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Compression Molding Method

Compression molding is a production method that is applied by forcing the mold to take the shape of the mold cavity by compression method so that the material fills the geometry of the mold chamber volumetrically. The mold is heated by the hot plates of the presses or the heating resistance equipment to make it hot in order to fulfill the vulcanization conditions. The solid silicone compound mold to be used in compression molding is preformed according to the chamber volume and geometry. It forces the material inside the mold plates, which are closed by hydraulic pressure between the press plates, to take form. The material placed in the mold chamber is ensured to flow with pressure, thus filling the mold chamber volumetrically. The vulcanization reaction takes place within a certain period of time, depending on the silicone rubber process conditions under temperature and pressure. The material, which starts to form cross-links with vulcanization, takes the form of the mold chamber. After the curing is completed, the mold is opened and the part is removed from the mold by apparatus or manually. In this method, burrs are inevitable on the part, the burr of the produced part is then cleaned by secondary processes and made ready for use.

In compression molding, a mold temperature in the range of 150°C to 200°C is generally chosen. Temperature plays an important role. It should be as high as possible to shorten the vulcanization time and indirectly the production cycle times. However, excessively high temperatures may cause undesired scorching and breakage defects in the part during curing. The vulcanization time is determined by the temperature of the material, the mold temperature and the part thickness.

In solid silicone rubbers, there are mold release components used in the compound to remove the part from the mold after compression molding or mold release materials applied to the mold surfaces. Mold release materials should be used in appropriate proportions, when used excessively, they may cause unwanted staining and deposits on hot mold surfaces. It can also create unwanted errors on the part.
Although the compression molding method is the simplest and most primitive method, it is the most suitable molding method for the geometry of some parts. If the production numbers and quality of a part with a simple geometry that can also be produced by injection molding can be met by the compression method, it can be produced by the compression method without the need for extra press, equipment and mold costs.

Some advantages and disadvantages of production by compression molding can be listed as follows:
• Mold and machine investment costs are less than other molding methods.
• The annual production amount can be more efficient in the production of parts that can be costly in production with injection.
• It enables the production of some large geometry parts that cannot be produced by transfer or injection molding.
• Mold change on the press can be done faster.
• Before production, the material to be used must be preformed according to the mold cavity volume, part weight and mold cavity geometry.
• Operator labor, which is not very suitable for automation, is high.
• There are situations that extend the cycle times such as material preparation, operator labor, and in some cases, manual mold opening and closing.
• In compression molding, slightly more material than the weight of the part is used to properly fill the entire mold chamber. In this case, burrs form and material wastes occur. It also requires the secondary cleaning processes required to obtain the final part.
• Compared to the injection method, production can be made without requiring machine and mold expertise for the operator. By specifying very simple process conditions, sequential cycles can be achieved without any problems.

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