The plastic extrusion machine line refers to a complete production line for extruding plastic raw materials to end-shape products. It consists of an extruder(single screw or twin screw), head and dies, and downstream auxiliary machines.
The plastic extrusion machine lines mentioned in this article is based on a single screw extruder.
3. Coating extrusion line
4. Sheet extrusion line
5. Co-extrusion line
First of all, to describe the components of the plastic extrusion machine line, we must first understand the steps of the plastic extrusion process.
According to the above process steps, we divide the plastic extrusion machine line into several parts:
For the main part of the extruder, we have described each part in detail in another article- Single Screw Extruder. In this part, we start from the steps of extrusion molding to illustrate the equipment required for an extrusion line.
In the production of certain plastic products, plastic resins and auxiliary materials can easily absorb moisture in the air during storage.
The presence of moisture in the raw materials will cause performance problems such as bubbles or markings on the products, and when it is more serious, the performance indicators of the products will be reduced. Therefore, raw materials such as ABS, polycarbonate, polyamide, polymethacrylate, and cellulose acetate in the production of molded products must be dehumidified and dried before production.
But for the production of resins such as polyethylene, polypropylene, and polyoxymethylene, if the packaging bag is intact, it is generally not necessary to carry out drying treatment.
Drying conditions before plastic extrusion
| Raw Material | Drying temperature/℃ | Drying time/h |
| Polyethylene, polypropylene | 70-80 | 1-2 |
| Polystyrene, ABS | 70-80 | 2-4 |
| Polymethylmethacrylate | 70-90 | 4-6 |
| Polycarbonate | 120-130 | 6-8 |
| Polyamide | 80-100 | 12-16 |
| Cellulose acetate | 70-80 | 2-4 |
| Hard PVC | 80-90 | 1-2 |
There are many drying methods used in the modern plastic product production industry. Common methods include hot air drying and microwave oven drying of raw materials.
For some raw materials that are dried under high-temperature conditions (such as polyamide), the raw materials are easy to oxidize and change color when exposed to air. At this time, we will use vacuum drying for drying, and this type of raw materials are not suitable for long-term storage after drying, it is best to use continuous production after the raw materials are dried and directly put into the hopper of the plastic extruder for production.
If the extruder is equipped with a normal hopper, another dryer is required to dry the raw materials first, and then put them into the hopper. If extruders are equipped with drying hoppers, which can be directly produced after drying, so as to prevent the raw materials from absorbing moisture after storage.
Dring Hooper with Fan and Electric Heating
As one of the important parts of the extruder, the screw is the part of the extruder to transport solid plastics, plasticized plastics, and melts. The changes in the geometry of each part will directly affect the quality and output of plastic products.
The performance of the machine plays a decisive role. When designing the screw, various factors must be considered to adapt to the characteristics of different plastics. In the plastics industry, it is impossible to satisfactorily produce various plastic products with just one "universal" screw.
Nowadays, with the increase of plastic varieties, ordinary screws can no longer meet the requirements of various plastic extrusion. In response to this situation, the manufacturers designed a variety of new screws on the basis of ordinary screws.
The new type of screw is mainly used to set some pins or open a part of unconventional thread sections in the normal direction of the thread to change the conveying state of the material, thereby improving the mixing capacity and melting speed of the material, so that it can be plasticized better. The new type of screw has a variety of structural forms. Currently, barrier screws, separation type screws, and split type screws are widely used.
Poor plasticization of plastic extruder occurs in the form of:
(1) The surface of the plastic layer has a toad skin phenomenon.
(2) The temperature control is low, the meter pointer reflects the low temperature, and the actual measured temperature is also low.
(3) The plastic surface is black with tiny cracks or small particles that are not well plasticized.
(4) The plastic glue is not well stitched, and there is an obvious mark.
The head is the transitional part of the die and the barrel, die is the molded part of the product. The head and the die are basically integrated, collectively referred to as the head of the extruder. Extrusion Head and Die Classification: Film blowing head, Profile head, Sheet head, Rod head, Pipe head.
The Role of the Head:
The structure of the head:
Design Points of Extruder Head:
The product has a very high temperature when it is extruded from the die, and it does not have enough strength and hardness to bear its own weight and is easy to deform. In order for the product to quickly reach the precise size and geometry, when it leaves the die, it must be sizing and cooled immediately.
There are different sizing methods according to different products. In most cases, sizing and cooling are performed at the same time. There is an independent shaping device only when extruding pipes and profiles. When extruding sheets and plates, the extrudate often passes through a pair of pressure rollers to be shaped and cooled, and extruded film, monofilament, etc. do not need to be shaped, only by cooling.
Cooling generally adopts air and water cooling, and the speed of cooling has a greater impact on the quality of the product. For rigid products, it cannot be cooled too fast, which will easily cause internal stress and affect appearance. For soft or crystalline products, it needs to be cooled immediately to avoid product deformation.
Pipes cooling and sizing methods
Profile sizing methods
After the thermoplastic leaves the die, due to the dual effects of thermal shrinkage and mold release expansion, the cross-section of the extrudate is inconsistent with the cross-sectional shape and size of the die.
Therefore, when extruding thermoplastics, the extrudate should be pulled out continuously and uniformly. Haul- off the machine, also called puller machine is used for this, the purpose is:
1. Help the extrudate leave the die in time, and maintain the continuity of the extrusion process.
2. Adjust the extrusion profile’ section size and performance.
The pulling speed should match the extrusion speed. Usually, the pulling speed is slightly larger than the extrusion speed. It can eliminate the dimensional change of the product caused by the expansion of the mold, and on the other hand, it has a certain stretching effect on the product. The stretching effect of traction can make the product moderately oriented macromolecules so that the product can be improved in terms of strength.
There are mainly two types of traction equipment: Roller type and crawler type.
The pulling speed of various products is different, usually extruding film and monofilament requires a faster speed and a larger draft. The traction speed of extruded hard products is much smaller, and the traction speed is usually determined according to the size of the product not far from the die.
The shaped and cooled products can be wound or cut as needed. Soft products are cut after being wound to a set length or quality, and hard products are cut after being sent from the traction device to a certain length. Generally, stacker bases are used to temporarily store cut products.
Some products need to be processed after extrusion to improve the performance of the product.
For example, polyamide plastics with strong hygroscopicity will absorb moisture and expand when stored in the air. This expansion takes a long time to reach equilibrium. In order to accelerate the humidity balance of such plastic products, it is often necessary to immerse in an aqueous medium to heat the humidity after extrusion molding. In this process, heat treatment to eliminate internal stress is also carried out, which is very beneficial to the performance of the product.
1. Temperature (one of the important conditions to ensure the progress of extrusion)
2. Pressure
3. Extrusion speed
4. Pulling speed
According to the characteristics of extrusion production described above, let's summarize the general operation method. Some special raw materials and special requirements need to be adjusted according to the actual situation.
It is difficult to use one or two simple points to describe the complete plastic extrusion line, because there are many kinds of raw materials and products involved, which have very different characteristics and production requirements. We can only describe what equipment is required for an extrusion line from the steps of the extrusion process. Later, we will describe in detail the specific extrusion product classification.
