Melt processable rubber is an amorphous polymer with no distinct melting point, which can be produced by injection, extrusion, blow molding, or calendering using resin or rubber processing equipment. Melt flow properties are similar to rubber but different from thermoplastics. The polymer does not melt only from external heat and becomes very viscous, hard to process semi-liquid. A plasticizing screw must be used to melt and flow material under shear stress. When forming without shear, viscosity and strength of melt increase sharply. Even under conditions of shearing and a certain heating, when mold is filled and plasticizing screw is stopped or rotated, rate of increase in viscosity and strength of melt is still very high.
The melt rheology of melt-processed rubbers can be illustrated with Aleryn polymers. The combination of external heating and stored shear heat can cause material to melt at temperatures between 160 and 166°C. The melting point cannot be higher than 182°C. As technology of melt processing improves, more and more new polymers appear.
According to rheological law of melt-processed rubber, during injection molding, it can be quickly demoulded, and processing temperature can also be set to a lower temperature, which greatly shortens processing cycle and reduces production costs. Its higher melt strength can reduce or eliminate product warping and mold sticking, and machine is easier to clean. Melt processable rubbers typically consist of halogenated (chlorinated) olefins and copolymers of ethylene with reactive intermediates capable of donating H+.
Aleryn is a typical single-phase melt-processed rubber with general mid-range characteristics that can replace COPE thermoplastic elastomer. Single phase polymer blends are compatible while multi phase blends are incompatible and a compatibilizer must be added during blending. The relative density is 1.08~1.35. Various performance enhancers can be added, especially stabilizers, plasticizers and fire retardants.
Alcryn is commonly used in automotive glass seals and fuel tank gaskets, industrial door and window seals and protective film, wire and cable sheathing, and power hand tool handles, non-slip soft grip hand tools. It has good weather resistance, chemical resistance and cushioning performance. The translucent Aleryn is extruded into thin films that are used to make masks, hoses and ports with soft buttons for computers or phones through injection molding. Some categories can be painted directly without need for a primer. Typical DuromeThe benchmarks are 60A, 76A and 80A Shore hardness.
The halogen content of melt-processed rubbers requires use of anti-corrosion equipment and cavity-type tool steels, as well as proper ventilation during processing. The influence of melt viscosity and strength must be taken into account when designing products, machines and tools (wall thickness gradient, radius, screw parameters, gate shape and size, and impeller dimensions). The processing temperature and pressure parameters must be calculated according to rheological formula.
In order to melt hard granules evenly, a medium-sized screw with a shallow groove must be used. The melt maintains uniform flow in mold (small gate for high shear), large bore and large gate for stable mold filling. The closure must be symmetrical and rounded to ensure a uniform and stable melt flow. Symmetrical and rounded runners are part of most traditional mold designs, but they are very important for melt rubber mold designs. The molds have large ejector pins or flat plates to ease removal of rubber during demoulding. The mold must be cooled down to 24°C. Mold temperatures vary with different product categories and applications, and hot molds are typically used to achieve smooth surfaces and reduced orientation.
When using extrusion and blow molding, requirements for injection molding are same, that is, it is necessary to maintain a uniform and appropriate flow of melt. Shallow knurled screws can improve shear and mixing. Extrusion using a screw, a long barrel and a screw can provide a more uniform melt flow. From feeding section to dispensing section, temperature setting has increased, temperature setting of feeding section is relatively high, and temperature of dispensing section and head is relatively low. The extrusion die has a conical shape, horizontal pressure bearing surface is small, and die size is close to size of molded part. This is due to fact that Alcryn has a small coefficient of thermal expansion.
The melt rheological behavior of this polymer is favorable for formation of preforms during blow molding, since preforms are not subjected to shear stress, and solidification begins at 166°C. The high viscosity of melt provides blowdown ratios above 3:1.
Technological requirements for thermocompression molding and calendering of melt-processed rubber elastomers are similar to injection molding and extrusion molding.