How UG plastic mold design structure analysis is extrusion molding
Overview
Plastic extrusion molding is to use heating to make the plastic flow, and then pass it through a mold under a certain pressure, and then make a continuous profile after shaping. There are many types of plastic products processed by extrusion, such as pipes, films, rods, plates, cable coatings, monofilaments, and profiles with special-shaped cross-sections. The extruder can also perform preparatory processes such as mixing, plasticizing, dehydrating, pelletizing and feeding the plastics or semi-finished products processing. Therefore, extrusion molding has become one of the most common plastic molding processing methods.
Most plastic products produced by extrusion method use thermoplastics, and some use thermosetting plastics. Such as polyvinyl chloride, polyethylene, polypropylene, nylon, ABS, polycarbonate, polysulfone, polyformaldehyde, chlorinated polyether and other thermoplastics, and phenolic, urea-formaldehyde and other thermosetting plastics.
Extrusion molding has the advantages of high efficiency, low investment, simple manufacturing, continuous production, less floor space, and clean environment. Plastic products produced by extrusion molding have been widely used, and their output accounts for more than one-third of the total plastic products. Therefore, extrusion molding occupies a very important position in the plastics processing industry.
Analysis of typical structure of extrusion molding die
The head is the main part of the extrusion mould, it has the following four functions.
(1) Make the material change from spiral motion to linear motion.
(2) Generate necessary molding pressure to ensure compactness of the product.
(3) The material is further plasticized through the machine head.
(4) Products with the required cross-sectional shape are formed by the machine head.
Now take the pipe extrusion head as an example, analyze the composition and structure of the head, as shown in the figure
1. Die and mandrel
The outer surface of the molded product of the die and the inner surface of the molded product of the core rod, so the shape and size of the cross-section of the product are determined by the shaped part of the die and the core rod.
2. Porous plate (filter plate, grid plate)
As shown in Figure 8-2, the function of the porous plate is to change the material from spiral motion to linear motion, and at the same time 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 compact.
3. Splitter and splitter bracket
The shunt is also called torpedo head. The plastic is turned into a thin ring through the diverter, which is convenient for further heating and plasticization. There is also a heating device inside the splitter of the large extruder.
The splitter bracket is mainly used to support the splitter and the mandrel, and also split the stream to strengthen the stirring effect. The shunt bracket of the small head can be designed as a whole with the shunt.
4. Adjusting screw
It is used to adjust the gap between the die and the core rod to ensure the uniform wall thickness of the product.
5. Nose body
Used to assemble the parts of the head and connect the extruder.
6. Sizing sleeve
Make the product get good surface roughness, correct size and geometric shape through the sizing sleeve.
7. Blockage
Prevent the leakage of compressed air and ensure a certain pressure in the pipe.
Classification and design principles of extrusion molding die
1. Classification
Due to the different shapes and requirements of the extruded products, there must be a corresponding head to meet the requirements of the product. There are many types of heads, which can be roughly classified according to the following three characteristics:
(1) Classified according to the purpose of the machine head
Can be divided into extruder head, blower head, extruder head, etc.;
(2) Classification by product export direction
It can be divided into a straight head and a horizontal head. The direction of the material flow in the former is consistent with the axial direction of the extruder screw, such as a hard tube head; the direction of material flow in the latter is the same as the axial direction of the extruder screw. A certain angle, such as the cable head;
(3) According to the pressure in the head
It can be divided into low pressure die (material flow pressure is MPa), medium pressure die (material flow pressure is 4-10MPa) and high pressure die (material flow pressure is above 10MPa).
2. Design Principles
(1) The flow channel is streamlined
In order to make the material fill and be evenly squeezed out along the flow channel of the machine head, and at the same time to prevent the material from overheating and decomposition, the flow channel in the machine head should be streamlined and cannot be expanded or contracted sharply, let alone dead angles and stagnant areas. The runner should be processed very smoothly, and the surface roughness should be below Ra 0.4um.
(2) Sufficient compression ratio
In order to make the product compact and eliminate the joint seam caused by the shunt bracket, a sufficient compression ratio should be designed according to the different types of products and plastics.
(3) Correct section shape
The design of the molding part of the die should ensure that the material has the specified cross-sectional shape after extrusion. Due to the physical properties of the plastic, pressure, temperature and other factors, the cross-sectional shape of the molding part of the die is not the corresponding cross-sectional shape of the product. There is a considerable difference between the two, this factor should be considered when designing, so that the forming part has a reasonable cross-sectional shape. Since the change of the product cross-sectional shape is related to the molding time, controlling the necessary molding length is an effective method.
(4) Compact structure
Under the condition of meeting the strength, the structure of the machine head should be compact, and its shape should be as regular and symmetrical as possible, so that the heat transfer is uniform, the loading and unloading is convenient, and there is no leakage.
(5) The selection of materials should be reasonable
Because the machine head wears a lot and some plastics are highly corrosive, the material of the machine head should be carbon steel or alloy steel with high wear resistance and hardness, and some even have to be chrome plated to improve the corrosion resistance of the machine head. .
In addition, the structural size of the head is also related to the shape of the product, heating method, screw shape, extrusion speed and other factors. Designers should apply the above principles flexibly according to specific circumstances.
Typical extruder head and design
Common extruder heads include pipe extrusion heads, blown film heads, wire and cable coating heads, and special-shaped material extrusion heads.
Pipe extrusion head and design
1. The structure of the pipe extrusion head
The common structural forms of pipe extrusion heads are as follows:
(1) Straight tube machine head Figure 8-3 shows the straight tube machine head. The structure is simple and has a shunt bracket. The core mold is difficult to heat, and the shaping length is long. It is suitable for the extrusion of thin-walled and small-diameter pipes such as PVC, PA, PC, PE, PP and other plastics.
(2) Elbow type machine head Figure 8-4 shows the elbow type machine head. Its structure is complex, there is no shunt bracket, the core mold is easy to heat, and the shaping length is not long. Suitable for large and small diameter pipes, especially suitable for the molding of PE, PP, PA and other plastic pipes with fixed inner diameter.
(3) Side-type machine head Figure 8-5 shows the side-type machine head, which has a complicated structure. There is no shunt bracket, the core mold can be heated, and the shaping length is not long. Suitable for large and small diameter pipes.
2. Design of the head parts of pipe extruder
(1) Die
The die is a part that forms the outer surface of the pipe, and its structure is shown in Figure 8-6. The inner diameter of the die is not equal to the outer diameter of the plastic pipe, because the pressure of the pipe blank extruded from the die is suddenly reduced, and the plastic expands due to elastic recovery. At the same time, the pipe diameter will shrink under the cooling and traction of the pipe blank. . The size of these expansion and contraction is related to the plastic properties, extrusion temperature and pressure and other molding conditions and the structure of the sizing sleeve. At present, there is no mature theoretical calculation method to calculate the expansion and contraction value. Generally, it is based on the required pipe cross-sectional size. The stretch ratio determines the cross-sectional size of the die. The so-called stretch ratio refers to the ratio of the cross-sectional area of the annular gap of the die forming section to the cross-sectional area of the pipe.
