Plastics moulding processes

As already stated, the shaping of plastics materials is achieved by the application of heat, pressure or both. There are many ways in which this can be done, depending on the nature of the polymer, the type and size of product and the quantity and dimensional accuracy required. The methods to be described here are moulding by compression, transfer, injection and low-pressure processes. 

Metals have a definite melting point and in general tend to be free-flowing in a molten state. Polymers, on the other hand, have no definite melting point but are softened by the application of heat, which renders them plastic . In this state they may be considered as very viscous fluids and, as a result, high pressures are required for moulding. 

The viscosity of a polymer is reduced by the application of heat, but there is an upper temperature limit at which the polymer begins to break down in some way. This breakdown is known as degradation. All polymers are bad conductors of heat and are therefore susceptible to over-heating. If a polymer is subjected to excessive temperature or to prolonged periods in the mould, degradation will occur. 

There is also a lower limit of temperature below which the polymer will not be soft enough to flow into the mould. The temperature for moulding must be between the upper and lower limits and will directly affect the viscosity of the polymer. 

The moulding techniques to be discussed essentially differ only in the way the moulding material is heated and delivered to the mould. 

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Surface texture or moulded-in configurations can be provided by many plastic moulding processes. Raised surfaces can additionally be kiss blocked or coated. 

Name three types of plastics moulding process. 

The plastics moulding process also allows ecomonic production of improved reflector designs in conventional circular shapes, and gives improved control and reproducibility of performance. This latter is particularly important with the increased light output of halogen bulbs, to avoid glare. 

Polynorbomene is also of interest to the plastics processor since by using a dry blending process for mixing and a modified injection moulding process for fabricating, the use of conventional rubber-processing equipment may be avoided. 

Such lenses may be made by machining from rod. More recently processes have been developed where the monomers are cast polymerised in tiny plastics moulds whose cavity corresponds to the dimensions of the lens and using procedures very reminiscent of those described for the manufacture of acrylic sheet (see Section 15.2.2). 

Compression moulding is one of the most common methods used to produce articles from thermosetting plastics. The process can also be used for thermoplastics but this is less conunon - the most familiar example is the production of LP records. The moulding operation as used for thermosets is illustrated in Fig. 4.62. A pre-weighed charge of partially polymerised thermoset is placed in the lower half of a heated mould and the upper half is then forced down. This causes the material to be squeezed out to take the shape of the mould. The application of the heat and pressure accelerates the polymerisation of the... 

In some compositions the PIB is plasticized with ethyl oleate to improve low temp properties. .. Since the propint is a stiff paste it can be inserted into a rocket motor by a type of injection moulding process (Refs 3 5)... 

In the case of paints and printing inks, the initial preparations will be in the semi-solid state because solvents are needed both in the process of dispersing the pigment in the paint or ink medium and for application purposes. These solvents dry out after the paint or ink is applied. When making coloured plastic articles, both heating and solvents may be used to aid dispersion in the plastic medium as part of the moulding process. However, from the viewpoint of the optical properties in all of these pigment uses, what is most important is that each of these media has a refractive index close to 1.5. 

Journal of Cellular Plastics 37, No. 1, Jan. 2001,p.21-42 THERMAL CONDUCTIVITY OF A POLYETHYLENE FOAM BLOCK PRODUCED BY A COMPRESSION MOULDING PROCESS Martinez-Diez J A Rodriguez-Perez M A De Saja J A Arcos y Rabago L O Almanza O A... 

In the method of producing a hollow foamed plastic body from polyethylene or polypropylene by means of an extrusion blow-moulding process using a single-screw... 

Fluorination of polyethylene surfaces leads to an increase in the surface energy, some degree of cross-linking and a reduction of the free volume of the polymer. All of these effects impart on the surface of the polymer a barrier that is very impermeable to hydrocarbon solvents. A blow-moulding process, in which a low concentration of fluorine in nitrogen is used as the blow-moulding gas, is used for the production of plastic fuel tanks for the automotive industry (Airopak , Air Products) [51]. Post-treatment of hydrocarbon surfaces with fluorine is an alternative technology and techniques for the surface fluorination of natural and synthetic rubber have been described [52]. 

Three types of moulding processes are available - compression, transfer and injection moulding. In compression moulding the rubber blank is placed directly into the cavity of the mould where it is heated by conduction which causes rubber flow by application of pressure. Transfer moulding uses prewarmed rubber which is heated during transfer and forced through small orifices into the mould cavities in a three-part mould. In the injection moulding process, the rubber compound is pushed under pressure from an injection head where it has been heated and plasticized into a closed heated mould where cure is completed. 

Copper/Constantan 400-62QK tol = 0.0041 T-2731K 1073 5566 10741 food processing, stack gases, plastic moulding, lubricating oils, producer gas plants. 

Even yet experts say that the best combs are saw-cut from flat uniform pieces of bone (and such combs remain on sale in more-expensive shops). Combs can be cut in the same way from plastic sheet, achieving comparable quality, but the stock must be prepared suitably and the work as a whole is more costly than the commercial method of making plastic combs in large numbers—injection moulding using materials such as nylon or polypropylene from the moulding process, after removal of the sprues, fully finished, saleable products (teeth included) can be obtained in a few seconds. 

Mould To shape plastic parts or finished articles by heat and pressure. The cavity or matrix into which the plastic composition is placed and from which it takes its form. The assembly of all the parts that function collectively in the moulding process. 

Plastic imitations may show marks finm the moulding process, and possibly air bubbles. 

In order to reduce the volume stream and to optimize the efficiency of the following processing units the plastics have to be compacted mechanically before heat/pressure treatment starts. Bales of compacted wet plastics reach bulk densities of about 450 kg/ m3, they are put into moulds which will be transported into autoclaves. During the autoclave process plastics are heated and plastified under saturated steam conditions. Autoclave process is within a wide range independent from changing compositions of the plastics. The process temperature (about 170°C) does not provoke decomposition of plastics, used for household packaging materials. Hydrothermal autoclave conditions lead to further compaction and a stable product (bulk density up to 700 kg/m3) of the mixed different kinds of plastics. Fig. 1 shows a compacted plastic block after finishing the autoclave process. 

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