Plastic Material Selection Guidelines for Injection Molding

Plastic material selection for injection molding is a very critical part of plastic product design. Type of plastic material selected affects plastic part design, its performance and cost. In this article we will discuss plastic material selection process and common thermoplastic used in injection molding. This guide will help mechanical design engineers in the selection of best thermoplastic material during plastic part design.

Thousands types of plastic material grades are available in the market. But only a few commercial grade plastic materials are widely used to manufacture plastic parts using injection molding. Following are some of the widely used plastic materials in the injection molding.

  • Acrylonitrile Butadiene Styrene (ABS)
  • Polycarbonate (PC)
  • ABS + PC Blend
  • HDPE (High Density Polyethylene)
  • Polypropylene (PP)
  • Nylon (PA 6, PA 66, PA 11)
  • PEEK
  • Polystyrene (PS)
  • Delrin (POM)

All of the above materials are available in various grades. Each of these grades have different properties and characteristics.

Factors affecting Selection of Plastic Material For Injection Molding

Various factors have an impact on the selection of plastic material for required application. You should consider the following points during the selection of plastic material for a given application.

  1. Product working Temperature.
  2. Structural Strength Requirements : Mechanical Properties Requirement
  3. Cost Constraints.
  4. Weight Constraints.
  5. Plastic Part Design.
  6. Product Aesthetic Requirements.
  7. Assembly Process.
  8. Type of application : Indoor or Outdoor.

To fulfill above plastic part design requirements. Following properties of plastic material are analyzed.

Material Thermal Properties

Plastic materials are shortlisted based on product following thermal requirements. 

  • Maximum and Minimum Working Temperature
  • Heat dissipation during product operation
  • HDT Temperature (Heat Deflection Temperature)
  • Melting Temperature

For example if the product working temperature is 50°C and it is dissipating heat. ABS is not a good choice. PC or glass filled PC can be recommended for this application.

Material Physical Properties

Following physical properties of a plastic materials are considered during plastic material selection:

  • Impact Resistance
  • Tensile Strength
  • Wear Resistance
  • Material Strength and Stiffness etc.
  • Creep Resistance

Above physical properties of a plastic material are analyzed on the following  parameters.

  • Static and Dynamic Loads Acting on Part
  • Wear Resistance
  • Continuous Loading etc.

For example, for moving parts such as gears, screws Nylon or POM is recommended because of their self lubrication properties.

Injection Molding Capability

Following Thermoplastic material processing or mold-ability properties also has an impact on plastic material selection.

  • Melting Temperature
  • Melt Flow Rate
  • Shrinkage
  • Dimensional Stability etc.
Injection Molded Part Aesthetics and Dimensional Accuracy

Plastic material selection also has an impact on part aesthetics and dimensional accuracy. For example, ABS material gives good dimensional accuracy and Finish. Therefore ABS is recommended to manufacture indoor consumer electronics products. Click this Link!! to know plastic material recommendations for required SPI surface finish.

Exposure to Chemicals

It’s very important to consider the exposure of the product to different chemicals. For example Nylon, acetal are generally suitable for industrial environments. 

Exposure to outside Weather (Weather Resistance Properties)

Outside Temperature, moisture and UV exposure affect plastic part properties and appearance. Therefore for outdoor products it is recommended to select a material that can withstand outside weather conditions. For example, UV Radiation causes yellowish color on plastic parts or they can degrade plastics. Therefore If part is exposed to UV radiations, it is recommended to use UV additives or UV-stabilized plastic material.

Assembly Process

Part assembly process also has an impact on plastic material selection. For example, For plastic part assembly using adhesives, ABS and PC are recommended. Whereas for Nylon and HDPE special adhesives are required. PP and ABS are easy to ultrasonic weld compared to Nylon and PC.

Injection Molding Materials

Here is the list of commonly used plastic materials for injection molding with their properties.

  • High Impact Resistance
  • Low Cost
  • High Dimensional Accuracy
  • Good Aesthetics
  • Vulnerable to Solvents
PC (Polycarbonate)
  • Heat Resistant
  • High Impact Resistant
  • Food Grade material is Available
  • UV Resistant
  • Moderate Moldability
  • Stress Cracking
  • High Cost
  • Heat Resistant
  • High Impact Resistant
  • High Toughness
  • Vulnerable To Solvents
PP (Polypropylene)
  • High Chemical Resistance
  • Food Grades material is Available
  • Low Dimensional Accuracy
Nylon (PA 6, PA 66, PA 11)
  • Self Lubricating
  • High Strength
  • High Chemical Resistance
  • Water Absorption
Delrin (POM)
  • Self Lubricating
  • High Strength and stiffness
  • Prone to Warpage
  • Water Absorption
HDPE (High Density polyethylene)
  • Fine Details can be Produced
  • High Impact resistance
  • Low Strength
  • Excellent Mechanical Properties
  • Good Chemical Resistance
  • High Cost
PS (Polystyrene)
  • Low Cost
  • Food Grade material is Available
  • Low Mechanical Strength

To sum up, Plastic material selection process for injection molding is a very critical part of product design. Type of material used ensures product quality, performance and cost. All product functional and certification parameters need to be considered before selection of material.

We will keep updating more information on plastic material selection for injection molding. Please add your suggestions, comments or questions on plastic material selection in the comment box.

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