By: Tristine Nubla, Date: 02/13/2018

Compression molding is a manufacturing process that is often used to shape thermoplastics and rubber compounds by applying heat and high pressure. Thermoplastics are ideal for plastic molding due to their pliability at high temperatures and their stiffness once cooled. Compression-molded products are manufactured in high volumes and therefore, are common in daily applications. Examples of compression molding can be seen in household items such as kitchenware and buttons. Additionally, compression molding can be utilized to develop electrical products and car parts such as rubber tires. 

Compression molding is one of first plastic-molding processes that has been identified. Alfred Critchlow is credited with its invention. In the 1800s, he created union cases with a compound that comprised of shellac resin, called Florence Compound. In 1847, Critchlow founded the Pro Corporation, which is known as the first plastics molding company.

• Used to mold high strength products including kitchenware, rubber tires, electrical housings, switches, and buttons 
• Advantages
  • Low cost
  • High volume production
  • High strength
  • Lighter, more pliable, and more resistant to corrosion compared to metals 
  • Wide range of part sizes
• Disadvantages
  • High capital
  • Time to set mold
  • Simpler geometries compared to injection molding
  • Scrap can not be reprocessed
• Intermediate Tolerances
  • A1 (High Precision) to A4 (Basic)
  • Shrinkage: Product shrinks in size as it cools
  • Mold: More expensive to produce a mold with more precision

1. Heating
   1. The cavity mold is heated to around 300 degrees Farenhiet depending on the material. 
   2. A "slug" or "charge" is placed in the mold and heated.
2. Compression
   1. The mold is closed with pressures up to 3000 PSI.
3. Cooling
   1. Pressure is maintained as the product cools. 
4. Removal/Trimming
   1. The product is removed from the mold.
   2. Excess material may be trimmed off at the end. 

Labor: Most plastic molding processes are fully automated. Only 1-2 people are required in order to operate equipment. They should have good mechanical and technical skills. 

• Equipment Costs - Hydraulic Press
  • $800 to $2000
• Tooling Costs - Mold
  • Depends on material (steel, aluminum)
  • Depends on complexity of tool
  • Example: It costs $300,000 per tool for a bumper beam.
• Material Costs - Various

• The unit cost will vary greatly depending on the item.
  • Example: A bumper beam will cost about $25.00 and a plastic washer will cost about $0.55. 
  • High production volumes will lower costs.