GB2095611A

GB2095611A - Heating thermoplastic preforms

Info

Publication number: GB2095611A
Application number: GB8209286A
Authority: GB
Original assignee: Milacron Inc
Current assignee: Milacron Inc
Priority date: 1981-03-30
Filing date: 1982-03-30
Publication date: 1982-10-06
Also published as: GB2095611B; DE3210676A1; FR2502540B1; JPS57174222A; CA1193640A; FR2502540A1; JPS615894B2; DE3210676C2

Abstract

In a method and apparatus for radiant heating of hollow thermoplastic article preforms to the molecular orientation temperature, radiant energy is both converted to heat at the article surface and penetrates the interior of the article wall and is therein converted to heat. The principal limitation of radiant heating is the capacity of the material to conduct heat away from the surface where extreme temperatures would result in crystallisation of the material. To achieve the desired temperature uniformly throughout the article wall, brief periods of intense infra-red radiant heating are alternated with brief periods of negligible heating to take advantage of the combined effects of radiant and internal conductive heating. The article preforms (10) are rotatably supported parallel to a plane of quartz lamps (22) so that the combined patterns of radiant emission of all the lamps are distributed in contiguous segments over the preform surfaces. The articles (10) are rotated while the quartz lamps (22) are alternately energised with high and low levels of power for presettable periods. <IMAGE>

Description

SPECIFICATION Improvements relating to methods of and apparatus for heating thermoplastic articles The invention relates generally to heating of thermoplastic article preforms in preparation for expanding the preforms to produce molecularly oriented thermoplastic articles. More particularly, the invention relates to controlled radiant of preforms using tungsten filament quartz lamps.

Radiant heating has long been recognized as the most efficient method of heating when compared to convection and conduction. Radiant heating methods differ from convection and conduction heating in that the bulk of energy is transferred from the source to the object by electromagnetic waves at the speed of light and the object material converts the incident radiant energy to heat. Convection heating transfers heat by circulation of a heated medium, such as air, between the heat source and the object and conduction transfers heat by the incremental flow of heat through a highly conductive medium such as aluminium or copper. These methods, unlike radiant heating, require that the medium convert the energy to heat to achieve energy transfer and consequently exaggerate heat loss to the environment.

In order to maximise the heating efficiency of radiant heating, substantial quantities of energy must be radiated having wave lengths in the bands that are transmitted by the material and thereby penetrate beyond the preform surface.

However, since a black body radiator produces all wave lengths, operating a source to produce high levels of transmission band energy necessarily results in radiating elevated levels of absorption band energy. One approach in the prior art to overcome the attendant adverse surface overheating that would otherwise result, has been to exchange surface heat with a cooling gas. This method has the drawback that suitable means has to be provided to produce a flow of gas at an acceptable temperature.

Additional problems are encountered when dealing with preforms having non-uniform wall thicknesses over the length of the orientable portions. One prior art solution for this has been to lower the power input for the sources having outputs directed primarily towards the thinner wall sections. That solution, however, has the disadvantages of an attendant shift of the spectral peak of radiated energy toward longer wave lengths and a substantial reduction of the total level of the emitted radiation.

Furthermore, where the prior art has proposed operation of quartz lamp sources at continuous high power input levels, acceptable lamp life has been achieved only by cooling the lamp surfaces with a heat exchanging gas.

Therefore, one object of the present invention is to control the heat transfer from radiant sources to thermoplastic preforms in such a way as to take advantage of the efficiency of radiant heat transfer without resorting to a heat exchanging medium for the source or the preform.

A further object of the invention is to provide flexible control of the preform heating cycle so that variations of material thickness and preform size can be easily accommodated.

In accordance with one aspect of the invention we provide a method of heating a thermoplastic article to the required temperature and over a predetermined depth inwardly of a heat receiving surface comprising the steps of (a) establishing incidence of radiant heat wave energy from a source at the receiving surface on the article, (b) energising the heat source to a relatively high temperature over a first period during a heat source cycle and to a relatively low temperature over a second period of the cycle, (c) selecting the high temperature and the first period to be compatible with heat absorption and transmission characteristics inwardly of the article from the receiving surface to avoid heat induced degradation, (d) selecting the low temperature and second period to be compatible with attainment of the desired temperature at the predetermined depth inwardly from said receiving surface.

From a further aspect the invention resides in the provision of an apparatus for radiant heating of a thermoplastic article to a required temperature and over a predetermined depth inwardly of a heat receiving surface on the article comprising (a) a heating means having a heat source energisable to provide radiant heat wave energy, (b) means for supporting the heating means and the article in such spaced relation as to establish incidence of said heat wave energy on said surface, (c) means for energising the heat source, (d) means for varying the energy input to said heat source over respective first and second time intervals during a heating cycle to vary at least one of the parameters of heat source temperature and time interval concerned.

Primarily the method and apparatus are provided for radiant heating of polyethylene terephthalate beverage bottle preforms. These thermoplastic preforms must be heated to what is known as the molecular orientation temperature prior to blow molding to the final shape. The preferred embodiment incorporates tungsten filament tubular quartz lamps and heating cycle control timers. The lamps are arranged in a single plane and the preforms are supported with their longitudinal centerlines parallel to the plane of the lamps. The lamps are energised alternately with a high and low level power input for programmable first and second periods which may be determined independently of each other while the preforms are simultaneously rotated about their longitudinal centerlines. The overall heating period may be determined independently of said periods.

The invention will now be described by way of example with reference to the accompanying drawings wherein:~ FIGURE 1 is a schematic representation of apparatus for radiant heating of thermoplastic article preforms; FIGURE 2 shows the cross-section of an article preform in the spatial relationship of the preform to the radiant heating elements; FIGURE 3 is a graph of the absorption of PET with respect to wave length of incident radiation; FIGURE 4 is a representation of a wall section of an article preform; FIGURE 5 includes graphs of the spectral energy distribution of black bodies at several temperatures and the thermal response times of various radiant sources.

Figures 1 and 2 illustrate apparatus for radiant heating of thermoplastic article preforms. In the oven design of the preferred embodiment, article preforms 10, each of which as can be seen is of a form to afford a longitudinally extending centerline, are carried by an assembly comprising a carrier 1 8 and a number of preform gripper assemblies 15 including cups 14 and fingers 16.

In Figure 1, the carrier 18 is shown with a partial section removed to show the gripper assemblies 1 5, one of which is shown cut away revealing the formed portion 12 (typically threaded for capping) of a preform 10. The cups 14 shield the formed portion 12 of the preforms 10 from the heat source while the remaining or further portions which are to be heated to the molecular orientation temperature of the material are irradiated from the heat source over the heat receiving surfaces.The grippers 15 are attached to the spindles 20 riding in spindle bearings 21 to provide for rotation of the preforms during the time they are exposed to the radiant heat source, so that although in any given position of rotation part only of the heat receiving surface of each article (that presented towards the heat source) is exposed to radiation, the whole of the heat receiving surface is irradiated in consequence of the rotation imparted. Rates of rotation vary depending on the preform wall section thickness and the distance of the preforms from the heat sources. In applicants preferred embodiment, the rotation rate is between about 15 rpm and about 120 rpm.

The heater assembly 25 consists of a heat source in the form of heating elements 22 mounted in an oven frame 20 which aligns the elements 22 such that their longitudinal centerlines lie in a vertical plane parallel to one another. The heating elements 22 chosen by applicants are commercially available and are of the type having a tungsten filament 88 enclosed in a quartz tubular envelope 84. The selected lamps have a continuous power rating of 3800 watts at an operating voltage of from 550 to 600 volts. At this power level, the lamps reach a maximum temperature of about 2500 degrees K (approximately 22270C).

Power to energise the quartz elements is provided by an alternating current power source 24, one side of which is connected to the terminals 23 of one end of all of the heating elements 22. The other side of the alternating current power source 24 feeds amplifiers 26 which are in turn connected individually to the heating elements 22 at connections 27. Each of the amplifiers 26 is capable of producing a high and low power output over first and second periods in each heating cycle. Applicants have selected commercially available phase controlled switching amplifiers that include overcurrent limiting circuitry to protect the switching devices against damage that could otherwise result from cold filament in-rush currents characteristic of the chosen lamps.The high power output level is adjusted to deliver approximately 80% of continuous rated power of the lamps to desensitise the lamp circuits to voltage variations from the source 24 and reduce stress on the lamps. The adjustment is made using a continuously variable input device 32 such as a potentiometer. The low power output is set to deliver between about 1% and 10% of the continuous rated power of the lamps. The amplifiers also accept inputs for selecting whether the output shall be at the high or the low level. In applicants preferred embodiment, these inputs are provided by independently presettable timers 28 and 30, one for a high power output period and the other for the low power output period.The low power level is adjusted to a point at which the radiant energy output of the quartz elements is insignificant, with respect to heating of the preforms, while at the same time maintaining sufficient energy input to the lamps to reduce the adverse effects of full switch thermocycling as would result if the current were completely cut off which, however, may be practicable in certain forms of the apparatus and which would thus be within the scope of the invention.

Although in the applicants preferred embodiment, independent timers 28 and 30 control the high power output period and low power output period for each amplifier 26, thus allowing the adaptability of the control system to a larger variety of article preform sizes, equally suitable control could be achieved by using a single percentage timer for each amplifier 26. In such an arrangement, the total cycle time is fixed by the time of the percentage timer and the high power or low power period is adjusted as a percentage of that total period. Applicants have found that for best results the high power period should not be less than the period of a full rotation of the preform.

As will be explained in greater detail subsequently, the rate of radiant heating is limited by the thermal conductivity of the preform material. Applicants invention takes advantage of the fact that the quartz elements produce substantial quantities of radiation at the wave lengths that are transmitted by the preform material. During the high power output periods, the radiant energy is both adsorbed and transmitted by the preform material. Because the energy density reaching the preform is highest at the surface, the period of exposure of any portion of the preform wall section is limited by the ability of the preform to conduct the heat generated at the surface into the interior of the wall section.

The interior wall section is heated by the combined effects of the penetration of radiant energy in the transmission band of the material and the conduction of heat to the interior from the surface. During the low power period, the energy density of the radiant emission at the preform section is insignificant and heating of the interior portion of the wall section is accomplished primarily by conduction. The independent adjustment of the high power and low power periods is used to accommodate variations of wall thickness. The independent control of power for each of the heating elements provides a means of accommodating thermal profiling of the article preform bodies.

Referring to Figure 2, the quartz lamps 22 are shown in an array with the centerlines of the lamp cross-sections, comprised of envelopes 86 and filaments 88, located in a plane P, at a fixed distance D from the longitudinal centerline of the partial cross-sections 80 of a preform 10 lying in a parallel plane P2. The distance D and the centre spacing between elements 22 are arranged to distribute the energy radiated in the direction of the preform 10 in discrete segments which overlap at parallel plane P2. The overlap is indicated by the intersection of the phantom lines such as 22a of the patterns of radiant energy with plane P2. This results in distributing these radiant energy patterns at least contiguously over the surface of the orientable portion of the preform 10.The portion of the preform wall section at 82 compared to the portion at 84, is seen to be substantially thinner and effectively nearer to the centerline of the quartz lamps 22 because of the tapered profile of the preform 10. To take advantage of the combined heating effects of conduction and radiation, while at the same time accommodating such variations of effective distance and preform wall thickness, the high power output and low power output period timers, 28 and 30, for each of the amplifiers 26 controlling the energy input to the quartz lamps 22 are made independently adjustable. Thus, the radiant energy density at any segment of the unshielded portion of the preform is controlled.

To best explain the combined effects of radiant and conductive heating within the preform wall section, reference is had now to Figures 3 and 4.

Figure 3 is a graph of the percent transmittance of polyethylene terephthalate for radiation in the range of wave lengths from about 2 microns to about 14 microns. In the range of wave lengths from about 2 microns to about 5.5 microns, with the exception of an absorption band just below 3 microns, the percent transmittance is above 80%.

Referring to Figure 4 and considering the preform wall section to be comprised of slabs 210 of a thickness equal to that for which the transmission of 2 micron wave length radiation is about 80%, it can be seen that nearly 10% of the incident radiation at the surface 200 is transmitted through a wall section having a thickness equal to ten of these slabs. Therefore, with high energy densities incident at the surface of the preform, and provided that a substantial portion of this incident energy is at a wave length that can be transmitted by the preform material, the transmittance of these wave lengths by the material produces significant heating effects within the material itself.At the same time, the thermal conductivity of the material determines the rate at which the incident energy being absorbed at the surface and which is being converted into heat there, is transferred on into the interior of the wall section.

The advantage of high temperature quartz lamps with regard to the quantity of near infra-red wave length radiation (wave lengths in the range of from about 1 microns to about 2 microns) is best appreciated by reference to Figure 5. The curves 42, 44, 46 and 48 of graph 40 plot the radiant energy (WF) as measured along the vertical axis against the wave length (A) of radiation as plotted along the horizontal axis.These curves correspond to the relationship defined by the equation developed by Plank set forth below:

A5(e- 1) Where: WF is the radiant emittance, is the constant = 3.14159, C is the speed of light = 2.9979 (108) m/sec, H is Plant' constant = 6.62 (10-34) joule sec, K is Boltzman's constant = 1.38 (10-23) joule/degrees K, T is the temperature of the body in degrees K (degrees Kelvin), A is the wavelength of radiant emission in microns, Curve 42 is plotted for a black body at a temperature of approximately 2500 degrees Kelvin (2227 C). Curve 44 is plotted for a black body at a temperature of approximately 2000 degrees Kelvin (1 7270C). Curve 46 is plotted for a black body at a temperature of approximately 1800 degrees Kelvin and curve 48 is plotted for a black body at a temperature of approximately 1000 degrees Kelvin (7270C). While metal sheath heating elements typically operate at a temperature of approximately 1000 degrees Kelvin (7270C), quartz lamps of the type used in applicants preferred embodiment operate at a peak temperature of approximately 2500 degrees Kelvin (22270C). As can be seen from graph 40, the peak radiant energy for the black body of curve 42 is in excess of 100 times the peak radiant energy for a black body of curve 48; and significantly, the peak radiant energy for the higher temperature black body occurs at much shorter wave lengths.Specifically, for a black body at a temperature of 2500 degrees Kelvin (22270C), the peak of radiant energy occurs at a wave length of approximately 1.1 5 microns while the peak radiant energy for a black body at a temperature of 1000 degrees Kelvin (7270C) occurs at a wave length of approximately 2.9 microns.

The graphs 41 and 43 show the thermal responses of a variety of heating elements. The vertical axes measure the percentage of maximum output and the horizontal axes measure time.

Graph 41 plots the response for heating elements to go from a cold start to their maximum output and graph 43 plots the response time for heating elements to go from their maximum output to the cold condition. Curves 54 and 60 correspond to the response for metal sheath heaters. Curves 52 and 58 correspond to the response time for quartz tube elements and curves 50 and 56 correspond to the response times for the quartz lamp elements of applicants preferred embodiment. It is to be noted that the quartz lamp curves 50 and 56 show that a cold quartz lamp attains an 80% level nearly instantaneously and a hot quartz lamp reduces its output to a 20% level nearly instantaneously.As a consequence when the amplifiers 26 of Figure 1 are operated with high power output periods substantially less than one minute, the quartz elements will nevertheless be operating at temperatures in excess of about 2000 degrees Kelvin (1 7270C) during the high power output period. Referring to the graphs 40, this temperature results in radiant emission corresponding to that of a black body as plotted by curve 44. As can be seen, even in these short high power output periods, substantial quantities of near infra-red wave length radiation are available from the quartz lamp heating elements. A further characteristic of these quartz lamps is that the quartz envelope is essentially opaque to radiation from the tungsten filament of wave lengths greater than about 7 microns.Therefore, the lamps tend to further reduce the energy density of some of the absorption band emittance at the preform surface.

Referring again to Figure 1, and keeping in mind the feasibility of using high power output periods of significantly less than one minute, it can be seen that the timed control of the high power output for each of the heating elements provides the most efficient means for achieving the desired heating of the article preform. In particular, if the heating elements 22 in closest proximity to the thinnest wall section 82 of Figure 2 were operated at a reduced temperature, then the radiant emission would be degraded as indicated by the curves of graph 40 and the spectral distribution of the radiant energy would suffer an attendant shift toward longer wave lengths.Using applicants process to accommodate the thinner wall sections and their closer proximity to the heating elements, while at the same time achieving the desired molecular orientation temperature for the material in the thicker wall section, it is only necessary to adjust the high power output period and low power output period for the respective amplifiers to maintain the desired temperature in the thinner wall section while achieving desired temperature in thicker wall sections. Thus, the overall or total heating time for a preform is determined by the wall section thickness and the distance from the heating elements.

Applicants have found that for preforms suitable for producing a finished bottle having an approximate volume of 0.5 litre, in which the wall section thickness is about 0.125 inches (3.175 mm) that the most suitable distance D is from about 1 inch (25.4 mm) to about 3 inches (76.2 mm). The lamp centre spacing in this application is about 1 inch (25.4 mm). The timers are set so that the low power output period is set below about 10 seconds and the high power output period is set below about 10 seconds and the rate of rotation is about 18 rpm.

While the invention has been illustrated in some detail according to the preferred embodiments shown in the accompanying drawings, and while the preferred illustrated embodiments have been described in some detail, there is no intention to thus limit the invention to such detail. On the contrary, it is intended to cover all modifications, alterations and equivalents falling within the spirit and scope of the appended

Claims

claims. CLAIMS

1. A method of heating a thermoplastic article to a required temperature and over a predetermined depth inwardly of a heat receiving surface comprising the steps of (a) establishing incidence of radiant heat wave energy from a source at the receiving surface on the article, (b) energising the heat source to a relatively high temperature over a first period during a heat source cycle and to a relatively low temperature over a second period of the cycle, (c) selecting the high temperature and the first period to be compatible with heat absorption and transmission characteristics inwardly of the article from the receiving surface to avoid heat induced degradation.

(d) selecting the low temperature and second period to be compatible with attainment of the desired temperature at the predetermined depth inwardly from said receiving surface.

2. A method according to claim 1 wherein the heat source is a filamentary heating lamp and the filament is energised by the supply of electrical current thereto.

3. A method according to either of claims 1 and 2 wherein some energy is applied to the heat source during both the first period and during the second period.

4. A method according to any one of the preceding claims wherein the arrangement of the heat source and the article is such that radiant heat energy is incident at any instant on part only of the heat receiving surface, and wherein the article and the heat source are moved relatively to each other to bring about irradiation by said heat source of the whole of said heat receiving surface.

5. A method according to claim 4 wherein the relative movement effected between the article and the heat source is such that the whole of the heat receiving surface is subjected to irradiation from the heat source a plurality of times in each heating cycle.

6. A method according to any one of the preceding claims wherein during the first period the temperature of said heat source is between 2000 degrees Kelvin (1 2270C) and 2500 degrees Kelvin (1 7270C) and during the second time interval the temperature of the heat source is no more than about 1000 degrees Kelvin (727 C).

7. An apparatus for radiant heating of a thermoplastic article to a required temperature and over a predetermined depth inwardly of a heat receiving surface on the article comprising (a) a heating means having a heat source energisable to provide radiant heat wave energy, (b) means for supporting the heating means and the article in such spaced relation as to establish incidence of said heat wave energy on said surface, (c) means for energising the heat source, (d) means for varying the energy input to said heat source over respective first and second time intervals during a heating cycle to vary at least one of the parameters of heat source temperature and time interval concerned.

8. Apparatus according to claim 7 wherein the heating means comprises a filamentary heating lamp and wherein the means for energising the heat source and for varying the energy input comprises an electrical supply circuit including means for establishing a higher value of electrical current through the lamp filament during the first period and a lower value of current during the second period.

9. Apparatus according to either of claims 7 and 8 including means for effecting relative movement between the supporting means for the heating means and the supporting means for the article in a manner such that the whole of the heat receiving surface of the article is irradiated by the heat source during operation of the apparatus.

10. Apparatus for heating a thermoplastic article substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

10. Apparatus according to claim 9 wherein the manner of relative movement is such that the whole of the heat receiving surface of the article is subjected to irradiation by the heat source a plurality of times in each heating cycle.

11. An apparatus for radiant heating a thermoplastic article preform having a longitudinal centerline to the molecular orientation temperature, the preform having a formed portion and a further portion to be heated, the apparatus comprising: (a) a heater assembly including quartz lamps mounted such that filaments thereof lie in a first plane for causing the individual patterns of radiant energy of the lamps to be distributed over segments of a further plane parallel to the first plane; (b) means for rotatably supporting the preform by its formed portion with its longitudinal centerline lying in said further plane which is so spaced apart from the first plane as to cause the patterns of radiant energy to be distributed over the surface of the further portion no less joined than contiguously; and (c) means for alternately energising the lamps with high power and low power for presettable periods of time less than about thirty seconds to heat the further portion to the molecular orientation temperature.

12. Apparatus of claim 1 1 wherein the means for alternately energising each lamp with high and low power further comprises an independently presettable control means for energising each lamp.

13. Apparatus of claim 2 wherein the means for alternately energising each lamp with high power and with low power further comprises a phase controlled power switching amplifier having overcurrent limit circuitry.

14. Apparatus according to any one of claims 7 to 13 wherein means for energising the heat source and for varying the energy input is arranged to produce a heat source temperature between about 2000 degrees Kelvin (12270C) and 2500 degrees Kelvin (1 7270C) during the first period or the high power period and enough power during the second period or low power period to produce a heat source temperature of no more than about 1000 degrees Kelvin (7270C).

15. A method of heating a thermoplastic article substantially as hereinbefore described with reference to the accompanying drawings.

16. Apparatus for heating a thermoplastic article substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

17. A method including any novel feature or novel combination of features herein disclosed.

18. Apparatus including any novel feature or novel combination of features herein disclosed.

New claims or amendments to claims filed on 1st June, 1982 Superseded claims 1-18 New or amended claims:~

1. A method of heating a hollow thermoplastic article, preparatory to expansion, to the molecular orientation temperature of the material of the article and throughout the wall thickness inwardly of a heat receiving external surface of that portion of the article which is to be expanded comprising the steps of (a) establishing incidence of radiant heat wave energy on a part of the receiving surface on the article from a source comprising a filamentary quartz electric lamp, (b) energising the heat source to a relatively high temperature over a first period during a heat source cycle or less than about one minute and to a relatively low temperature over a second period of the cycle, (c) selecting the high temperature to be in a range providing radiation at wave lengths establishing a high percentage transmittance through said material and the first period to be such as to be compatible with heat absorption and transmission inwardly of the article from the receiving surface to avoid heat induced degradation, (d) selecting the low temperature to be compatible with attainment of the molecular orientation temperature throughout the wall thickness while reducing thermal shock in respect of said lamp, (e) moving the article and the heat source relatively to each other to bring about irradiation by said heat source of the whole of the heat receiving surface a plurality of times in each heating cycle.

2. A method according to claim 1 wherein the relatively high temperature is between 2000 degrees Kelvin (1227 0C) and 2500 degrees Kelvin (1 7270C) and the first period is less than about thirty seconds.

3. A method according to either of claims 1 and 2 wherein the low temperature is no more than about 1000 degrees Kelvin (7270C).

4. An apparatus for radiant heating of a hollow thermoplastic article, preparatory to expansion, to the molecular orientation temperature of the material of the article and throughout the wall thickness inwardly of a heat receiving external surface on that portion of the article which is to be expanded, the apparatus comprising (a) a heating means comprising a filamentary quartz electric lamp energisable to provide radiant heat wave energy, (b) means for supporting the heating means and the article in such spaced relation as to establish incidence of said heat wave energy on a part of said surface, (c) means for energising the lamp, (d) means for varying the energy input to said lamp over respective first and second time intervals during a heating cycle, and comprising an electrical supply circuit including means for establishing a higher value of electrical current through the lamp filament during the first period of less than about thirty seconds and a lower value of current during the second period of such duration that the first and second periods total less than about one minute, the higher value of current producing a temperature of between 2000 degrees Kelvin (1 2270C) and 2500 degrees Kelvin (17270C), and the lower value of current producing a temperature of not more than about 1000 degrees Kelvin (7270C), (e) means for effecting relative movement between the supporting means for the heating means and the supporting means for the article in a manner such that the whole of the heat receiving surface of the article is irradiated by the lamp a plurality of times in each heating cycle.

5. An apparatus for radiant heating a thermoplastic article preform having a longitudinal centerline to the molecular orientation temperature, the preform having a formed portion and a further portion to be heated, the apparatus comprising: (a) a heater assembly including quartz lamps mounted such that filaments thereof lie in a first plane for causing the individual patterns of radiant energy of the lamps to be distributed over segments of a further plane parallel to the first plane; (b) means for rotatably supporting the preform by its formed portion which its longitudinal centerline lying in said further plane which is so spaced apart from the first plane as to cause the patterns of radiant energy to be distributed over the surface of the further portion no less joined than contiguously; and (c) means for alternately energising the lamps with high power and low power for presettable periods of time less than about thirty seconds to heat the further portion to the molecular orientation temperature.

6. Apparatus of claim 5 wherein the means for alternately energising each lamp with high and low power further comprises an independently presettable control means for energising each lamp.

7. Apparatus of claim 6 wherein the means for alternately energising each lamp with high power and with low power further comprises a phase controlled power switching amplifier having overcurrent limit circuitry.

8. Apparatus according to any one of claims 5 to 7 wherein means for energising the heat source and for varying the energy input is arranged to produce a heat source temperature between about 2000 degrees Kelvin (1 2270C) and 2500 degrees Kelvin (1 7270C) during the first period or the high power period and enough power during the second period or low power period to produce a heat source temperature of no more than about 1000 degrees Kelvin (7270C).

9. A method of heating a thermoplastic article substantially as hereinbefore described with reference to the accompanying drawings.