Injection Molding Materials: Plastic Selection and Comparison Guide

Selecting an injection molding material affects strength, appearance, tolerance, chemical resistance, tool design, cycle time and product cost. Two plastics that look similar can behave very differently during molding and in service.

This guide compares common injection molding materials, explains the properties that matter and provides a practical selection process for prototypes, bridge production and full-scale manufacturing.

How to Choose an Injection Molding Material

Begin with product requirements rather than a familiar resin name. Define mechanical loads, operating temperature, chemicals, UV exposure, flame performance, appearance, regulatory needs and expected product life.

The final grade matters as much as the polymer family. Fillers, impact modifiers, flame retardants, UV stabilizers and processing packages can change performance and molding behavior.

Key Material Selection Questions

• What loads, impacts and repeated stresses will the part experience?

• What is the continuous and short-term operating temperature?

• Will the part contact fuels, cleaners, oils, acids or outdoor weather?

• Does the assembly require stiffness, flexibility or low friction?

• Are transparency, color, gloss or texture important?

• Must the material meet flame, food-contact, medical or electrical standards?

• How tight are the dimensional and warpage requirements?

• What production volume and unit-cost target apply?

Thermoplastics vs Thermosets

Most injection molded parts use thermoplastics. They soften when heated, flow into the cavity and solidify during cooling. In principle, thermoplastics can be reheated, although degradation, contamination and performance requirements limit reprocessing.

Thermosets cure through a chemical reaction and do not melt again after curing. They can provide excellent heat resistance and electrical properties, but require different processing and tooling considerations. This guide focuses primarily on thermoplastics.

Amorphous vs Semi-Crystalline Plastics

ABS: Balanced Performance and Appearance

Acrylonitrile butadiene styrene is widely used for consumer housings, equipment covers, automotive interior components and prototypes. It offers a useful balance of toughness, stiffness, surface quality and processability.

ABS accepts color, texture, painting and printing well. It does not provide the highest heat, weather or chemical resistance, so outdoor and demanding chemical applications require careful grade selection or a different resin.

Common ABS Applications

• Electronic enclosures

• Control panels and appliance parts

• Automotive interior trim

• Handheld-product housings

• Cosmetic prototype components

Polycarbonate: Impact Strength and Transparency

Polycarbonate, or PC, is known for high impact strength, dimensional stability and optical potential. It is used for protective covers, lighting components, electrical housings and structural transparent parts.

PC requires proper drying before molding and can be sensitive to certain chemicals and stress cracking. Thick optical parts need controlled tool polish, gate design, cooling and processing to minimize bubbles, flow marks and residual stress.

PC/ABS: A Practical Housing Material

PC/ABS blends combine the processability and appearance of ABS with improved heat and impact performance from PC. They are common in electronics, transportation interiors and business equipment.

Different blend ratios and flame-retardant packages produce different properties. Specify the actual commercial grade rather than writing only “PC/ABS” on the drawing.

Polypropylene: Lightweight and Chemical Resistant

Polypropylene, or PP, has low density, excellent fatigue resistance and broad chemical resistance. It is commonly used for living hinges, containers, caps, fluid-handling parts and consumer products.

PP has relatively high shrinkage and lower stiffness than many engineering plastics. Mold design must account for warpage and dimensional change. Fillers can increase stiffness and reduce shrinkage but also change impact performance and surface appearance.

Polyethylene: Tough and Moisture Resistant

Polyethylene is available in multiple density ranges, including LDPE and HDPE. It offers low moisture absorption, good chemical resistance and useful toughness.

LDPE is flexible and common in squeezable products. HDPE is stiffer and used in caps, containers, industrial components and outdoor products. Both have significant shrinkage compared with many amorphous plastics.

Nylon: Strength, Wear and Moisture Considerations

Nylon, also called polyamide or PA, provides good strength, fatigue resistance and wear performance. Common grades include PA6, PA66, PA12 and reinforced versions.

Nylon absorbs moisture, which changes dimensions and mechanical properties. Drawing tolerances and testing conditions should define whether parts are measured dry-as-molded or after conditioning. The resin must also be dried correctly before molding.

Glass-Filled Nylon

Glass fiber significantly increases nylon stiffness, strength and heat resistance while reducing overall shrinkage. It is useful for structural brackets, under-hood components and load-bearing housings.

Fibers align with melt flow, causing directional shrinkage and strength. Gate location becomes important for warpage and weld-line performance. Glass fiber is also abrasive, so the tool and processing equipment must be selected for wear resistance.

Acetal: Low Friction and Dimensional Performance

Acetal, also known as POM, offers low friction, good wear, fatigue resistance and dimensional stability. Gears, bearings, latches, clips and moving mechanisms frequently use acetal.

POM is semi-crystalline and can shrink significantly. It also needs careful processing because overheating can cause degradation. Adhesive bonding and painting are difficult due to its low surface energy.

PMMA Acrylic: Optical Clarity and Weatherability

PMMA, commonly called acrylic, provides excellent transparency, surface gloss and outdoor weather resistance. It is used for lenses, light guides, display covers and decorative components.

Acrylic is more brittle than polycarbonate. Sharp corners and high assembly stress should be avoided. Optical surfaces require polished tooling and controlled handling to prevent scratches.

Polystyrene and HIPS

General-purpose polystyrene offers clarity, rigidity and economical processing but has limited impact resistance. High-impact polystyrene, or HIPS, includes rubber modification to improve toughness at the expense of transparency.

These materials are common in packaging, disposable products, appliance liners and low-load housings. Heat, solvent and outdoor performance should be checked for the intended environment.

PBT and PET Polyester

PBT is an engineering polyester with good electrical properties, chemical resistance and dimensional performance. Connectors, switches, sensor housings and automotive electrical parts commonly use reinforced PBT grades.

PET can also be injection molded, often with reinforcement, but processing and crystallization require close control. Both materials must be dried properly to prevent hydrolytic degradation during molding.

TPE and TPU Elastomers

Thermoplastic elastomers provide rubber-like flexibility while using thermoplastic processing. TPE is a broad family, while TPU is known for abrasion resistance, toughness and a wide hardness range.

Soft-touch grips, seals, feet, flexible covers and overmolded features use these materials. Bonding to a rigid substrate depends on compatible grades, surface condition, melt temperature and mechanical interlocks.

PEEK and High-Performance Polymers

PEEK provides exceptional temperature, chemical, wear and mechanical performance. It is used in aerospace, medical, energy and demanding industrial components.

The material is expensive and requires high processing and mold temperatures. Tool steel, heaters, insulation, gating and molding equipment must be appropriate. Other high-performance options include PEI, PPS and LCP, each with different strengths and limitations.

PPS: Heat and Chemical Resistance

Polyphenylene sulfide offers high temperature capability, flame resistance, dimensional stability and chemical resistance. It is commonly glass- or mineral-filled for automotive, electrical and fluid-handling applications.

PPS compounds can be abrasive and may produce brittle thin features if the design is not appropriate. Material handling, tool venting and corrosion or wear protection need review.

PEI: Heat-Resistant Amorphous Plastic

Polyetherimide is an amorphous high-temperature resin with strength, dimensional stability and inherent flame performance. It is used in aircraft interiors, electrical components, medical equipment and reusable devices.

PEI requires high melt and mold temperatures and thorough drying. Chemical compatibility and color requirements should be evaluated with the actual grade.

LCP: Thin Walls and Precision Features

Liquid crystal polymer flows well into very thin sections and offers dimensional stability, heat resistance and electrical performance. It is widely used for miniature connectors and electronic components.

LCP is highly directional. Gate position and flow orientation influence strength and warpage. Very thin tool features require precise machining and careful venting.

Material Property Comparison

Unfilled vs Reinforced Plastic

Glass fiber, carbon fiber, minerals and impact modifiers are added to adjust properties. Reinforcement can increase stiffness, strength and heat resistance while reducing shrinkage.

Tradeoffs may include reduced surface quality, higher viscosity, anisotropic behavior and increased tool wear. A filled grade is not automatically better; it should solve a defined product requirement.

How Fillers Affect Molded Parts

Mechanical Properties

Tensile strength alone does not define part performance. Modulus, elongation, impact strength, fatigue, creep and stress concentration all matter. Temperature and moisture can also change published values.

Use datasheet values to screen candidates, then test molded parts under realistic loads. Fiber orientation and weld lines may make local properties different from standard test specimens.

Temperature Performance

Heat-deflection temperature, glass-transition temperature, melting temperature and continuous-use rating describe different behaviors. Do not select a material using only one high number in a datasheet.

Consider applied stress, exposure duration, thermal cycling and assembly constraints. A material can remain solid at a temperature but still creep or lose stiffness under load.

Chemical Resistance

Chemical compatibility depends on concentration, temperature, exposure time and mechanical stress. A plastic that tolerates brief cleaning may not withstand continuous immersion.

Review the exact fluid and test molded samples when failure would be costly. Colorants, flame retardants and residual molding stress can change performance.

UV and Outdoor Exposure

Sunlight, heat, moisture and pollutants can fade color and reduce strength. Acrylic, selected ASA grades and stabilized engineering plastics are common outdoor candidates.

A UV additive improves resistance but does not guarantee indefinite service. Define the expected geography, exposure duration, color change and mechanical retention requirements.

Flame-Retardant Materials

Electrical and transportation products may require a specified flammability classification at a defined wall thickness. The certification applies to a particular commercial grade, color and thickness range.

Verify the supplier documentation rather than assuming that all grades of one polymer meet the same rating. Flame-retardant packages can influence flow, impact strength and tool deposits.

Food-Contact and Medical Grades

Regulated applications require more than a generic polymer name. Resin certification, colorant, traceability, change control, biocompatibility and manufacturing conditions may all be relevant.

The product manufacturer is responsible for validating the final application. Communicate all regulatory and documentation requirements before quotation and material purchase.

Color and Appearance

Natural resin can be colored with pre-compounded material or masterbatch. Pre-colored resin offers strong consistency, while masterbatch can provide flexibility for smaller volumes.

Color appearance changes with texture, wall thickness, lighting and material grade. Use a controlled physical standard and approve molded samples rather than relying only on a screen or color code.

Transparent Material Selection

PC, PMMA, clear ABS, PET and selected copolyesters can provide transparency. The best choice depends on impact, optical clarity, chemical exposure, UV and temperature.

Transparent parts reveal flow marks, weld lines, bubbles and contamination. Tool polish, drying, gate design and clean handling are essential to cosmetic quality.

Material Shrinkage and Tolerance

Every resin shrinks as it cools. Semi-crystalline materials generally shrink more than amorphous materials, and filled grades may shrink differently along and across the flow direction.

Mold dimensions use an estimated shrinkage allowance, but actual results depend on geometry and processing. Tight features should be reviewed for steel-safe correction after trial measurements.

Moisture Absorption and Conditioning

Nylon and certain other polymers absorb moisture after molding. Moisture can change dimensions, toughness and stiffness. Product testing must reflect the expected service condition.

This is different from pre-molding drying. Drying removes moisture that could degrade resin in the barrel, while conditioning establishes the intended post-mold moisture state.

Resin Drying Before Molding

Hygroscopic materials such as PC, PA, PET, PBT, TPU, PEEK and PEI require controlled drying. Moist resin can cause splay, bubbles, molecular degradation and lower mechanical performance.

Drying temperature, time, dew point and material handling should follow the resin supplier's instructions. A part that looks acceptable may still have degraded properties if processing moisture was excessive.

Recycled Resin and Regrind

Recycled content can reduce virgin material use, but source consistency, contamination and property retention must be managed. Post-industrial regrind from a controlled molding process differs from mixed post-consumer material.

Define the permitted percentage, number of heat histories, appearance limits and required testing. Regulated or critical products may restrict regrind.

How Material Choice Affects Mold Cost

Abrasive filled materials may require hardened or coated tool components. Corrosive or flame-retardant compounds may justify corrosion-resistant steel. High-temperature polymers require capable machines, heaters, insulation and cooling systems.

Material also influences gate style, venting, shrinkage allowance, draft and expected tool maintenance. Finalize the grade before tool design whenever possible.

How Material Choice Affects Part Cost

Resin price matters, but it is not the only driver. Density determines how much mass is needed for a given volume. Cycle time, scrap, drying energy, runner waste and inspection also affect delivered cost.

A more expensive resin can sometimes reduce wall thickness, eliminate an insert or extend product life. Compare total product value instead of cost per kilogram alone.

Prototype Material vs Production Material

Prototype processes may not offer the exact production resin. A machined or printed substitute can validate geometry but may not predict molded strength, shrinkage, surface or environmental response.

When material performance is critical, use rapid injection tooling to produce parts in the intended commercial grade before final validation.

Material Substitution Risks

Changing from one grade to another can alter shrinkage, color, impact strength, flame performance and regulatory status. Even similar materials from different suppliers may require process and dimension review.

Maintain the approved manufacturer and grade on controlled documentation. Use a formal change process for alternates rather than accepting an undocumented equivalent.

Material Selection Workflow

1. Define functional, environmental and regulatory requirements.

2. Prioritize must-have properties and acceptable tradeoffs.

3. Screen polymer families using reliable datasheets.

4. Select specific commercial grades.

5. Review moldability, shrinkage and tooling effects.

6. Prototype and test the complete part or assembly.

7. Approve color, texture and supplier documentation.

8. Freeze the grade before final tool dimensions are released.

Information to Send an Injection Molding Supplier

• 3D CAD and controlled drawing

• Approved resin manufacturer and grade

• Permitted alternatives, fillers and recycled content

• Color and surface-finish standard

• Operating temperature and chemical exposure

• Flame, food, medical or other compliance needs

• Critical dimensions and conditioning state

• Annual quantity and program life

• Mechanical and environmental test plan

Frequently Asked Questions

What is the most common injection molding material?

There is no single material for every market. PP, ABS, PE, PC and nylon are all widely used. The correct choice depends on product function, environment, appearance and cost.

Which plastic is best for a strong molded part?

Strength depends on the type of load and operating condition. PC offers impact toughness, nylon offers fatigue and wear performance, and reinforced polymers provide high stiffness. Geometry and weld-line position are also critical.

Which injection molding plastic has the lowest shrinkage?

Many amorphous and filled grades have relatively low shrinkage, but actual dimensional behavior depends on flow direction, wall thickness, packing and cooling. Select a specific grade and validate it in the intended geometry.

Can the resin grade be changed after the mold is built?

Sometimes, but different shrinkage and flow may change dimensions and quality. Review the new grade with the molder and perform a controlled trial before approving the substitution.

Are glass-filled materials always better?

No. They improve stiffness, strength and heat resistance but can increase warpage, tool wear and surface roughness. Use reinforcement only when its benefits match the product requirement.

When is PEEK worth the cost?

PEEK is appropriate when high temperature, chemicals, wear or demanding mechanical performance justify its material and processing cost. Less expensive engineering plastics should be evaluated when requirements are moderate.

Request an Injection Molding Material Review

Successful material selection connects product requirements with moldability, dimensions, appearance and total cost. Choosing the exact grade before tooling reduces trial changes and long-term supply risk.

Explore Aluprototype's plastic injection molding services, evaluate early-stage alternatives with our vacuum casting capabilities, or upload your CAD files on the request-a-quote page for a material and DFM review.