Brief Introduction of TPE Secondary Injection Molding
Source：德创化工Time：2019-12-22Hits：1966
Whether insert injection molding or secondary injection molding is selected, thermoplastic elastomer (TPE) is injected on a rigid substrate compatible therewith. This has enhanced the functionality of "soft-coated hard" goods, such as insulation, chemical resistance, better fit for human engineering, better hand feel, grip and top-notch aesthetic feeling.
Insert molding: In insert molding, a rigid part (usually a relatively hard plastic part) is first made and embedded into a mold cavity, and then TPE is injected on this component to finally obtain a good product. Conventional injection molding equipment can be used for insert molding. The insertion of rigid parts can be completed by manual or mechanical arms.
Secondary injection molding: Secondary injection molding is also called secondary injection molding, two-color molding or multi-material molding. The special machine used in this case has a plurality of cartridges for writing different data into the same mold.
Adhesion in compound injection molding In compound injection molding, the adhesion between solid substrate and TPE is the key to the success of processing. Poor adhesion can lead to doubts such as peeling, bending, abrasion or delamination. The compatibility between the two data and the processing temperature are important factors that affect the bonding strength.
Data Compatibility: Bond Strength of Different Hard/Soft Data Combinations
Processing temperature: It reflects the relationship between TPE melting temperature, rigid substrate temperature and bonding strength. The application of TPE in compound injection molding thermoplastic elastomer (TPE-E or COPE) based on polyester is produced by DSM engineering plastics. This kind of polyester elastomer has the strength, processing characteristics and thermosetting elastomer function of engineering plastics, improving processing, yield and chemical resistance.
Most soft-touch compound injection molding applications involve injecting a thin layer of soft material (TPE) onto a hard substrate. Because TPE generally needs to flow through a longer path and a thin-walled area to fill the mold, TPE must have higher fluidity. The low viscosity commodity specification optimizes the crucial cohesiveness for compound injection molding and is the preferred data for this processing and application.
Chemical Bonding: TPE is very well bonded to polar polymers (such as PC, PC/ABS, ABS) used as rigid substrates through compound injection molding. Because of this outstanding adhesion, even under very high torque, it will not show the appearance of delamination with the substrate.
In the process of selecting exterior elastomers, TPE has excellent layout characteristics and excellent exterior sensory function, and is linked with TPE's outstanding processing function in compound injection molding. Physical combination or application of adhesive: mechanical interlocking depiction is often used to improve the combination force with the substrate during compound injection molding between TPE and nonpolar polymer (such as PP) or PA. In addition, adhesive or primer can be used to improve the adhesion between flexible TPE and nonpolar polymer.
(1) Thermal elastomer (TPR) has high melt viscosity, even at low shear rate, it also shows the properties of non-Newtonian fluid and has high sensitivity to shear. When injection molding TPR, the molding cycle is short, which is similar to common thermoplastics. TPR can replace natural or synthetic rubber, PE, PVC and PU, improve thermal stability and low temperature flexibility, and is widely used in automobile manufacturing and other departments.
(2) styrenic thermal elastomers styrenic thermal elastomers are mostly ternary block polymers, i.e. polymers formed by combining the tails of more than two polymer segments with different compositions, which can be homopolymers or copolymers and can be made into various block polymers, such as polybutadiene, polystyrene, polyisoprene or their copolymers; Can be different isomers of homopolymers; It can also be binary block, triblock or multiblock arranged in various different sequences; It can be all elastic segments or polymers linked together with soft elasticity.
Styrene thermoelastic bodies are mainly synthesized by taking styrene, butadiene and isoprene as monomers and can be divided into three categories. Triblock ABC, star (AB)n or (aba) n; ABC and multiblock cba (abc) n. These block thermal elastomers have good green rubber strength and elasticity without vulcanization, small permanent deformation after stretching, good flexibility and elasticity, and large surface friction coefficient. The thermal performance is also very good, and remains flexible at low temperatures. The melt index of general purpose polystyrene blended with butadiene/styrene thermal elastomer increased from 3.5 to 4.0.
Thermal elastomers used for injection molding are usually mixed and granulated by an extruder. As the viscous flow activation energy of block thermal elastomers such as SBS is 2.5~3 times larger than that of rubber, the viscosity is more sensitive to temperature than rubber. Rheological properties are more special than rubber. Its viscosity is dozens of times larger than that of rubber with similar molecular weight, and it has Newton region and non-Newton region. When the shear stress exceeds 3×103Pa, the shear rate increases rapidly. Repeated processing of block thermoelasticity on injection molding machine will significantly reduce melt index and rebound performance.
(3) polyester thermal elastomer
Polyester thermal elastomers are prepared by transesterification. The raw materials are dimethyl terephthalate and polytetramethylene glycol ether (molecular weight 60-3000) to obtain hard segments containing crystallized dibutyl terephthalate and amorphous soft segments of polyalkylene terephthalate. The crystalline phase plays a chemical crosslinking role and is reversible when heated. While the amorphous phase imparts elasticity to the polymer. Adjusting the ratio of the two phases can change the thermoelastic hardness, modulus, melting point, chemical corrosion resistance, air tightness and other properties. When polyester thermal elastomer is injected, it has good thermal stability, low melt viscosity and wide molding temperature range.
(4) Thermoelastic polyurethane thermoplastic elastomer (TPU) Thermoelastic polyurethane is a branch of solid polyurethane elastomer. It is a product made by reacting diisocyanate with polyether or polyester polyol with terminal hydroxyl and low molecular weight diol long chain agent. Hardness is controlled by the proportion of segment (diol) and soft segment (polyol); The degree of crosslinking is controlled by controlling the ratio of diisocyanate to hydroxyl (diol or polyol). Thermoplastic polyurethane is injected at a higher temperature, and its sensitivity to shear rate will also decrease due to its low viscosity.
