when thermoplastic materials are processed into products which are required in large quantities , it is necessary to make full use of the properties of the material . it is desirable to obtain the maximum strength and service life using a minimum of material . in the case of certain plastics materials , special advantages can be achieved gy orientation of the material . infra - red radiation is eminently suitable for re - heating . it enables the plastics material to be heated not only at its surface but simultaneously over its entire depth in a few seconds . through the absorption characteristic of thermoplastics a more intense heating occurs in the surface layers which face the radiation source . as can be seen from fig1 a temperature distribution develops which is determined by the radiation temperature and the specific absorbing properties of the plastics material . the more intense the irradiation , the higher is the temperature peak at 2 . depending on the material that is to be processed , this can cause an undesired change in or damage to the affected layer , as is shown in fig1 at 5 and 6 , since the thermal conductivity of thermoplastics is relatively low and the temperature balance within the wall is not achieved immediately but occurs only after some time , as is evident from the lines 1 &# 39 ;, 1 &# 34 ;, 1 &# 39 ;&# 34 ; which show the temperature distribution at predetermined time intervals after the irradiation process has been completed . this phenomenon is of particular significance in the case of relatively large wall thickness , for example of more than 3 mm . the improvement of the properties of the material in thermoelastic processing increases considerably with an increase in the extent by which the material can be stretched ; for example , stretching by more than ten times is possible in the case of saturated polyethylene terephthalate ( pet ) and materials which behave in a similar manner . in order to achieve specific properties , more especially in the case of pressure tanks , it may be necessary to start with a blank having a wall thickness that is more than 3 mm . as is known , pet is highly transparent in its amorphous and orientated state . through crystallisation pet becomes opaque and brittle . the rate of cyrstallisation reaches its maximum at temperatures around 150 ° c . during the heating process by infra - red radiation , in which the thermoelastic processing stage is reached , the critical temperatures in the vicinity of the surface can be present for such a length of time that an at least partial crystallisation as indicated at 5 and 6 occurs on the surface of the originally amorphous workpieces . the consequences of this are as mentioned above , and further processing by orientation is no longer practicable to the desired or otherwise possible extent . for example in the case of polyvinyl chloride ( pvc ), polyacrylonitrile , polystyrene ( ps ) and materials which behave in a similar manner during processing , the overheating in the irradiated surface layer brings about a release of dissolved substances and / or , by chemical changes , a release of constituents of the macromolecules . this can damage the material and / or lead to the formation of gas bubbles which impair the appearance of the workpiece that is to be produced . in order to prevent the above - mentioned consequences of the temperature peak 2 in the irradiated surface layer , the peaks are , according to the invention , reduced by cooling the surface layer during the heating process . in fig2 the infra - red radiation is designated by 8 . the temperature peak 4 of the temperature profile 3 is now considerably lessened ; the period before which an approximate equalization of the temperature profile 3 is achieved is likewise shorter ( lines 3 &# 39 ;, 3 &# 34 ;). consequently , the undesired changes in the surface do not occur . in the case of pet , it is now possible to re - heat semi - finished products or blanks that have a fairly large wall thickness so that they reach the thermoelastic processing temperature within a few seconds . biaxial orientation of blown hollow bodies is becoming of increasing importance , but it is only an economical process , involving large quantities of materials , if an improvement of the properties of the material -- i . e ., higher strength , reduced gas permeability and better transparency -- can be obtained . these properties are necessary in the case of pressure bottles , for example for drinks containing carbon dioxide and for aerosols . pet is now being used as a particularly suitable thermoplastic material for this purpose . the starting material for the process comprises cold premoulded blanks 9 with closed bottoms , which can have been manufactured according to any one of a number of known methods . for the purpose of being heated , the blanks 9 are disposed in a heating chamber which contains a plurality of infra - red radiators 10 , which are arranged on one side or on several sides of the chamber . preferably , the blanks 9 pass continuously through the heating chamber ( as shown by the symbol • in fig3 ) whilst being rotated simultaneously about their own axes . a cooling device 11 for the blanks is fastened to a support 12 on which the blanks are mounted and by means of which the blanks are guided past the radiators 10 . during the heating process , the blanks are fanned by the cooling device 11 , which is stationary in relation to the axes of rotation of the blanks . in the embodiment illustrated in fig4 the outflow of the preferably gaseous cooling medium that is directed on to the blanks occurs through a gap 13 in the tubular cooling device 11 . in another embodiment , shown in fig5 the outflow of the cooling medium can occur through a plurality of holes 14 . if heating of the blanks in the longitudinal direction is to be effected in a predetermined non - uniform manner , then the holes 14 can , as shown in fig6 be arranged in groups , in order to vary the intensity of the cooling medium stream in a likewise predetermined non - uniform manner . in order to increase the cooling effect of the gaseous medium , the latter can be enriched with or replaced by a liquid . in particular small droplets of liquid may be incorporated in the gas stream . in a further embodiment , which is not shown , the heating device is in the form of a tunnel , through which a gas stream flows which cools the surface of the blanks . furthermore , the cooling process can also be carried out using a cooling element which resiliently bears against the irradiated blank and through whose contact with the surface of the blank during irradiation heat is transmitted out of the surface layer . to prevent any irregular cooling flow distribution and consequently irregular cooling caused by inaccurate alignment of the cooling medium stream in relation to the axes of the individual blanks , the discharge of the coolant fluid can be effected in such a way that , as shown in fig7 the stream of coolant widens considerably after leaving the cooling device 11 or is , as shown in fig8 blown out obliquely at an angle α . with these arrangements , slight deviations in the alignment of the blanks and the cooling device hardly become noticeable . processing of the plastics material occurs in the thermoelastic moulding temperature range ( approximately 100 ° c in relation to a melt point of approximately 200 ° c ).