Molding of ultra high molecular weight polyethylene (UHMW-PE)
Since the viscosity of the ultrahigh molecular weight polyethylene (UHMW-PE) in the molten state is as high as 108 Pa*s, the fluidity is extremely poor, and the melt index is almost zero, so it is difficult to process by a general mechanical processing method. In recent years, the processing technology of ultra-high molecular weight polyethylene (UHMW-PE) has been rapidly developed. Through the transformation of ordinary processing equipment, ultra-high molecular weight polyethylene (UHMW-PE) has been developed from the initial press-sintering forming to Extrusion, blow molding and injection molding as well as other special methods of forming.
2.1 General processing technology
(1) Press sintering
Press sintering is the most primitive processing method for ultra high molecular weight polyethylene (UHMW-PE). This method is relatively inefficient and prone to oxidation and degradation. In order to improve production efficiency, direct electric heating method can be used [1]; in addition, Werner and Pfleiderer have developed an ultra-high-speed sintering process using a vane mixer with a maximum blade rotation speed of 150 m/s. The material can be raised to the processing temperature in just a few seconds.
(2) Extrusion molding
Extrusion molding equipment mainly includes a plunger extruder, a single screw extruder and a twin screw extruder. The twin-screw extrusion uses a co-rotating twin-screw extruder.
In the 1960s, most of the plunger extruders were used. In the mid-1970s, Japan, the United States, and West Germany developed a single-screw extrusion process. Japan’s Mitsui Petrochemical Co., Ltd. first achieved the success of round bar extrusion technology in 1974. At the end of 1994, China developed a single-screw extruder for Φ45 ultra-high molecular weight polyethylene (UHMW-PE). In 1997, it achieved the success of the industrial production line of Φ65 single-screw extruded pipe.
(3) Injection molding
Japan’s Mitsui Petrochemical Co., Ltd. developed the injection molding process in 1974 and commercialized it in 1976, and later developed reciprocating screw injection molding technology. In 1985, Hoechst Corporation of the United States also realized the screw injection molding process of ultra high molecular weight polyethylene (UHMW-PE). In 1983, China’s domestic XS-ZY-125A injection machine was modified, and the ultra-high molecular weight polyethylene (UHMW-PE) roller and water pump bushing for beer canning production line were successfully injected. In 1985, it was successfully injected. Medical artificial joints, etc.
(4) Blow molding
When ultra-high molecular weight polyethylene (UHMW-PE) is processed, when the material is extruded from the die, it is retracted due to elastic recovery, and there is almost no sagging phenomenon, so it is a hollow container, especially a large container, such as The blow molding of fuel tanks and vats creates favorable conditions. Ultra-high molecular weight polyethylene (UHMW-PE) blow molding can also lead to high-performance films with balanced strength in the longitudinal and transverse directions, thus solving the problem that HDPE films have long been inconsistent in the longitudinal and transverse directions and are prone to longitudinal damage.
2.2 Special processing technology
2.2.1 jelly spinning
The preparation of high-strength, high-modulus polyethylene fibers by the jelly-spinning-super-stretching technique is a novel spinning method that emerged in the late 1970s. The Dutch DSM company first applied for a patent in 1979, and then the industrialized production was realized by Allied Company of the United States, Toyobo-DSM Company established by Japan and the Netherlands, and Mitsui Company of Japan. The Research Institute of China Textile University began research on the project in 1985 and gradually formed its own technology to produce high-performance ultra-high molecular weight polyethylene (UHMW-PE) fiber.
2.2.2 Lubrication extrusion (injection)
Lubrication extrusion (injection) molding technology is to form a lubricating layer between the extrusion (injection) material and the mold wall, thereby reducing the difference in shear rate between the points of the material, reducing the deformation of the product, and at the same time achieving low temperature. Increase the extrusion (injection) rate of high viscosity polymers under low energy conditions. There are two main methods for producing a lubricating layer: self-lubricating and co-lubricating.
2.2.3 Roll forming
Roll forming is a solid-state processing method in which a large pressure is applied to the ultra-high molecular weight polyethylene (UHMW-PE) below the melting point, and the particles are effectively fused by the particles. The main equipment is a rotating wheel with a groove and a bow slider with a tongue groove. The tongue groove is perpendicular to the groove. The friction between the material and the wall is effectively utilized during processing, and the pressure generated is sufficient to deform the ultrahigh molecular weight polyethylene (UHMW-PE) particles. A heating abutment is installed at the end of the base to extrude the material through the die. If this rolling device is used in combination with an extruder, the machining process can be continuous.
2.2.4 Press forming after heat treatment
The ultrahigh molecular weight polyethylene (UHMW-PE) resin powder was subjected to short-term heating at 140 ° C to 275 ° C for 1 min to 30 min, and it was found that some physical properties of the ultrahigh molecular weight polyethylene (UHMW-PE) were unexpectedly greatly improved. Compared with the unheated UHMPWE product, the heat-treated ultra-high molecular weight polyethylene (UHMW-PE) powder has better physical properties and transparency, smoothness of the surface of the product and low-temperature mechanical properties. Greatly improved.