Section 1. Intro to Plastic Extrusion Machine Line
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.
Types of Plastic Extrusion Machine Line
3. Coating extrusion line
4. Sheet extrusion line
5. Co-extrusion line
Section 2. Components of single screw plastic extrusion machine line, used to produce pipes/tubes/hoses, profiles
First of all, to describe the components of the plastic extrusion machine line, we must first understand the steps of the plastic extrusion process.
Steps of the plastic extrusion process and important factors
- The preparation stage of raw materials
- Plasticization stage
- Molding stage
- Cooling and Sizing stage
- Pulling, crimping, or cutting stage of the extrudate
According to the above process steps, we divide the plastic extrusion machine line into several parts:
- a single screw main extruder (prepare raw materials and plasticization stage)
- head and die (molding stage)
- cooling tank (cooling and sizing stage)
- haul-off machine(pulling)
- cutting or stackers(crimping or cutting stage)
Components of single screw plastic extrusion machine line
2.1. Single Screw Main Extruder (Prepare Raw Materials and Plasticization Stage)
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.
- Prepare raw material stage -Dry pretreatment of raw materials
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|
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
- Plasticization stage
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.
2.2. Head and Die (Molding Stage)
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:
- Make the viscous fluid material change from spiral motion to parallel linear motion, and then import it into the die for forming
- Generate backpressure, accurately, and quantitatively transport melt materials to feed the machine head, improve product quality.
- Generate the necessary molding pressure to ensure that the product structure is tight and becomes the required cross-sectional shape.
The structure of the head:
- Die: Used to shape the outer surface of the product
- Mandrel: Used to shape the inner surface of the product
- Screen: Composed of filter plate and filter screen. The function is to make the material change from spiral motion to linear motion, and build a certain pressure along the screw direction, increase the degree of plasticization of plastic, and block impurities and unplasticized materials
- Head die set: Equivalent to the mold base, used to support the parts of the machine head
- Clamping ring: The head and the extruder are connected by screws and flanges
- Shunt: Make the plastic melt split into a thin ring to smoothly enter the molding area while further plasticizing
- Shunt bracket: Support shunt and mandrel
- Sizing sleeve: Through cooling, the cross-section of the plastic part extruded from the die is stabilized and trimmed to obtain a more accurate cross-sectional shape and size
Design Points of Extruder Head:
- The flow path of the machine head should be smooth and streamlined to reduce the flow resistance so that the material is filled and extruded evenly along the flow path of the machine head, and at the same time, the material is prevented from overheating and decomposition.
- In order to make the product compact and eliminate the diversion marks caused by the diversion bracket, the material should have a sufficient compression ratio.
- It is necessary to consider the different physical properties of each plastic and the influence of production on the shape and size of the product section to design the correct and reasonable shape and size of the head and die.
- Set up appropriate adjustment and control devices, such as melt flow adjustment, molding temperature adjustment, and adjustment of the gap between the die and the mandrel.
- Under the condition of meeting the strength, the head structure should be compact and easy to assemble and disassemble, the connection should be tight to prevent leakage, and its shape should be as symmetrical as possible to make the heat transfer uniform.
- Due to the large wear of the machine head, materials should be selected reasonably, and steel and alloy steel with high hardness, wear resistance, corrosion resistance, and high-temperature non-deformation are often selected. Some require chrome plating to improve their wear resistance and corrosion resistance.
2.3. Cooling Tank (Cooling and Sizing Stage)
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
2.4. Haul-Off Machine (Pulling), and Cutting or Stackers (Crimping or Cutting Stage).
2.4.1 Haul-Off Machine
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.
2.4.2 Cutting or Stackers
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.
The Factors Affecting the Quality and Output of the Extrudate:
1. Temperature (one of the important conditions to ensure the progress of extrusion)
- From the feeding port to the die head, the temperature needs to be gradually increased. The temperature of the screw feeding section should be not too high, and the temperature of the compression section and the homogenization section can be higher.
- The head temperature needs to be controlled between the material flow temperature and the decomposition temperature.
- The temperature of the die is slightly lower than the temperature of the machine head, but it is necessary to ensure that the plastic has good fluidity.
- The pressure of the machine head is also a part that needs to be strictly controlled. If the pressure is too high, the extruded material will be tight and reduce the output.
- Reasonably control the screw speed to ensure the accuracy of the temperature control system to reduce pressure fluctuations
3. Extrusion speed
- The speed of the motor and the screw need to be controlled in the best state to achieve the optimization of product quality and output. If the speed is too high, it is easy to make the surface of the extrudate rough and wrinkles appear. If the speed is too low, it is easy to have a poor plasticizing effect and low output.
4. Pulling speed
- The pulling speed is slightly larger than the extrusion speed to ensure the extrudate moves forward under the best molding conditions.
Section 3. Extrusion Line Operations
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.
- The plastic raw materials needed for extrusion production are best to be dried and reheated if necessary.
- Before starting up, replace the clean screen and filter, install the head, and die.
- Then heat the part of the extruder that needs to be heated, and open the cooling jacket at the bottom of the hopper.
- When each part reaches the set temperature, the joints and bolts of each part of the machine head should be checked and tightened while hot, to avoid leakage of raw materials during operation
- Start the extruder and start feeding. If using a volumetric dosing system, manual intervention is required, pay attention to the current meter, pressure gauge, and feed. But if using a gravimetric dosing controller, it will be controlled by the PLC computer.
- Before the plastic is extruded, no one should be in front of the machine head and die part.
- After the plastic is extruded, the extrudate needs to be slowly led to the cooling and puller equipment, and then the various parts are adjusted accordingly according to the indicator value of the instrument and the requirements for the end-shape products. Then the following extrusion process can work most efficiently.
- Before stopping the machine, it is generally necessary to extrude all the materials in the extruder to facilitate the next operation. Especially when using materials with poor thermal stability, the material must be used up to shut down the machine.
Section 4. Conclusion
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.