Preparatory work before compression molding

As a wet premix, BMC molding compounds contain volatile active monomers. Do not remove the packaging prematurely before use, otherwise these active monomers will volatilize from the BMC material, reducing its fluidity and even causing performance degradation or even scrap. Of course, for BMC molding compounds that have been unpacked and not used up, they must be resealed for the next pressing.

1. Calculation and weighing of the charging amount

Generally speaking, first of all, it is necessary to know the volume and density of the pressed product, plus the loss of burrs and flash, and then calculate the charging amount. The accurate calculation of the charging amount is directly related to ensuring the accuracy of the geometric dimensions of the product, preventing shortages or waste of materials due to excessive materials, especially for BMC, a thermosetting composite material that cannot be recycled after molding, which is of great practical significance for saving materials and reducing costs.

In practice, because the shape and structure of the molded product are relatively complex, the calculation of its volume is both complicated and not necessarily accurate, so the charging amount is often estimated. For automatic machines, the feeding amount can be controlled within ±1.5% of the total amount of material used, while reaching 5% or more will definitely cause flash on the closing surface of the mold. This thin layer of excessive material will quickly solidify and form flash under the action of high mold temperature in the heated state.

2. Preheating of the mold

BMC molding compounds are a kind of thermosetting reinforced plastic. For thermosetting plastics, before molding, the mold should be preheated to the required temperature. This actual temperature is related to the type and formula of the BMC molding compound to be pressed, the shape and wall thickness of the product, the molding equipment used, and the operating environment. It should be noted that do not charge the mold cavity until the mold temperature reaches the set value and is uniform.

Generally speaking, the following issues should also be considered during the molding operation:

1. The temperature of the BMC molding compound should generally be above 15℃.

2. The position of placing the material should be selected according to the shortest flow path that can be obtained during pressing, preferably to ensure that the material can reach all corners of the mold cavity at the same time. For places where material stagnation or "dead corners" may occur, the material can be placed in advance.

3. The placed material should be distributed as evenly as possible.

4. Because the curing time required for general BMC molding materials at 150℃ is less than 1min/mm, the feeding should be quick. If manual weighing is used, the speed is slow, which is not conducive to improving production efficiency. Therefore, when pressing smaller products, a mold with a common charging chamber is used.

5. For products with relatively complex shapes, the material can be pre-pressed into blanks similar to the product, which can avoid shortages or weld lines in the protruding parts of the pressed product.

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