The design of plastic extrusion molds for extrusion thermoplastic elastomers includes the design of geometric parameters (such as wall thickness, ribs, radii, hollows, and hinges), and at the same time, they have to consider how they will affect product processing and its performance. Listed below are general guidelines for extrusion design.
Uniform or near-uniform cross-section thickness will be more easy to process, reduce costs, better error control, better surface finish and more complex shapes. The wall thickness is between 0.5 mm (0.02 ") and 9.5 mm (0.375"). Thinner wall thicknesses are possible, but the santoprene 8000 thermoplastic elastomer series is required. The change in wall thickness should be smooth and smooth, and should be as small as possible, because it will help the stamping die to be balanced.
During the change of wall thickness, if the thickness is changed too drastically, problems may occur in the process of balancing the flow field. The thickness of the ribs should be 50% of the nominal wall thickness, and the radius should be based on this design.
Where there is a sudden change, the fillet should be used instead of the transition. The radius of the extruded part is not less than 0.20mm (0.007 ").
There may be hollow sections in the cross section. The extrusion die may have the shape of a hollow section from the beginning. When cooling, compressed air can be used to maintain the shape in the hollow section. Another method is to use a vacuum outside the extruder to help maintain the shape of the hollow section. More hollow sections make the design of the mold more complicated, and it is more difficult to maintain the contour shape. Unless it is a design requirement, the hollow section should be minimized or completely avoided. Blowing in during the extrusion process is a means of cooling the inner wall of the part. This requires air to circulate along the cutting line or punching direction.
The thermoplastic elastomer tpv can be foamed by chemical and mechanical methods. For chemical foaming, a blowing agent such as a heavy hydrochloride salt can be used. The achievable specific gravity of the foam density ranges from 0.97 (typically unfoamed tpv) to 0.70. Lower density is affected by patents. The blowing agent will degrade at 180 ° C to 190 ° C, because most of the tpv basis is performed at 195 to 215 ° C.
For mechanical methods, water is the working medium. Here, the technology called "Water Foaming" is a patented technology. Special equipment is required to obtain a consistent foam structure and density. The density was reduced from 0.97 to 0.20. Densities in this range can be obtained by controlling the processing process. The decrease in density affects the mechanical characteristics, so this is classified as the design in the application.
Coextrusion is a technique that combines two materials into one part in an extrusion process. The two extruders are connected in series to provide a bottom die and co-extrud the respective polymer materials along the corresponding channels to obtain the extruded glue of the two materials. Even materials such as tpv and polypropylene can be fused together. Multi-layer extrusion is a good way to mix hard and soft materials. More typically, high-hardness section sections, such as thermoplastic elastomers tpv, are generally used as support structures for components, while low-hardness materials provide flexibility. This is common in sealing applications because the sealing area is soft and the soft material can be crushed to obtain a good sealing effect. In the process of balancing the flow field, it is simpler to use a higher hardness thermoplastic elastomer tpv as a rigid material instead of polypropylene.
Thermal welding is a popular method for joining extruded glue made of tpv. Heat is introduced to the connection surface to melt the surface, then the surfaces are bonded together, and a slight pressure is applied to ensure that no gas enters the contact surface. After cooling, the joint is almost as strong as the part itself. Another method of joining the extruded parts is to use an adhesive system. Some fillers are required, depending on the joint material's joint and bond strength requirements.
Hinge is a method to eliminate stress at a point or concentrate deflection for a particular point. If there is a bend at a point, the stress is concentrated at the corner. A hinge is a notch in a section that is thinner than the adjacent section. As the adjacent walls are thicker, the thin section (hinges) will bend first when the edges are deformed. Thus, the hinge will help control the deflection of the lips. Because of the bending at the thinner interface, the forces that cause edge deformation will be eliminated, but the corresponding thickness will be readjusted to meet the needs. Also, since stress occurs locally, the elastic recovery capacity should become better.
Note that it is important to avoid designing the wall thickness too thinly at the hinge, as it requires sufficient thickness to eliminate the stresses that occur and to avoid the tendency of the components to buckle. In order to ensure the proper thickness, the size of the hinge needs to be reasonably selected. Finite element analysis (FEA) will help determine whether the thickness is reasonable. In order to obtain a more optimized design, actual machining errors and cross-section geometry should also be considered.
Lip and ball seals are common seal applications. Generally speaking, ball seals are better because of their superior resilience compared to lip seals. Compared to lip seals, ball seals provide a higher sealing force. This is because the ball seal can provide sealing force on each side like a lip seal. Of course, things are always fair. Ball seals require more force than lip seals, and these forces translate into higher sealing forces.
When the extruded part is installed and bent with a certain radius, an unfavorable phenomenon may occur at this time, that is, the twist of the extruded part. Kink may cause poor sealing or restrict water flow. Generally, the larger the radius of the bend, the less likely it is that the extruded part installed next to the corner will twist. In order to avoid twisting, two feasible methods can be adopted. One is to increase the wall thickness of the part, which will reduce kink. Increasing wall thickness has more influence on the inner diameter bending than the outer diameter bending. Another solution is to foam the part. The foamed parts allow the material to compress as it flexes inside. It is now known that the hardness of the material has a certain effect on the kink.