GB2042966A

GB2042966A - Method and Apparatus for Processing Synthetic Raw Material

Info

Publication number: GB2042966A
Application number: GB8001132A
Authority: GB
Original assignee: Individual
Current assignee: Individual
Priority date: 1979-01-13
Filing date: 1980-01-14
Publication date: 1980-10-01
Also published as: NL8000119A; BR8000191A; IT8019161A0; IT1130855B; AU5451180A; FR2446159A1

Abstract

Granular plastics material 4 is heat plasticised by shear in a melt chamber 1 under the action of a rotor 10 having radially projecting legs 11, and is delivered under pressure towards an injection moulding or extruding die 8, the major portion of the material 4 being melted by heat generated by friction and shear forces arising from the material eddying in the melt chamber 1, and the pressure being generated by a feed means 3 upstream of the melt chamber 1, which may comprise a pair of screws 3, or a single screw which may carry the rotor 10 at its downstream end. The rotor 10 may reciprocate radially and/or axially e.g. to effect injection and an adjustable or replaceable throttle may be provided between the chamber 1 and the die. The plasticised material arrives at the die 8 under a pressure slightly less than that generated by the feed means 3, and the heat generated by the rotor 10 is substantially equal to that required for melting the material 4. <IMAGE>

Description

SPECIFICATION Method and Apparatus for Processing Synthetic Raw Material This invention relates to a method of, and apparatus for, processing synthetic raw material.

It is known to process synthetic raw material by the use of thermal vortices induced in a stream of the raw material. In this case, grains of the pulverulent or granular raw material are melted by heat generated by the friction and shear forces caused by a rotating member. The Table below shows the high specific performance achievable as compared with earlier methods in which fusion heat was applied to the raw material as it flowed, in a relatively thick layer, along the threads of a feed screw or through the gap between rollers or friction discs, said heat passing through laminated plates heated from the exterior. Alternatively, or in addition, said heat was generated by frictional resistance to flow.

The decisive factor affecting output is the minimum period of dwell of the grains of raw material in the inlet zone, that is to say the period within which the grains of raw material can be penetrated by the heat and brought to melting temperature. The duration of this period depends upon the required amount of heat (which in turn is determined by the volume of the individual grains to be melted), and upon the length of the heat-conduction path from the heated surface of the grain of raw material to its core. The period of dwell for any given grain is equal to the square of the diameter of the grain.

Table Melting of Granular Raw Material Using Heat Generated by Friction and Shear Forces During Eddying Melting capacity per litre Grain thickness Melting Time of melting space (mllllmetres) (seconds) (litres of raw material/hour) PVC PE PVC PE 1 6 3.5 600 1029 0.5 1.5 0.86 2400 4230 0.3 0.54 0.23 6660 15600 0.2 0.24 0.14 15000 25700 0.1 0.06 0.035 60000 102000 where PVC is polyvinylchloride and PE is polyethylene.

The production of heat directly at each grain permits the use of a melt chamber having practically any required cross-section and thus permits heating of correspondingly large throughput capacities while still retaining an acceptable flow velocity. The length of the melt chamber can be kept very short (4-5 times its diameter). Since the individual grains are of necessity uniformly heated up, subsequent homogenisation of the entire plasticised mass is not necessary.

The disadvantage of this method is that it is difficult to extrude or shape the resultant high-grade plasticised mass without reducing the quality of the product. This is particularly the case where a shaping or extrusion tool offers high resistance to flow, since the method is based on the assumption that rapid heating of the grains of raw material could be achieved and controlled only if the force necessary for producing the friction and shear forces during eddying, and for creating the pressure head in the melt chamber, is of such magnitude (for example 5 bars) that it results only in movement through the melt chamber and in the uniting of fused grains of raw material to form a deformable or extrudable mass. Therefore, for the purpose of extruding the plasticised mass, compression means (for example a discharge compression screw) has to be provided downstream of the melt chamber.This compression means raises the plasticising pressure for example from 5 bars to 50-600 bars. Such discharge compression screws require a large number of threads, and, therefore, have to be of considerable length, which is disadvantageous. This is so even if they are designed only to provide a favourable transmission effect particularly for highly fluid masses. However, a much greater disadvantage that occurs during this compression is the harmful rise in temperature of the mass.

The invention is based on the realisation that the above-mentioned limitation of the pressure head in the melt chamber is not necessary. Tests have shown that even at a pressure of 50 to 600 bars, as a result of suiting the amount of raw material fed into the apparatus to the resistance offered by its exit die, the rapid heating up of the eddying grains of raw material can be controlled without difficulty if the circumferential speed of the eddy-producing member is limited just sufficiently to ensure that the heat generated by friction and shear forces is of a magnitude necessary for melting that throughput quantity that is to flow through the shaping die. The shaping or extrusion tool (die) can, therefore, be arranged directly at the outlet of the melt chamber.

Accordingly, the present invention provides a method of processing pulverulant or granular synthetic raw material, the method comprising the steps of plasticising the raw material in a melt chamber to form a plasticised compact mass under the action of heat and pressure, and passing the plasicised compact mass to a downstream shaping or extruding tool, the major portion of the raw material being melted by heat generated by friction and shear forces arising from the raw material eddying in the melt chamber, and the pressure being generated by feed means upstream of the melt chamber, wherein the pressure generated by the feed means is so high that the thoroughly plasticised particles of raw materials arrive at the shaping or extruding tool immediately after they have united to form said plasticised compact mass, wherein said plasticised compact mass arrives at said tool under a pressure slightly less than that generated by the feed means, and wherein the heat generated by the friction and shear forces is substantially equal to the heat required for melting the raw material.

In this method, the high pressure required for plasticising or extrusion is built up in the feed means (for example a single or multiple feed screw) disposed upstream of the melt chamber, that is to say in the cold raw material. The small amount of frictional heat generated, by the feed means, during the movement and compression of the still particulate raw material, has an advantageous effect since the raw material is dried and pre-heated. Since this pressure has already been built up in the cold porous stream of raw material and since the plasticised compact mass forms, starting at the tool (die), in the direction opposite to that of the flow of the raw material, air and steam can be displaced counter to the flow of the raw material and towards the feed means. This air and steam can, therefore, be easily extracted.The method permits exploitation of the above-tabulated values for the specific melting capacity up to the limits of the capacity of the drives of the eddy-forming member and the feed means.

In the known methods, the theoretical excess capacity results in the plasticising pressure being kept as low as possible. Tests involving the method of the invention have shown, however, that no theoretical excess capacity occurs, nor is it present when high plasticising pressures are used. Thus, no difficulties arise if the peripheral speed of the eddy-forming member, and the rate of flow of the ! stream of the raw material through the melt chamber, are such that only the permissible melting temperature occurs and is maintained. The occurrence of excess capacity is avoided by an automatic extension of the period of dwell. This extension of the period of dwell can be readily achieved at a temperature that is still permissible.If, for example, a raw material consisting of polyvinylchloride having a grain size of 0.1 millimetre thickness is thoroughly plasticised within 0.06 seconds at a melting temperature of 1 800 C, the plastic mass can be masticised without damage on a test rolling mill at 1 800C for a further 20 minutes. In the case of polyvinylchloride material, which calls for critical treatment during its working, the period of dwell is then 1 :20,000 for example.

The method of the invention can be carried out with the apparatus mentioned above, but without the compression means which is essential to the operation of that apparatus.

Preferably, however, the invention also provides apparatus for processing pulverulant or granular synthetic raw material, the apparatus comprising a melt chamber for plasticising the raw material to form a plasticised compact mass under the action of heat and pressure, a tool downstream of the melt chamber for shaping or extruding the plasticised compact mass, and feed means upstream of the melt chamber for feeding pressurized raw material to the melt chamber, the melt chamber housing a rotable eddy-producing member, the raw material being melted by heat generated by friction and shear forces arising from the raw material eddying in the melt chamber, wherein the apparatus is such that the pressure generated by the feed means is so high that the thoroughly plasticised particles of raw material arrive at the tool immediately after they have united to form said plasticised compact mass, said plasticised compact mass arrives at the tool under a pressure slightly less than that generated by the feed means, and the heat generated by the friction and shear forces is substantially equal to the heat required for melting the raw material.

Advantageously, the eddy-producing member is constituted by a rotatable shaft provided with a plurality of radially-extending lugs. With a fixed peripheral rotational speed, the lugs do not cause movement of the raw material in the direction of flow through the melt chamber. The cross-section of the melt chamber is such that the raw material carried through it does not undergo any appreciable change in state due to the heat generated by friction between the raw material and the inner wall of the melt chamber, even with maximum throughflow, without the aid of the friction and shear forces produced by said member. To achieve the eddying effect, the surface area of the inner wall of the melt chamber that is swept by the eddy-producing lugs is somewhat greater than the surface area of the eddy-producing member itself.For a given pressure of the raw material, the total pressure applied to the (larger) surface of the inner wall is always greater than the total pressure applied to the surface of the eddy-producing member. Presuming the coefficienc. of friction is the same between the raw material and these surfaces, the greater pressure on the inner wall acts as a braking friction-force component, which inhibits the tendency of the eddy-producing member to move the raw material in a peripheral direction. The retarded rotation of the raw material along the inner wall then results in the eddying effect.

The feed means may be constituted by a pair of feed screws or by a single feed screw. The feed means may be arranged vertically, in which case the feed screw has a particularly large intake capacity, since the introduced raw material fills up the first intake thread substantially completely, and the initial pressure caused by the weight of the raw material builds up rapidly, in the following threads.

Where the feed means is a single feed screw, this feed screw may be horizontal, in which case an auxiliary charging screw can be provided for feeding the raw material to the intake thread of the feed screw. The feed screw and the eddy-producing member may have independent drive motors.

Preferably, however they are both fixed to a common shaft which is provided with a drive motor. If the conveying capacity of the auxiliary charging screw is so controlled that it slightly increases the pressure of the dead-weight of the introduced raw material (say to between 1 and 2 bars), this increase in pressure acts to increase the throughput capacity of the feed screw. The provision of such an auxiliary charging screw, which has a variable feed and requires only a low-powered drive, enables the throughput capacity of the feed screw to be controlled even when the speed of rotation of the feed screw is fixed. Thus, the adaptation of the apparatus to suit the speed of rotation of the eddy-producing member is simplified.

If the apparatus is used in conjunction with a continuous injection-moulding machine, the eddyproducing member may be mounted so as to be axially displaceable.

The provision of the feed screw and the eddy-producing member on a common shaft results in the apparatus being adaptable for different operating conditions, that is to say for different throughput capacities. This is particularly important where a temperature rise occurs during movement of the raw material along the feed screw, as it is important to avoid undesirable premature using of the material.

The amount of heat generated during this movement is kept low, despite a high temperature rise, because a suitably short period of dwell of the raw material during its movement can be ensured. The high melting capacity in the melt chamber itself permits this short period of dwell.

Another advantage of providing the feed screw and the eddy-producing member on a common shaft, is that the period of dwell in the region of the feed screw is so much greater than the period of dwell in the melt chamber (which is determined by the grain size), that sufficient time is provided for carrying out technical measures for obtaining different throughput capacities. This will be clear from the above-mentioned Table. The period of dwell for fusing polyvinylchloride raw material of grain thickness 0.1 millimetre in the melt chamber is 0.06 seconds, and for polyvinylchloride of grain thickness of 2 millimetres this period is 24 seconds. In the case of polyethylene the period of dwell is 0.035 seconds for a grain thickness of 0.1 millimetres and is 17 seconds for a grain thickness of 2 millimetres.The corresponding periods of dwell for the grains of raw material in the region of the feed screw are considerably greater, and the difference in time thus obtained enables the procedures to be influenced by shortening the time during which the material is moved.

As mentioned above, even if the temperature rise in the region of the feed screw takes the temperature of the raw material to above its melting temperature, this cannot greatly affect the raw material since the period during which it is being moved is too short for the amount of heat, which would be necessary for premature melting, to be created and to take effect. It is thus, possible, while tolerating high temperatures, to operate the feed screw at much higher speeds than has hitherto appeared possible. In this connection, a dwell period which is approximately 50% less than the period required for fusion has been found suitable.Since, in the case of a fixed throughput capacity per hour, the periods of dwell are dependent upon the volume of the screw threads, and for a fixed depth of thread, this volume is determined by the diameter of the thread and the length of the threaded portion, the magnitude of the permissible period of dwell can be calculated as a function of the diameter of the feed screw. If the volume of the melt chamber is sufficiently large, correspondingly long periods of dwell are possible during movement of the raw material, which periods in turn make it possible to operate the apparatus with variable throughput capacities and speeds.

The amount of heat to be supplied to the stream of raw material grains by means of the eddying action must be large enough to raise the raw material to the desired final temperature, as well as for melting the grains.

The provision of an oversize melt chamber which results in an adequate period of dwell is not accompanied by any disadvantages. Synthetic compounds that have already been completely plasticised can withstand very excessive periods of dwell during the eddying process when the desired working temperature is maintained. Thus, the period of dwell of 0.06 seconds required for fully melting grains of polyvinylchloride having a thickness of 0.1 millimetres at a temperature of 1 800C, can be extended to at least 120 seconds without damaging the product when further eddying of the plasticised mass takes place. For a fixed throughput capacity, the period of dwell in the melt chamber is dependent upon the volume of this chamber. For a fixed ratio of chamber-diameter to chamber-length, this volume can be maintained by selecting an appropriate length of chamber.Different throughput capacities can also be achieved, if the pressure that builds up on the feed screw is controlled as a determining factor for the creation of heat, by using a suitable counter-pressure at the zone where the composition emerges from the melt chamber. If the apparatus is designed for injection-moulding, this counter-pressure can be achieved by pressure control during the retraction of the common shaft which acts as a piston.

The arrangement of the feed screw and the eddy-inducing member on a common shaft permits, in conjunction with the possibility of favourably matching the lengths of these two members, the use of a shaft whose total length corresponds to that of a known simple plasticising screw (with or without a homogenising mixing head) in an existing screw housing, so that this known type of shaft can be replaced by the arrangement in accordance with the invention.

The invention will now be described in greater detail, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a part-sectional side elevation of one form of processing apparatus constructed in accordance with the invention: Figures 1 a and 1 b are graphs showing changes in pressure in parts of the Figure 1 apparatus; Figure 2 is a part-sectional side elevation of a second form of processing apparatus constructed in accordance with the invention; Figures 2a and 2b are graphs showing changes in pressure in parts of the apparatus of Figure 2; Figure 3 is a view similar to that of Figure 2 and shows a modification of the second form of apparatus; Figure 4 is a part-sectional side elevation of a third form of processing apparatus constructed in accordance with the invention;; Figures 4a, 4b and 4c are graphs showing changes in pressure in parts of the apparatus of Figure 4: Figure 5 is a part-sectional side elevation of a fourth form of processing apparatus constructed in accordance with the invention; Figures 5a and 5b are graphs showing changes in pressure in parts of the apparatus of Figure 5; and Figure 6 is a part-sectional side elevation of a fifth form of processing apparatus together with pressure and area graphs for this apparatus.

Referring to the drawings, Figure 1 shows processing apparatus having a melt chamber into which raw material 4 is fed from a charging funnel 2 by way of a pair of feed screws 3. The deadweight pressure of the raw material 4, designated by the reference numeral 6 is raised over the course of the curve 5 (see Figure 1 a) to a plasticising pressure 7 by the displacement action of the feed screws 3 which rotate in opposite directions. The raw material 4, which is slightly warmed up by the compression work of the screws 3 (but whose structure is as yet unchanged), is forced into the melt chamber 1 under the pressure 7 (see Figure 1 b), and is forced through this chamber towards a die 8.

As this happens, the pressure 7 is reduced to the renament but still adequate extrusion pressure 9 (see Figure 1 b) by the resistance to flow in the melt chamber 1. An eddy-producing tool 10 having radiallyextending lugs 11 rotates in the melt chamber 1. The area of the inner surfaces of the housing wall 1 2 of the melt chamber 1 is somewhat larger than the sum of the areas of the surfaces of the eddyproducing tool 10 including the lugs 11. The eddy-producing tool 10 is driven by a motor 13 to rotate at a peripheral speed which is such, at the pressures 7 and 9, to generate sufficient heat to fuse the amount of raw material 4 flowing through the melt chamber 1. This heat is generated by the friction and shear forces caused by the rotation of the tool 10.In Figure 1 b, the reference number 14 indicates cold raw material in the entry zone of the melt chamber 1, the reference number 15 indicates fused raw material, and reference number 16 indicates a thoroughly heated stream of material which, forms, a compact flowable mass. This compact flowable mass is forced, under the extrusion pressure 9, through the die 8 acting as an outlet or shaping tool. If the apparatus is to be used for injection-moulding, the eddy-producing tool 10 may execute additional radial and/or axial movements, indicated by the arrows 18a and 18b, the tool 10 thus acting as a piston.

The processing apparatus of Figure 2 has a single, vertical feed screw 3 in place of the two inclined screws of the Figure 1 apparatus. Apart from this, the two forms of apparatus are identical. The length of the screw 3, and the pitch and depth of its thread, are dimensioned for simply moving the raw material 4 and building up pressure. The vertical arrangement of the screw 3 results in the first intake thread being very well filled, and it thus also ensures the build-up of a high pressure, despite its short length. The pressure varies as shown by the curve 5 in Figures 2a and 2b.

Figure 3 arrangement shows the apparatus of Figure 2 used for injection-moulding. Thus, its eddy-producing tool 10 is movable radially and axially as indicated by the arrows 1 8a and 18b.

Figure 4 shows a processing apparatus similar to that of Figures 2 and 3, but having a single, horizontal feed screw 3. Here, for the purpose of filling the intake thread 1 9 of the feed screw 3, there is provided an upstream auxiliary charging screw 20, which is vertically disposed, and which increases the dead-weight pressure 6 of the raw material 4 (see Figure 4a) to the charging pressure 21. This results in the intake screw-thread 19 being well filled, irrespective of the angle at which the raw material 4 is poured in. Moreover, the pressure build-up of the feed screw 3 can easily be controlled. As illustrated, the axis of the eddy-producing tool 10 in the melt chamber 1 is vertical. Alternatively, it may be horizontal and at 900 to the axis of the auxiliary charging screw 20.The cherffesin pressure in the various parts of the apparatus are shown by the curves 5 in Figures 4a, 4b and 4c.

Figure 5 shows a processing apparatus similar to that of Figure 4, but having the feed screw 3 and the eddy-producing tool 10 arranged on a common shaft driven by a motor 13. The materialmoving capacity of the feed screw 3 is so determined, by means of the depth and pitch dimensions of the screw thread, that, despite the fact that the feed screw and the tool rotate at the same speed, the tool produces the heat necessary for melting the amount of raw material 4 advanced by the feed screw. The changes in pressure within the apparatus are indicated by the curves 5 in Figures 5a and 5b.

With the aid of a vertical, auxiliary charging screw 20, which is arranged in the charging port, it is possible to regulate the charging pressure 21. This regulation of the charging pressure 21, together with the adjustability of the feed screw 3, results in adjustability of the throughput capacity to suit the generation of heat produced by the eddy-producing tool 10.

The dimensions of the feed screw 3 are such that, even at the speed of rotation of the eddyproducing tool 10 that is necessary for the heat-producing eddying motion, the structure of the compressed raw material 4 which is subjected to the plasticising and extrusion pressures 7 and 9, is not appreciably altered, and the grains of raw material are heated up to melting temperature only in the melt chamber 1 by friction and shear forces.

Figure 6 shows a processing apparatus having a housing 11, a single feed screw 33, and an eddy-producing tool 44. The feed screw 33 and the eddy-producing tool 44 are arranged on the same shaft 22. The inside diameter of the housing 11 is D, the length of the feed screw 33 is LS and the length of the eddy-producing tool 44 is LW.

The raw material enters the apparatus at 55, and is moved by the feed screw 33 to the eddyproducing tool 44. As this happens, the raw material is brought to a pressure which suffices to force the raw material, plasticised by means of the tool 44, through an exit die 66 which shapes the plasticised material. As was the case with the earlier forms of apparatus, the shaft 22 may be axially displaceable for carrying out injection-moulding. In this case, the die 66 is replaced by transfer ducts leading to an injection-moulding chamber.The length LS of the feed screw 33 and the length LW of the eddy-producing tool 44 are such that, for a selected minimum throughput capacity (that is to say at a minimum speed of rotation), a complete and thorough plasticisation is achieved by means of the eddy-producing tool 44; and, with a maximum throughput (that is to say a maximum speed of rotation), the period of dwell of the raw material on the feed screw 33, which extends over the path of travel from the charging port 55 to the eddy-producing tool 44, is shorter than would be necessary for causing premature fusing of the raw material before entry into the eddy-producing tool 44.

The adjustment of the most useful periods of dwell of the raw material, during movement of the material along the feed screw 33, to suit the necessary relative large generation of heat during its eddying motion in the region of the tool 44, can be facilitated or rendered possible in the following way (in the case where the subsequent shaping does not require the maximum pressure that can be obtained in the region of the feed screw). For different throughput capacities, a counter-pressure is set up in the emerging stream of material by means of adjustable or replaceable throttle devices (not shown) arranged in the zone of the die 66, and this counter-pressure determines the level of the pressure occurring along the feed screw 33 and in the region of the eddy-producing tool 44.If the apparatus is used for injection-moulding, and the shaft 22 (which together with the feed screw 33 and the eddy-producing tool 44, acts as a piston), is designed to be moved backwards and forwards in the axial direction, then the axial movement during charging of the injection-moulding chamber can be retarded to such an extent that the counter-pressure occurring in the injection-moulding chamber determines the level of the pressure occurring in the feed screw 33 and in the region of the eddyproducing tool 44.

The curve 77 (see Figure 6) shows the rise in temperature of the raw material along the feed screw 33, from room temperature tR to the temperature tT. The curve 88 shows the further rise in temperature as the raw material moves along the eddy-producing tool 44 to the final temperature tF of the product. The area 99 indicates the amount of heat QT/h generated during movement along the feed screw 33, and corresponds to the temperature rise from tR to tT. Similarly the area 110 indicates the amount of heat QW/h which corresponds to a further rise in temperature to tE, and the area 111 indicates the required amount of heat QS/h for melting the raw material. The area 112 indicates the period of dwell ZET of the raw material in the region of the feed screw 33 that would be necessary to fuse the raw material prematurely during its movement.The area 113 indicates the selected permissible period of dwell ZAT that occurs, and this can be approximately 50% of ZET. The area 1 14 indicates the period of dwell ZAW occurring in the eddy-producing tool 44, which period must be greater than the time required for fusing grains of the raw material by means of the eddy-generated heat.

In a particular example, the apparatus of Figure 6 has a nominal diameter D of 100 millimetres, the length LS of the feed screw 33 is 1 5xD=1 500 millimetres, and the length LW of the eddyproducing tool 44 is 4.5xD=450 millimetres. The smallest depth of thread in the feed screw 33 is 5 millimetres, and the entire thread volume corresponds to a weight of raw material of 3.5 kilograms.

The melt chamber has a free volume which corresponds to a weight of raw material of 2 kilograms.

Let it be supposed that the raw material is polypropylene a) in powder form and having a grain thickness of 0.1 millimetres, and b) in granular form having a grain thickness of 2.2 millimetres, and that the raw material is to be fused and extruded with throughput capacity of 200 kilograms/hour in the apparatus.

The thermal data of the raw material (mean values, WE=Kcal) are 1) c=specific heat=0.4 WE/kg C 2) S=melting or conversion heat=40 WE/kg 3) tS=physical melting temperature=1 300C 4) tT=highest permitted temperature during movement along the feed screw=1 400C 5) tE=temperature of product at the die=2200C 6) tD=difference in temperature for the heat gradient 0C 7) tR=discharge temperature=20 C I Data Relating to the Feed Screw 33 (1) The amount of heat QT/h for a tD of 200C to 1 400C and a throughput capacity of 200 kilograms/hour, is: QT/h=200xc;;(1 40-20)=200x0.4x 120=9600 WE/h.

This is represented by the area 99.

(2) The impermissible period of dwell ZET which leads to undesirable (harmful) fusion of the raw material, rolling forward in the thread of the screw, for a minimum thickness of layer of 5 millimetres is 120 seconds for a) and b). This is represented by the area 112.

(3) The permissible period of dwell ZAT that should be maintained in order to prevent premature fusing of the raw material, when the length LS=1 500 millimetres, the melt chamber volume contains 3.5 kilograms of raw material, and the throughput capacity is 200 kilograms/hour, is 3.5x3x600/200=63 seconds. This is represented by the area 113. Thus, the ZAT/ZET ratio is approximately 50%.

II Data Relating to the Eddy-producing Tool 44 (1) The amount of heat QW/h for a tD of 1 400C to 2200C and a throughput capacity of 200 kilograms/hour, is 200x cx (220-1 40)=200x0.4x 80=6400 WE/h.

This is represented by the area 11 0.

(2) The amount of heat QS/h for melting a throughput capacity of 200 kilograms/hour, is 200 x 5=200 x40=8000 WE/h.

This is represented by the area 111.

(3) The period of dwell ZAW occurring in the melt chamber for a throughput capacity of 200 kilograms/hour and a chamber volume of 2 kilograms, is 2x3600/200=36 seconds. This is represented by the area 114.

(4) The necessary period of dwell for thoroughly melting individual grains (as shown in the table provided at the beginning of this specification), is: (a) 0.05 seconds for powder having a grain thickness of 0.1 millimetres (b) 24 seconds for a granulate having a grain thickness of 2.2 millimetres By causing the raw material to eddy and by maintaining the product temperature of say 2200 C, it is possible to subject the plasticised polypropylene compounds to an equalising period of dwell which may extend over 300 seconds or more, without causing damage to the product.

The areas 99, 110, 111 and 112 representing amounts of heat and shown in graph form in the drawings, as well as the areas 11 3 and 114 representing periods of dwell are illustrated to scale in relation to each other.

Claims

1. A method of processing pulverulant or granular synthetic raw material, the method comprising the steps of plasticising the raw material in a melt chamber to form a plasticised compact mass under the action of heat and pressure, and passing the plasticised compact mass to a downstream shaping or extruding tool, the major portion of the raw material being melted by heat generated by friction and shear forces arising from the raw material eddying in the melt chamber, and the pressure being generated by feed means upstream of the melt chamber, wherein the pressure generated by the feed means is to high that the thoroughly plasticised particles of raw material arrive at the shaping or extruding tool immediately after they have united to form said plasticised compact mass, wherein said plasticised compact mass arrives at said tool under a pressure slightly less than that generated by the feed means, and wherein the heat generated by the friction and shear forces is substantially equal to the heat required for melting the raw material.

2. A method as claimed in Claim 1, wherein the pressure generated by the feed means is controlled by the counter-pressure of the raw material acting on throttle means positioned adjacent to said tool.

3. A method as claimed in Claim 1 or Claim 2, wherein additional materials are contained in, or subsequently added to, a premix of the pulverulant or granulated raw material, these additional materials being finely distributed in the melt chamber during the eddying movement.

4. A method as claimed in any one of Claims 1 to 3, wherein the temperature rise of the raw material during its passage through the feed means is sufficient to raise the temperature of the raw material to above its melting temperature, but the period of dwell occurring during said passage is so short that the particles of raw material are not substantially melted before reaching the melt chamber.

5. A method of processing synthetic raw material substantially as hereinbefore described with reference to the accompanying drawings.

6. Apparatus for processing pulverulent or granular synthetic raw material, the apparatus comprising a melt chamber for plasticising the raw material to form a plasticised compact mass under the action of heat and pressure, a tool downstream of the melt chamber for shaping or extruding the plasticised compact mass, and feed means upstream of the melt chamber for feeding pressurised raw material to the melt chamber, the melt chamber housing a rotable eddy-producing member, the raw material being melted by heat generated by friction and shear forces arising from the raw material eddying in the melt chamber, wherein the apparatus is such that the pressure generated by the feed means is so high that the thoroughly plasticised particles of raw material arrive at the tool immediately after they have united to form said plasticised compact mass, said plasticised compact mass arrives at the tool under a pressure slightly less than that generated by the feed means, and the heat generated by the friction and shear forces is substantially equal to the heat required for melting the raw material.

7. Apparatus as claimed in Claim 6, further comprising throttle means positioned in the melt chamber adjacent to the tool, the throttle means controlling the pressure generated by the feed means in dependence upon the counter-pressure of the raw material acting thereon.

8. Apparatus as claimed in Claim 6 or Claim 7, wherein the tool is a die arranged at the exit of the melt chamber.

9. Apparatus as claimed in any one of Claims 6 to 8, wherein the feed means is constituted by a pair of feed screws.

10. Apparatus as claimed in any one of Claims 6 to 8, wherein the feed means is constituted by a single feed screw.

11. Apparatus as claimed in Claim 10, wherein the feed screw and the eddy-producing member having independent drive motors.

12. Apparatus as claimed in Claim 10, wherein the feed screw and the eddy-producing member are fixed to a common shaft.

1 3. Apparatus as claimed in Claim 12, wherein said common shaft is provided with a drive motor.

14. Apparatus as claimed in any one of claims 10 to 13, wherein the feed screw is horizontal, an auxiliary charging screw being provided for feeding the raw material to the intake thread of the feed screw.

15. Apparatus as claimed in Claim 14, wherein the pressure of raw material in the intake thread of the feed screw arising from the dead-weight of the raw material entering said intake thread is so adjusted to the working pressure in the melt chamber, by means of the auxiliary charging screw, that the conveying capacity of the feed screw can be adjusted to control said working pressure without changing the speed of rotation of the feed screw.

16. Apparatus as claimed in any one of Claims 10 to 1 S, wherein the pitch of the screw-thread of the feed screw is such that the surface area of the screw shank plus the surface area of the thread flanks is at least 3% smaller than the area of the inner wall of the housing that surrounds the feed screw.

17. Apparatus as claimed in Claim 13, wherein the length of the feed screw depends upon the period of dwell of the raw material when maximum throughout takes place.

18. Apparatus as claimed in Claim 17, wherein the length of the feed screw is less than 18 times the diameter of the feed screw.

1 9. Apparatus as claimed in Claim 1 7 or Claim 18, wherein the length of the eddy-producing member is such that the period of dwell of the raw material in the melt chamber is greater than is theoretically required with a constant processing temperature.

20. Apparatus as claimed in Claim 19, wherein the length of the eddy-producing member is greater than 2.5 times its diameter.

21. Apparatus as claimed in any one of Claims 6 to 20, wherein the eddy-producing member is provided with a plurality of radially-extending lugs.

22. Apparatus as claimed in Claim 21, wherein the area of the core surface of the eddy-producing member plus the surface area of the lugs is at least 3% smaller than the inner surface area of the melt chamber housing that is swept by the eddy-producing member.

23. Apparatus as claimed in Claim 22, wherein the area of the core surface of the eddy-producing member plus the surface area of the lugs is at most 10% smaller than the inner surface area of the melt chamber that is swept by the eddy-producing member.

24. Apparatus as claimed in any one of Claims 6 to 23, wherein the eddy-producing member is such that it provides no appreciable conveying action when it rotates.

25. Apparatus as claimed in any one of Claims 6 to 24, wherein the eddy-producing member is mounted so as to be axially displaceable.

26. Apparatus for processing synthetic raw material substantially as hereinbefore described with reference to, and as illustrated by Figure 1, Figure 2, Figure 3, Figure 4, Figure 5 or Figure 6 of the accompanying drawings.