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What is Plastic Injection Molding?


The Basics of Plastic Injection Molding

Plastic Injection Molding is a manufacturing method to obtain plastic molded product by injecting molten plastic resin into a mold, the resin then cools below its melting temperature and ejected, retaining the desired shape.

It is the most common used manufacturing process for creating plastic parts. It is widely used in a variety of industries in producing from small medical part to large automotive and aerospace parts.

Plastic Injection Molding Processes
Courtesy of researchgate.net

Benefits of Choosing Plastic Injection Molding Process

  1. Suitable for high production runs
  2. Consistent quality of the manufactured plastic parts
  3. Price per part decreases with higher order quantity
  4. Can manufacture details features or complex geometry
  5. High quality of surface finishes
  6. Minimal post processing
  7. Low labor costs

Common Materials Used in Plastic Injection Molding

There is a wide range of material selections for Plastic Injection Molding process. The selection of material used depends on each application. Each material grades or types vary in terms of materials, mechanical, thermal, chemical, electrical and optical properties. Hence, always check with the manufacturer and obtain the material data sheet prior to production.

Plastic Injection Molding resin can also mixed with additive or colorant to modify their mechanical or physical properties.

An additive is commonly used to improve the stiffness of the injection-molded part. A very common additive is glass fiber. The glass fiber can be mixed with resin pellets at ratio 10%, 20% and 30% to achieve the different mechanical properties.

Colorant can be added to the mixture to create a variety of colored part. Common colors used include red, yellow, blue, green, white and black in a wide range of different shades.

  Most Common Available Plastic Resin
ABS Acrylonitrile Butadiene Styrene
PA (Nylon) Polyamide
PA + GF Polyamide + Glass Fiber
PMMA (Acrylic) Polymethyl Methacrylate
PC Polycarbonate
PC + GF Polycarbonate + Glass Fiber
PC +PBT Polycarbonate + Polybutylene Terephthalate
PC +ABS Polycarbonate + Acrylonitrile Butadiene Styrene
PE Polyethylene
HDPE High Density Polyethylene
LDPE Low Density Polyethylene
PEEK Polyether Ether Ketone
PEEK + GF Polyether Ether Ketone + Glass Fiber
PEI (Ultem) Polyetherimide
PEI + GF Polyetherimide + Glass Fiber
PP Polypropylene
PP + GF Polypropylene + Glass Fiber
POM (Acetal) Polyoxymethylene
PS Polystyrene
PVC Polyvinyl Chloride
TPU Thermoplastic Polyurethane
TPV Thermoplastic Vulcanizate

Tooling Types for Plastic Injection Molding

1. Rapid Tooling
  • Using aluminum mold.
  • Short Life: 5,000 – 10,000
  • Typically machined within 1-2 weeks depends on the complexity of the mold design.
2. Production Tooling
  • Normally using steel mold such as P20 (up to 50,000 shot life), NAK80 (up to 100,000 shot life), S136 (up to 200,000 shot life) or H13 (up to 500,000 shot life).
  • Typically machined in 3 weeks.
  • Able to integrate side-pulls or cam-actions.
3. Multi-Cavity or Family Molds
  • Multiple identical cavities or family of parts are cut into the same mold.
  • Allow producing more parts per production cycle, result in increasing the productivity and efficiency.
4. Insert Molding
  • Inserts are placed into the mold, able to extend tool life for critical features.
  • Allows inserts such as helicoils to be molded in the design.
5. Over Molding
  • Allows different material to be molded over on top of another premade parts.
  • Sometimes referred to as two-shot molding.

Surface Finishes for Injection Molded Part

There is a wide range of surface finishes that can be done on plastic injection molded part as shown in the following:

Available Surface Finishes Description
As Molded • No secondary process such as polishing or grinding on the mold surface.
• Part will show tooling marks.
Standard Finish • Type of finish: Paper finish/ Stone Finish/ Blasted Finish
• Typically SPI B-1 to SPI D-3 (depends on geometry and draft angle.
SPI Finishes • Wide range of surface finishes from high polish to blasted finish (SPI A-1 to SPI D-3)
Surface Texture • Mold-Tech Finishes (swirls, lines, patterns)
• Common Mold-Tech Texture: MT110XX, MT112XX, MT113XX and MT114XX series.
Pad Printing or Silk Printing • Transfer 2D images into a 3D object.
Laser Engraving • Using laser beam to engrave names, logos or numbers onto your parts.
Chrome Plating • Chrome plating on ABS part to enhance aesthetic appearance, structural integrity, strength, durability and abrasion resistance.

Cost Drivers in Plastic Injection Molding

1. Tooling Cost
  • Determined by the total cost of designing and machining the mold.
  • This cost is independent of total number of manufactured parts.
  • For small production run, the tooling cost has a great impact on the overall cost. Always simplify the mold design to reduce the time spending on machining the mold.
  • For large volume production run, the tooling cost contribution for each part will be reduced when the part volume increases. Hence, effort should be focusing on reducing the total cycle time in order to reduce the cost per manufactured part.
2. Material Cost
  • Determined by the volume of material used and its price per kilogram.
  • It is dependent on production volume.
3. Production Cost
  • Is determined by total cycle time and also time spent on the post processing processes.