Analyze the design criteria for plastic extrusion dies

Wall thickness

Uniform or nearly uniform section thickness will be easier to process, lower cost, better error control, better surface finish and more complex shapes. The minimum wall thickness is 0.5 mm (0.02") and the maximum wall thickness is 9.5 mm (0.375"). Thinner wall thicknesses are possible, but the santoprene 8000 thermoplastic elastomer series is required. The wall thickness should be smooth and smooth, and should be as small as possible, as this will help to equalize the stamping die.

Rib

In the process of changing the wall thickness, if the thickness changes too much and too much, problems may occur in the process of balancing the flow field. The thickness of the rib should be 50% of the nominal wall thickness and the radius should be designed based on this.

radius

Where the change is sharp, use rounded corners instead of transitions. The minimum radius of the extruded part is 0.20 mm (0.007").

Hollow

There may be a hollow section in the cross section. The extrusion die may have a hollow cross-sectional shape from the beginning, and compressed air may be used to maintain the shape in the hollow section when cooling, and another method is to use a vacuum on the outside of the extruder to help the hollow section maintain the shape. More hollow sections make the design of the mold more complicated and the maintenance of the contour shape becomes more difficult. Unless it is a design requirement, the hollow section should be minimized or even avoided altogether.

Air blowing in the process of extrusion is a means of cooling the inner wall of the component. This requires air to circulate along the cutting line or punching direction.

Foam extrusion

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 foam density specific gravity that can be achieved is 0.97 (typically unfoamed tpv) to 0.70. Lower density is affected by patents. The blowing agent degrades at 180 ° C to 190 ° C because most of the tpv is based on 195 to 215 ° C.

For mechanical methods, water is the medium of action. Here, the technology called "water foaming" is a patented technology. Special equipment is required to achieve consistent foam structure and density. The density was reduced from 0.97 to 0.20. The density within this range can be obtained by controlling the processing process. The reduction in density affects the mechanical properties, so this is categorized as the shape design in the application.

Multi-layer extrusion

The coextrusion process is a technique in which two materials are combined into one part in one extrusion process. Two extruders are connected in series to provide a bottom mold and to cause the respective polymeric materials to be coextruded together along the corresponding channels to obtain an extruded glue of the two materials. Uniform materials such as tpv and polypropylene can be fused together. Multi-layer extrusion is a good way to mix hard and soft materials. Typically, high hardness sections, such as thermoplastic elastomers tpv, are generally used as support structures for the 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 for good sealing. In the process of balancing the flow field, it is simpler to use the thermoplastic elastomer tpv with higher hardness as the rigid material instead of polypropylene.

Welding node

Heat welding is a popular method for joining extruded glue made with tpv. Heat is introduced into the connection surface, so that the surface melting, surface then bonded together, and applying a slight pressure to ensure that no gas into between the contact surfaces. After cooling, the joint is almost the same strength as the part itself. Another method of joining the extruded parts is to use a gluing system. Some fillers are required, depending on the combination of joint materials and bond strength requirements.

Hinge

A hinge is a way to eliminate stress at a point or to focus deflection at a particular point. If there is a bend at a certain point, the stress will concentrate on the corner. The hinge is a notch in the section that is thinner than the adjacent section. Since the adjacent walls are thicker, the thinnest section (hinged) will bend first when the edges are deformed. Thus, the hinge will help control the deflection of the lip. At the interface due to the bending of the thinner, the force will cause edge deformation is eliminated, but the thickness will be appropriate to meet the needs readjustment. Also, since stress occurs locally, the elastic recovery capability should be better.

Note that it is important to avoid designing the wall thickness too thin at the hinge because it requires sufficient thickness to eliminate the stresses that occur and to avoid the tendency of the part to kink. In order to ensure proper thickness, it is necessary to properly select the size of the hinge. Finite element analysis (fea) will help determine if the thickness is reasonable. In order to achieve an optimized design, the actual machining error and the final cross-section geometry should also be considered.

Lip seal and ball seal

Lip and ball seals are common sealing applications. In general, a spherical seal is preferred due to its superior elastic recovery capability relative to the lip seal. The ball seal provides a higher sealing force than the lip seal. This is because the spherical seal can provide a sealing force on each side like a lip seal. Of course, things are always fair, ball seals require more force than lip seals, which translate into higher sealing forces.

Kink

When the extruded part is mounted and bent at a certain radius, an unfavorable phenomenon may occur at that time, that is, the kink of the extruded part. Kinking may result in poor sealing or restrictions on water flow. In general, the greater the radius of the bend, the less likely the extruded component mounted next to the bend will be kinked.

In order to avoid the occurrence of kink, two possible methods can be used. The first is to increase the wall thickness of the part, which will reduce the kink. The wall thickness becomes larger, and the inner diameter is more curved than the outer diameter. Another solution is to foam the part, which foams the material so that it can compress when bent inside. It is now known that the hardness of the material has a certain effect on the kink.