Plastic moulding

In Plastic moulding, the moulding process selected depends upon two main factors: The geometry of the component to be moulded and the material from which it is to be made. There are two main groups of plastic moulding materials: Thermoplastic materials that soften every time they are heated, and thermosetting plastic materials that undergo a chemical change during moulding and cannot be softened again by heating.

Thermoplastics

Since this group of plastic can be softened every time they are heated, they can be recycled and reshaped any number of times. This makes them environmentally attractive. However, some degradation occurs if they are overheated or heated too often and recycled materials should only be used for lightly stressed components.

Thermoplastic moulding

These are usually made by the injection moulding process which is suitable for quantity production of both large and small components and is the most widely used moulding process. Small components can be made in multi-impression moulds and left on the sprue until required to prevent loss. Examples of typical components made by injection moulding can range from model kit parts made from polystyrene, and small nylon gears for office machinery, to motor vehicle rear light clusters made from transparent acrylic plastic and even complete motor vehicle bumpers moulded from impact-resistant plastics.

Thermosetting plastics

This group of plastics differ from thermoplastic materials in that polymerization is completed during the moulding process and the material can never be softened again. Polymerization during the moulding process is called curing.

Thermosetting plastic moulding (Positive die moulding)

Only in very special circumstances can thermosetting plastics be injection moulded. Almost invariably, thermosetting plastics are moulded by compression or transfer techniques. Since the plastic resin can be readily blended with a wide variety of filler materials and pigments, mouldings made from thermosetting plastics can be given a wide range of properties and appearances. Compression mouldings are used for components such as:

• meter cases
• electric fan bodies and blades
• electrical insulators for switch gear, contactors and distribution equipment
• tableware

In all these examples rigidity and strength are required, coupled with good surface finish and scratch resistance. Only thermosetting plastics have all these properties at the same time.

Forms of supply

Both thermoplastic and thermosetting plastic moulding materials are normally available as powders or as granules packed in bags or in drums. Thermoplastic powders and granules are homogeneous materials consisting of the polymer together with the coloring agent (pigment), lubricant and die-release agent. Thermosetting plastic materials are unsuitable for use by themselves, and the thermosetting plastics come in powder or granule form mixed with additives to make them more economical to use, to improve their mechanical properties, and to improve their moulding properties.

Composition

A typical thermosetting plastic moulding material could consist of :

• Resin powder or granules, 38% by weight
• Filler, 58% by weight
• Pigment, 3% by weight
• Mould-release agent, 0.5% by weight
• Catalyst, 0.3% by weight
• Accelerator, 0.2% by weight

The low-cost filler not only bulks up the powder or granules and makes the material more economic to use, but also has a considerable influence on the properties of moulding produced from a given resin. For example, fillers improve the mechanical strength, electrical insulation properties and heat resistance. Fillers also reduced shrinkage during moulding. Typical fillers and their properties are:

• Glass fiber, High strength and good electrical insulation
• Wood flour, Low cost, high bulk, low strength
• Calcium carbonate, Low cost, high bulk, low strength
• Rock wool, Heat resistance
• Aluminium powder, Wear resistance and high strength
• Shredded paper, Good strength but inclined to absorbe moisture
• Shredded cloth, Higher strength but also inclined to absorb moisture
• Mica granules, Heat resistance with good electrical insulation properties

Compression moulding conditions

The moulding materials may be fed into the mould as a powder, as granules or compacted into a preformed shape. The latter is used to ensure uniform filling of the mould cavity, particularly when the cavity has a complex form.

Loading

Correct loading of the mould is critical, insufficient material resulting in voids and porosity through the cavity not being properly filled. A slight excess of material is preferable as it ensures complete and uniform filling of the mould with any excess being allowed to form a 'flash'. Excessive overcharging must be avoided as the powder is incompressible and damage could be done to the mould and to the press. Automatic metering and filling of moulding material results in more uniform results than hand filling, as well as being more productive.

Preheating

The moulding material can be loaded either cold or preheated. Preheating reduces the curing time and also reduces erosion of the mould cavity, since the partially plasticized materials are in a less abrasive condition. During curing, volatile gases are released and these must be allowed to escape, either through the mould clearances, through vents or by momentarily opening the mould part-way through the cure.

Lubrication

To prevent sticking, a release agent (lubricant) must be spread into the mould cavity immediately prior to loading because plastic resins are also very good adhesives.

Curing time

The correct curing time and temprature are also critical, as over-curing produces a dull and blistered surface with some crazing, internal cracking, and poor mechanical properties. Under-curing may produce a component with the correct appearance but with poor mechanical properties. Moisture in the moulding powder can also cause blisters and porosity. The correct curing conditions are generally determined by trial and error, based upon previous experience with similar moulding.

Injection moulding

The compression moulding process is usually used in conjunction with thermosetting plastics. Injection moulding is usually used in conjunction with thermoplastic material. In the injection moulding process, a measured amount of thermoplastic material is heated until it becomes viscous, whereupon it is injected into the mould cavity under high pressure. In this respect a curing has to take place and the moulds can be opened as soon as the moulding has cooled sufficiently to become rigid and self-supporting. Injection-moulding materials are generally arranged with the mould parting-line vertical and the axis of injection horizontal. As an alternative to the ram feed, large-capacity machines may use a screw feed mechanism for filling the dies.

References