Plastic moulding
Encyclopedia
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.
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.
is completed during the moulding process and the material can never be softened again. Polymerization during the moulding process is called curing
.
materials and pigment
s, mouldings made from thermosetting plastics can be given a wide range of properties and appearances. Compression mouldings are used for components such as:
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.
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.
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:
) must be spread into the mould cavity immediately prior to loading because plastic resins are also very good adhesive
s.
Thermoplastic
Thermoplastic, also known as a thermosoftening plastic, is a polymer that turns to a liquid when heated and freezes to a very glassy state when cooled sufficiently...
materials that soften every time they are heated, and thermosetting plastic
Thermosetting plastic
A thermosetting plastic, also known as a thermoset, is polymer material that irreversibly cures. The cure may be done through heat , through a chemical reaction , or irradiation such as electron beam processing.Thermoset materials are usually liquid or malleable prior to curing and designed to be...
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 mouldingInjection moulding
Injection molding is a manufacturing process for producing parts from both thermoplastic and thermosetting plastic materials. Material is fed into a heated barrel, mixed, and forced into a mold cavity where it cools and hardens to the configuration of the cavity...
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
Polystyrene
Polystyrene ) also known as Thermocole, abbreviated following ISO Standard PS, is an aromatic polymer made from the monomer styrene, a liquid hydrocarbon that is manufactured from petroleum by the chemical industry...
, and small nylon
Nylon
Nylon is a generic designation for a family of synthetic polymers known generically as polyamides, first produced on February 28, 1935, by Wallace Carothers at DuPont's research facility at the DuPont Experimental Station...
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 polymerizationPolymerization
In polymer chemistry, polymerization is a process of reacting monomer molecules together in a chemical reaction to form three-dimensional networks or polymer chains...
is completed during the moulding process and the material can never be softened again. Polymerization during the moulding process is called curing
Curing (chemistry)
Curing is a term in polymer chemistry and process engineering that refers to the toughening or hardening of a polymer material by cross-linking of polymer chains, brought about by chemical additives, ultraviolet radiation, electron beam or heat...
.
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 fillerFiller (materials)
Fillers are particles added to material to lower the consumption of more expensive binder material or to better some properties of the mixtured material...
materials and pigment
Pigment
A pigment is a material that changes the color of reflected or transmitted light as the result of wavelength-selective absorption. This physical process differs from fluorescence, phosphorescence, and other forms of luminescence, in which a material emits light.Many materials selectively absorb...
s, 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 flourWood flourWood flour is finely pulverized wood that has a consistency fairly equal to sand or sawdust, but can vary considerably, with particles ranging in size from a fine powder to roughly the size of a grain of rice. Most wood flour manufacturers are able to create batches of wood flour that have the...
, Low cost, high bulk, low strength - Calcium carbonateCalcium carbonateCalcium carbonate is a chemical compound with the formula CaCO3. It is a common substance found in rocks in all parts of the world, and is the main component of shells of marine organisms, snails, coal balls, pearls, and eggshells. Calcium carbonate is the active ingredient in agricultural lime,...
, Low cost, high bulk, low strength - Rock wool, Heat resistance
- AluminiumAluminiumAluminium or aluminum is a silvery white member of the boron group of chemical elements. It has the symbol Al, and its atomic number is 13. It is not soluble in water under normal circumstances....
powder, Wear resistance and high strength - Shredded paperPaperPaper is a thin material mainly used for writing upon, printing upon, drawing or for packaging. It is produced by pressing together moist fibers, typically cellulose pulp derived from wood, rags or grasses, and drying them into flexible sheets....
, Good strength but inclined to absorbe moisture - Shredded cloth, Higher strength but also inclined to absorb moisture
- MicaMicaThe mica group of sheet silicate minerals includes several closely related materials having highly perfect basal cleavage. All are monoclinic, with a tendency towards pseudohexagonal crystals, and are similar in chemical composition...
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 (lubricantLubricant
A lubricant is a substance introduced to reduce friction between moving surfaces. It may also have the function of transporting foreign particles and of distributing heat...
) must be spread into the mould cavity immediately prior to loading because plastic resins are also very good adhesive
Adhesive
An adhesive, or glue, is a mixture in a liquid or semi-liquid state that adheres or bonds items together. Adhesives may come from either natural or synthetic sources. The types of materials that can be bonded are vast but they are especially useful for bonding thin materials...
s.