GB2069920A

GB2069920A - Heating device for a mould inlet

Info

Publication number: GB2069920A
Application number: GB8006586A
Authority: GB
Original assignee: Palmer R M
Current assignee: Palmer R M
Priority date: 1980-02-27
Filing date: 1980-02-27
Publication date: 1981-09-03

Abstract

A heating device for a mould injection inlet or runner is described which comprises a jacket for surrounding the runner or inlet passage consisting of an inner sleeve 1 and an outer sleeve 2. The space between the inner and outer sleeves is occupied by a plurality of quartz glass tubes 4, through which a helically coiled wire element 3 connected between electrical leads 5, 6 is passed in turn, forming a zig-zag path extending in alternate directions lengthwise of the jacket. In each tube, the patch of the element is varied to provide a region 8 at each end of the tube wherein the element is tightly wound at a turn ratio of 2:1 and a middle region 9 where the element is loosely wound with a turn ratio about 20:1, and transitional zones 10 where the pitch changes. The effect of this is to prevent over-heating of the middle part of the device, keeping the inlet or runner more evenly heated over its length whereby heat damage in an over-heating middle part can be avoided. <IMAGE>

Description

SPECIFICATION Radial coil sprue bush heater This invention relates to temperature control of mould inlets for moulding synthetic plastics materials, such as for example thermoplastics.

In modern moulding systems where heating is required there is often the need to supply high temperatures in very confined areas of a controllable state, whereby the accurate flow temperature of plastics material injected into a mould or passed through any processing device must be maintained in order to prevent solidification of the plastics in the tooling or burning and damaging of the material by scorching or partial carbonisation. This is especially the case in injection mould hot runner systems where the tool itself must remain cold in order to set the material after injection whilst the material runner system must remain at the flow temperature of the relevant plastics material.

Various tools and devices exist to help in this field, however, the major drawback in all these systems is difficulty in obtaining, maintaining for a reasonable period of time and controlling the heat input.

Due to the limited space available, conventional heaters which are small enough to fit within hot runners or mould inlet passages either cannot obtain the operative temperature ranges between 2000C-5000C due to the limits of their construction, or having achieved the temperature by over running the element wire they have a limited lifespan.

The temperature over the surface of these elements can only be carried by reducing the actual element area and subsequently the wire diameter, to obtain the same resistance, thus running the element hotter, for example with a conventional area of 6 sq. ins. The maximum wire size that will fit is say a 39 g wire due to conventional methods of fitting the element, i.e. two strands run through a tube 381' diameter which is then wrapped in a coiled section. Maximum wattage obtainable is anywhere between 25 to 30 watts per square inch. In order to alter the heat spread over the given area the coils are then stretched open into given areas thus reducing the available length of element carrying tube and element wire within.

With elements of evenly spaced rods, the passage experiences a very high peak temperature near its middle, and where the end regions are kept hot enough to keep the plastics melted, the temperature in the middle may be high enough to scorch the plastics.

These methods also do not enable close tolerance control of the heat output at any point along the unit and are prone to production of hot and cold spots, the control method is difficult as the thermocouples must either run through the tube alongside the element where they cannot be positioned accurately, or must be positioned outside with sacrifice of tools strength in such confined spaces.

An object of the invention is to overcome some of these problems.

The invention provides a heating device in a mould inlet runner comprising at least one spiral element wound substantially completely around the inlet runner, the element having its pitch varied to provide a region or regions where the element is more tightly wound than elsewhere, and a further region or regions where the element is less tightly wound than elsewhere.

This construction allows a much greater amount of wire to be placed in the end regions and thus lowers the working surface temperature towards the middle of each coil whilst retaining the same ohmage and therefore the same (and in most case enable greater) wattage densities to be obtained and work safely.

The element pitch required is determined by firstly determining the heat curve requirement of the tool or device and then calculating the required wattage per zone of area, this is then interpreted by ratio of coils/inch and the ratio then achieved in practice on a winding machine with a fixed rotation speed and a variable feed ratio either achieved mechanically by a cam turning at a fixed ratio with the machine varying a resistor or clutch system or alternatively by processor sensing.

For example, the machine may be programmed to increase the feed ratio the required amount for a number of turns then reducing the feed in the same method, e.g. a coil with a ratio starting at 2:1 can be increased slowly or rapidly to any 12:1-32:1 and then returned to 2:1 for the next run. The finished element has numerous runs for passing one through each tube which must be an even number in order to return the element to the same point of entry when fitting.

The element is then threaded through Quartz tubes or some type of ceramic either as individual tubes or a series of holes running through the periphery of a block.

The whole assembly is terminated with high temperature cables and slid into an inner case tube, an outer case is slid over, and one end insulated and welded or brazed sealed. The thermo-couple is located at the required position outside the insulator and against the case and the whole unit is then vibration filled with a refractory of magnesia or similar high temperature insulator, the termination tube is fitted over the cables attached in place and the other end of the casing sealed. The termination tube has a fire bore hole running through it to enable vacuum exhausting of any remaining air and the tube is then sealed.

A preferred embodiment of the invention will now be described by way of example with reference to the accompanying drawings.

Wherein:~ Fig. 1 is a sectional view of a heating element according to the invention for a mould inlet passage.

Fig. 3 is a sectional view on line A-A of Fig. 1 showing the electrical connections of the heating element; Fig. 3a is a diagrammatic heat curve of a heating element for a mould inlet passage with a heating element with a constant pitch coil; and Fig. 3b is a similar curve of the heating effect of the heating element according to the invention.

In Figs. 1 and 2, a heater for a mould inlet passage comprises an inner tube 1, and an outer tube 2 of greater diameter. The space between outer tube 1 and 2 is occupied by a plurality of quartz glass tubes 3, and an electric wire heating element 4 is passed along each tube 3 in series extending between a first electrical supply lead 5, and a second electrical supply lead 6 in a zig-zag all around the sleeve, passing through each tube in turn.

At each tube end, the element passes through an end insulator 7 of mica or other refractory insulating material, and has a first end region 8 of relatively tightly wound coils, e.g. turn ratios 2 :1 where the spaces between coils are twice the diameter of the element, a central zone 9 where the coils are very loosely wound with a turn ratio in the order of 20:1 (say from 15:1 to 25:1) and a transition zone 10 at each end where the winding changes continuously close e.g. 1:7-3:2 to the widely spaced winding. A thermo-couple 1 1 having leads 12 is located in the jacket outside the tubes 4 and senses the temperature of the sleeve.

The effect of this variation of the pitch of the winding of the helical heating element 3 is illustrated in Figs. 3a and 3b.

In Fig. 3a is shown schematically the typical temperature curve of a heating element of constant pitch, while in Fig. 3b similarly shown the curve of a heating element as described above.

It will be noted immediately that the curve of Fig. 3b is flatter than that of Fig. 3a. Any heating element has a tendency to reach a peak temperature near its middle, and this is evident in Fig. 3a. For the temperature at the ends of the inlet channel to be above the melting point of the plastics material being injected, the average temperature may have to be so high that the peak temperature in the middle may cause heat damage to some plastics - such as scorching or even choring. With the heating element of the invention the filament 3 still experiences the same central temperature peak, but because the coils of the element near the middle of the element are more widely spaced that at the ends, the concentration of its heat is lessened and the temperature effect becomes more evenly distributed. The average temperature can be kept significantly above the melting point of the plastics material, enabling it to be free flowing in all parts of the runner, but the peak temperature does not risk becoming so high that heat damage is risked. Lower average temperatures can in many cases be used with consequent savings of power costs.

Claims

CLAIMS 1. A heating device in a mould inlet runner comprising at least one spiral element wound substantially completely around about the inlet runner, the element having its pitch varied to provide a region or regions where the element is more tightly wound then elsewhere, and a further region or regions where the element is less tightly wound than elsewhere. 2. A device according to claim 1 wherein the first said regions, wherein the element is more tightly wound are located one at each end of the runner and the further region wherein the element is less tightly wound in a region intermediate the length of the tube. 3. A device according to claim 1 or 2 wherein the element is enclosed in a jacket comprising an inner and an outer sleeve. 4. A device according to claim 1, 2 or 3, wherein the element is passed in a zig zag path to and fro along the length of the sleeve in turn supported by each of a plurality of insulating supports which extend longitudinally of the jacket between an outer and an inner sleeve completely around the jacket, the element in each tube being more tightly wound in the end regions of the tube and loosely wound in a central region of the tube. 5. A device according to claim 4 wherein the turn rate of the element in the end regions of the tubes in the range from 1.7 to 3.2, the space between turning being twice the thickness of the element wire. 6. A device according to claim 4 or 5 wherein the turn ratio of the element in the middle parts of the tubes is in the range of from 12:1 to 27 :1. 7. A device according to any one of claims 3 to 6 including end insulation or ceramic material at each end of the jacket. 8. A device according to any preceding claim having a temperature versus length curve substantially in accordance with Fig. 3b of the accompanying drawings. 9. A heating device for a mould inlet runner substantially as hereinbefore described with reference to and as illustrated in Figs. 1 and 2 of the accompanying drawings. New claims or amendments to claims filed on 16th April, 1981. Superseded claims 1 to 9. New or amended claims:~

1. A heating device in or for a mould runner receptor bore inlet comprising at least one spiral element passed in a zig-zag path to and fro in the longitudinal direction of a sleeve in turn supported by each of a plurality of supports of electrically insulating material extending longitudinally of a jacket formed by an outer sleeve wherewithin the firs.-mentioned sleeve is carried, the spiral turns extending side by side around the second, outer, sleeve and having their pitch varied from one zone to the next zone in longitudinal direction of the device and the jacket having an outer dimension corresponding to said bore whose inlet/runner is formed by its inner dimension.

2. A device according to claim 1 wherein each run of the element is supported in an insulator tube.

3. A device according to claim 2 wherein the insulator is formed of quartz glass.

4. A device according to claim 1, 2 or 3 wherein the jacket is evacuated.

5. A device according to any preceding claim wherein the element is tightly wound at end zones of each rund and their spiral turns are pitched on from the next with fewer per unit length in a zone intermediate the device.

6. A device according to any of the preceding claims having at least one thermocouple within the outer sleeve.

7. A device according to any of the preceding claims wherein the jacket is charged with refractory pulverulent material.

8. A device according to claims 2 or 3 and any of claims 4 to 7 wherein the turn rate of the element in the end regions of the tubes in the range of from 1.7 to 3.2, the space between turning being twice the thickness of the element wire.

9. A device according to claim 2 and any of claims 3 to 8 wherein the turn ratio of the element in the middle parts of the tubes is in the range of from 12:1 to27:1.

10. A device according to any preceding claims having an even number of runs whereby terminal element leads are at the same end of the device without a conductor passing separately along the jacket as would otherwise be the case.

11. A device according to any preceding claim including insulation of ceramic material sealed at each end of the jacket.

12. A device according to any preceding claim having a temperature versus length curve substantially in accordance with Fig. 3b of the accompanying drawings.

13. A heating device for a mould inlet runner substantially as hereinbefore described with reference to and as illustrated in Figs. 1 and 2 of the accompanying drawings.