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How To Choose Materials for Injection Molding?

Written by

Arterex Medical

Published on

June 30, 2025

Read time

12 minute read

Injection molding is one of the most widely used manufacturing methods for producing high-volume plastic parts. While the process itself is well-established, the success of any molded product depends heavily on selecting the right material. A poor material choice can lead to product failure, manufacturing defects, high costs, or compliance issues. On the other hand, the right material enhances performance, ensures durability, reduces waste, and supports smooth production.

Material selection is a critical step in the product development lifecycle. With hundreds of available thermoplastics and thermosets, each offering unique mechanical, thermal, chemical, and visual properties, the decision-making process can become complex and time-consuming.

This guide provides a structured, step-by-step approach to help engineers, designers, and manufacturers choose the best material for their specific application. It begins with identifying key part requirements, including mechanical strength, environmental resistance, and aesthetic needs. It then reviews common plastic categories used in injection molding, explains essential moldability factors like melt flow and shrinkage, and outlines how to compare materials based on datasheets, cost, and processing conditions.

You will also learn how to test shortlisted materials, work with suppliers, and address regulatory and sustainability considerations. By following this process, you can select a material that meets your technical goals, supports production efficiency, and delivers long-term product success.

Why Material Choice Matters?

Material selection affects:

  • Performance: Strength, durability, flexibility, or rigidity, depending on application.
  • Cost: Raw material cost, cycle time, and tooling impact the bottom line.
  • Manufacturability: Ease of molding, shrinkage behavior, mold filling, part ejection.
  • Aesthetics: Surface finish, color, clarity, and texture contribute to the consumer experience.

A structured material selection process ensures alignment between your product’s design intent, operational demands, regulatory constraints, and cost objectives.

Understanding Your Part Requirements in Injection Molding

Before you dive into comparing material datasheets or consulting with resin suppliers, the first and most critical step in material selection is understanding what your part needs to do. This process ensures the material aligns with both functional performance and manufacturing feasibility. It must involve close collaboration between your design, engineering, and manufacturing teams to identify all relevant operational, environmental, and production factors.

A. Functional Requirements

Your part must perform reliably throughout its life cycle. This section focuses on the key mechanical, thermal, electrical, and environmental factors that influence material choice.

1. Mechanical Properties

Mechanical stresses and strains are often the primary drivers of material performance. Understanding the following properties will help ensure your part can withstand operational forces:

  • Tensile Strength
     The maximum pulling force a material can withstand without breaking.
     Essential for load-bearing parts such as brackets, mounts, or housings.
  • Flexural Strength
     The ability to resist bending forces.
     Critical for components like clips, enclosures, and covers.
  • Impact Strength
     Indicates resistance to sudden force or shock loading.
     Measured using Izod or Charpy impact tests. Important for consumer goods, automotive parts, and drop-prone applications.
  • Stiffness / Modulus of Elasticity
     Defines how much a part resists elastic deformation under load.
     Higher modulus = less flexible. Crucial for parts requiring dimensional precision or rigidity.
  • Hardness
     Indicates surface resistance to scratching, denting, or abrasion.
     Measured using Shore or Rockwell hardness scales.
  • Fatigue Resistance
     The ability to resist failure under repeated cyclic loading.
     Needed for components like handles, living hinges, or latches.
  • Creep Resistance Refers to a material’s ability to resist gradual deformation under long-term loading, especially at elevated temperatures.

Important in parts under constant load, like structural mounts or support beams.

2. Thermal Properties

If your part will operate in varying temperature conditions, you must evaluate the following:

  • Operating Temperature Range
     The range within which the part is expected to perform safely and consistently.
  • Heat Deflection Temperature (HDT)
     The temperature at which the material begins to deform under a specified load.
     Helps determine performance under real-world heat exposure.
  • Thermal Conductivity
     The rate at which a material transfers heat.
     Important for parts involved in heating or cooling systems.
  • Coefficient of Thermal Expansion (CTE)
     Describes how much a material expands or contracts with temperature change.
    Low CTE = better dimensional stability, crucial for precision components.

3. Electrical Properties (if applicable)

For components used in electrical or electronic applications, consider:

  • Dielectric Strength
     The maximum electric field the material can withstand before breaking down.
  • Volume Resistivity
     Measures a material’s resistance to electrical current across its volume.
     Higher values = better insulator.
  • Dissipation Factor
     Indicates energy lost as heat. Lower values are ideal for insulating materials in high-frequency applications.

4. Chemical Resistance

Exposure to chemicals can degrade materials and compromise part integrity. Analyze potential exposure to:

  • Acids, Bases, Solvents, Greases
     Materials must resist corrosion, softening, swelling, or cracking.
  • Environmental Stress Cracking (ESC)
     Cracking caused by the combination of mechanical stress and chemical exposure.
     Often difficult to detect in early design stages, but critical in industrial and automotive settings.

5. Environmental Resistance

Your part’s durability is often tested by the elements. Consider:

  • UV Stability
     Required for outdoor applications to prevent discoloration, embrittlement, or cracking.
  • Moisture Absorption
     Materials like Nylon are hygroscopic, absorbing water that can impact dimensional stability and strength.
  • Flame Retardancy
     Regulatory compliance with fire safety standards such as UL 94, particularly for consumer electronics, automotive, and building materials.

6. Other Functional Requirements

Some applications may demand additional performance capabilities:

  • Biocompatibility
     For medical devices and implants, materials must comply with standards like USP Class VI or ISO 10993.
  • Food Contact Safety
     Materials must be approved by agencies like FDA, NSF, or the EU Commission for contact with consumables.
  • Abrasion Resistance
     Vital in moving or sliding parts such as gears, bushings, and wear strips.
  • Optical Clarity
     Required for lenses, light covers, or transparent consumer packaging.
     PMMA and PC are common choices.
  • Sound Dampening
     Used in electronics housings or automotive interiors to reduce noise and vibration transmission.

B. Aesthetic Requirements

Especially for consumer-facing or brand-critical products, how a part looks and feels is just as important as how it functions.

Key Considerations:

  • Color Consistency
     Uniform appearance across batches and parts is important for brand identity and customer perception.
  • Surface Finish
     Glossy, matte, or textured finishes can be achieved through material choice or mold texturing.
  • Transparency / Translucency / Opacity
     Choose materials based on whether your part needs to be see-through, light-diffusing, or completely opaque.
  • Surface Quality
     Material and mold design affect cosmetic defects like sink marks, flow lines, weld lines, or warpage.

C. Geometric and Design Considerations

The physical form and complexity of your part significantly influence material suitability.

Design-Driven Factors:

  • Complex Shapes
     Thin walls, deep ribs, sharp corners, or undercuts make mold filling more difficult. Select materials with suitable flow behavior.
  • Wall Thickness
     Uniform wall thickness helps prevent issues like warpage, sink marks, and inconsistent cooling.
  • Tolerances
     Parts requiring tight tolerances should be molded from dimensionally stable materials, such as amorphous thermoplastics.
  • Living Hinges / Snap Fits
     These features require materials with high fatigue resistance and flexibility, such as polypropylene (PP) or PE.

D. Production Volume & Cost Targets

Cost and scalability must be factored into early-stage material selection, especially as production ramps up.

Volume Considerations:

  • Low-Volume Production:
     It may be cost-effective to use a higher-performance (and higher-cost) material if tooling or processing costs are low.
  • High-Volume Production:
     Requires materials that are cost-effective per unit and support high-speed, high-efficiency molding.

Total Cost Analysis:

Evaluate not just the cost per kilogram of material, but also:

  • Tooling costs
  • Cycle time
  • Waste and scrap rates
  • Regrind or recycling potential
  • Labor and secondary operations

What Are The Key Material Categories for Injection Molding?

Choosing the right material is crucial to successful injection molding. Among the broad spectrum of materials available, plastics are the most commonly used due to their flexibility, cost-effectiveness, and range of mechanical properties. The two major categories of plastics used in injection molding are thermoplastics and thermosets, each with their own subcategories and application-specific characteristics.

A. Thermoplastics (Primary Choice for Injection Molding)

Thermoplastics are polymers that soften when heated and harden upon cooling, making them ideal for high-volume, repeatable production. They can be remelted and reshaped without significantly degrading their properties, which also allows for recycling and efficient manufacturing.

1. Amorphous vs. Semi-Crystalline Thermoplastics

Thermoplastics can be broadly categorized based on their molecular structure: amorphous and semi-crystalline. This classification has a significant impact on processing behavior, mechanical performance, and end-use suitability.

Property

Amorphous

Semi-Crystalline

Structure

Disordered, random polymer chains

Highly ordered crystalline regions

Clarity

Often transparent

Usually opaque

Shrinkage

Low and uniform

Higher and more directional (anisotropic)

Impact Resistance

Generally higher

Lower (unless modified with additives)

Examples

PC (Polycarbonate), ABS, PMMA

Nylon (PA), PBT, HDPE, POM

  • Amorphous plastics are easier to process and are ideal when dimensional stability and aesthetic surface finish are important.
  • Semi-crystalline plastics tend to offer better chemical resistance, wear resistance, and higher strength—but they require more careful mold design due to higher shrinkage and warpage.

2. Commodity Plastics (Low Cost, General-Purpose)

Commodity plastics are inexpensive, easy to mold, and suitable for high-volume production of non-critical parts.

Key Commodity Thermoplastics:

  • Polyethylene (PE)
    • Types: HDPE, LDPE, LLDPE
    • Properties: Durable, chemical-resistant, low cost
    • Uses: Bottles, packaging, toys, tanks

  • Polypropylene (PP)
    • Properties: Excellent fatigue resistance, low density, food safe
    • Unique Feature: Ideal for living hinges
    • Uses: Caps, containers, medical syringes

  • Polystyrene (PS)
    • Types: GPPS (General Purpose), HIPS (High Impact)
    • Properties: Easy to mold, rigid but brittle (GPPS), impact-modified (HIPS)
    • Uses: Disposable cutlery, cases, packaging inserts

  • Polyvinyl Chloride (PVC)
    • Properties: Tough, chemical and flame-resistant, good insulation
    • Uses: Pipes, cable insulation, window frames

  • Acrylonitrile Butadiene Styrene (ABS)
    • Properties: Strong, impact-resistant, good surface finish
    • Uses: Consumer electronics, automotive trim, appliances

3. Engineering Plastics (Enhanced Mechanical and Thermal Performance)

Engineering plastics offer superior strength, thermal resistance, and dimensional stability compared to commodity plastics. They are commonly used in demanding industrial, automotive, and consumer applications.

Notable Engineering Thermoplastics:

  • Polycarbonate (PC)
    • Properties: High impact resistance, optical clarity, UV-resistant grades available
    • Uses: Safety goggles, electronics housings, light diffusers

  • Nylon (Polyamide – PA6, PA66)
    • Properties: Tough, abrasion-resistant, self-lubricating
    • Uses: Gears, bushings, automotive parts, textiles

  • Acetal (POM – Polyoxymethylene)
    • Properties: Low friction, high dimensional stability, chemical resistance
    • Uses: Bearings, gears, conveyor components

  • Polybutylene Terephthalate (PBT) / Polyethylene Terephthalate (PET)
    • Properties: Excellent electrical insulation, good wear resistance
    • Uses: Electrical connectors, switch housings, automotive sensors

  • Polyphenylene Oxide/Ether (PPO/PPE)
    • Properties: Excellent dimensional stability, hydrolytic resistance
    • Uses: Electronics, HVAC components, water system parts

  • Polyphenylene Sulfide (PPS)
    • Properties: Chemical and thermal resistance up to 260°C, high strength
    • Uses: Automotive under-hood, electrical insulation, pump components

  • Polyether Ether Ketone (PEEK)
    • Properties: Exceptional strength, fatigue resistance, biocompatible, can withstand 250°C+
    • Uses: Aerospace, medical implants, semiconductor equipment

4. Specialty Plastics

These high-performance materials are used in extreme environments or highly specialized applications. While they are more expensive and difficult to process, they offer unmatched properties in terms of temperature, chemical resistance, or electronic behavior.

Examples:

  • Liquid Crystal Polymer (LCP)
    • Properties: Excellent for thin-wall flow, dimensional stability, and heat resistance
    • Uses: Microelectronic connectors, medical devices

  • Fluoropolymers (PTFE, PVDF)
    • Properties: Outstanding chemical resistance, high temperature endurance, non-stick behavior
    • Uses: Chemical handling, electrical insulation, seals, gaskets

B. Thermosets

Unlike thermoplastics, thermosetting plastics undergo a chemical reaction during molding that irreversibly cures the material into a solid state. They cannot be remolded or reshaped. Thermosets are used when heat resistance, rigidity, and dimensional stability are more important than recyclability or transparency.

Common Thermoset Materials:

  • Phenolics
    • Properties: Excellent electrical insulation, heat and flame resistance, high rigidity
    • Drawback: Brittle and non-ductile
    • Uses: Electrical switchgear, appliance handles, automotive components

  • Epoxies
    • Properties: Strong, high-temperature stability, excellent adhesion
    • Uses: Structural adhesives, electronics encapsulation, aerospace components

  • Silicones
    • Properties: Highly flexible, heat-resistant, biocompatible, weather-stable
    • Uses: Medical implants, baking molds, gaskets, baby products

What Are The Factors Influencing Material Processability (Moldability)?

Material processability—or moldability—refers to how easily and effectively a plastic material can be processed through injection molding. It plays a critical role in determining the efficiency of production, the quality of molded parts, and the longevity of your mold tooling.

Understanding the key processability factors can help avoid common molding defects such as warpage, flash, voids, splay marks, sink marks, and inconsistent part dimensions.

Below are the most important factors influencing material moldability:

1. Melt Flow Index (MFI) / Melt Flow Rate (MFR)

MFI or MFR is a measure of the ease with which molten plastic flows under a specified pressure and temperature. It’s typically reported in grams per 10 minutes (g/10 min).

Impact on Moldability:

  • High MFI: The material flows easily, making it suitable for thin-walled or intricate parts. However, very high MFI can sometimes lead to weaker mechanical properties due to lower molecular weight.
  • Low MFI: Indicates higher viscosity and tougher flow, which can make mold filling more difficult but results in stronger, more impact-resistant parts.

Best Practices:

  • Use higher MFI materials for small, detailed, or multi-cavity parts.
  • Choose lower MFI materials for stronger, load-bearing components.

2. Shrinkage

Shrinkage occurs when a material contracts as it cools from melt temperature to solid state. This can impact the final size, fit, and tolerance of the part.

Impact on Moldability:

  • Amorphous Plastics (e.g., ABS, PC): Shrink less and more uniformly, offering better dimensional stability.
  • Semi-Crystalline Plastics (e.g., Nylon, POM): Exhibit greater and less predictable shrinkage due to crystalline structure formation.

Controlling Shrinkage:

  • Add fillers (e.g., glass fibers): Reduce shrinkage and enhance rigidity.
  • Optimize mold design and processing conditions (cooling rate, packing pressure).
  • Incorporate draft angles and adjust tool dimensions to compensate for shrinkage behavior.

3. Moisture Sensitivity

Some plastics are hygroscopic, meaning they readily absorb moisture from the air. If not properly dried, moisture can cause processing issues and material degradation.

Common Hygroscopic Materials:

  • Nylon (PA)
  • PET, PBT
  • PC, ABS (moderately)

Impact on Moldability:

  • Undried material can cause:
    • Splay marks (visible streaks on surface)
    • Brittleness
    • Loss of tensile strength
    • Porosity in molded parts

Best Practices:

  • Use dehumidifying or vacuum dryers before molding.
  • Monitor ambient humidity in storage and handling environments.
  • Ensure proper drying time and temperature per manufacturer’s recommendation.

4. Processing Temperatures

Each plastic resin has specific temperature requirements for melting, mold filling, and solidification.

Key Temperature Factors:

  • Melt Temperature: Temperature at which the plastic becomes flowable.
  • Mold Temperature: Affects surface finish, shrinkage, and part warpage.
  • Processing Window: Range between minimum and maximum recommended melt temperatures.

Impact on Moldability:

  • Too low: Incomplete filling, short shots, poor weld lines.
  • Too high: Material degradation, increased cycle time, flashing.

Best Practices:

  • Choose materials with a wide processing window for easier and more forgiving processing.
  • Ensure molding equipment and mold tooling can handle the required temperatures consistently.

5. Thermal Stability

  • Thermal stability refers to a material’s resistance to degradation at elevated temperatures or during extended residence time in the barrel.

    Concerns with Poor Thermal Stability:

    • Discoloration
    • Burn marks
    • Gas generation
    • Loss of mechanical properties

    Important For:

    • Thin-walled parts
    • Complex mold geometries
    • High-speed injection molding

    Best Practices:

    • Minimize residence time in the barrel.
    • Avoid over-shearing during screw rotation.
    • Use appropriate barrel temperatures and backpressure settings.

6. Viscosity and Shear Thinning Behavior

  • Viscosity is the resistance of molten plastic to flow.
  • Shear thinning means viscosity decreases with increased shear rate (as seen in injection molding under high pressure).

Impact on Moldability:

  • Low-viscosity materials flow easily and are suitable for complex, thin, or long flow path parts.
  • Shear-thinning plastics perform well in high-speed molds, as they flow better under pressure but thicken once cooled, helping hold shape.

Examples:

  • ABS, PC, and PP exhibit good shear-thinning behavior.
  • Amorphous materials typically flow more uniformly than semi-crystalline ones.

Best Practices:

  • Use proper gate design and injection speed to take advantage of shear-thinning.
  • Avoid sharp corners and thick-to-thin transitions that can cause flow hesitations or weld lines.

The Material Selection Process: A Step-by-Step Guide

Selecting the right plastic for injection molding is not just a matter of picking a material with high strength or low cost. It requires a structured, data-driven approach that aligns material performance with product functionality, manufacturability, regulatory compliance, and budget constraints. Below is a detailed, step-by-step guide to help engineers, designers, and product developers make well-informed material decisions.

Step 1: Define Requirements

The foundation of smart material selection is a clear and thorough definition of your part’s requirements. This includes both performance expectations and application constraints.

What to Document:

  • Mechanical stress conditions: Will the part experience tension, compression, impact, fatigue, or abrasion?
  • Thermal environment: What is the operating temperature range? Any exposure to heat cycling?
  • Chemical exposure: Will the part contact oils, fuels, acids, cleaning agents, or moisture?
  • Electrical needs: Does it need insulation or conductivity?
  • Aesthetic expectations: Does it need to be transparent, glossy, matte, or color-matched?
  • Regulatory demands: Is FDA, RoHS, or UL compliance required?
  • Production volume and cost targets: What is the expected lifecycle volume and cost per part?

Defining these early helps filter materials that meet both technical and business objectives.

Step 2: Broad Material Screening

Once the requirements are outlined, the next step is to narrow the field of potential materials to 2–5 realistic candidates based on general material classes.

How to Categorize:

  • Commodity Plastics – Low cost, suitable for non-critical parts (e.g., PP, PE, PS)
  • Engineering Plastics – Better strength, durability, and performance (e.g., PC, ABS, Nylon)
  • High-Performance Plastics – For extreme environments (e.g., PEEK, PPS, LCP)

Tips:

  • Start by eliminating entire families that don’t meet critical requirements (e.g., exclude PVC if chemical resistance is insufficient).
  • Consider initial recommendations from similar use cases or industry benchmarks.

Step 3: Property Comparison

Now it’s time to dive into the data. You’ll compare the shortlisted materials based on actual physical, thermal, chemical, and optical properties using material selection tools.

Recommended Tools:

  • MatWeb (www.matweb.com): Free searchable database of material property data.
  • UL Prospector: Trusted by engineers to access certified plastics datasheets.
  • Granta CES Selector (Ansys): Advanced tool for plotting performance against cost or weight.

Key Properties to Compare:

  • Tensile Strength and Modulus
  • Impact Resistance (Izod, Charpy)
  • Heat Deflection Temperature (HDT)
  • Melt Flow Index (MFI)
  • Shrinkage Rate
  • UV, Chemical, and Moisture Resistance
  • Transparency or Optical Clarity
  • Flame Retardancy (UL 94 rating)

Plot these values against your requirements and eliminate materials that fall short in critical categories.

Step 4: Consider Moldability

Even the perfect material can become a problem if it’s difficult or costly to mold. Work closely with your tooling engineer and molder to validate compatibility between your material choice and the design.

Key Moldability Factors:

  • Melt Flow Index (MFI): Affects how easily the material fills the mold—critical for thin walls or complex geometries.
  • Shrinkage and Warpage: High shrinkage materials require mold compensation; inconsistent shrinkage leads to dimensional inaccuracy.
  • Drying Requirements: Hygroscopic materials (like Nylon or PET) need pre-drying to prevent defects like splay or bubbles.
  • Processing Window: Evaluate melt temperature, mold temperature, cooling time, and thermal degradation risk.

Tip:

If the material requires high mold temperatures or tight tolerances, ensure the tooling is designed accordingly to handle those needs.

Step 5: Conduct Cost Analysis

It’s not enough to look at the cost per kilogram. You need to evaluate the total cost per part, including material processing, tooling, and waste.

Cost Considerations:

  • Raw Material Cost per Pound/Kilogram: High-performance plastics are much more expensive.
  • Tooling Costs: Some materials require hardened steel molds or more complex gating, which increases tooling investment.
  • Cycle Time: Materials with fast cooling cycles reduce production time.
  • Waste and Scrap Rates: Certain plastics are more prone to warpage or defects.
  • Regrind/Recycling Potential: Can you reuse scrap material in the same process without degrading quality?

Example:

A lower-cost resin with high scrap rates and longer cycle times may actually cost more per part than a slightly more expensive engineering plastic.

Step 6: Supplier Consultation & Testing

Once you’ve narrowed your selection to 1–2 top choices, consult with material suppliers and molders. Their expertise can help validate your decision and ensure you’re working with the latest formulations or additives.

What to Do:

  • Request technical datasheets and processing guides.
  • Ask for material samples or trial pellets.
  • Conduct prototyping:
    • 3D Printing (e.g., FDM or SLS): For early-stage fit/function validation.
    • Pilot Injection Molding Runs: Use aluminum or soft steel tools for short production runs to test part performance under real molding conditions.

Why It Matters:

Even with strong data, real-world testing is critical to uncover material behavior that doesn’t show up in spreadsheets—like surface finish issues, part sticking, or weld line weaknesses.

Step 7: Regulatory & Sustainability Checks

Before locking in your material, ensure it meets all compliance and environmental requirements relevant to your product and target market.

Key Regulatory Considerations:

  • RoHS (Restriction of Hazardous Substances): Required for most electronic products.
  • REACH Compliance: European standard for chemical safety.
  • FDA or NSF Certification: Essential for medical or food-grade applications.
  • UL Certifications: Required for flame-retardant or safety-critical components.

Sustainability Considerations:

  • Is the resin recyclable (e.g., PP, PE, PET)?
  • Is there a bio-based or biodegradable version (e.g., PLA)?
  • Can you reduce waste by reusing regrind?

A growing number of manufacturers now prioritize sustainable materials as part of environmental and corporate responsibility goals.

What Are The Common Plastics for Injection Molding?

The common plastics for injection molding are listed below:

1. ABS (Acrylonitrile Butadiene Styrene)

  • Type: Amorphous thermoplastic
  • Key Properties: Toughness, impact resistance, good surface finish
  • Applications: Automotive parts, electronic housings, consumer goods, LEGO bricks
  • Pros: Easy to mold, aesthetic surface, relatively low cost
  • Cons: Poor UV and chemical resistance

2. Polypropylene (PP)

  • Type: Semi-crystalline thermoplastic
  • Key Properties: Lightweight, fatigue-resistant (hinges), chemical resistance
  • Applications: Medical devices, food containers, automotive parts, living hinges
  • Pros: Low cost, good chemical resistance
  • Cons: Poor UV resistance without additives

3. Polyethylene (PE)

  • Types: HDPE (High-Density), LDPE (Low-Density), LLDPE (Linear Low-Density)
  • Key Properties: Flexibility, chemical resistance, water resistance
  • Applications: Containers, caps, toys, piping, liners
  • Pros: Impact-resistant, cost-effective
  • Cons: Limited rigidity and temperature resistance

4. Polycarbonate (PC)

  • Type: Amorphous thermoplastic
  • Key Properties: High impact strength, transparency, heat resistance
  • Applications: Safety glasses, optical lenses, electrical housings
  • Pros: Excellent toughness and clarity
  • Cons: Sensitive to chemicals, expensive

5. Nylon (Polyamide – PA6, PA66)

  • Type: Semi-crystalline thermoplastic
  • Key Properties: High strength, abrasion resistance, good wear properties
  • Applications: Gears, bearings, automotive components, cable ties
  • Pros: Tough, self-lubricating
  • Cons: Absorbs moisture, requires drying

6. Polystyrene (PS)

  • Types: GPPS (General Purpose), HIPS (High Impact)
  • Key Properties: Easy to mold, rigid, glossy
  • Applications: Disposable cutlery, packaging, appliance parts
  • Pros: Inexpensive, good dimensional stability
  • Cons: Brittle (GPPS), not chemical-resistant

7. Acetal (Polyoxymethylene – POM)

  • Type: Semi-crystalline thermoplastic
  • Key Properties: Low friction, stiffness, dimensional stability
  • Applications: Gears, valves, precision parts
  • Pros: Good wear resistance, machinable
  • Cons: Difficult to bond or paint, high shrinkage

8. Polyethylene Terephthalate (PET)

  • Type: Semi-crystalline thermoplastic
  • Key Properties: Strength, dimensional stability, clarity (when amorphous)
  • Applications: Bottles, food containers, automotive parts
  • Pros: Recyclable, FDA approved for food use
  • Cons: Requires drying, sensitive to processing moisture

9. PVC (Polyvinyl Chloride)

  • Type: Amorphous thermoplastic (can also be compounded as thermoset)
  • Key Properties: Chemical resistance, flame retardancy
  • Applications: Pipes, fittings, electrical housings, window frames
  • Pros: Flame-retardant, chemically resistant
  • Cons: Can release toxic fumes when heated, processing limitations

10. PEEK (Polyether Ether Ketone) – High-Performance

  • Type: Semi-crystalline thermoplastic
  • Key Properties: High temperature and chemical resistance, strength
  • Applications: Aerospace, medical implants, high-end electronics
  • Pros: Exceptional mechanical and thermal properties
  • Cons: Very expensive, requires special molding equipment

Injection Molding Plastics Comparison

Property / PlasticABSPolypropylene (PP)Polyethylene (PE)Polycarbonate (PC)Nylon (PA)Polystyrene (PS)Acetal (POM)PETPVCPEEK
Cost$$$$$$$$$$$$$$$$$$$$$
Tensile StrengthMediumLowLowHighHighLowHighMediumMediumVery High
Impact ResistanceHighMediumHighVery HighMediumLowMediumMediumMediumVery High
Heat ResistanceMediumLowLowHighHighLowMediumMediumMediumVery High
Chemical ResistanceModerateHighHighModerateModerateLowModerateModerateHighHigh
UV ResistancePoorPoorPoorFairPoorPoorFairPoorGoodExcellent
TransparencyNoNoNoYesNoYes (GPPS)NoYes (Amorphous)NoNo
StiffnessMediumLowLowHighMediumMediumHighMediumMediumHigh
Ease of MoldingEasyEasyEasyModerateModerateEasyModerateModerateModerateHard
ApplicationsHousings, toysPackaging, living hingesBottles, filmsLenses, helmetsGears, bearingsCups, casingsGears, valvesContainers, autoPipes, fittingsAerospace, implants


 

What is The Strongest Plastic for Injection Molding?

When strength and performance are non-negotiable, PEEK (Polyether Ether Ketone) stands out as one of the strongest plastics used in injection molding. It offers an exceptional combination of:

  • High tensile strength (up to 100–140 MPa)
  • Excellent heat resistance (continuous use up to 260°C)
  • Outstanding chemical resistance
  • Superior wear and fatigue resistance

PEEK is commonly used in aerospace, automotive, and medical implants—industries where parts must perform in extreme conditions. It’s ideal for structural components that must endure high stress, elevated temperatures, or harsh chemicals.

Other strong engineering plastics include:

  • Nylon (PA 6/6): High strength and toughness, widely used in gears and mechanical parts.
  • Polycarbonate (PC): High impact resistance with good dimensional stability.
  • Acetal (POM): Strong, rigid, and low-friction for high-precision parts.

What is The Weakest Plastic for Injection Molding?

At the other end of the spectrum, Polystyrene (PS)—especially General Purpose Polystyrene (GPPS)—is considered one of the weakest injection molding plastics in terms of mechanical properties. It has:

  • Low tensile strength
  • Poor impact resistance
  • Limited flexibility and elongation

While it’s very easy to mold and cost-effective, polystyrene is brittle and can crack or shatter under stress, making it unsuitable for load-bearing or high-impact applications.

Other lower-strength plastics include:

  • Low-Density Polyethylene (LDPE): Very flexible but not suitable for structural parts.
  • PVC (Rigid): While durable and chemically resistant, it has relatively low tensile strength compared to engineering plastics.

What Are The Types of Plastic Used in Injection Molding?

Injection molding utilizes a wide variety of plastics, each offering distinct properties tailored to different applications. These plastics are generally categorized into two major groups: thermoplastics and thermosets. Here’s an overview of the main types used in injection molding:

1. Thermoplastics (Most Common)

Thermoplastics are the most widely used plastics for injection molding. They can be melted, shaped, and cooled repeatedly without significant chemical change, making them recyclable and versatile.

a. Commodity Thermoplastics

These are affordable, easy-to-process materials used for high-volume, general-purpose products.

  • Polypropylene (PP): Lightweight, chemical-resistant, and fatigue-resistant (used in packaging, caps, living hinges)
  • Polyethylene (PE – HDPE, LDPE): Flexible, moisture-resistant, used in containers and liners
  • Polystyrene (PS): Rigid and low-cost, ideal for disposable items and packaging
  • Polyvinyl Chloride (PVC): Strong and flame-retardant, used in pipes and profiles
  • Acrylonitrile Butadiene Styrene (ABS): Tough with good surface finish, common in automotive and electronics

b. Engineering Thermoplastics

These plastics offer better mechanical, thermal, or chemical performance than commodity plastics.

  • Polycarbonate (PC): Transparent and impact-resistant, used in lenses, enclosures
  • Nylon (Polyamide – PA6, PA66): Strong and abrasion-resistant, ideal for gears and mechanical parts
  • Acetal (POM): Low-friction and dimensionally stable, used in precision components
  • PET / PBT: Excellent wear resistance and dimensional stability for automotive and industrial use
  • PPE / PPO (Polyphenylene Oxide/Ether): Good electrical properties and heat resistance

c. High-Performance Thermoplastics

Used in demanding environments where standard engineering plastics fail.

  • PEEK (Polyether Ether Ketone): Exceptional mechanical, thermal, and chemical resistance
  • PPS (Polyphenylene Sulfide): High-temperature resistance and chemical durability
  • LCP (Liquid Crystal Polymers): Excellent flow and high strength, suitable for electronic connectors

2. Thermosetting Plastics (Less Common in Injection Molding)

Thermosets harden permanently after curing and cannot be remelted. While less common in injection molding, they’re essential for high-temperature and high-performance applications.

  • Phenolic Resins: Heat and flame-resistant; used in electrical parts
  • Epoxy Resins: High strength and adhesion properties
  • Silicone: Flexible, biocompatible, and heat-resistant; used in medical and kitchenware applications

3. Bioplastics and Sustainable Options (Emerging)

As industries move toward sustainability, bio-based and biodegradable plastics are gaining popularity in injection molding.

  • PLA (Polylactic Acid): Derived from corn starch; biodegradable and used in packaging and prototyping
  • PHA, PBS, and starch-based blends: Compostable plastics for single-use products

What Are The Raw Materials Used in Injection Molding?

Injection molding relies on a wide range of raw materials, primarily plastics in pellet or granule form, along with additives and fillers that enhance specific material properties. These raw materials determine how the molded parts perform in terms of strength, appearance, flexibility, durability, and cost.

1. Thermoplastics (Most Common Raw Material)

Thermoplastics are polymers that can be melted, molded, and solidified repeatedly without significant chemical change. These materials dominate injection molding due to their recyclability and versatility.

Common Thermoplastics Used:

  • Polypropylene (PP) – Lightweight, fatigue-resistant
  • Polyethylene (PE) – Flexible, moisture-resistant
  • Acrylonitrile Butadiene Styrene (ABS) – Tough, impact-resistant
  • Polystyrene (PS) – Rigid, easy to mold
  • Polycarbonate (PC) – Transparent, strong, high impact resistance
  • Nylon (PA) – Strong, wear-resistant
  • Acetal (POM) – Rigid, low-friction
  • PET & PBT – High dimensional stability
  • PEEK – High-performance, heat-resistant

2. Thermosetting Plastics (Less Common)

Unlike thermoplastics, thermosets cure irreversibly. Once set, they cannot be remelted, making them ideal for high-temperature or electrical applications.

Examples:

  • Phenolic resins – Heat-resistant, used in electrical components
  • Epoxies – Strong, chemically resistant
  • Silicones – Flexible, heat-tolerant, biocompatible

3. Additives and Modifiers

To enhance or customize material performance, raw plastics are often blended with additives such as:

  • Colorants – For custom colors and finishes
  • UV stabilizers – Improve resistance to sunlight degradation
  • Plasticizers – Add flexibility to rigid materials
  • Flame retardants – Reduce flammability (e.g., UL 94 rated)
  • Antioxidants – Prevent thermal or oxidative degradation

4. Fillers and Reinforcements

To improve mechanical properties or reduce cost, base resins can be compounded with:

  • Glass fibers – Increase strength, stiffness, and dimensional stability
  • Minerals (e.g., talc, calcium carbonate) – Improve rigidity and reduce shrinkage
  • Carbon fibers – Used in high-strength, lightweight applications

5. Bioplastics and Sustainable Materials (Emerging Options)

Eco-conscious manufacturing is driving the use of biodegradable and bio-based plastics, such as:

  • PLA (Polylactic Acid) – Compostable, used in prototyping and packaging
  • PHA, PBS, and starch blends – Degradable under industrial composting conditions.
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