Application Review: Compression Molding

Process:
Compression Molds

Compression molds vary considerably in their shape, size, and complexity. This process utilizes a verticle, 2 post or 4 post, press. There is often a high number of cavities in the mold. 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.

Compression molded parts have several advantages. They have the ability to mold materials that have more colors than any other molding process. Also, compression molding can provide products that can tolerate the most extreme environmental conditions. Parts with heavy wall sections can be molded without the excessive sink as seen in injection molded parts.

HPDC has achieved compression molding in epoxy tooling. By using epoxy tools the tooling cost was dramatically reduced giving the ability to test the part design and tolerances before committing to hard tooling. HPDC has achieved as many as 500 cycles from an epoxy tool.

Compression Molding Advantages:
Thick Wall Sections Epoxy Tooling use Material Strength Low Volume Value Rubber Like Parts
Paintable Molded In Inserts

Compression Molding Disadvantages:
Longer Cycle times
Some Excessive Flash
Higher Part Cost
Part Trimming Required
Shortened Tool Life