Overview

Plastic extrusion molding is to heat the plastic to a flowing state, and then make it pass through the plastic mold under a certain pressure, and then make it into a continuous profile after shaping. There are many kinds of plastic products processed by extrusion, such as pipe, film, bar, plate, cable coating, monofilament and profiled section, etc. The extruder can also be used for mixing, plasticizing, dehydration, granulation, feeding and other plastic preparation processes or semi-finished product processing. Therefore, extrusion molding has become one of the common plastic molding processing methods.

Most plastic products produced by extrusion method use thermoplastic, and some also use thermosetting plastics. For example, PVC, polyethylene, polypropylene, nylon, ABS, polycarbonate, polysulfone, polyformaldehyde, chlorinated polyether and other thermoplastics, as well as phenolic, urea formaldehyde and other thermosetting plastics.

Extrusion molding has the advantages of high efficiency, low investment, simple manufacturing, continuous production, small floor area, clean environment and so on. The plastic products produced by extrusion are widely used, and the output accounts for more than one third of the total plastic products. Therefore, extrusion molding plays an important role in the plastic processing industry.

Analysis of typical structure of extrusion head

The machine head is the main part of the extrusion mold, which has the following four functions.

(1) Change the material from spiral motion to linear motion.

(2) Generate necessary forming pressure to ensure the compactness of products.

(3) The material is further plasticized through the machine head.

(4) The product with the required section shape is formed through the machine head.

Now take the pipe extruder head as an example to analyze the composition and structure of the head.

1. Die and mandrel

The outer surface of the die forming product and the inner surface of the mandrel forming product, so the shaping part of the die and mandrel determines the cross-sectional shape and size of the product.

2. Perforated plate (filter plate, grid plate)

The function of perforated plate is to change the material from spiral motion to linear motion, and at the same time, it can prevent unplasticized plastic and mechanical impurities from entering the machine head. In addition, the perforated plate can also form a certain head pressure, making the product more dense.

3. Diverter and diverter support

Diverter is also called torpedo head. The diverter turns the plastic into a thin ring for further heating and plasticization. A heating device is also installed inside the diverter of the large extruder.

The diverter support is mainly used to support the diverter and mandrel, and also to separate the logistics and enhance the mixing effect. The diverter bracket of the small handpiece can be designed as a whole with the diverter.

4. Adjusting screw

It is used to adjust the gap between the die and the mandrel to ensure the uniform wall thickness of the product.

5. Nose body

It is used to assemble all parts of the machine head and connect the extruder.

6. Calibrating sleeve

Through the sizing sleeve, the product can obtain good surface roughness, correct size and geometry.

7. Blocking

Prevent compressed air leakage and ensure a certain pressure in the pipeline.

Classification and design principle of extrusion head

1. Classification

Due to the different shapes and requirements of extruded products, there should be corresponding machine heads to meet the requirements of products. There are many kinds of machine heads, which can be roughly classified according to the following three characteristics:

(1) Classification according to the purpose of the machine head

It can be divided into pipe extruding machine head, pipe blowing machine head, plate extruding machine head, etc;

(2) Classification according to the export direction of products

It can be divided into straight head and horizontal head. The flow direction of the material in the front head is consistent with the axial direction of the extruder screw, such as the hard tube head; The direction of material flow in the rear head forms a certain angle with the axial direction of the extruder screw, such as the cable head;

(3) Classification according to the pressure of the machine head

It can be divided into low-pressure head (material flow pressure is MPa), medium pressure head (material flow pressure is 4-10mpa) and high-pressure head (material flow pressure is above 10MPa).

2. Design principles

(1) The flow passage is streamlined

In order to ensure that the material can be filled and evenly extruded along the flow channel of the machine head, while avoiding overheating and decomposition of the material, the flow channel in the machine head should be streamlined, without sharp expansion or contraction, and there should be no dead corners or stagnant areas. The flow channel should be processed smoothly, and the surface roughness should be lower than ra0.4um.

(2) Sufficient compression ratio

In order to make the product compact and remove the joints caused by the shunt support, sufficient compression ratio should be designed according to different types of products and plastics.

(3) Correct section shape

Due to the physical properties, pressure, temperature and other factors of plastic, the design of the head forming part should ensure that the material has the specified section shape after extrusion. The section shape of the head forming part is not the corresponding section shape of the product. There is a considerable difference between the two. This factor should be considered in the design to make the formed part have a reasonable section shape. Because the change of product section shape is related to the forming time, it is an effective method to control the necessary forming length.

(4) Compact structure

Under the condition of meeting the strength, the structure of the machine head should be compact, the shape should be regular and symmetrical as far as possible, so that the heat transfer is uniform, the loading and unloading is convenient, and there is no material leakage.

(5) Material selection should be reasonable

Due to the large wear of the machine head and the strong corrosivity of some plastics, the material of the machine head should be carbon steel or alloy steel with high hardness and wear resistance, and some even need chromium plating to improve the corrosion resistance of the machine head.

In addition, the structure size of the die is also related to the shape of the product, heating mode, screw shape, extrusion speed and other factors. Designers should flexibly apply the above principles according to specific conditions.

Typical extruder head and design

Common extruder heads include pipe extruder head, pipe blowing film extruder head, wire and cable coating extruder head, special-shaped material extruder head, etc.