Moulded Articles

In this process the dispensing head incorporates an on-off device enabling 

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In 1930 BASF, then part of IG Farhen, installed a plant for producing 100 tonnes of polystyrene per annum and in 1933 the first injection moulded articles were produced. In the US semi-plant-scale work at the Dow Chemical Company showed promise of commercial success in 1934. As a consequence there became available shortly before World War II a material of particular interest because of its good electrical insulation characteristics hut otherwise considerably inferior to the polystyrene available today. Because of these excellent electrical characteristics prices were paid of the order of several dollars per pound for these polymers. 

An important subdivision within the thermoplastic group of materials is related to whether they have a crystalline (ordered) or an amorphous (random) structure. In practice, of course, it is not possible for a moulded plastic to have a completely crystalline structure due to the complex physical nature of the molecular chains (see Appendix A). Some plastics, such as polyethylene and nylon, can achieve a high degree of crystallinity but they are probably more accurately described as partially crystalline or semi-crystalline. Other plastics such as acrylic and polystyrene are always amorphous. The presence of crystallinity in those plastics capable of crystallising is very dependent on their thermal history and hence on the processing conditions used to produce the moulded article. In turn, the mechanical properties of the moulding are very sensitive to whether or not the plastic possesses crystallinity. 

Weathering. This generally occurs as a result of the combined effect of water absorption and exposure to ultra-violet radiation (u-v). Absorption of water can have a plasticizing action on plastics which increases flexibility but ultimately (on elimination of the water) results in embrittlement, while u-v causes breakdown of the bonds in the polymer chain. The result is general deterioration of physical properties. A loss of colour or clarity (or both) may also occur. Absorption of water reduces dimensional stability of moulded articles. Most thermoplastics, in particular cellulose derivatives, are affected, and also polyethylene, PVC, and nylons. 

A wide variety of thermoplastics have been used as the base for reinforced plastics. These include polypropylene, nylon, styrene-based materials, thermoplastic polyesters, acetal, polycarbonate, polysulphone, etc. The choice of a reinforced thermoplastic depends on a wide range of factors which includes the nature of the application, the service environment and costs. In many cases conventional thermoplastic processing techniques can be used to produce moulded articles (see Chapter 4). Some typical properties of fibre reinforced nylon are given in Table 3.2. 

This expression therefore enables the thickness of the moulded article to be calculated from a knowledge of the die dimensions, the swelling ratio and the mould diameter. The following example illustrates the use of this analysis. A further example on blow moulding may be found towards the end of Chapter 5 where there is also an example to illustrate how the amount of sagging of the parison may be estimated. 

The process is attractive for a number of reasons. Firstly, since it is a low pressure process the moulds are generally simple and relatively inexpensive. Also the moulded articles can have a very uniform thickness, can contain reinforcement, are virtually strain free and their surface can be textured if desired. The use of this moulding method is growing steadily because although the cycle times are slow compared with injection or blow moulding, it can produce very large, thick walled articles which could not be produced economically by any other technique. Wall thicknesses of 10 mm are not a problem for rotationally moulded articles. 

Rigid, brittle structure Structurally stable Only moulded articles (injection moulding possible with RIM)... 

The addition level for the different applications varies from 5-10 phr for moulded articles to 15-30 phr for extrusions. Roller coverings, dependant upon hardness, are often made using factices to facilitate grinding operations. Special products such as erasers use factice, usually white factice, at levels between 200-400 phr. 

A multi-layer expansion-moulded article is obtained by moulding a multi-layer parison comprising a foamed resin... 

Patent Number EP 963827 A2 19991215 MOULDED ARTICLE OF FOAMED AND EXPANDED BEADS OF PROPYLENE RESIN... 

The moulded article, which has an apparent density of 0.11 to 0.45 g/cu.cm. and an average number of cells within the range of 5 to 100 cells/sq.mm., is obtained by heating and moulding the foamed beads. 

The iQ automotive battery incorporates a micro-chip which monitors and controls the electrochemical process of the lead-acid battery, as well as its physical environmental and operating conditions. The thermal insulation of the battery also plays an important role. Neopolen P, a thermoplastic particle foam from BASF which can be processed completely without blowing agents, was chosen for this application. The foamed battery container calls for a solid frame that can be mounted onto the car body, and this frame is configured as a solid PP injection-moulded article, and fused to the container. 

Patent Number US 5747549 A 19980505 FOAMED PARTICLES OF POLYPROPYLENE HOMOPOLYMER AND MOULDED ARTICLE OF THE FOAMED PARTICLES... 

Patent Number US 5292465 A 19940308 PROCESS FOR PREPARING LIGHTWEIGHT, RIGID COMPOSITE FOAMED MOULDED ARTICLE... 

The multi-layer article is made using moulds between which a cavity clearance is freely set. A skin material lined with foam is placed between the upper and lower moulds and molten PP containing a chemical blowing agent is supplied through a resin melt conduit in the lower mould when the cavity clearance is between (C plus 15) mm and (C plus 50) mm, where C is the thickness of the skin material lined with the foam. The upper mould is lowered at a specific rate and the molten resin is pressed at a specific pressure to fill the cavity ends with the molten resin to complete the moulding of the resin body. The body is pressed for a certain time to form a skin layer, the upper mould is lifted up to decrease the compression pressure of the skin material lined with the foam to a pressure lower than the blowing pressure of the PP resin to form and solidify the foamed body, the upper mould is lowered to apply pressure to the moulded article and finally the article is cooled in the mould. 

From the solutions of polyimides in tetrahydrofuran (THF) strong, flexible and colourless films can be cast. Pressing of the polyimides carried out at temperatures 100 °C higher than Tg and 10 MPa leads to the formation of yellowish transparent moulded articles. 

High labour cost since each cavity of the mould should be loaded individually. Moulds are heavy and operator involvement is high in loading and unloading. Operator only unloads the moulded articles. 

Tearing Bad mould design, low hot tear strength of compound, overcure and careless removal of moulded article. Avoid sharp edges, improve compounding technique, cure at optimum level and use simple devices for removal. 

Moulded articles can be thermally treated to improve the physical properties, particularly the notched Izod impact strength, but removal of the volatile by-products of the solid-phase polymerisation is a greater problem than for fibres and films 112). Use of internal dielectric heating has been suggested as a means of avoiding limitation of further polymerisation to the surface. 

Polybutylene terephtalate (PBTP) differs in its chemical structure only slightly from PETP its melting point is somewhat lower and its processability is better. The applications in injection moulded articles are similar to those for PETP. 

The surface structure of a plastic may exert a considerable influence on the friction an example is given by injection moulded articles of PP, which show, in touch with each other, a coefficient of 0.7, while for sand-blasted surfaces a value of only 0.3 is found. For injection-moulded nylon // = 0.65, for machined surfaces 0.47. As a matter of fact, lubrication has a strong influence the value of 0.47 for nylon is reduced to 0.19 with water lubrication and to 0.08 with oil. 

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