VARI moulding

MCCB (moulded case circuit breaker) or MCB (miniature circuit breaker) and contactor] and the overload relay. These recommendations permit damage of components on fault to varying degrees as noted below ... 

One consequence of this is that in a thermoplastic injection moulding the molecular orientations will vary from place to place according to the flow pattern. This may well set up sufficient stresses in the moulding to cause it to distort from its intended shape. Factors affecting the amount of orientation were discussed briefly in the previous chapter and by the author elsewhere. Sometimes the conditions required to minimise distortion may be such that the moulding cycle becomes too long to be economic. In these cases it may be necessary to modify the moulding or at least the position and number of feed... 

Distortion in mouldings can be worse in crystalline polymers than in amorphous plastics. This is because additional stresses may be set up as a result of varying crystallinity from point to point in the moulding so that the shrinkage on cooling from the melt also varies from point to point. This uneven shrinkage sets up stresses which may lead to distortion. 

Butadiene and styrene may be polymerised in any proportion. The Tfs of the copolymers vary in an almost linear manner with the proportion of styrene present. Whereas SBR has a styrene content of about 23.5% and is rubbery, copolymers containing about 50% styrene are leatherlike whilst with 70% styrene the materials are more like rigid thermoplastics but with low softening points. Both of these copolymers are known in the rubber industry as high-styrene resins and are usually used blended with a hydrocarbon rubber such as NR or SBR. Such blends have found use in shoe soles, car wash brushes and other mouldings but in recent times have suffered increasing competition from conventional thermoplastics and to a less extent the thermoplastic rubbers. 

The properties of the nylons are considerably affected by the amount of crystallisation. Whereas in some polymers, e.g. the polyacetals and PCTFE, processing conditions have only a minor influence on crystallinity, in the case of the nylons the crystallinity of a given polymer may vary by as much as 40%. Thus a moulding of nylon 6, slowly cooled and subsequently annealed, may be 50-60% crystalline, while rapidly cooled thin-wall mouldings may be only 10% crystalline. 

Example 2.22 A certain grade of acrylic has a Kc value of 1.6 MN and the fatigue crack growth data as shown in Fig. 2.77. If a moulding in this material is subjected to a stress cycle which varies from 0 to 15 MN/m, estimate the maximum internal flaw size which can be tolerated if the fatigue endurance is to be at least 1(P cycles. 

The study on commercial HDPE samples could not provide a correlation of the izod impact test with the field performance test, i.e., drop impact resistance on moulded products [113]. It was found that the sample of highest density and lowest izod impact strength passed the drop impact test, but other samples of lower density and higher izod impact strength could not withstand shock loading by drop impact and failed in brittle manner. This may be due to the fact that velocities and modes of loading vary widely in different impact tests. It has been reported that even the qualitative agreement between the different impact tests is poor because the test bars and moulded products often have different orientation characteristics, particularly near the surface [115]. 

The range of tlwrinoplaslic materials that can be CMiuded ot uyection moulded is too large and varied tor covciage in this boo s. 

This example shows how the distribution of filler particles can vary in the channel cross-section during injection moulding. 

The optimum temperature varies widely from species to species but in general the common moulds will grow better at 22-25 °C than most human pathogenic and commensal bacteria. It is customary, therefore, to incubate mould cultures at lower temperatures than bacterial cultures. 

Moulds and yeasts show varying responses to biocides. These organisms are often important in the pharmaceutical context because they may cause spoilage of formulated products. Various types of protozoa are potentially pathogenic and inactivation by biocides may be problematic. Viral response to biocides depends upon the type and structure of the virus particle and on the nature of the biocide. 

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. 

Most tests will be made on standard test pieces which may be pieces cut from a component or a sheet, or they may have been moulded separately from the same material. Where test pieces or sheet are produced for the trials it is important that they are produced in as near as possible the same way as the product and that the processing conditions are recorded. Different results can be expected from compression and injection moulding or from extrusion (where a choice is possible). Directional properties can result from the conditions of flowing and cooling in a mould. For example, in a study at ERA, the creep strain of unfilled HDPE, either individually moulded or cut from square plaques, varies by up to a factor of two depending on the orientation of flow [40]. This difference becomes even more marked with short fibre reinforcement. 

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