Mould melt distribution

In the wall thickness fluctuations up to 5 % may occur. As a result of the uneven temperature in the molten polymer during rotation, and also by the not always exactly reproducible rate of cooling, deviations in the dimensions of the finished product may amount to 5 %. Requirements are, that the materials can be molten completely, that the melt is sufficiently low-viscous, and that the molten polymer does not degrade too rapidly. Besides plasticised PVC, HDPE and LDPE are often used, as well as copolymers of PE such as EVA (ethylene - vinyl acetate copolymerj.Because the shear stresses in this process are extremely low, a narrow molar mass distribution is to be recommended, as discussed in 5.4. Cycle times vary between 3 and 40 minutes, dependent on the wall thickness. Cycle times can be reduced considerably by using machines with multiple moulds, since the cycle time... 

Properties.—Succinic acid has been known for a long time. It is quite widely distributed in nature. It is found in unripe fruits, especially in grapes, also in lignite, in peat and in many plants. Its most important occurrence is in amber from which it may be obtained by distillation. It is also a constituent of wines where it is the product of the alcoholic fermentation of the sugars of grape juice. Another source, which will be considered later, is from malic and tartaric acids by bacterial or mould fermentation. Succinic acid crystallizes in plates or columns which melt at 182°. It sublimes when it is heated below its melting point. When heated rapidly to 235° it loses water and forms an anhydride. It is soluble in 14 parts of water. 

This equation is used alternately with the flow equation, to update the melt temperature distribution. If the injection pressure is below the limit of the machine, the new position of the melt front is computed. The programmes output the melt front shape and melt pressure isobars at various times, hence predict whether a mould can be filled satisfactorily. If not, modifications can be made to the CAD file for the mould geometry, and the analysis repeated, before the mould cavity is machined. Figure 5.27 shows the predicted flow fronts for an instrument panel moulding. 

Melt degradation is used commercially to narrow the molecular weight distribution, from a starting value of Mw/Mn = 5, to make it more suitable for fibre spinning and blow moulding. Random chain scission at tertiary C—H bonds eventually produces a most probable molecular weight distribution with M /Mn = 2. In practice, the distribution, produced by deliberately degrading the polypropylene with added peroxides, is broader, but... 

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