Due to its glossy appearance, excellent transparency, high mechanical strength and good storage of articles in the container, the waste does not pollute the environment when it is burned and disposed of, so it is very fast in the packaging bottle. It is most commonly used in pressure-resistant plastic food containers. Due to the improvement of its resin modification and processing technology, the use of polypropylene has also increased year by year.
First, the main raw materials and typical formulations
1. Polyethylene: A polyethylene resin having a melt index of l to 6 is generally used.
2. Polypropylene: A polypropylene resin with a melt index of 2 to 4 is generally selected.
3. Polystyrene: General purpose or impact type injection grade polystyrene resin is generally used.
4. Polyvinyl chloride: a hard transparent bottle formula.
Second, the main equipment and characteristics of plastic barrel production
1. Extruder and machine head:
It is basically the same as the ordinary extrusion-blowing method.
2. Parison disposal installation:
The tubular parison extruded by the extruder is subjected to the installation to stop cutting, bottom fusion and neck processing to form a test tube-shaped bottomed parison.
3. Parison heating installation:
The parison is heated and installed with an oven, a heating sleeve, a heating passage, etc., and the heating method can be heated by electric heating or infrared. However, the heating installation should be able to adjust the temperature and balance the heating to minimize the temperature difference between the inside and the outside of the parison.
4. Stretch installation: There are two kinds of tensile mandrel and stretching jig.
1 stretch mandrel
The tensile mandrel is inserted from the upper part of the parison, is pressed against the bottom of the parison under the action of hydraulic pressure, stops longitudinal stretching, and then the air hole on the mandrel is inflated by the contracted air to stop the radial stretching. Most stretch forming uses this stretch mounting.
2 stretching fixture
The stretching jig is clamped from the outside to both ends of the tubular parison, and the longitudinal stretching is stopped under the action of hydraulic pressure, and then inflated to stop the radial stretching. There are flash trims on the neck and bottom, and the fly edges can be recycled for recycling.
5. Extrusion-pulling-blowing special molding machine:
There are many types of extrusion-pull-blowing special molding machines in foreign countries. At present, there is no such special equipment in China.
Third, the production process of plastic barrels
Process control of injection molding parison
1 Injection temperature
The injection temperature is related to the type and characteristics of the original data, the type of the injection molding machine, and the thickness of the product. For crystalline resins such as polyethylene, polypropylene, etc., the injection temperature should be higher than the melting point. For amorphous polymers such as polystyrene and polyvinyl chloride, the injection temperature is higher than the viscous flow temperature. Resins with low melt batches have higher injection temperatures than resins with higher melt indices.
The use of a plunger type injection molding machine is higher than that of a screw type injection molding machine. Thin wall parisons require a higher injection temperature than thick wall parisons. Ordinary polystyrene injection temperature is 140 ° C ~ 240 ° C, polyethylene is 150 ° C ~ 280 ° C, polypropylene is 205 ° C ~ 285 ° C, PVC is 160 ° C ~ 190 ° C injection mold temperature is generally 40 ° C ~ 80 °C.
2 injection pressure
Ordinary resins have a low melt index, a thin parison wall, and a higher injection pressure, and vice versa. Plunger injection molding machines require higher pressure than screw injection molding machines. Under the condition of ensuring the quality of production, a lower injection pressure should be used as much as possible. The injection pressure of ordinary polystyrene is 58.8 to 107.9 MPa, the polyethylene is 58.8 to 98.06 MPa, the polypropylene is 54.9 to 98.06 MPa, and the polyvinyl chloride is 78.4 to 127.5 MPa.
Fourth, product specifications
Ordinary consumer plastic drums are mainly low-density polyethylene resin or high- and low-density polyethylene resin materials, while high-density polyethylene and high-molecular-weight polyethylene resins are mainly used for consumption of large containers.
Main equipment and features: The extruder and blow molding installation of large containers are the same as those for plastic drums, but a hopper should be added between the extruder barrel and the machine head.
Since the parison of the large-sized product is also large, if the extruder is slowly extruded, the parison will exhibit a serious vertical stretching phenomenon caused by its own weight, resulting in uneven longitudinal wall thickness of the product. At the same time, due to the long extrusion time, the parison is too low in temperature due to cooling, which also affects the quality of blow molding.
After the additional hopper is installed, the extruder continuously extrudes the molten material into the hopper. When the stored material reaches a certain amount, the material is extruded once and quickly by the press material to form a parison with an average thickness and proper temperature. . Therefore, the addition of the hopper not only reduces the parison sag, adheres to the parison temperature, but also consumes large products with a smaller extruder.
Five, plastic barrel product standards
There is no national or ministerial standard for extrusion-blow molding of large hollow containers. The necessary corrections can be made with reference to the GB 13508-92 polyethylene blow molding drum standard. PETP soft drink bottles perform the QB1868-93 standard.
Extrusion-stretching-blow molding, in which the plastic bottle is first formed into a tubular parison by extrusion, and then the bottom is fused to form a bottomed parison, and then the parison is processed to a desired stretching temperature of the plastic used. The mechanical force of the inner (stretched mandrel) or the outer (stretching jig) is subjected to longitudinal stretching, and at the same time or later, compressed air is blown to perform radial stretching to obtain a hollow article.
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