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An Overview of Plastic Material Selection For Injection Molding

Plastic material selection is very critical part of product design. Type of material selected affects product design, performance and its cost. Lot of thermoplastic grades are available in market. In this article we will discuss plastic material selection process and common thermoplastic used for injection molding.

Plastic Material Selection Process For Injection Molding

First step in plastic material selection is to understand product requirements. You need to list down required plastic material properties. For example

  • Maximum and minimum working Temperature.
  • Compliance and certification requirements such as Reach, UL or RoHS certified.
  • Is the parts are going to rub with each other ?
  • Type of application : Indoor or Outdoor.
After understanding the requirements. Next step is fulfilling those requirements. 
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 product working temperature is 50°C and dissipating heat. ABS is not a good choice. PC or glass filled PC can be used for this application.

Material Physical Properties

Material physical properties includes :

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

Required material physical properties is selected based on following  product requirements.

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

For example, for moving parts such as gears Nylon or POM is used

Injection Molding Capability

Thermoplastic processing and drying conditions also affect the material selection. These properties includes material :

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

Plastic material also affect part aesthetics and dimensional accuracy. For example ABS material gives good dimensional accuracy and  Finish. Therefore various indoor consumer electronics products are manufactured in ABS. 

Refer this article gives material recommendations for required SPI surface finish.

Chemical Exposure and Weather Resistance

This is very important factor during material selection. For example Nylon, acetal are generally suitable for industrial environments. 

Temperature, moisture, and UV sun exposure affect plastic part properties and appearance. The end-use of a product determines the type of weather resistance required like outdoor parts must withstand continuous outdoor exposure and perform in the full range of weather conditions. For example, UV Radiation can yellow or degrade many plastics. If part will be exposed to a radiation source, we can consider painting or using a UV-stabilized resin.

Assembly Process

Assembly process also affects material selection. For example, adhesives works best with ABS and PC. Whereas for Nylon and HDPE we require special adhesives. Its easy to ultrasonic weld PP and ABS compared to Nylon and PC.

Injection Molding Materials

Material Properties
ABS High Impact Resistance
Low Cost
High Dimensional Accuracy
Good Aesthetics
Vulnerable to Solvents
PC (Polycarbonate) Heat Resistant
High Impact Resistant
Food Grades are Available
UV Resistant
Moderate Moldability
Stress Cracking
High Cost
ABS+PC Heat Resistant
High Impact Resistant
High Toughness
Vulnerable To Solvents
PP (Polypropylene) High Chemical Resistance
Food Grades are 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
PEEK Excellent Mechanical Properties
Good Chemical Resistance
High Cost
PS (Polystyrene) Low Cost
Food Grades are Available
Low Mechanical Strength

Conclusion

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

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