GB2166078A - Apparatus for heating plastics compositions - Google Patents

Abstract

The apparatus is intended for the controlled heating of plastics compositions, particularly molten polymers, during flow between a plasticiser 1 and a nozzle 3. The plastics composition is heated by dissipation due to the fact that there is between the plasticiser 1 and nozzle 3 a device 5 formed with radial slots (8), Fig. 2 (not shown) which produces a pressure drop in the plastics. The slots may instead be axially disposed, Figs. 3, 4 (not shown), or of conical configuration, Fig. 5 (not shown). <IMAGE>

Description

SPECIFICATION Apparatus for heating plastics compositions Field of application of the invention The invention relates to an apparatus for the controlled heating of plastics compositions, particularly batches of molten polymers such as for example batches of molten polyvinyl chloride as they flow during the forming process.

Characteristics of the known technical solutions It is known that during forming processing of polyvinyl chloride (PVC) by injection moulding, the composition is protectively plasticised by a screw plasticiser with a high length:diameter ratio, flow into the tool taking place so that the rate of flow of the composition in the mould cavity isgreater than in the sprue (BRD OS 2651532, Japanese Patent No. 7139276).

It is also known that small injection mouldings of PVC can be produced by using the dissipated energy occurring in the gate/runner system in order to heat the material and so improve the flow properties (BRD OS 2425621, US 3 674 401).

It is furthermore known that controlled thermal and material homogenisation can be achieved by disposing shearing and mixing devices on the worm periphery and/or in the plasticising cylinder or in front of the nozzle.

Various constructions are known (BRP OS 2648348, BRD OS 2412779, US 3785620, DDR PS 147 648).

Arrangements referred to as static blenders are also known which operate without any moving parts and which are located at the end of the plasticising cylinder or in the region of the nozzle or runner. In these apparatuses, the material is thermally and materially homogenised, the plasticised composition,# during the flow process, passing through the static mixing elements and then flowing into the tool (GB 1173353, BRD OS 2708200, DDR PS 0153 197, CH PS 547120, Austrian Synthetics Journal 14, 1983, 9/10, p. 126).

In a manner similar to that of the static blenders, so also fusion filters have a homogenising effect (CB 1515647, US 3767056, BRD OS 2263660).

The method proposed in Japanese Patent No. 71392-76 is only applicable to injection moulding machines with a considerable L/D ratio; generally speaking, however, the L/D ratios in injection moulding machines are up to about L/D 20. Furthermore, due to the relationship of the speeds in the runner system and in the tool, either very large and thus unfavourable runner cross-sections occur or the products are so constructed that the conditions cannot be observed. Furthermore, pressure losses in the tool are high so that property gradients are occasioned.

If any attempt is made to use the dissipative heating method in the gate when producing large components, then the temperature gradients are high over the cross-section which results under these conditions. This type of heating is thus considerable restricted and even with relatively small pressure losses or low speeds in the region of nozzle, runner and gate, decomposition phenomena occur in the plastics composition in the high shear zones; at least, considerable thermal inhomogeneity occurs. The main task of the shearing and mixing equipment is homogenising. A controlled increase in temperature is not intended. Thus it is also not possible to reduce the thermal loading on the material to be plasticised by lowering the plasticising temperature.

In the case of static mixing equipment at the end of the plasticising cylinder, considerable quantities of material will pass through the mixing element in a short time. In order to avoid a high additional pressure loss in the mixing elements, which might for example have a negative influence on the injection output of the injection moulding machine, this means that the mixing apparatus must have large cross-sections which in turn means that the overall length is considerable. This results in long dwell times in the static mixer and heat sensitive materials, such as for example PVC, become damaged. Furthermore, static mixers must be externally heated, resulting in additional expense for heating and regulating equipment.

When using shearing and mixing elements on the screw periphery, the dwell time of specific areas of material in the molten batch is considerable at the high temperatures generally needed for moulding. Thermally sensitive materials are damaged. This can in fact happen to such an extent that the technological process becomes completely unstable. On the other hand, the regularity ofthe thermal and material homogenisation process is low because during the plasticising process in the injection moulding machine, the conditions are changed due to axial movement of the screw.

Object of the invention The object of the invention is to develop an apparatus which makes it possible to process heat sensitive materials, particularly PVC, by moulding, e.g. injection moulding. At the same time, it is intended to achieve a high technological stability of the process with low economical expenditure on the machinery, technological and materials side while attaining high quality in the products to be manufactured and particularly also a high surface quality while avoiding at the same time any substantial and adverse effect on the health of the operating staff.

Statement of the substance of the invention The invention is based on the problem of developing an apparatus which makes it possible thermally to prepare plastics compositions, particularly heat sensitive materials such as PVC, that the plastics composition acquires high flow properties, the thermal action time being as short as possible by virtue of the limited thermal stability. Under these conditions, it is also intended to ensure that the molten batch flows in the tool under such conditions that no surface structures occur along the lines of markings and the like.

According to the invention, the problem is resolved in that by reason of the apparatus, the composition supplied by the plasticiser, for example the injection moulding machine, becomes heated in a zone described as the dissipating system and disposed between the plasticiser and the tool as the material passes through it and becomes thermally equalised, so ensuring that the composition (molten batch) is fed to one or a plurality of dissipators comprising slots with gap-like apertures, for example in the form of broad slot nozzles, or which by virtue of their disposition give rise to the formation of slots which are rotationally symmetrical, radial or axial or any combination thereof, rises in temperature equivalent to the pressure drop resulting, after which the flow of composition passes into the tool, e.g. the injection moulding tool.The dissipation system consists of at least one of the said nozzle systems, whereby with the serial disposition of a plurality of these systems preferably the exit zones of the slots will be offset in relation to the subsequent inlet zones, so achieving a further uniformity of composition. Particularly to compensate for thermal losses, the apparatus is heated or temperature controlled.

In another embodiment, it may for rheological or thermal reasons, be a good idea for the slots to be of conical shape and/or for the slots to be adjustable or variable for dissipation.

The molten batch coming from the plasticiser and which, if the dissipation system is employed, can have its temperature level lowered, is passed through the dissipators according to the invention, which have slots disposed in them, the choice of geometrical conditions of the slots and the viscosity of the molten batch which is to be controlled via the temperature giving rise to a pressure loss which remains in the composition predominantly as dissipated energy, resulting in other words in a controlled rise in temperature, following which the batch can then be fed either directly to the tool or to one or more further dissipators and then to the tool.

The advantages of the solution according to the invention are in particular those which are described hereinafter. Applying the apparatus means that the temperature of the composition is increased under control, the viscosity is lowered and thus the flow properties are improved. Controlled raising of the temperature in a very short time permits of a very low thermal level in the plasticiser so that the risk of thermal damage or complete breakdown of the composition is substantially reduced and trouble in the technological process is avoided. By virtue of the design of the dissipator, a high thermal homogeneity is achieved and in conjunction with the mechanical and technological conditions, favourable adaptation to a further range of applications is provided for. Furthermore, the apparatus is of simple design, operates reliably and expenditure is minimal.

Example of embodiment It is intended to explain the invention in greater detail hereinafter with reference to three embodiments which are shown in the accompanying drawings, in which: Figs. 1 and 2 show the diagrammatic construction of the dissipation system with radial dissipation gaps; Figs. 3 and 4 show the diagrammatic construction of the dissipation system with axial dissipation gaps, and Fig. 5 shows the diagrammatic construction of the dissipation system with rotationally symmetrical dissipation gaps.

Corresponding to Figs. 1 and 2, in the case of the radial embodiment, the flow of molten material from the plasticiser 1 passes through the single or multi-part dissipator 5 which in the embodiment is shown as being a two-part dissipator is sub-divided into a plurality of partial flows which correspond to the number of dissipator gaps 8. By virtue of the selected design of the support 4 and of the dissipator 5, it is first and foremost a favourable flow which is guaranteed, together with the possibility of satisfactory manufacture and easy cleaning of the dissipators 5.

Both parts of the dissipator 5 are centred on each other by a centring pin 6 which is constructed as a multi-slot or multi-tooth profile so making it possible, by an intentional offset of the two parts of the dissipator 5 to influence the temperature gradients of the emerging flow of molten mass. Secure clamping of the two holders 4 and of the two parts of the dissipator 5 is guaranteed by screwingin of the nozzle 3. After leaving the dissipator 5, the strands of molten material are fed together again and passed through the nozzle 3 into the tool, in the case of the embodiment this is the injection moulding tool. The receiving member 2 of the apparatus is heated by an electrical heating sleeve 7.

In accordance with Figs. 3 and 4, in the case of the axial embodiment, the flow of molten mass from the plasticiser 1 is passed through the one or multi-part dissipator 5, which in the embodiment illustrated is a twopart dissipator, and is divided into a plurality of partial flows corresponding to the number of dissipator gaps 8. The dissipator 5 consists of a package of panels 9 comprising longitudinal apertures and held together by an annular holder 4. Both parts of the dissipator 5 are braced in respect of each other by the spacer ring 10 and clamped in the receiving member 2 of the apparatus by the nozzle 3.

The selected design of the holder 4 in conjunction with the dissipator 5 in the form of a package permits of uncomplicated manufacture, simple cleaning and what from the flow aspect is a favourable behaviour. Both parts of the dissipator 5 can be twisted in respect of each other so that the temperature gradient in the emerging molten batch can be influenced.

After it leaves the dissipator 5, the mass of strands of molten material are fused together again and passed through the nozzle 3 into the injection moulding tool. The receiving member 2 of the apparatus is heated by the heating sleeve 7.

As shown in Fig. 5, in the case of the rotationally symmetrical embodiment of dissipator gap 8, the flow of molten mixture from the plasticiser 1 is passed through the one or multi-part dissipator 5 which in the example illustrated is a one-part dissipator, and broken up. The dissipator 5 is held and centred in the receiving body 2 by the holders 4 so that the annular dissipation gap 8 is created between the dissipator 5 and the receiving body 2. By appropriately choosing the diameter of the dissipator 5, it is possible to control the variation in dissipator gap 8 and adapt it to the processing problem. By flow-wise favourable constructing the dissipator 5 and the holders 4, it can be guaranteed that no "dead corners", shadows and sharp edges result.

Furthermore, manufacture and cleaning of the system are readily possible. After flowing through the dissipator gap 8, the molten mass is fused together again and passed through the nozzle 3 into the injection moulding tool.

The apparatus is equipped with an electrical heater 7. The dissipation system can also be so devised that the conically convergent and divergent zones are jointly or exclusively activated. The dissipator 5 can thereby also be axially displaceably and externally adjustably disposed, for particularly simple adaptation to whatever are the current demands and conditions. In order to avoid unintentional outflow of plastics composition from the nozzle, the apparatus can also be constructed as an occluding nozzle.

The aforementioned examples of embodiment all have the following features and advantages in common: the apparatus according to the invention is used in injection moulding machines instead of the ordinary nozzle the apparatus consists of a receiving body provided with a heater, a screw-in nozzle and corresponding fittings (dissipator) the apparatus is easy to fit, dismantle and clean the apparatus can be easily adapted to working circumstances by changing the dissipator or by adjusting it without additional adaptation works, the apparatus can be easily installed between the plasticiser of the injection moulding machine and the tool the apparatus is disposed immediately in front of the runner system of the injection moulding tool so that the dissipatively heated mass flows directly into it, and the apparatus has a favourable flow design in which "dead corners", sharp edges or flow shadows are avoided so that the processing of thermally and rheologically sensitive mixtures is possible.

Claims (6)

1. Apparatus for heating plastics compositions, utilising dissipation in flow, applied to forming, e.g. during injection moulding, characterised in that the apparatus is disposed between the plasticising equipment (1) and nozzle (3) which in the case of injection moulding is the injection moulding nozzle, the apparatus consisting of one or a plurality of dissipators (5) which comprise slots with gap-like apertures (8) or the presence of which gives rise to slots which are rotationally symmetrical, radial, axial or combinations thereof.

2. Apparatus according to para. 1, characterised in that the slots with gap like apertures (8) in the dissipator (5) are adjustable or variable.

3. Apparatus according to para. 1, characterised in that the slot (8) which forms when the dissipator (5) is fitted is adjustable or variable.

4. Apparatus according to paras. 1 to 3, characterised in that the slots (8) extend conically.

5. Apparatus according to paras. 1 to 4, characterised in that with serial connection of the dissipators (5) the slots (8) are offset in relation to one another and are adjustable or variable.

6. Apparatus for heating plastics compositions as claimed in Claim 1, substantially as described herein with reference to and as illustrated by any one of the examples shown in the accompanying drawings.