Compression Molding Machine

Compression molding is a method of molding in which the molding material, generally preheated, is first placed in an open, heated mold cavity. The mold is closed with a top force or plug member, pressure is applied to force the material into contact with all mold areas, and heat and pressure are maintained until the molding material has cured. The process employs thermosetting resins in a partially cured stage, either in the form of granules, putty-like masses, or preforms. Compression molding is a high-volume, high-pressure method suitable for molding complex, high-strength fiberglass reinforcements. Advanced composite thermoplastics can also be compression molded with unidirectional tapes, woven fabrics, randomly orientated fiber mat or chopped strand. The advantage of compression molding is its ability to mold large, fairly intricate parts. Compression molding produces fewer knit lines and less fiber-length degradation than injection molding.

Processing Steps

• Step 1

A piece of uncured rubber is placed in the mold.

• Step 2

The mold is closed up and held under hydraulic pressure while the rubber cures.

• Step 3

When the mold opens the part can be removed. The excess rubber, called flash, needs to be trimmed off the part.

Types of Compression Molding

sheet molding compound (SMC)

sheet molding process involves placing a reinforcement, such as a glass mat, between sandwiching layers of a thermoplastic and heating the materials to produce a single sheet of material. The single sheet of material is then cut to the desired size and then reheated to molding temperature before being placed in a compression molding press. This process has the disadvantage of higher cost because of the apparatus required, the material handling costs incurred in making the sheet, handling and cutting the sheet, and the like. The material used to make the sheet is also subject to three thermodynamic cycles, a first cycle when the thermoplastic sheet is formed, a second cycle when the thermoplastic sheets and glass mat are molded together, and a third cycle when the resulting sheet is heated to molten temperature prior to molding the part.

bulk molding compound (BMC)

bulk molding compounds by producing a billet of molten material that is placed into a compression molding press which molds the molten material into a part. Effectively placing and distributing long reinforcing fibers in the billet has heretofore required complex machinery. For example, U.S. Pat. No. 5,165,941 issued to Ronald C. Hawley on Nov. 24, 1992, discloses an extruder apparatus and process for compounding thermoplastic resin and fibers. The Hawley extruder includes an apparatus for compounding thermoplastic resin and reinforcing fibers incorporating a resin extruder in which thermoplastic resin pellets are melted in a second, compounding, extruder in which the molten thermoplastic resin is mixed in intimate contact with long reinforcing fibers. The melted thermoplastic resin is not fed into the device with the fibers, but rather is introduced into the compounding extruder at a point downstream of the inlet point for reinforcing fibers, so that the fibers are mechanically worked and heated before coming into contact with heated, molten thermoplastic resins.

thick molding compound (TMC)

low pressure molding compound (LPMC)

• Straight Compression Molding

resin transfer molding (RTM)

• Hydraulic Transfer Presses
• Vacuum Molding
• Hydraulic Vacuum Presses

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 Cost is usually less than steel stamping tools and injection molding tools & part cost depends on the application - if a product has structural requirements and there is lot of intergration of features then this process can produce cheapest products as compared to steel, injection molding or aluminium products.

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

Automotive exterior panels especially for commercial vehicles,Radio & appliance knobs, ash trays & electrical parts.

compression molding machine

Types of compression molding machine – hand operated, automatic,. single and multi daylight machines, bulk factor, preheating of molds