FR2465585A1 - ADDITIVE INJECTION INTO THE CYLINDER OF AN EXTRUDER - Google Patents

Abstract

IN THIS EXTRUDER, TO MAKE THE MIXTURE OF THE BASIC MATERIAL AND THE INJECTED ADDITIVE VERY HOMOGENEOUS, THE ADDITIVE IS INJECTED BY A PUMP THAT FLOWS INTO A CROWN OF INJECTORS 11 INDEPENDENTLY AND SUCCESSIVELY AT A HIGH FREQUENCY VERY SMALL INDIVIDUAL DOSES. THESE INJECTORS DELIVER IN THE MIDDLE OF THE THICKNESS OF THE BASIC PLASTIC MATERIAL VEIN, IN A 10 INJECTION CHAMBER SITUATED BETWEEN TWO MIXING AREAS 6 AND 8 OF WHICH THE LAST IS LOCATED JUST UPSTREAM OF LAFILIERE9.

Description

The present invention relates to extruders

of plastics.

  Various methods are already known for injecting an additive into the cylinder of an extruder for plastics. The additive may be substituted by an agent which causes foaming, coloring, vulcanization

  or any other desired modification of the plastic material

than basic.

  Usually the injection takes place near the feed end of the extruder, in the so-called feed area, in the compression area or in

  a special decompression zone.

  In this case, the injection is easy to perform, thanks to the low pressure, and the screw has sufficient time to do the mixing work. On the other hand, the additive must then remain in the fillets for a long time.

  of the screw and, for this reason, the temperature of the

  melted plastic should be kept low

  because most additives start to react

  favorably as soon as the temperature of the

  plastic material is higher.

  If you want to keep the temperature of the plastic as low as possible, and especially

  in the case of polyethylene, a low viscosity must be given

  rotation size with the extruder screw. This low rotation speed in turn results in a low

  quantity of plastic material extruded per unit of time.

  By postponing the point of injection of the additive plus

  near the extrusion point, the time is reduced during the

  which the mixture remains in the cylinder before the extrusion and this makes it possible to bring the basic plastic material to a higher temperature which is obtained, for example by increasing the speed of the screw; in other words, we

  thus obtaining an improvement in the yield of the extruder.

  The following factors affect the operation of the cylinder end of the extruder:

  1 - the pressure reaches its maximum in the end -

of the extruder cylinder.

  2 - the mixing capacity of a normal screw is mixed

  poor, especially if the mixing work only begins at the end of this screw. 3 - the internal temperature differences of the mixture reach

  their maximum in the end of the extruder cylinder.

  4 - the Sie mixture has only a short time to absorb

  ber the shock caused by the additive.

  5 - the operation of the injector is impaired and the

  nozzle clog if the additive causes plasticization

of the basic plastic.

  6 - if we aim to obtain higher production,

  that is, to get a warmer mixture, the particles

  Mixers have less time to leave the extruder.

  Each of these factors must be taken into account and, above all

  all of their combined effects. The process according to the invention

  tion which will be described below completely solves the ef-

  combined disadvantages of these factors, which the other methods fail to achieve. In addition, it offers new application possibilities

  in the plastics industry.

  The invention relates to an extruder for plastics comprising a cylinder containing a screw, a first mixing zone situated at the end of the screw,

  - an injection zone comprising several injectors is-

  on the circumference of the cylinder used for injecting

  ter a liquid additive in the stream of plas-

  melted tick, a second mixing zone and a die.

  The extruder according to the invention is characterized in that the injectors are arranged to be actuated by deu i- ir r -r D r'an; tm.nirP ntn;. K-

  One can easily obtain for this applica-

  tion of high pressure pumps capable of delivering

  approximately 1000 discharges per minute at each injector., For example, if the pump rotates at a speed of 600 revolutions / min and is equipped with six injectors and if a request is made

  addition of 2% additive for a flow rate of plas-

  tick 500 liters / hour, the flow of a single discharge will be about 0.05 ml. Injection shocks exerted on

  the plastic flow will therefore be very attenuated-

  and, thanks to the individual operation of the injectors, the variations in the concentration of the additive produced in the longitudinal direction of the extruded plastic are reduced to a minimum. Since the injectors tend to get clogged, the individual operation of the injectors makes it easier to find the clogged injector and

  this allows better monitoring of the whole work.

  In addition, the injection area is preferably

rence formed by a cylindrical annular chamber, the injectors opening-approximately in the middle of the chamber. In this embodiment, the additive is injected into the center of the stream of molten plastic and immediately mixed well with the base compound. Yes

  the additive is only injected at the surface of the ma-

  plastic, it frequently tends to slide directly along the internal surface of the extruder cylinder, thus causing the formation of high concentrations

  local additive in the finished product.

  The negative effect of these high local concentrations of additive is particularly detrimental when vulcanized products are manufactured by injecting a peroxide as an additive. In the process of

  vulcanization, the peroxide forms bridges between the chains

  nes of the polymer. If there is too much peroxide locally, this substance reacts on itself

  instead of reacting on the basic plastic,

  to say that with regard to the vulcanization process,

the peroxide is lost.

  The invention further brings about an important simplification.

  bearing treatments in which one seeks to obtain

  vulcanized and foam products. If the vulcani-

  sant and the foaming agent are dissolved in each other, they can be injected using the same pump. Examples of such substances are DTX peroxide and Frigen 11.

  Other features and advantages of the invention

  tion will appear during the description which follows.

  In the accompanying drawings, given only by way of example; Fig. 1 is an overall view of the extruder in longitudinal section,

  - Fig. 2 is an enlarged view of the former

  extrusion cylinder tread and showing the dosing cheek;

  Fig.3 is a sectional view along line III-

III of Fig. 2.

  The drawings are schematic and should not be

  considered as an illustration of the principle of

  vention. Reference 1 designates the extruder cylinder and reference 2 designates the conveyor screw housed in

  this cylinder. Reference 3 designates a hopper from which the ma-

  plastic is introduced into the extruder. The re-

  references 4 and 5 designate approximately the so-called zones

  supply and compression systems in which the injection

  tion of the additive is frequently performed in the ex-

  classic extruders. The head of the extruder, located at

  the end of the screw 2 includes a first zone of ma-

  laxage 6, an injection zone 7, a second metallization zone

  lange 8 and a die 9. The kneading and mixing zones

  ge 6 and 8 are not shown in detail because they are known. A manufacturer of this type of machine is

  Union Carbide Corporation. The same observation is valid-

  ble for the die 9. The injection zone 7 comprises a

  cylindrical body which connects the mixing and mixing zones

  lange 6 and 8 and leaves free between itself and the wall 1 of the cylinder an annular space 10 of relatively large width. Several injectors 11 open into space 10

  extending approximately to the center of this space.

  The additive to be injected is taken from a reservoir 12 and conveyed by a pump 13 to each of the injectors 11. The pump 13 is preferably a piston pump whose pistons are respectively and individually connected to the injectors so that a single injector is open at the same time but you can also use a pump

membrane.

  The mass of plastic material pushed by the screw 2 is kneaded in the first kneading zone 6, this kneading the

bringing to a uniform temperature, which eliminates the risk

  than forming hot spots. In the absence of such

  mixing, temperature deviations from the tem-

  average mass temperature can be condidible and

  mixing excessively hot parts of the mass with the addi-

  tif causes an adverse reaction. On the other hand, the presence of the first mixing zone ensures that the injection zone will not receive unmelted mass. The mass

then being homogeneous.

  The relatively large space which is provided in the

  injection zone 7 following the mixing zone 6 tran-

  quillizes the mass and also makes it possible to give the

  tes of the injectors 11 a certain height of projection on the internal surface of the cylinder 1 or a certain depth of

  penetration into the injection area. Since the ad-

  ditif is thus injected into the center of the current of plas-

  melted tick, it is immediately mixed with the mass of

  basic material, which eliminates the possibility for the addi-

  tif to exit directly from a space without pressure

  by sliding along the internal surface of the cylinder. All-

  However, it should be noted that the injection zone 7 is not subjected to a decompression which it is desired to maintain as stable as possible the state of pressure of the mass.

  flowing to the right of the injection site. The flow of

  jection can be conveniently adjusted by varying the speed

  of rotation of the pump 13 and thus a good pre-

adjustment cision.

  The incorporation of the additive is further improved by the second zone 8 for kneading the mixture which turns with the screw and which is analogous to the first zone 6 in its working principle. In some prior extruders, this mixing is carried out by a static mixer but this solution increases the risk of detrimental lengthening of the distance traveled by each particle of the mass and of lengthening of time. In a static mixer, each particle of the mass circulates around bars and, in general, when a current meets an angle, there is always a point where the speed is zero, that is to say in in other words, the interested particle of the mass is immobilized. The immobilized particles agglomerate then finally detach themselves to be entrained by the current, forming a defect in the product. In one

  rotary mixer, this drawback is reduced.

Claims (3)

  1 - Extruder for plastics including   a cylinder containing a screw, a first zone of ma-   laxation located at the end of the screw, an injection zone   tion comprising several injectors spaced at the circumference   ference of the cylinder and used to inject a liquid additive into the stream of molten plastic, a second mixing zone and a die, an extruder characterized in that the injection zone has the form of an annular cylindrical chamber, the injectors are extending roughly   to the center of this room, and that the injec-   teurs (11) are arranged to be put into action by inter-   mittence and alternately by a pump (13) in such a way   re only one injector is open at a time.   2. Extruder according to claim 1, character-   sed in that the pump (13) is a piston pump whose pistons are respectively and individually connected to the different injectors (11).   3. Extruder according to claim 2, character-   sée in that the injection rate is adjusted by action on the pump speed <13), has the effect of varying   the working frequency of the injectors (11).