GB2059864A - A plastics extruder with means for injecting a liquid additive into the flow of molten plastics - Google Patents

Abstract

A plastics extruder has means 11 for injecting a liquid additive at the region of the end of the extruder barrel 1. The additive is injected at an injection section 10 situated between two mixing sections and of relatively large cross sectional flow area for the molten plastics, through a plurality of injection nozzles 11 arranged around the circumference of the extruder. The injection nozzles which preferably extend to the center of the melt flow, are operated individually by a pump P.

Description

SPECIFICATION A plastics extruder with means for injecting a liquid additive into the flow of molten plastics Different methods for injecting an additive into the barrel of a plastics extruder are known. The additive may comprise an agent causing foaming, colouration, vulcanization or any other desired change of the base plastics.

Usually the injection takes place close to the infeed end of the plastics extruder, to the so-called feeding zone, to the compression zone, or to a special decompression zone.

In this case the injection is, due to the low pressure, easy to carry out and the screw has time to perform the mixing work. On the other hand, the additive then has to stay longer in the screw, and for this reason the temperature of the molten plastics must be kept low because most additives the sooner start to react in an undesired manner the higher the temperature of the plastics is. In order to keep the plastics temperature as low as possible, especially in the case of polyethylene, the speed of rotation of the screw in the extruder must be low. This again results in a small quantity of extruded plastics per unit of time.

By transferring the injection point for the additive closer to the extrusion head the time the mixture stays in the barrel before extrusion is reduced, and this again permits a higher temperature of the base plastics which is achieved, for example, by increasing the speed of the screw, in other words, the output of the extruder is improved.

The following factors affect the operation in the end of the extruder barrel: 1. The melt pressure is at its highest in the end of the extruder barrel.

2. The mixing ability of a normal screw is poor, especially, if the mixing starts in its end only.

3. The internal temperature differences of the compound are at their biggest in the end of the extruder barrel.

4. The compound flow has only little time to recover from the shock caused by the injection of the additive.

5. The operation of the injection nozzle will be hampered and the nozzle will be clogged if the additive causes plasticization of the base plastics.

6. If a higher output, i.e. a hotter plastics compound is aimed at, the compound parti cles have less time to leave of the extruder.

Each of these factors must be taken into account and, above all, their combined effect.

The method to be described now expressly solves the combined effect of said disadvantages, which other methods fail to do. In addition, it offers new possibilities for the application of the method in the plastics industry.

The present invention thus relates to a plastics extruder, comprising a barrel with a screw therein, a first mixing section at the end of the screw, an injection section having a plurality of injection nozzles spaced around the circumference of the barrel, for injecting a liquid additive into the flow of molten plastics, a second mixing section, and a tip.

The extruder of the invention is characterized in that the injection section is formed as a cylindrical annular chamber, the injection nozzles extending approximately to the center of said chamber, and that the injection nozzles are arranged to be intermittently operated in turn by a pump.

In a preferred embodyment the operating pump is a piston pump each piston of which is connected to a respective injection nozzle.

High-pressure pumps capable of applying up to about 1000 impacts per minute to each nozzle are readily available for the purpose.

For example, if the pump rotates at 600 rpm and is provided with six nozzles and if an addition of 2 % additive is required for a plastics quantity of 500 litres/hour, the volume of one impact will be about 0.05 ml. The injection shocks to the flow of plastics will thus be very small and due to the individual operation of the injection nozzle the additive concentration changes occuring in the longitudinal direction of the extruded plastics are minimized. As the injection nozzles have a tendency to clog, the individual operation of the nozzles makes it easier to establish which nozzle in each case is clogged and the whole process can thus be better supervized.

As the additive is injected into the center of the flow of molten plastics it will immediately be well admixed with the base compound. If the additive is injected only to the surface of the plastics flow it oftens tends to slip directly out along the inner wall of the extruder barrel thus causing locally high additive concentrations in the final product.

The negative effect of such locally high concentrations are damaging in particular when producing vulcanized products by injecting peroxide as additive. In the vulcanization process, peroxide forms bridges between the polymer chains. If there is too much peroxide locally, it will react with itself and not with the base plastics, i,e, as far as the vulcanization process is concerned, the peroxide will be destroyed.

The invention further offers a substantial simplification in processes where it is desired to produce vulcanized and foamed products. If the vulcanizing and foaming agents are dissolved in each other, they can be injected by means of the same pump. For example, DTX peroxide and Frigen 11 are such materials.

In the following, a preferred embodyment of the invention will be described with reference to the drawing attached.

Figure 1 is a general view of the extruder, in a longitudinal section.

Figure 2 is an enlargened section of the end of the extruder barrel, the so-called metering zone.

Figure 3 is a section taken along the line 111-111 in Fig. 2.

The drawing is schematical and is only intended as an illustration of the inventive idea. Reference numeral 1 designates the extruder barrel and numeral 2 the transport screw arranged therein. Numeral 3 indicates a hopper wherefrom the plastics material is fed into the extruder. Numerals 4 and 5 approximately show the so-called infeed and compression zones, at which the injection of additive often is carried out in conventional extruders. The metering zone at the end of the screw 2 comprises a first mixing section 6, an injection section 7, a second mixing section 8 and a tip 9. The mixing sections 6 and 8 are not shown in detail since they are known as such, one manufacturer of such equipment is the Union Carbide Corporation. The same applies to the tip 9.The injection section 7 comprises a cylindrical body connecting mixing sections 6 and 8 and leaving an annular space 10 of relatively large width between itself and the barrel wall 1. A plurality of injection nozzles 11 open into the space 10, extending approximately to the center of the space. The additive to be injected is taken from a reservoir 12 and is fed by a pump 13 to each of the nozzles 1 1. The pump 13 is preferably a piston pump with each piston connected to a respective injection nozzle whereby only one nozzle at a time is opened, but a memrane pump may also be used.

The plastics mass forwarded by the screw 2 is in the first mixing section 6 mixed to a uniform temperature whereby the risk of the occurence of so-called hot spots is eliminated.

Without such mixing, the fluctuations from the average temperature of the mass may be quite considerable, a too hot mass particle, when admixed with the additive, causes an undesired reaction. On the other hand, the first'mixing section ensures that no unmolten mass will get into the injection section. The mass is homogenous when it comes to the injection section 7.

The retatively large space 10 in the injection section 7 following the mixing section 6 quiets the mass and permits the penetration of the tips of the nozzles 11 out of the inner wall of the barrel 1. Because the additive thus is injected into the centre of the flowing molten plastics mass, it will be immediately be better admixed with the base mass and the problem of the additive slipping direct out to a pressureless space along the inner walls of the barrel is eliminated. However, it should be noted that the injection section 7 is not subjected to an underpressure because it is desired to keept the pressure state of the mass flowing at the injection point as stable as possible. The injection rate is regulated by varying the rotational speed of the pump 13 and in this way a good regulating accuracy is easily achieved.

The admixture is further ensured by means of the second mixing section 8 which rotates along with the screw and as to its operational principle is similar to the first mixing section 6. In some earlier extruders this mixing is carried out by a static mixer, whereby the likelihood of a harmful lengthening of the distance travelled by a mass particle and of the time increases. In a static mixer the mass particle travels around various bars, and if the flow in general meets a corner, there will always be a point where the flow rate is zero, in other words, the mass particle will stop.

These stopped particles are gathered together and are ultimately loosened along with the flow, while spoiling the product. In a dynamic rotating mixer this disadvantage is minimized.

Claims (5)

1. A plastics extruder, comprising a barrel with a screw therein, a first mixing section at the end of the screw, an injection section having a plurality of injection nozzles spaced around the circumference of the barrel, for injecting a liquid additive into the flow of molten plastics, a second mixing section, and a tip, characterized in that the injection section is formed as a cylindrical annular chamber, the injection nozzles extending approximately to the center of said chamber, and that the injection nozzles are arranged to be intermittently operated in turn by a pump.

2. A plastics extruder according to claim 1, characterized in that the operating pump is a piston pump each piston of which is connected to a respective injection nozzle.

3. A plastics extruder according to claim 2, characterized in that the rate of injection is regulated by regulating the rotational speed of the pump thereby changing the working frequency of the injection nozzles.

4. A plastics extruder, substantially as hereinbefore described with reference to, and as shown in, the accompanying drawings.

5. Any novel feature or combination of features disclosed herein.