Vacuum Forming


Vacuum forming  is the term used to describe a number of thermoplastic sheet shaping techniques which employ reduced air pressure to impart the final detail to a moulded article. With some of these techniques, pneumatic or mechanical pre-stretching of the material may be employed during the moulding cycle so as to obtain acceptable thickness distribution of the material.
The process consists of inserting a thermo-plastic sheet into the machine in cold state, heating it to its shaping temperature, then moulding it to the desired shape. Sheets in the thickness range of 0.025 mm-6.35 mm can be shaped in this manner.
Heaters: the heaters generally consist of ceramic IR elements of the dull emitter type mounted in polished aluminium reflectors. The sheet has to be heated uniformly over its surface area and throughout its thickness. This uniformity can best be achieved by the zone control of the heaters. By controlling the areas of heat intensity, heat losses at the periphery of the sheet caused by convection air current and absorption by metal frames can be fully compensated for, resulting in uniform heating. Fast movement of the heaters is important to avoid differential heating of the material at the beginning and at the end of the heating cycle. A uniform temperature throughout the thickness of the material can generally be achieved with single sided heating for material thickness upto about 2.4 mm. For greater thickness, double-sided heaters are employed. Apart from uniform heating, double sided heating permits a time saving factor which can be as much as 50% for material thicker than 2.4 mm. 
According to the type of material being formed, the electrical loading of the top heater should vary  from 10-30 KW/m2. The bottom heater should have a loading of approximately one-half of that at the top. 
Where zone control of the top heater is not possible, the efficiency of the top heater can be improved by the addition of reflectors at the sides of the heaters and the clamping frame. The reflectors can be made from high purity aluminium, which compensates for heat losses at the periphery of the sheet. Trial and error variation of the width of the reflectors and the angle with regard to the clamping frame may be necessary to provide uniform heat distribution over the sheet. To compensate for heat losses at the periphery, aluminium foil can be placed over the inner faces of the clamping frame to act as a reflector.
By replacing conventional elements with Ceramic Infrared Heating Elements, companies have benefited by reduced electrical loadings, better production quality, higher production rates and lower maintenance costs.
Power savings to the extent of 25% can be made with the use of our elements. Some of the reasons why energy is saved are:
1.Plastic will absorb the IR radiation emitted by ceramic elements more readily than that by tubular elements.
2.Ceramic elements are less dependent than tubular heaters on the condition of the reflector behind them. In case of tubular heaters, if the reflector is dirty, a lot of heat is lost. 
3.The emissivity of a tubular heater is 0.8 whereas that of a ceramic element is 0.96(very near the black body emissivity of 1).
4.It is very difficult to divide a heater using tubular heaters into zones. With ceramic elements though, it is very easy to start in the centre of the heater with low wattage and gradually move to higher wattage elements towards the periphery. Alternatively, the same wattage elements could be used throughout but the power can be regulated to individual zones.
5.Rod elements because of their nature tend to give local lines of heat. To overcome this, heating must be slower. This would allow heat to conduct between the lines so as to give uniform heating. The longer the heating cycle, the more the energy used.  
For a forming area of 300mmx500mm, the heaters should be used as shown in the sketch and different zones (numbered 1 to 4) should be formed. If the heaters are zoned as suggested, there should be ample heat at the edges and corners. You would have the facility of turning down the centre area, which almost invariably becomes hotter because the sheet is unable to dissipate as much heat from the centre as from the edges. Also the clamping frame holding the sheet in place at the edges will take away a lot of heat. In addition there is a tendency for the heater itself to become hotter in the centre because the middle elements are surrounded by elements from which they pick up heat; that is why it is important to either have lower wattage elements in the centre or provide the possibility of reducing the heat output of the centre. It is also important to extend the heating elements over the clamping frame to ensure that the sheet is heated right upto its edges. Moreover, by using zone ceramic elements, the cycle time would certainly be reduced. 
Dotted line in the figure below indicates clamp frame (300mmx500mm).