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Guide to compression moulding process, advantages, limitations and applications

Compression moulding process, advantages, limitations and applications

Compression molding is among the oldest materials processing techniques. In commercial use, compression moulding is the most simple and reliable process. Due to its simplicity and availability, it is most widely used process.

Compression molding is the process of molding in which a preheated polymer is placed into an open, heated mold cavity. The mold is then closed with a top plug and compressed in order to have the material contact all areas of the mold.

It is a high pressure forming process in which the molten plastic material is squeezed directly into a mould cavity, by the application of heat and pressure to conform to the shape of the mold

This process is able to produce parts with a wide array of lengths, thicknesses, and complexities. The objects it produces are also high in strength, making it an attractive process for a number of different industries.

Thermoset composites are the most common type of material used in compression molding.Compression molding is thus commonly used for thermosetting plastics such as phenolics, urea, melamine, an alkyds; it is not ordinarily used for thermoplastics.

Process parameters

Important factors to be considered before compression molding process are:

  • Amount of plastic material (charge).
  • Heating time
  • Melting temperature of plastic material.
  • Pressure required to squeeze the material in to the mold cavity.
  • Cooling time.

Advantages of compression molding

Compression molding is a popular technique for a number of reasons. Part of its popularity stems from its use of advanced composites. These materials tend to be stronger, stiffer, lighter, and more resistant to corrosion than metal parts, resulting in superior objects. Manufacturers accustomed to working with metal parts find that it is very simple to convert an object designed for metal into a compression molding part. Because it is possible to match metal part geometry with this technique, in many circumstances one can simply drop-in and replace the metal part altogether.

Another advantage of compression molding is its ability to create very complex parts. While this technique can not quite reach the production speed of plastic injection molding, it does offer more intricacies in geometry than typical laminated composites. It also allows for longer fibers than plastic injection molding, resulting in stronger and stiffer materials. Therefore, compression molding can be seen as a middle ground between plastic injection molding and laminated compound fabrication.

With its ability to create complex parts with a huge variety of applications while keeping part cost and turnaround time a priority, compression molding is an advantageous process for manufacturers in a wide range of industries. Compression molding process is one of the low cost molding methods as compared to injection molding and transfer molding.

  • Low initial setup cost and fast setup time.
  • Heavy plastic parts can be molded.
  • Good surface finish of the molded parts.
  • Wastes relatively little material as compared with other methods.

Limitation:

Offers least product consistency

  • Not suitable for fragile mold features, or small molds
  • 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.

Problems occuring in compression moulding process

While performing the process, there can be some issues regarding the part:

  • Bulge on opposite side of insert.
  • Cure blister
  • Dull appearance
  • Excessive flash
  • Flow lines
  • Procure or hard spots
  • Mould stains
  • Mottled surface appearance
  • Short shot
  • Orange peel
  • Shrinkage
  • Sticking in mould
  • Burn mark
  • Warpage

Applications

Compression molding is used for manufacturing: Electrical and electronic equipments, brush and mirror handles, trays, cookware knobs, cooking utensils, dinnerware, appliance housings, aircraft main power terminal housing, pot handles, dinnerware plates, automotive parts, flatware, buttons, and large container.