UHMWPE sheet is widely used in various fields such as fenders, carriage slides, coal bunker linings, roller skates, railway, highway and bridge bearing gaskets; Cnc Machining UHMWPE profiles can be prepared by machining the sheet, or obtained by injection molding
5.1 Compression molding
(1) Compression molding process
Compression molding is the earliest processing method for UHMWPE machining, and the process flow is shown in Figure 12. The method is not affected by the molecular weight and fluidity of the resin raw materials, and produces products with small internal stress and warpage deformation, simple equipment, low production cost, and low production efficiency.
In addition to the traditional compression molding process, the preparation methods of the sheet include:
- Extrusion molding. Use a plunger extruder for reciprocating batch extrusion or a screw extruder for continuous extrusion;
- Gas-assisted extrusion molding. During the screw extrusion process, a stable gas is injected between the die and the melt material to provide full-slip boundary conditions on the wall surface and reduce the frictional resistance between the melt and the flow channel;
- Ultrasonic powder molding method. Use ultrasonic vibration to quickly heat and plasticize the powder itself and then mold it;
- High-speed impact molding. At the temperature near the melting point, the mold filled with powder is subjected to high-speed impact and then sintered.
All of the above methods can improve the production efficiency of plates, but they are not easy to form larger products, and are limited in actual production.
(2) Plate modification technology
UHMWPE sheet has low surface hardness, easy fatigue wear, poor heat resistance and creep resistance, which restrict its application in some specific environments. In order to improve these deficiencies, domestic and foreign scholars have done a lot of research on molding resin or product modification, the main methods are polymerization modification, filling modification, cross-linking modification and so on.
Using new catalysts or in-situ polymerization to modify UHMWPE resin from the catalytic polymerization stage can prepare resins with specific molecular chain structures or compound modified resins that are uniformly mixed at the molecular scale, but the technical barriers are high; filling modification is simple and easy to implement However, there are problems of filler agglomeration or poor compatibility; cross-linking modification can better improve wear resistance and heat resistance. Among them, chemical cross-linking is more sensitive to the amount of cross-linking agent, and the residual instability after radiation cross-linking The degradation of free radicals and oxidative components has an effect on the properties of composite materials.
The author’s research group in this paper prepared UHMWPE composites with good self-lubrication, thermal conductivity, wear resistance, creep resistance and load-bearing capacity through solid-phase grafting, organic-inorganic hybridization and other methods. The large amount of frictional heat accumulated in the friction process cannot be conducted and plastic deformation occurs, which brings about potential safety hazards. Successfully prepared high wear resistance, high load bearing, and long cumulative displacement that can meet the needs of highway and railway bridge bearings. UHMWPE profiles.
5.2 Injection molding
(1) Injection molding process
Injection molding is the most efficient production method for large-scale preparation of various UHMWPE wear-resistant profiles. Compared with ordinary plastics, UHMWPE injection molding has the following difficulties: resin is difficult to feed, difficult to plasticize; melt viscosity is large, easy to block; injection molding is difficult to compact; product cooling is easy to deform.
Japan’s Mitsui Petrochemical Company realized the injection molding and commercialization of UHMWPE in the 1970s. Its injection molding technology is actually a combination of plunger injection and compression molding, which has the defects of poor plasticization, slow heat transfer and long molding cycle. ; Later, the reciprocating screw injection molding technology was developed, and the melt was sheared and stretched by the reciprocating screw, which solved the problems of poor plasticization.
In the 1980s, Beijing Plastics Research Institute improved the reciprocating screw to complete UHMWPE injection molding using high pressure and high speed injection technology. However, due to the low clamping force of domestic injection molding machines and the backward operating system, the stability of injection molding products was poor. In addition, UHMWPE injection molding is carried out by plunger push injection machine, multi-plunger injection machine, and plunger stamping injection machine, but there are disadvantages such as poor plasticizing effect, long molding cycle, and small melt amount.
In recent years, some companies have used reciprocating screw injection molding machines to inject flow-modified UHMWPE, but so far, injection molding technology has not been applied on a large scale in the field of UHMWPE.
The research shows that the screw groove of the feeding section is of equal depth, the screw groove of the compression section is gradually shallower, the homogenization section is divided into two sections, the deep screw groove and the shallow screw groove, and the nozzle is divided into a cylindrical cavity, a conical cavity, a nozzle hole and an inverted conical cavity. The design of the machine can promote the material to be gradually plasticized in the barrel and achieve smooth injection. The movable cavity technology is adopted, the cavity is expanded when filling the mold, and the cavity is compressed when the mold is clamped, so that the melt compaction can effectively improve the shrinkage problem of the product. . The order of the influence of various process parameters of the screw injection molding machine on the tensile properties of the product is injection pressure > injection speed > temperature in the metering section > screw speed.
The author of this paper believes that UHMWPE is easy to slip and can be fed with a deep screw groove design; a reasonable distance between the screw and the barrel can achieve full shear melting of high-viscosity melt and avoid screw locking; appropriate nozzle diameter can provide sufficient shear And avoid poor discharge; appropriate injection pressure and clamping force can ensure that the mold cavity is filled without overflowing; the variable mold temperature technology of high mold temperature filling and gradually decreasing mold temperature and pressure can also effectively solve the problem of product shrinkage.
(2) Modification technology of injection molding raw materials
Much of the focus on modification of injection-molding grade resins is to improve fluidity, including polymerization modification and blending modification. For example, bimodal polyethylene, solvent oil-containing resin, lubricating modifier-containing resin, etc. are prepared by new catalysis and polymerization processes; in addition, the processing performance of 3d printing and UHMWPE materials is improved by blending with components that improve the fluidity of the matrix.
HDPE and UHMWPE have good compatibility, UHMWPE/HDPE blending is a more convenient and effective method for UHMWPE flow modification; nanoparticles make composite materials have the characteristics of organic, inorganic and nanomaterials, and nanoparticles are introduced by in-situ polymerization , which can prepare injection-molding grade composite materials with uniform dispersion; use new catalytic polymerization technology such as single active center to develop new resins with both narrow molecular weight distribution and long-chain branched structure and low entanglement degree, which can solve the problem of processing performance and mechanical properties. The contradiction between them is also a research direction worth looking forward to.
Improving the fluidity of the resin can avoid problems such as melting glue and injection difficulties. The filling method also needs to solve the problems of loose product structure and serious shrinkage. Only when the fluidity modification is matched with the injection molding process and equipment can the final injection molding products with stable comprehensive performance and excellent quality be formed.
5.3 Artificial joints
At present, more than 90% of the artificial joint materials on the market are made of UHMWPE material. There are about three million joint replacement surgeries in the world every year. The wear and damage of UHMWPE components are the main factors affecting the life of artificial joints.
(1) Preparation process
UHMWPE artificial joints can be prepared by two methods of plunger extrusion and molding, and then modified and processed to obtain artificial knee joints, hip joints and other products. Compression-molded UHMWPE has an isotropic crystal orientation, while plunger extruded material has a slightly different crystal morphology with distance from the centerline. These crystal morphology differences can lead to subtle differences in fatigue crack growth behavior, Thereby affecting the phenomenon of osteolysis. Therefore, the use of molded UHMWPE molding or pom cnc machining is currently the mainstream of artificial joints.
(2) Research progress
UHMWPE was first used to make artificial joint liners and pads in 1960. At present, the wear resistance of UHMWPE base resin cannot fully meet the long-term use of joints. Research institutions and enterprises such as Massachusetts General Hospital have developed cross-linked UHMWPE, which greatly reduces joint wear and tear. FDA (FDA) approved for clinical use.
Subsequently, European and American countries invented vitamin E polyethylene, which has both anti-oxidation and wear resistance properties, and was approved by the FDA for clinical use in 2009. my country has gradually used UHMWPE to make joint liners since 1990, but the molecular weight, molding process and joint manufacturing process of domestic UHMWPE are relatively backward, the clinical life of artificial joints is short (about five years on average), and complications caused by wear and tear serious.
Increasing the entanglement density of UHMWPE by melt annealing can improve its amorphous region microstructure, thereby reducing the osteolysis of UHMWPE (i.e., grinding slag due to wear); however, the effect of melt annealing is limited and cannot effectively maintain high entanglement. state, the use of radiation crosslinking to reduce the chain mobility required for large-scale plastic deformation can slow down the formation of surface debris; however, secondary thermal processing after or during radiation melting can lead to oxidative degradation of the material , causing a decrease in crystallinity and strength, and using vitamin E as an antioxidant for medical-grade UHMWPE can eliminate residual free radicals in highly cross-linked UHMWPE artificial joints.
Recently, researchers found that the flavonoid natural antioxidants rutin, quercetin and tea polyphenols are more antioxidant than vitamin E. The author’s research group cooperated with key domestic artificial joint research units and enterprises to first obtain UHMWPE resin with reasonable molecular weight, distribution, particle size and distribution, and metal residue and comprehensive properties that meet the requirements of implant-grade standards; Research on modification of cross-linking, antibacterial and anti-oxidation.
