min) should be selected instead.
• Pitfall 3: Not conducting sample testing and proceeding directly to mass production
  • Material properties must be verified through actual injection molding (such as testing sample strength, temperature resistance, and dimensional stability) to avoid mass scrap.

2. Five-Step Selection Process (Directly Implementable)

1. Clarify Requirements: List the product’s core indicators (such as tensile strength > 50 MPa, temperature resistance > 100°C, compliance with FDA certification).
2. Preliminary Screening: Select 2-3 candidate materials based on requirements (such as PA66 + glass fiber, PC, ABS + PC alloy).
3. Assess Adaptability: Check the compatibility of the materials with equipment and molds (such as clamping force, barrel temperature).
4. Cost Calculation: Compare the unit price, processing cost, and subsequent cost of the candidate materials to select the most cost-effective option.
5. Sample Verification: Produce samples and test their performance, appearance, and processing stability to finalize the material selection.

VI. Conclusion

Material selection for injection molding is essentially a “multidimensional balancing act”—it must meet the product’s performance and functional requirements, adapt to processing conditions, and control costs. There is no “best” material, only the “most suitable” one: PP, although ordinary, is the cost-effective king for daily necessities; PEEK, although expensive, is irreplaceable in high-end industrial fields.

For practitioners, a “requirement-oriented” selection mindset must be established: first clarify “what the product is for and where it will be used,” then screen materials based on core indicators, and finally optimize through sample verification. At the same time, staying informed about new developments in material technology (such as biodegradable plastics and modified engineering plastics) can provide more possibilities for product innovation. Only by selecting the right material can injection molding transition from “qualified production” to “high-quality and efficient production.”