Mould Weights

RIM of PUs will include the cost of consumables, e.g. up to 25% of total moulding weight, release agent, tools and wash solvents. 

Each machine will have a maximum permitted mould weight for the movable mould halves. These values should not be exceeded for any reason as production problems and premature wear would be the result. Examples of some commercially available machines from the manufacturer ARBURG are shown in Table 3.4, the type of supports are shown in Figure 3.19. 

Support for vertical forces such as mould weight... 

With injection times greater or equal to solidification time, the injection moulded component weight remains practically the same (does not increase). With injection times less than solidification time, the injection moulded component weight decreases. If the flow stops before the mould has fully filled, the final moulding weight will be lower. The occmrence of sink marks is also a sure indication that the injection time (duration of effect of pressure) is shorter than the solidification time. 

It is found that a force F will inject a given weight of a thermosetting polymer into an intricate mould in 30 s at 177°C and in 81.5 s at 157°C. If the viscosity of the polymer follows an Arrhenius Law, with a rate of process proportional to calculate how long the process will take at 227°C. 

A third type of suspension is that in which polymer particles are suspended in monomer which is then polymerised. This is, however, rather more a variant of the casting process in which monomer or low molecular weight polymer is cast into a mould and then polymerised in situ. 

Polymerisation casting involves mixing monomer or low molecular weight polymer with a polymerisation initiator, pouring the mix into the mould and allowing polymerisation to occur in situ. A variation is to impregnate fibres with initiated monomer or other low molecular weight material and polymerise to produce composite structures. The main problem is due to the heat of polymerisation. Unless heat transfer distances are kept short or unless the reaction is carried out very slowly it can easily get out of hand. 

Fligh molecular weight, hroad MWD polymer—used primarily for blow moulding and pipe. 

Low molecular weight, narrow MWD polymer—used widely for injection moulding and rotational moulding. 

High molecular weight HDPE (HMW-HDPE) is used for blown film and for demanding moulding and structural uses. Examples are blow moulded drums for packing dangerous chemicals and pressure piping. 

In recent years rotational casting methods have made the slush moulding process virtually obsolete. In these processes an amount of material equal to the weight of the finished product is poured into a mould. The mould is then closed and rotated slowly about two axes so that the paste flows easily over the cavity walls in an oven at about 200-250°C. When the compound has gelled, the moulds are cooled and the moulding removed. Compared with the slush moulding process there is no wastage of material, little flash, and more even wall thickness. Completely enclosed hollow articles such as playballs are most conveniently made. 

The above process is limited to simple shapes whose principal dimension is not more than four times, and preferably less than twice, that of the next largest dimension. More intricate shapes must be made by machining or in some instances by a coining operation which involves stamping a sintered moulding of the same weight and approximate dimensions as the finished part at 320°C. 

Poly(vinyl acetate) is too soft and shows excessive cold flow for use in moulded plastics. This is no doubt associated with the fact that the glass transition temperature of 28°C is little above the usual ambient temperatures and in fact in many places at various times the glass temperature may be the lower. It has a density of 1.19 g/cm and a refractive index of 1.47. Commercial polymers are atactic and, since they do not crystallise, transparent (if free from emulsifier). They are successfully used in emulsion paints, as adhesives for textiles, paper and wood, as a sizing material and as a permanent starch . A number of grades are supplied by manufacturers which differ in molecular weight and in the nature of comonomers (e.g. vinyl maleate) which are commonly used (see Section 14.4.4)... 

The average molecular weight of most bulk polymerised poly(methyl methacrylates) is too high to give a material which has adequate flow properties for injection moulding and extrusion. 

Cast material is stated to have a number average molecular weight of about 10. Whilst the Tg is about 104°C the molecular entanglements are so extensive that the material is incapable of flow below its decomposition temperature (approx. 170°C). There is thus a reasonably wide rubbery range and it is in this phase that such material is normally shaped. For injection moulding and extrusion much lower molecular weight materials are employed. Such polymers have a reasonable melt viscosity but marginally lower heat distortion temperatures and mechanical properties. 

The properties of three types of poly(methyl methacrylate) (sheet based on high molecular weight polymer, lower molecular weight injection moulding material and a one-time commercial copolymer) are given in Table 15.1. 

The major nylon moulding materials are each available in a number of grades. These may differ in molecular weight but they may also differ in the nature of additives which may be present. 

Lubricants may also be used to enhance flow and mould release. Materials used are usually of low moleeular weight, contain a hydrocarbon component and an amide component, and are typified by ethylene bis(stearamide). 

Because of their light weight, ability to be moulded into intricate shapes in one piece, low coefficients of friction and absence of slip-stick behaviour, acetal resins find use as bearings. 

Unlike other water-soluble resins the poly(ethylene oxide)s may be injection moulded, extruded and calendered without difficulty. The viscosity is highly dependent on shear rate and to a lesser extent on temperature. Processing temperatures in the range 90-130°C may be used for polymers with an intrinsic viscosity of about 2.5. (The intrinsic viscosity is used as a measure of molecular weight.)... 

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