Compression Moulding: An Overview

After Leo Hendrick Bakeland successfully created the first mouldable material in 1905, people comfortably ditched celluloid for Bakelite compounds in the production of domestic products. His discovery paved the way for not only numerous plastic materials in use today but also a significant number of moulding techniques used in the industry to date.

From 2008 to 2018, statistics show that the sales value of injection and compression-type moulds exceeded 6.2 million British pounds [1].

Here, you will learn about:

Different moulding techniques for producing plastic
What compression moulding is
Process of compression moulding
Materials and application of compression moulding process
Advantages of compression moulding over other moulding techniques

Plastic moulding techniques

Plastics are generally produced through moulding. It is the process of shaping liquid or pliable material using a rigid frame called mould. There are quite a several moulding processes available today, and the advantages of one over another are mainly dependent on the end-product requirements. Nevertheless, here are some of the most common processes in use today:

Rotational Moulding
Injection Moulding
Blow Moulding
Compression Moulding
Extrusion Moulding

Table 1 shows the moulding processes, their descriptions and a host of end-products obtained from the processes.

Compression moulding technique was initially developed specifically to mould Bakelite compounds. It now processes varieties of thermoset plastics, thermoplastics, elastomers, rubbers and the list goes on. This moulding technique produces desired shapes when plastic raw materials are compressed in a cavity by using mould, heat and pressure. It is typically used for the manufacture of a small-to-medium randomly aligned fibre-reinforced thermoset composite [2].

Table 1. Different Moulding Processes and descriptions

Moulding Technique	Description	Products
Rotational Moulding	It is more suited for producing hollow parts. Moulded products are achieved by placing powder or liquid resin in the mould, and then rotating it in an oven.	Bulk containers, storage tanks, road cones.
Injection Moulding	It has to do with injecting molten plastic into a mould at very high pressure. They are quite expensive to use.	Car parts, plastic parts for surgical operation.
Blow Moulding	It is more suited for making hollow parts with thin walls. The process is achieved by heating plastic material before injecting air to blow up the hot plastic part like a balloon.	Plastic bottles
Extrusion Moulding	Extrusion moulding is similar to the injection moulding except that a die is used in place of mould. Also, it is more suited to the production of long continuous shapes.	PVC pipes, hoses.
Compression Moulding	As the name suggests, plastic material is heated in a mould before being pressed into specific shapes	Auto parts

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Figure 1. Cap compression moulding machine. Courtesy of Kaipu.

The Process of Compression Moulding

Compression moulding requires a mould made up of two parts- a cavity (female) part of the mould and a plunger (male) part of the mould. The moulding process requires a press with heated platen (upper and lower). Because the mould parts are attached to the upper and lower platens, the parts are designed to align when assembled. This alignment attribute forms the basis of the compression moulding technique.

It is worth mentioning that before commencing any moulding operation, the positioning of the mould parts relative to the upper and lower platen of the press must be determined. When moulding compounds are used, the cavity part of the mould is usually attached to the lower platen of the press, while the plunger part is mounted on the upper platen.

On the contrary, if a plastic impregnated material is to be used, the cavity part of the mould is attached to the upper platen, while the plunger part is attached to the lower platen. Nevertheless, you must ensure that the two mould parts aligns and matches each other.

The plastic moulding material is charged to the cavity part of the heated mould after it is weighted out and preheated. Next, the plunger and cavity parts of the moulds are brought together by closing the press. This closing action forces the moulding material to melt and flow, thereby filling the cavity between the two parts of the mould and displacing entrapped air within the system.

The plastic is left in the mould at some specified temperature, pressure and period to allow it cure completely. When thermoset plastic is used, preheat temperature of moulding material may be as high as 300°F, mould heat temperature falls in the range of 250°F to 350°F, and the mould pressure may vary between 1000psi to 2000psi. Nevertheless, the moulding material is usually charged in the cavity slightly in excess to ensure it is completely filled.

After the moulding process, the excess material, also known as the flash, is often seen squeezed out between the mating surfaces of the mould parts. While filing and sanding are good enough to remove the flash, you'll often find people freezing the plastics to aid easy removal of the flash.

The process of compression moulding is quite different for thermoplastics. Contrary to the thermoset plastics moulding, the mould needs to be cooled at the end of the moulding operation for thermoplastics. As plastics become hardened and cannot flow after moulding, the thermosets need not be cooled after moulding.

The moulding process can be as quick as three minutes to complete and as long as two hours: it all depends on the type of plastic used and the size of the charge. Even with the numerous stages or processes highlighted, it is essential to note that the curing or cooling stage consumes most of the time in the moulding operation, and three types of moulds are used for compression Moulding:

Positive Mould
Flash Mould
Semi Positive Mould

Positive Mould

In the positive mould, all the mould material is charged into the mould cavity, which is usually very deep. As there is very little escape for the plastic material, the smallest possible volume is formed when the plunger compresses the material. By altering the weight of plastic material, the thickness of the moulded parts increases or decreases proportionately. Hence, for multi-cavity moulds, a variation of the part thickness or density may occur if one cavity is charged more than the other.

Flash Mould

This is the most used mould type. It is simple to construct and holds the thickness and density of parts within close limits. These attributes are possible in flash compression moulding because of a unique feature called the pinch-off line. The pinch-off line allows excess material to escape the mould cavity as a flash, thereby allowing the material to be compressed to a density that is proportionate to the force applied.

Semipositive Mould

The semi-positive mould is just as popular as the flash mould. Besides, it combines features of the positive mould and the flash mould to produce even better-moulded parts. It has a large cavity that allows the material to be trapped positively, and it has sidewalls that allow excess material to escape the cavity as a flash.

Materials and Applications

Products manufactured by the compression moulding process are strong and durable. These unique attributes make them suitable for the automotive industry, where metallic parts are to be replaced with plastic parts to achieve vehicle lightweight.

Because the cost of producing single parts is substantial, end-users prefer mass production of individual parts to save cost. The number of cavities, the complexity of parts, and surface finish among others are factors that determine the cost. Nevertheless, you'll still find many manufacturers opting for compression moulding because of its ease of operation.

Asides the simplicity of compression moulding, here are other advantages that keep this moulding process popular to date:

Little material is wasted in compression moulding because of the absence of sprues and runners, which are predominant in the injection moulding
Compression moulding contributes to little mastication of material. Therefore, mechanical properties are unaltered or remain high.
Clamping pressure requirements is usually low. Hence, cheaper power tools can be used.
The compression moulding tool is quite inexpensive and readily available.

Plastic products manufactured from the compression moulding technique also have the benefit of being lighter and resistant to corrosion, which makes them suitable in the automotive industry. With the advent of computer technology, it has become much easier to achieve moulding objectives that were difficult. Computer-Aided Engineering (CAE) technology has been pivotal in flow analysis, temperature field analysis, and residual stress analysis in plastic moulding [3].

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Figure 2. Courtesy of Thomas Insights.

Fuentes

[1] Statista. (2020). UK: sales of injection & compression moulds for metal 2018 | Statista. [online] Available at: https://www.statista.com/statistics/511757/uk-sales-injection-compression-moulds-metal/ [Accessed 1 Mar. 2020].

[2]V. Goodship, B. Middleton, and R. Cherrington, Design and Manufacture of Plastic Components for Multifunctionality. Oxford: Elsevier, 2016

[3]Thomas, S. (2009). Advances in polymer processing. Cambridge: Woodhead Publishing Ltd.