Moulding faults

Presses with vacuum extraction fitted to extract gases from moulding cavities eliminate moulding faults arising from air entrapment, or gases generated during vulcanisation. These presses are... 

The relationship between these variables is complex and needs to be fully understood through use of factorial experiment design for each type of machine used in the factory. The unit must allow the rubber to be processed uniformly without scorch. There must be a smooth pathway for the rubber, free from dead zones where there is little or no rubber flow. Rubber that collects in these zones will soon cure to form hard nibs that will grow in size until they re-enter the material flow, impede the injection, and produce moulding faults. This problem exists in many existing machines where the first rubber to pass into the injection barrel is the last to be injected, and may be partially retained for many cycles. The design of the injection unit is therefore crucial to the operation of the machine. 

In the ideal world moulding faults do not exist. Unhappily companies continue to live with problems, either because the problem is thought to be impossible to solve or it has not been recognised as a production cost that can be saved. If company rejects are zero, then skip this section ... 

Figure 2.15 Common moulding faults and some corrective measures...

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 ... 

To obtain a strong busbar mounting system, suitable to withstand the electrodynamic forces arising out of a system fault, modern practice is to make use of thermosetting plastics, such as DMC (Dough Moulding Compounds)... 

Warping, difficulty of moulding to close tolerances and wavy or fibre-patterned surfaces or faults arising from the high shrinkage during cure. 

These products are applied to the mould cavities and do not cause product faults and generally do not cause bond problems. 

Defects are mentioned in other chapters in connection with plastics in a variety of forms, manufactured in various ways (extrusions, films, mouldings, sheet, and so forth). As demands for quality and uniformity become more stringent it is important to limit and to localize all defects, and as a first step it is necessary to identify and to classify them. Figure 1 is an attempt to do so for seven types of fault, as follows ... 

To produce mouldings with high surface gloss the finishes of the cavities and cores of moulds must be excellent and free from faults. Since the temperature affects the rate of output and the finish on mouldings, precise control of this also is very important. 

Moulded-in busbars 6 Isolator blades with moving contacts 7 Hinged front door 8 Door interlocked with isolator 9 Flameproof window for instruments, indicator lights and operation counter 10 Flameproof windows for visual isolation 11 Fault reset and/or earth fault test push-buttons under covers 12 Static protection unit ... 

Mould material — Most moulds for rubber moulding are made of steel. The metal should be close grained and of uniform and fault free texture. A carbon content of 0.25% to 0.45% is preferable. Moulds are sometimes made of aluminium alloys or manganese bronze with a high silicon content. Moulds difficult to machine by conventional methods can be made using these alloys. 

Switchgear tends to be operated infrequently, whereas motor control centres operate frequently as required by the process that uses the motor. Apart from the incomers and busbar section circuit breakers, the motor control centres are designed with contactors and fuses (or some types of moulded case circuit breakers in low voltage equipment) that will interrupt fault currents within a fraction of a cycle of AC current. Circuit breakers need several cycles of fault current to flow before interruption is complete. Consequently the components within a circuit breaker must withstand the higher forces and heat produced when several complete cycles of fault current flow. 

The outgoing switching device in a high current, high fault level, switchboard will usually be a power circuit breaker if it feeds more that about 400 amps to the load. Below 400 amps the circuit could have a fuse-contactor combination, see sub-section 7.3.2 for comments on contactors and Chapter 8 on fuses. Therefore if the outgoing device must be a circuit breaker then the comments and discussion in sub-section 7.2.3 above apply. Low voltage switchboards often use moulded case circuit breakers... 

Moulded case circuit breakers are available in two basic modes of operation, current limiting and non-current limiting. It is difficult to design a moulded case circuit breaker to have a cut-off characteristic that is less than 0.01 second when a fully asymmetrical short-circuit current flows. However, there are such circuit breakers available, and care is needed when selecting these devices for a circuit that has a high prospective fault current. Some manufacturers are able to provide a cut-off in the order of 0.006 second. 

Moulded case circuit breakers are also available for incoming and busbar section purposes, with ratings up to 6000 A and service voltages between 220 V and 660 V. (At 415 V a 4000 A circuit breaker would satisfy the duty of a 2500 kVA feeder transformer with about 15% spare capacity.) These are also available as 4-pole units. Circuit breakers having ratings of 800 A and above are often provided with several adjustments that widely modify the shape of the complete protection curve, as described in Chapter 12. This enables the curve to coordinate with almost any other protective device or equipment that is immediately upstream or downstream of the circuit breaker. Some circuit breakers with the higher rated currents are also provided with integral earth fault protection facilities. 

When fuses or moulded case circuit breakers are applied to a circuit it is necessary to ensure that their /-squared-t characteristics coordinate properly with the thermal capabilities of the downstream equipment, especially the cables. In order to determine the /-squared-t characteristics of a protective device it is assumed that the current in the device suddenly changes from a normal load value to the fault value in a very short period of time, i.e. similar to a step change in a control system. Hence for each value of current along the x-axis of the device s time-current characteristic the value of the current squared multiplied by the corresponding time can be plotted. For cables and busbars the /-squared-t function equals a constant (k) for each cross-sectional area of conductor, as explained... 

Figure 7.7 Clearing time versus fault current for fuses and moulded case circuit breaker curves.

Prolonged storage or cold storage Increased viscosity Dispersion onto sand grains difficult. Sand mixture flows less well and is difficult to compact. Low-strength moulds Erosion, exogenous (sandy) inclusions firming and dimensional faults... 

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