CN1115280A - Extrusion molding machine for ultra-high molecular weight polymers - Google Patents

Abstract

The invention is an extrusion molding machine for ultra-high molecular weight polymers. It can be used for extrusion molding of ultra-high molecular weight polyethylene or high-viscosity materials. The machine includes combined screw, barrel and extruder head. The surface of the screw and the inner wall of the barrel of the extruder are covered with a layer of polytetrafluoroethylene filled modified material; the screw adopts small lift angle and large diameter structural parameters; the threads of the combined screw are staggered and discontinuous. Therefore, this extruder overcomes the problem that the ultra-high molecular weight polymer cannot be extruded due to its great adhesion and easy degradation by shearing, and makes the continuous extrusion molding of the ultra-high molecular weight polymer a reality.

Description

Extrusion molding machine for ultra-high molecular weight polymers

The invention relates to plastic processing machinery, in particular to a screw extruder for ultra-high molecular weight polymers and high-viscosity materials. It can be used for extrusion molding of ultra-high molecular weight polyethylene.

Ultra-high molecular weight polymers (such as ultra-high molecular weight polyethylene) have superior mechanical and physical properties and chemical stability, but their viscosity is high, the melt index is extremely low, close to zero, and their fluidity is poor. It is easy to degrade, so the processing of ultra-high molecular weight polyethylene has long been one of the hot spots in the plastics processing industry.

At present, most of the processing of ultra-high molecular weight polyethylene is formed by sintering method, and a plunger extruder is also used to push and produce products, but these products are mostly rods with simple shapes. It is rare to extrude such products with a screw extruder. Because it is very difficult to extrude ultra-high molecular weight polyethylene with the usual screw, and generally it cannot be extruded, even if the power consumed after extrusion is quite large. The reason is that the viscosity of this material is extremely high, and it is easy to adhere to the contact surface of the metal screw and barrel, and the frictional resistance is too large. However, the common screw parameters based on the theory of shear plasticization make UHMWPE very easy to degrade under the action of high shear rate, and the material extrusion is not easy to form. In the EP007851 patent, a high-viscosity and low-density polymer extrusion method is proposed, which is mainly coated with a low friction coefficient coating layer on the feeding section of the screw, such as polytetrafluoroethylene, and coated with metal 101 alloy on the barrel. cover. Similar patents include JP53-6366, JP4-18321 and so on. The above patents are not technical solutions for the processing requirements of ultra-high molecular weight polyethylene, but to solve the problems of poor feeding fluidity and low conveying efficiency of linear low-density polyethylene in solid granular or powder form, so the main purpose is to reduce the material The friction coefficient of the screw, especially the friction coefficient of the feeding section to the screw, is far from the processing characteristics of ultra-high molecular weight polyethylene, because:

(1) As recorded in the literature, the melt index of linear low-density polyethylene is 1, and the flow rate is ≥22 to ≤32, while ultra-high molecular weight polyethylene is a special material of polyolefins, and the melt flow rate is extremely low close to Zero, and its high viscosity is manifested in the melting or critical melting state. At this time, if the external friction force during processing is too large, especially the external friction force caused by adhesion is too large, the surface and the inside of the material will be strongly sheared. And cause thickening of the stagnant boundary layer. This is very easy to cause degradation of ultra-high molecular weight polyethylene that is not resistant to shear rates. If according to the scheme of reference document, only be coated with cladding on feeding section obviously still can't solve the above-mentioned processing problem.

(2) Although according to the principle of extrusion, it is known that the extrusion of materials is pushed by the flight of the screw by friction, but experimental research has proved that the key to the advancement of materials is not the absolute value of the friction coefficient of each surface, but the relationship between the material and the screw groove. The difference between the total friction force and the total friction force of the material on the barrel, that is, the friction coefficient of the two surfaces must have a certain difference to ensure that the material can be pushed against the inner wall of the barrel. For generally viscous linear low-density polymers, the greater the difference, the more conducive to improving the rate of material advancement. However, for ultra-high molecular weight polymers in the high viscosity of the plasticized state, this difference is too large. For example, in the comparative literature, only the coating is coated on the screw, and the inner wall of the barrel still maintains a large friction coefficient, which will cause machine friction. The excessively thick material on the inner wall of the cylinder stays in the boundary layer, which is not conducive to the advancement of the material and widens the distribution of the residence time of the material in the machine.

(3) The above-mentioned patents do not improve the screw parameters based on the theory of shear plasticization, and it is bound to be sheared and degraded when extruding ultra-high molecular weight polymers.

The task of the present invention is to provide a screw extruder that meets the processing requirements of ultra-high molecular weight polymers, to ensure that the high-viscosity materials in the melting and plasticizing process do not adhere to the contact surface of the screw and the barrel, and have good Excellent slidability, avoiding the high shearing effect of ultra-high molecular weight polymer materials, so that the materials can be fully plasticized, rolled, cut, mixed, and continuously extruded.

In order to achieve the above object, the present invention adopts the following technical solutions.

The main components of the present invention include a machine barrel and a combined screw rod installed in the machine barrel. It is characterized by:

(1) At least on the surface of the contact section between the screw and the plasticized material and the corresponding inner wall of the barrel, there are coatings of polytetrafluoroethylene filled modified composite materials, and the friction coefficient of the coating on the surface of the screw is smaller than that of the coating on the inner wall of the barrel. coefficient. The purpose of this measure is to simultaneously increase the sliding properties of the highly viscous material in the molten state to the screw and barrel, so that the material does not stick to the surface of the screw and barrel. For the extrusion of ultra-high molecular weight polymers, it is more important to solve the adhesion of the material to the surface of the machine than to improve the friction coefficient and power. In this way, the lump in the screw channel can be prevented from being degraded due to the difference in the up and down speed caused by the adhesion with the screw, which will generate additional shear. It also prevents the excessive adhesion of the material to the barrel to form a thicker boundary retention layer. It is very beneficial for continuous extrusion of ultra-high molecular weight polymers. The friction coefficient of the screw and barrel surface can be obtained by adjusting the proportion of filler in the coating.

(2) The screw parameters adopt a structure with a small lift angle and a large diameter. The lift angle is 8-12°, the diameter is φ200-φ400mm, and the length-to-diameter ratio of the screw is 4-6. Due to the large diameter, the screw speed should be low, which is 5-20rpm. The selection of the above parameters is quite different from the usual plasticizing shear theory. Under the premise of not reducing the line speed, it increases the thrust of the screw flight on the material, reduces the cross flow, and prevents the material from being degraded by transverse shearing. A small lift angle ensures stable extrusion with slight changes in the coefficient of friction.

(3) The screw rod is composed of a plurality of threaded elements, and when combined, each threaded element is staggered at an angle, so that the threads on the entire screw rod are discontinuous. In this way, the dough in the front screw groove is divided into two halves when it enters the follow-up thread, exposing the core of the dough, and the original surface that is about to be melted is combined by the follow-up thread to become the core, so the material advances through the entire screw. During the process, the plasticized and mixed state is maintained, and the core of the material is continuously updated through repeated "dividing" and "combining" flows to achieve the purpose of mixing.

The present invention is also characterized in that:

The cladding on the above-mentioned screw rod and machine barrel adopts polytetrafluoroethylene filled modified composite material, and its filler is composed of: (by volume ratio)

Bronze powder 35-60% glass fiber or glass powder 15-20%

Graphite, molybdenum disulfide ≤5%

The coatings on the surface of the screw and the inner wall of the barrel are all selected from the above components; as long as different component ratios are selected, the friction coefficient of the screw surface can be guaranteed to be smaller than that of the inner wall of the barrel. Generally speaking, choosing more glass fiber (or glass powder) or/and less graphite can get a larger friction coefficient, and vice versa, a smaller friction coefficient can be obtained. The addition of bronze powder can improve the thermal conductivity of the surface.

The screw groove on the above-mentioned screw has a nearly square cross-section with large rounded root corners, so that the materials roll and unite in the screw groove as a whole.

Effect of the present invention:

Due to the polytetrafluoroethylene filled modified composite coating on the surface of the screw and the inner wall of the barrel, the sliding property of the material on each surface is increased, and one of the difficulties in the extrusion of ultra-high molecular weight polymers with extremely high viscosity is solved. ——Extra shear caused by adhesive force and material retention on the inner wall of the barrel.

The choice of screw structure parameters effectively avoids and reduces cross-flow shear, and solves the problem that ultra-high molecular weight polymers are easily degraded by shear.

In a word, the present invention has the shear plasticization theory different from the usual extruder in the mechanism of the extruder, and adopts low-speed kneading, integral rolling and rubbing, split-flow plasticizing and mixing; the continuous Extrusion becomes a natural reality. At present, ultra-high molecular weight polyethylene has been successfully extruded.

The present invention is disclosed in detail below in conjunction with the accompanying drawings.

Figure 1 is a structural diagram of the extruder screw and barrel components.

Figure 2 is a schematic diagram of the combined screw structure of the extruder.

As shown in Figure 1-2: the main components of this screw extruder include a combined screw, a barrel 11 mounted outside the screw, and an extruder head 10, wherein the combined screw is composed of multi-segment thread elements 1, 2, 3, 4, 13 compositions. There is a hollow transmission shaft 5 in the middle, and each section of threaded elements is assembled on the hollow shaft 12 through the long guide key 6, and is driven to rotate by the hollow shaft, and the rotating speed is between 5-20rpm. A rotatable mandrel 7 is also movably worn in the middle of the hollow shaft, and its direction of rotation and rotating speed are regulated by another transmission system or motor.

In order to overcome the adhesion of the ultra-high molecular weight polymer (ultra-high molecular weight polyethylene) with extremely high viscosity in the instant melt state to the screw and the barrel, the extruder is at least on the surface of the contact section 1, 2, and 3 between the screw and the plasticized material. And on the inner wall of the corresponding barrel section, there is a covering layer of polytetrafluoroethylene filled modified material. Specifically, in addition to the feeding section of the screw, other parts of the screw and the corresponding inner wall of the barrel have this coating. The coating contains fillers such as bronze powder, glass fiber or glass powder, graphite, and molybdenum disulfide. It makes the surface of the screw and the inner wall of the barrel have good non-stick properties, and at the same time maintains good resistance to compression, creep and wear. The presence of bronze powder improves the thermal conductivity of the surface, which is necessary to dissipate the heat generated by the high power consumption during extrusion of high-viscosity materials. In order to ensure that the friction coefficient of the coating on the surface of the screw is lower than that of the coating on the inner wall of the barrel, the surface coating of the screw and barrel of this machine contains the following filler ratio: (according to the volume ratio of polytetrafluoroethylene)

Coating on the surface of the screw Coating on the inside of the barrel Bronze powder 30% 40% glass fiber 20% 20% and glass powder graphite, molybdenum disulfide 5% 3% According to different material requirements, the above ratio can be adjusted. The above cladding is covered by a general sintering method, and can also be covered by a pasting method, which can be implemented by ordinary technicians.

In addition, in order to meet the processing requirements that ultra-high molecular weight polyethylene is not suitable for shearing, this extruder has changed the conventional screw parameters established according to the theory of shear plasticization, and adopts a structure with a small lift angle and a large diameter. In the multi-head screw at 10°, the aspect ratio is 4-5, and the diameter is φ250mm. The thread lead angle gradually decreases from the feeding section to the extrusion section. The purpose of the small lift angle is to increase the forward thrust of the screw edge on the material and stabilize the extrusion. Reduce cross flow component or transverse shear. To a certain extent, increase production. The screw rod adopts a combination screw rod, and when the thread elements of each screw rod are combined, they are staggered by a certain angle, which can be staggered by half the width of the screw groove, so that the threads on the whole screw rod are not continuous. The dough coming out of the front screw groove is split into two by the follow-up screw edge, and the core and surface are continuously updated by the follow-up multi-thread, so that the material in the instant-melt state is always in a plasticized state, mixed, rolled and homogenized until it is extruded.

The screw groove on the screw rod is deep, and the depth is 2/3-1 pitch. It adopts a nearly square cross-section structure with large rounded root corners, so that the materials are rolled and rubbed as a whole, and are not subject to or reduce shear.

In addition to the above improvements, the extruder also has the following structure.

The mandrel is used as the mandrel for extruding the pipe, so that there is no split line and seam when the high-viscosity material is extruded into the pipe, and the mandrel rotates at a low speed during extrusion to make the molten material flow evenly.

The entire combination screw gradually reduces the flight angle from the feeding section to the extrusion section, and the screw groove gradually narrows to adapt to the performance change of the material. The front end of the screw is a transition cone 8 with a spiral groove, which forms a gap with the corresponding cone end 9 of the machine barrel. The end of the mandrel extends into the machine head 10 and also forms a gap therewith. In this way, there is a certain head resistance when the material passes through the rotating mandrel and the gap structure, which plays a role in regulating the material flow.

In order to ensure the advancement, tumbling and kneading of materials, longitudinal grooves are made on the inner wall of the barrel to increase the external friction such as mechanical microcosmic collision between the material and the barrel screw in the specified direction.

The end of the mandrel can also have a screw groove to supplement the plasticization of the material.

The machine head is made into tube or sheet, which is used to extrude products of different shapes. The inner cavity of the machine head can also be covered with polytetrafluoroethylene-filled modified composite materials to reduce material adhesion. The tubular head can be rotated to eliminate flow marks caused by the screw with the rotation of the mandrel.

Claims (6)

1, a kind of extrusion shaping machine of super high molecular weight polymer comprises assembled screw rod, is set in outer machine barrel (11) extruder head (10) of screw rod, it is characterized in that:

(1) layer that covering of the filling-modified material of one deck polytetrafluoroethylene (PTFE) is arranged at least on screw rod and beginning material contact surface of plastifying state and corresponding machine barrel (11) section inwall, and the coefficient of friction of screw surface coating is less than the coefficient of friction of machine tube inner wall face coating,

(2) screw rod adopts little lift angle, major diameter structure, and lift angle is at 8 °-12 °, and screw diameter is φ 200-φ 400mm, screw slenderness ratio 4-6,

(3) screw rod is combined by a plurality of screwing elements (1), (2), (3), (4), (13), and the fashionable angle that interlaces of the thread set on each screwing element makes the screw thread on the whole screw rod discontinuous.

2, extrusion shaping machine according to claim 1, it is characterized in that: on screw surface and the machine tube inner wall face to cover layer be the filling-modified material of polytetrafluoroethylene (PTFE) that contains following filling mixture ratio, (press the volume ratio of polytetrafluoroethylene (PTFE)) bronze powder 35-60% glass fibre or glass dust 15-20% stone are deceived, molybdenum bisuphide≤5%

3, extrusion shaping machine according to claim 1 is characterized in that, the screw channel on the screw rod is the square sectional that approaches with big fillet root angle, and spiral groove depth is a 2/3-1 pitch.

4, extrusion shaping machine according to claim 1 is characterized in that: screwing element adopts multiple thread.

5, extrusion shaping machine according to claim 1 is characterized in that: screw front end is a band spiral fluted transition conehead (8), with corresponding machine barrel tapering point (9) formation gap.

6, extruder according to claim 1 is characterized in that: head (10) inner chamber also has the layer that covering of the filling-modified material of polytetrafluoroethylene (PTFE).

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