Compression Molding

Compression molding is the oldest plastic molding method. A compression mold consisters of two halves into which are machined the mold cavities. The mold temperature is maintained using electric heaters, and the mold is held shut with a hydraulic cylinder, or toggle clamp.

Material is placed in the mold, and it is closed under 2000 to 6000 pounds per square inch of molding area. Contact with the heated mold surface softens the material, allowing it to fill in the entire cavity and initiating the chemical reaction which cures the part. Cure time is determined by the thickest cross section, mold temperature, material type and grade. After curing, the mold opens and the part is ejected.

There are 3 types of materials used in the compression molding process as described below:

• Sheet Molding Compound (SMC).
  With this method the material is purchased in Sheet form. Typically the material is a rubber or rubber like material. The material is cut to a near net shape, slightly larger than the mold area, in a predetermined sheet thickness, based on mass needed for finished part. This cut shape is placed into the bottom mold cavity. Both Top and Bottom cavities are kept at an elevated temperature. The press is then activated and the material is pressed into the cavities under high compression. The heated cavities activate the curing of the material. The part is then removed and sometimes post cured in a post cure oven.
• Bulk Molding Compound (BMC).
  With this method a bulk material is used. Often this material is molded in to a preformed slug that would meet the material mass requirements for the finished part. This preform is placed in the lower mold cavity and the press is then activated in the same manner as the SMC process. Again, the cavities are heated to cause the material to start it’s curing process.
• Liquid Molding Compound (LMC).
  With this method a 2 component thermoset liquid is used. The LMC material is kept at an elevated temperature as are the upper an lower molding cavities. During the molding process the 2 components are mixed and poured into both halves of the heated mold. When the proper gel occurs, the mold is closed and the press is activated. These parts are demolded and typically post cured to achieve full cure of product.

Materials
Thermoset plastics are used in this process. These materials cannot be remelted as can their thermoplastic cousins. The most widely used plastic is phenolic, commonly known as “Bakelite”.

Cost
Tooling & parts moderate.

Advantages

• Lowest cost molds
• Little “throw away” material provides advantage on expensive compounds
• Often better for large parts
• Lower labor costs
• Minimum amount of wasted material & Improved material efficiency
• Internal stress and warping are minimized.
• Dimensional accuracy & stability is excellent.
• Shrinkage is minimized and closely reprodcible.
• Thick sections and large parts are practical.
• Lower molding pressures allow molding of large parts on presses of lower tonnage

Disadvantages

• Offers least product consistency
• Not suitable for fragile mold features, or small holds
• Uneven parting lines present a mold design problem
• High impact composites make flash control & removal difficult.
• The depth of the molded holds is limited to 2 or 3 times their diameter
• Shot weight must be tightly controlled
• Dimension across the parting line may be difficult to hold but good accuracy may be obtained through tight process control

Examples of Applications
Radio & appliance knobs, ash trays & electrical parts.