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Compression Molding

Neslihan Yağmur11/12/2012

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Compression Molding Process

The process of molding a material in a confined shape by applying pressure and usually heat.Compression molding process is followed by two step first one preheating and pressurizing.

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Process Description

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Process Description

Charge is placed in cavity of matched mold in the open position,Mold is closed by bring the two halves together,Pressure is exerted to squeeze the resin so that it fills the mold cavity,While under pressure the mold is heated which cures the resin.

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Process Description

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Stages in Compression molding cycle

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Stages in Compression molding cycle

The various stages of the compression molding cycle time can be represented as a function of the force required to close the old at a constant rate. In the “plastication” stage the force increases rapidly as the polymer feed is compressed and heated.

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Stages in Compression molding cycle

The second stage flow commences, once the yield stress of the elastomer is exceeded. tc is the point at which the mold fills and compression of the melt occurs. Mold filling the majority of chemical reaction should take place after tc.

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Parameters1.The quantity of molding material

 2.Pressure of the molding process

Range of pressure 2000-3000 psi(13.8-20.7 MPa)

3.Mold temperature

Temperature range 300ºF to 375ºF (149⁰C- 191⁰C)

4.Cure time variables.

The period required to harden thermosetting material to partial and complete polymerization is called cure time

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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. 

• 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.

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  Materials used in this process

• Thermosetting polymers • Fiber reinforced composite• Thermoplastic:

-Ultra High Molecular Weight Polyethylene (UHMWPE)

- Long Fiber reinforced Thermoplastics

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Material properties affected by the process

Density

Strength

Anisotropic property (fiber orientation)

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Advantages

Low cost

Uniform density

Uniform shrinkage due to uniform flow

Improved impact strength

Dimensional accuracy

Internal stress and warping are minimized

Short cycle time

High volume production

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Disadvantages

• Curing time large• Uneven parting lines present • Scrap cannot be reprocessed• High initial capital investment

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Applications & products

• Dinnerware• Buttons• Knobs• Appliance Housings

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Applications & products

• Radio Cases• Automotive• ash trays & electrical parts

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ReferencesPolymer Processing Fundamentals, A. Tim, Carl Hanser Verlag, Munich, 1998Effect of Molding Parameter on Compression Molded Sheet Molding Compounds Part, Jin-Ho Jeong, Journal of Materials Processing , 1997