Analysis of several problems in extrusion process

1, the principle of extrusion

Extrusion molding machine working principle is: the use of specific shape screw, rotating in the heating barrel, will be sent by the hopper of plastic extrusion, plastic plasticizing evenly (melting), through the nose and the different shape of the mould, make plastic extrusion into the continuity needed for various shapes of plastic layer, extruded online core and the cable.

2, the composition of the extruder

The main machine of the plastic extruder is an extruder, which consists of an extruding system, a driving system and a heating and cooling system.

In the extrusion system, focus on the problem of the nose and mold, because the pressure of the nose and the ratio of mold directly determine the quality of the cable.

The nose is composed of alloy steel inner sleeve and carbon steel outer coat, and the nose is provided with a forming mold. The function of the head is to transform the rotating plastic melt into a parallel linear motion, which leads evenly and smoothly into the mold sleeve, and endows the plastic with the necessary molding pressure.

The plastic is plasticized and compacting in the barrel, and the porous filter plate flows into the molding mold through the nose neck along a certain flow channel. The mold core and mold sleeve are properly matched to form an annular gap with a decreasing cross section, so that the plastic melt forms a continuous dense tubular coating around the core wire. In order to ensure that the plastic flow channel in the nose is reasonable, eliminate the dead Angle of accumulated plastic, often placed with a shunt sleeve, in order to eliminate the pressure fluctuation of plastic extrusion, there is also a pressure equalization ring. The machine head is also equipped with a device for mold calibration and adjustment, which is easy to adjust and correct the concentricity of mold core and mold sleeve.

3, extruder auxiliary equipment

Auxiliary machines of plastic extruder mainly include line setting device, straightening device, preheating device, cooling device, traction device, meter meter, spark testing machine, line receiving device. The purpose of the extrusion unit is different

Assistive devices are also different. Such as cutting device, blow dryer, printing device, etc.

There are several problems worth studying in the extrusion process:

1) Core deviation problem

Eccentric core is one of the most common types of plastic extrusion waste products, and various types of bending of wire core is one of the important reasons for insulation eccentricity. In the sheath extrusion, the scratches on the sheath surface are often caused by the bending of the cable core. Therefore, the straightening device in various extruding machine groups is essential. The tension of the line should be realized through the straightening device, and the stability of tension is the premise to ensure core deviation. The ratio of mold core and mold sleeve should be strictly implemented in accordance with the process.

2) Moisture problems

Some conductors have moisture on the surface and need to be preheated. I think preheating of the cable core is necessary for both insulation and sheath extrusion. For the insulation layer, especially the thin layer insulation, can not allow the existence of air holes, wire core before extrusion through high temperature preheating can completely remove the surface of water, oil. For the sheath extrusion, the main function is to dry the cable core and prevent the possibility of air holes in the sheath due to the action of moisture (or moisture around the gasket). Preheating can also prevent the plastic extrusion due to sudden cooling and residual internal pressure.

In the extrusion process, preheating can eliminate the cold line into the high temperature head, in the die mouth and plastic contact formed when the great difference in temperature, avoid plastic temperature fluctuations caused by extrusion pressure fluctuations, so as to stabilize the amount of extrusion, ensure the quality of extrusion. Electric hot wire core preheating device is used in the extruder group, which is required to ensure rapid heating up, so that the wire core preheating and cable core drying efficiency is high. The preheating temperature is generally similar to the temperature of the nose. In actual production, sheath preheating is very little, the future to this aspect to develop.

3) Cooling problems

The molded plastic extruded cladding should be cooled and shaped immediately after leaving the nose, otherwise it will deform under the action of gravity. The cooling method is usually water cooling, and according to the different water temperature, divided into rapid cooling and slow cooling. Rapid cooling is the direct cooling of cold water, rapid cooling is good for plastic extruded coating, but for crystalline polymers

In terms of sudden heat cooling, it is easy to residual internal stress inside the extruded layer organization, resulting in cracking in the process of use, and the general PVC plastic layer is quenched.

Slow cooling is to reduce the internal stress of the products, in the cooling tank section placed with different temperatures of water, so that the products gradually cool and set, PE, PP extrusion using slow cooling, that is, through hot water, warm water, cold water three sections of cooling.

But in practice, considering the cost, we all use cold water cooling, but to get good products or to consider using slow cooling.

4) Screw cooling

The purpose of screw cooling is mainly to facilitate the conveying of materials in the feeding section, but also to prevent the decomposition of plastics due to overheating, which is conducive to the gas contained in the material can return from the cold mixture in the feeding section and discharge from the hopper. Therefore, the cooling of the screw should be strengthened.

The cooling medium through the screw can be water or air.

The following points should be noted when using screw cooling water:

1 screw cooling water flow should not be too large, to the appropriate amount, touch the water to feel the water warm.

Pay attention to the change of outer diameter when using screw cooling water. When the screw and traction speed are compatible, if the screw cooling water is used, it is easy to make the outer diameter of wire and cable smaller and the insulation thickness thinner.

During operation, water should be cut off to prevent accidents.

The use of screw cooling water should be made clear during the shift.

5) Temperature control

The temperature is zoned, and the temperature in each zone is different, so it should be implemented strictly according to the requirements.

High temperature: refers to the temperature control exceeds the best plasticizing temperature of a plastic, easy to make the plastic burning and aging, but also easy to produce air holes, bubbles, air holes, bad shape and other quality problems. The temperature is too high, but also cause the extrusion pressure fluctuation in the extrusion process, plastic in the barrel "slip", extrusion quantity is not stable, so that the extruding layer and product diameter size is uneven.

Too low temperature: refers to the temperature control is lower than the best plasticizing temperature of plastic, resulting in bad plasticizing of plastic, extrusion surface resin bumps or not plasticized good small particles. In particular, poor glue stitching not only affects product quality, but also easily causes disjointed plastic layer, cracks, glue breaking and other phenomena. Therefore, to strictly control the temperature in accordance with the provisions of the process, not too high or too low.

In the actual operation process, due to the new and old equipment, the size of the different outer diameter, extrusion process is different, temperature control is not the same, extrusion insulation and sheath used plastic, but because of the different additives in the resin, its temperature control is different. In addition, the level of environmental temperature will also affect the control of extrusion temperature, winter and summer will be 5 ~ 10℃.