Plastic science knowledge - extrusion molding process

What is extrusion molding?

Extrusion molding in plastic processing, also known as extrusion, in the process of the use of hydraulic press pressure on the mold itself extrusion said pressure out. It refers to the material through the extruder cylinder and the role between the screw, the side is thermoplasticized, the side is pushed forward by the screw, continuous through the head and made of a variety of cross-section products or semi-products of a processing method. Extrusion molding is a kind of efficient, continuous, low cost, suitable surface wide forming processing method, is a high polymer material processing technology appeared earlier, extrusion molding is the field of polymer processing in the production of the most variety, the most changes, high productivity, strong adaptability, wide use, output accounts for the proportion of the largest molding processing method. Extrusion molding is one of the most important forms of plastic material processing, it is suitable for most plastic materials in addition to some thermosetting plastics, about 50% of thermoplastic products are completed by extrusion molding, at the same time, also used in a large number of chemical fibers and thermoplastic elastomer and rubber products forming; Extrusion molding method can produce pipe, bar, sheet, profile, wire and cable sheath, monofilament and other forms of continuous products.

Extrusion molding process flow

Step 1 Add the ingredients

After the plastic is added to the hopper, it depends on its own weight or under the action of the forced feeder to enter the space of the screw groove and extrude forward under the push of the screw edge.

Step 2: Delivery

After the plastic enters the screw groove, every turn of the screw, all the plastic will transport forward a lead, then we call the transport rate of 1. However, for each screw, the forward throughput actually mainly depends on the friction factor fb of plastic to barrel and the friction factor fs of plastic to screw. The larger fb or fs is, the more solid plastic will be transported forward.

Step 3 Compress

It is absolutely necessary that the plastic be compressed during the extrusion process. First of all, plastic is a bad conductor of heat, if there is a gap between the particles, will directly affect its heat transfer, thus affecting the melting rate; Secondly, only under the increasing pressure along the length of the screw, will the gas between the particles be discharged from the hopper, otherwise, the product will become defective or waste products because of its internal bubbles.

Step 4 Melt

At the same time as the pressure rises, the moving solid plastics constantly contact and friction with the heated cylinder wall, and the temperature of the plastic material near the cylinder wall is constantly increased. After reaching the melting point, a thin layer of melt film is formed on the inner wall of the cylinder. After this, the heat source of solid plastic melting has two aspects: one is the conduction heat of the external heater of the cylinder. The second is the shear (internal friction) heat generated by different melt velocities in the melt film, namely the viscous dissipative heat referred to in rheology.

Step 5 Mix

In the extrusion process, under the action of high pressure, solid materials are generally compacted into dense solid plug, because there is no relative motion between the particles in the solid plug, so the mixing can only be carried out between the melt layers with relative motion.

Step 6 Vent

In the extrusion process, there are three kinds of gas need to be discharged. One is the air mixed with the powder particles. As long as the screw speed is not too high, generally speaking, this part of the gas can be discharged from the hopper under gradually increasing pressure. But when the rotational speed is too high, the material moves forward too fast, and the gas may not be fully discharged, thus forming bubbles in the product.

Step 7 Set the style

To the head after forming, after shaping products, completed

Advantages of extrusion process

1. Continuous production

According to the need to produce any length of pipe, plate, bar, profile, film, cable and monofilament.

2. High production efficiency

The single output of extruder is higher

3. Wide range of applications

This processing method is widely used in the processing of rubber, plastics, fibers, especially plastic products, almost the vast majority of thermoplastics and some thermosetting plastics can be processed with this method. Except for directly formed products. Extrusion can also be used for mixing, plasticizing, granulation, coloring, billet molding, etc

4. Multiple use of one machine

An extruder can process a variety of materials and a variety of products. As long as according to the material performance characteristics and product shape, size replacement of different screw and head, you can produce different products.

5. Simple equipment and less investment

Compared with injection molding and blow molding, extrusion equipment is relatively simple, easy to manufacture, low equipment cost, convenient installation and debugging. Equipment covers a small area, the workshop and supporting facilities requirements are relatively simple.

6. Simple structure and easy operation

Extrusion molding design need to pay attention to the problem

1. Uniformity of wall thickness

In the same product, the thickness difference of each part is too large, it will lead to uneven shrinkage, resulting in uneven distribution of stress in the product, making the product deformation serious.

2. Arc transition problem at acute angles.

Profiles are similar to the shape of Angle steel, channel steel and "working" steel in steel. At the right Angle of the profile must have a certain size radius of the arc transition, otherwise it will lead to stress concentration, products often crack in the process of use.

3. Thickness problem of profile.

There is a limit to the thickness of plastic products, otherwise, the internal stress of the forming process is enough to crack the products. Profile, due to the thickness is too large, whether the smooth forming is still a problem; Even if the molding out, the internal stress of the product is how much, this is a matter of great concern. With the permission of the user, these products can be changed into a reasonable shape. In this way, not only can ensure the smooth forming, but also can ensure that the internal stress of the product is not very large.

Extrusion molding common problems and solutions

1. Scorched plastic

Plastic coke is a common quality defect in the process of plastic extrusion, which is mainly manifested as: high temperature display; There are a lot of smoke, strong stimulation taste, serious popping sound; The extruded plastic layer has coke particles; There are continuous bubbles in the glue seam;

Solution: In the extrusion process should be strengthened to check whether the heating and cooling system work normally; Extrusion temperature should be set according to the process requirements and screw speed; Reasonable control of temperature and time, regular cleaning of extrusion system.

2. Poor plasticization of extruder

Poor plasticization mainly showed that the extruded cladding had toad skin; Plastic surface hair black, no luster, and there are small cracks; The extruded cladding has obvious seam at the gluing.

Solution: pay attention to the rationality of extrusion temperature control; The quality of the materials used should be confirmed; It is not possible to increase the extrusion speed by simply pursuing the yield. Strengthen the storage of raw materials, especially in the plastic drying process; Reasonable mold allocation to enhance extrusion pressure and screw reflux.

3. There are pores or bubbles in the extruded cladding section

Solution: the temperature of each section of screw should be controlled reasonably; Pre-dry the materials used in advance; Strict process operation requirements, improve the ability to evaluate the plasticization degree of plastics; Pay attention to the production environment and material storage conditions.

4. Extrusion bag size is not up to standard

The main performance is the deviation of core, sheath thickness or outside diameter

Solution: Should often measure the outer diameter of the sheath timely adjustment; Reasonable selection and adjustment of extrusion die; Pay attention to the tension change of the cable timely adjustment; Temperature control should be consistent with the specified requirements;

5. The bond strength of the longitudinal tape is not qualified

Solution: Pay attention to the mold requirements, and adjust according to the cable core if necessary; Can not allow the jacket to be cooled rapidly, in order to improve the bonding ability; The head temperature should not be lowered excessively while the sleeve shaping ability should be improved; Pay attention to the amount of filling so that you can scrape off a thin layer by touching the cable core with your fingers.