pre-pressing before compression molding

When compression molding thermoset molded materials, the preferred method of material handling is to use preforms. The process of producing preforms is called pre-pressing.

What is pre-pressing? Pre-pressing is pressing loose compression molding powder or fibrous at or slightly above room temperature in a pre-pressing mold to form a regular shaped material of a certain weight.

Usually, the plane size of the preform is slightly smaller than the product, while the thickness is greater than the product to ensure that the material reaches and fills all areas of the mold cavity during the compression molding process. It is difficult to calculate precisely how much volume to fill at one time during pre-pressing, especially for products with complex shapes. We usually simplify the shape of the product and multiply the resulting approximate volume by the density of that molded product and add a loss of 3%-5% to get the estimated charge volume. After a few test press corrections, the exact charge volume can be found.

In general, pre-pressing has the following effects.

• Fast, accurate and simple charging.
• Reduce the compression rate of the material, which can reduce the filling chamber of the mold, simplify the mold structure and reduce the weight of the mold.
• Avoid the fluttering of compression molding powder and improve labor conditions.
• Less air content in the billet makes the heat transfer fast, shortens the preheating time, can reduce the air bubbles in the products, and reduces the chance of scrap generation.
• Faster mold loading, reducing the unevenness of the quality of the material caused by the temperature difference between inside and outside.
• Convenient for molding larger or products with fine inserts.

Please note: Some preforms are made in advance. They should always be stored in sealed containers until they need to be molded. This will help prevent the preforms from gaining or losing moisture.

MDC often recommends compression moulding solutions if you need large or complex composite parts. This is an efficient option and we can handle every step of the manufacturing process, from start to finish. MDC will create a compression mold just for you, determine the correct composite formulation to meet your needs, pre-press and manufacture your part. We can process the composite material into almost any shape your product requires.