Low-voltage heating

The distributors in the two systems are usually heated by a supply of 220/230 V, (many EEC countries have raised their rated mains voltage to 230 V) but some systems operate at 5 V. Nozzles, on the other hand, are usually heated by a 220/230, 24 or 5 V supply. At 5 V and 24 V, special bar or tubular (high-supply) heaters are used, which assists miniaturisation and provides more uniform heating and more precise temperature control. This type of heating is known as low voltage heating, and the heaters as low voltage heaters. 

The most common heaters are 230 V coil heaters, but some companies prefer low voltage heating, which has led to a miniaturisation of heating elements. Low voltage heaters give very even heating and allow precision temperature control. 

Low voltage heating is particnlarly recommended for the injection of very small mouldings in multi-cavity moulds - for two reasons ... 

Low voltage heating is particularly recommended for injection moulding of such thermally sensitive materials as PA 66 and PA 46 (Stanyl, DSM), PBT and plastics with class VO flame-retardant additives. 

The advantage of manifolds with low-voltage heating is the rectilinear temperature distribution in the heating element, which facilitates processing of plastics with a narrow temperature window. The manifold heating time is very short. 

Figure 4.92 Manifold with internal low-voltage heating to 32-cavity mould...

Low-voltage heating requires the use of a transformer and special equipment to convert the current and control the temperature. 

Use of an HR system makes it possible to deliver the melt from the inside of the product, regardless of the core length. One restriction is the diameter of the core, since a space is needed to locate effective cooling between the nozzle and the shaping surface of the core. In these applications, nozzles with low voltage heating are slightly preferable because of the low external surface temperature of the nozzle. 

These ion lasers are very inefficient, partly because energy is required first to ionize the atom and then to produce the population inversion. This inefficiency leads to a serious problem of heat dissipation, which is partly solved by using a plasma tube, in which a low-voltage high-current discharge is created in the Ar or Kr gas, made from beryllium oxide, BeO, which is an efficient heat conductor. Water cooling of the tube is also necessary. 

Tia, as well as some other metals, can undergo a phenomenon where tiny metal filaments, called whiskers, form randomly on parts used ia electrical apphcafions. In low voltage, miniature circuitry, whiskers can cause short circuits. Alloys having 2% lead minimum or 0.5% bismuth or heat treatments are said to overcome the problem. A specification for electroplated fin coafings is available (133). 

Adequate single-phase protection is provided on low-voltage ac motor starters by three overload relays, which are now standard. Rotor heating is not particularly a problem on smaller motors which have more thermal capacity, but it is important to protect the stator windings of these machines against burnout. 

S Healing effect Low heat dissipation due to low voltage drop across the device (up to I V only)... 

All samples heated at 700 and 800°C show significant hysteresis that is, lithium is inserted in the materials near zero volts and removed at about one volt. We have shown that the amount of lithium which can be inserted in 700°C materials is directly proportional to their hydrogen (H) content. Table 4 shows that materials heated to 700 and 800°C retain substantial hydrogen. Upon heating to 900 C, the hydrogen is predominantly eliminated and so is the hysteresis. The samples then show substantial recharge capacity at low voltages. 

Low-voltage electric resistance heater cables fixed to the structural floor slab and then protected within a 50 mm thickness of cement and sand to give a suitable surface on which the floor vapour barrier can be laid. The heating is thermostatically controlled, and it is usual to include a distance reading or recording thermometer to give visual indication of the temperature of the floor at several locations below the insulation. 

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