Plastic extrusion, we usually think that it is only used in the construction industry as doors and windows. But in fact, plastic extrusion can also be used in many industries, such as lighting, electrical appliances, transportation and medical. Product structure design also needs to learn and master plastic extrusion, and in some occasions, extrusion may be an advantageous tool for cost reduction.
Extrusion Molding Process
1.1 Introduction to the process
Extrusion molding is the plastic materials processing field of the many variations, high productivity, adaptability, wide range of uses, and the largest proportion of molding process methods.
Extrusion molding is to make the polymer melt or viscous fluid in the extruder screw or plunger extrusion through a certain shape of the mouth of the mold and continuous molding, the resulting parts for a constant cross-section of the shape of the continuous profile.
The extrusion molding of rubber is usually called press-out. Rubber extrusion molding application earlier, the equipment and technology is more mature, extrusion is to make the rubber material through the extruder continuously made of a variety of different shapes of semi-finished products process, widely used in the manufacture of tire tread, inner tube, hose and a variety of cross-section of complex shapes or hollow, solid semi-finished products, but also can be used for the operation of rubber coating is an important process in the production of the rubber industry.
1.2 Process Advantages and Disadvantages
Extrusion Molding Advantages:
Simple operation, easy to control the process, can be continuous, industrialized, automated production, high production efficiency, stable quality.
Wide range of applications, widely used in plastics, rubber, composite materials molding and processing, but also commonly used in plastic coloring, mixing, plasticizing, granulation and plastic **** mixing modification.
Extrusion mold processing is easy and simple to replace.
Equipment is simple, less investment, quick results.
The production environment is hygienic and the labor intensity is low.
Low cost of parts, suitable for mass production.
Disadvantages of extrusion molding:
Only production line type parts, simple geometry.
Part section (or cross-section) can not be changed.
Relative to injection molding, parts are less accurate.
1.3 Applications
Extrusion molding is suitable for plastic films, nets, products with wrapping layers, pipes, plates, rods, sheets, films, monofilaments, cable wrapping layers, all kinds of profiles with a certain cross-section and length continuity. It can also be used for powder granulation, dyeing, resin blending, etc.; it is also widely used in the production of a variety of reinforced plastic tubes, rods, profiles of shaped sections.
Extrusion DFM Design Guide
2.1 Material Selection
Not all thermoplastics can be processed by extrusion molding.
Common base materials include hard and elastic materials.
Hard materials: PVC, PC, PETG, ABS, PP, HIPS, PMMA, LDPE, HDPE, POM, ASA, PA, AS, EVA, PC+ABS, etc.
Elastic materials: PVC, TPU, TPE, POE, TPR, TPV, TPEE, etc.
2.2 Uniform Wall Thickness
Like the design of injection molded parts, aluminum extrusion parts, etc., uniform wall thickness is crucial.
If the wall thickness of a plastic extruded part is not uniform, with some parts being thicker and others thinner, this allows the plastic to flow unevenly as it is extruded through the mold, resulting in varying cooling rates and ultimately deformation of the part.
If deformation needs to be controlled, additional cooling processes need to be added, resulting in lower line efficiency and higher production costs.
2.2 Avoiding Complex Hollow Structures
(1) Avoiding Complex Hollow Structures
Because plastic extrusion is a continuous process, the extruded part is low in strength at the time of extrusion, and it needs to be supported by air pressure, mandrels, etc., to maintain its shape and avoid deformation. The hollow part cannot provide support, and complex hollow structures can only be realized by opening up the section.
(2) Avoid hollow in the air
Hollow in the air will inevitably deform because it cannot provide support before cooling and curing.
(3) Provide openings for internal features
If the extrusion design has an opening, a groove, it is possible to design snaps or other features inside.
2.3 Design of rounded corners
The sharp corners on the extrusion can cause stress concentration, which can make the sharp corners into a weak point of the extrusion, which is prone to cracking or failure, and can reduce the impact resistance of the extrusion.
In extrusions, sharp corners should be avoided wherever possible, and rounded corners should be added at sharp corners with a radius equal to the wall thickness of the product, which helps the material flow more smoothly during the extrusion process and reduces the stresses in the contour corners.
2.4 Design to Avoid Shrinkage
Where multiple walls intersect, usually creating areas of thicker wall thickness, then the outer surface of the extrusion is susceptible to the undesirable appearance of shrinkage, much like injection molding.
If the extrusion is a cosmetic part, shrinkage can be avoided or masked by optimizing the design as shown below.
2.5 Avoid tight tolerances in the length direction
Whenever possible, try to avoid tight tolerances in the length direction. Thermoplastics shrink and expand with temperature, and cutting plastic extrusions to very critical lengths may unnecessarily increase costs.
Generally speaking, tolerances are +/-5 mm on a 1000 mm long extrusion. Of course, tolerance accuracy is somewhat related to length, and the shorter the length, the higher the accuracy that can be achieved.
2.6*** Extrusion
*** Extrusion refers to the simultaneous extrusion of two or more materials and combining them on the same extrusion between cooling of the extrusions.
It is possible to combine two different colors in the same extrusion, or to produce a two-color extrusion with a primary color and a second color stripe.
It is also possible to compose a hard material and a material with flexibility in the same extrusion, which allows a rigid extrusion to have a flexible lip or flap, or allows two rigid extrusions to be connected by a flexible hinge.