Moulding conditions

In Table 3.3 the machine sizes and conditions are given for these parts. It is of interest to compare the melt temperatures required for these polymers, as well as the cooling water temperature and the salient parts of the process cycle. 

Product Machine size tons closure force No. of tool impressions Melt temp C Cooling water C Inj. s Hold s Cool s 

PVC Bristle holder for combination nozzle 160 2 chrome plate (HCI attack) 172 25 1.5 22 39 

The mould for the PVC part is chromium plated to enable it to resist attack by hydrogen chloride, evolved in small quantities by molten PVC. 

The large motor housing, impellor and hood require single-impression tools, i.e. one is made at a time. The smaller bristle holder can be made two at a time in a two-impression tool. By comparison, the ears are made in a six-impression tool on a 150-ton machine. 

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With crystalline polymers the more orderly molecular packing leads to much greater shrinkage. Variations in moulding conditions can lead to large variations in shrinkage and need to be closely controlled. The main factors which cause an increase in shrinkage are ... 

Unlike polyurethane-RIM processes, nylon-RIM reactions are endothermic and require temperatures of 130-140°C. In contrast to the polyurethane-RIM systems, this enables thick wall parts to be made. Cycle times of 2-3 minutes are comparable to those for polyurethane-RIM. In the development stage, current work is concerned with reducing moulding times and optimising moulding conditions. 

As a result of these considerations, typical injection moulding conditions are ... 

Moulding conditions, i.e moulding pressure, temperature and time. 

Table 32.1 Range of values of physical properties of injection mouldings obtained by alteration of moulding conditions (after Allen and Van Putte, 1974) ...

Acetic acid and formic acid evolved (acetic acid may be used as an end-stopper) Formic acid evolved (if arduous moulding conditions have been used, the polymer may be corrosive at ambient temperatures)... 

The process of mould filling should not be considered as completely understood and controlled 98>, though some important features of gas-filled system flow were established by Fridman and co-workers 99 100) who have explained the formation of laminated structures in low-pressure moulding of foaming melts and found the correlation between moulding conditions and morphologic macro-structure and properties of foam articles. 

The mouldability index is the length of the elongated flow path that is filled before solidification of the injected molten polymer composition at specified moulding conditions, i.e., volumetric injection rate, mould temperature, polymer melt temperature. The mouldability index of the moulding compositions of this invention is characterised by spiral flow. 

It has been found that moulding conditions can affect almost every property of moulded parts. Among the properties affected are impact strength, crack resistance, and... 

Copper alloys such as the brasses and bronzes can be cast by either the sand-casting or permanent mould cast methods. The alloys casts in permanent mould condition have greater corrosion resistance than the sand-cast alloys.52 This difference has been attributed to the greater porosity of the alloys in sand-cast conditions. 

The influence of injection moulding conditions on the occurrence of surface defects on PVC pipe fittings was studied experimentally and by finite element analysis. Comparison of the experimental results with theoretical predictions allowed optimisation of the processing conditions for the production of parts having the required surface quality. 3 refs. 

Appearance of a plastic can be judged by a number of properties, i.e. gloss, or surface finish (a relationship of mould surface, moulding conditions, and material composition), haze, clarity, etc. 

The third aspect, potentially in conflict with the previous concept, involves the development of families of formulations based on specific chemicals all diphenylmethane diisocyanate (MDI), MDI/TDI in various percentages, all TDI and special polyols. This means dealing with very different demould times and moulding conditions, that render a generic seat plant , that was still so useable just a few years ago, obsolete. In this case, specific packages must be available, which conflicts with the concept of high flexibility expressed above. 

Use of factorial experiment design (FED) is especially beneficial for the determination of optimum moulding conditions. 

Wherever it is possible moulds should be provided with pockets for temperature measuring and recording instrumentation. This is essential if the moulding conditions are to be properly optimised and also provides a thermal finger-print for audit of the process during the production cycle. Surface temperature probes are better than nothing, but give results which may well differ between operators and are affected by the condition of the mould surface [5]. 

The cure time will vary according to the compound in use, the rubber sectional thickness, and the moulding conditions. The optimum value should be determined by the trials planned in step 6.3.4. For guidance see Table 6.2. The overall cure time will depend on the temperature of the injected rubber. The hotter that this can be sustained without incurring scorch penalties, the more even will be the crosslink density across the rubber section. The optimum value that is often quoted for crosslink density is 90%. This value was determined from a 2 mm thick sheet tested for tensile and related properties. 

This is the inherent shrinkage of a material during injection moulding. Moulding conditions, orientation of fillers, orientation of flow and other factors can all influence the specific level of immediate shrinkage but the inherent quality is material dependent. Crystallisation occurring after moulding influences dimensional stability . 

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