Blow molding method for hollow blow molding machine
In the two-step blow molding of the hollow blow molding machine, the material to be blown is pre-blown into particles of about 40 mm in the WLK-type consolidated single-axis main blow molding machine, and then further blown using the NZ-type subsequent blow molding machine. The plastic becomes an ending granule of about 3 to 10 mm, which is particularly depicted for secondary blow molding of pre-blowing materials.
In order to prevent the blow molding machine from appearing, it is necessary to continue to properly feed the crusher, and these requirements are generally difficult to ensure by manual operation. Once the feed is excessive, it will generally cause uneven blow molding, shaking, and accompanied by high-standard noise. The result is that the material that accumulates or is being destroyed is weakened by the conflict. In addition, the higher sensitivity of the agitation data, as well as the generally higher blade wear that forms the frequent replacement blades, remains to be seen.
In contrast, the two-step blow molding enables the main blow molding machine to be fed in a non-continuous manner, where the hopper exerts a buffer effect. The optimization of the entire production process by the feeding of this method is of great significance because those operators do not have to operate the blow molding machine discontinuously and then be able to carry out other operations.
Pre-blown particles are now excellently added to the NZ-type secondary blow molding machine after being controlled by an inter-stop switch. The placement of the two blow molding machines can be selected one above the other or one after the other. The secondary blow molding machine operates on the principle of the cutting machine and is designed for pre-blown particles. Compared to general blow molding machines, the second-generation blow molding machines are much smaller in size and require only a small amount of machine drive energy. Because of its uniform feeding, it worked at a speed of approximately 450 min-1, and there was no problem. When such a blow molding machine is used for the second destruction of pre-blowing data, the noise level of the attack is much lower than the one-step processing of the same speed (n=450-500 min-1).
Hollow blow molding is to put the tube blank obtained by extrusion or injection molding into a mold, injecting compressed air into the tube blank to inflate it, and then adhering it to the cavity wall, and then cooling to release the hollow product. Forming method. This molding method can produce various packaging containers such as bottles, pots and barrels, daily necessities and children's toys.
First, blow molding materials
Plastics used for hollow blow molding include polyethylene, polyvinyl chloride, polypropylene, polystyrene, linear polyester, polycarbonate, polyamide, cellulose acetate, and polyacetal resins.
High-density polyethylene consumes the most, and is widely used in the packaging of food, chemical and process liquids.
High molecular weight polyethylene is suitable for the manufacture of large fuel tanks and barrels.
Polyvinyl chloride is widely used in the packaging of cosmetics and detergents because of its good transparency and air tightness.
With the development of non-toxic polyvinyl chloride resins and auxiliaries, as well as the development of stretch blow molding technology, the use of polyvinyl chloride containers in food packaging has increased rapidly and has begun to be used in beer and other beverages containing carbon dioxide gas beverages. .
Linear polyester materials are new materials that have entered the field of hollow blow molding in recent years. Due to its glossy appearance, excellent transparency, high mechanical strength and good preservation of the contents of the container, the waste incineration treatment does not pollute the environment, so the development of the bottle is very fast, especially It is the most widely 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.
Second, process classification and characteristics
1. Different classification according to the tube blank forming method:
Extrusion-blow molding: simple production method, high output, low precision, and more applications
Injection-blow molding: high precision, good quality, high price, suitable for large batch products.
2, according to the different molding process:
Ordinary blow molding
Stretch blow molding: The product is stretched, high strength and good air tightness.
Extrusion-stretching-blow molding (referred to as extrusion-pull-blow)
Injection-stretching-blow molding (referred to as note-pull-blow)
3, according to the number of layers of the tube blank:
Single layer blow molding
Multi-layer blow molding: good overall performance, complex production, suitable for product packaging with high packaging requirements.
Third, the influencing factors
1, blowing pressure
In blow molding, compressed air has two functions. One is to make the tube swell and close to the cavity wall to form the desired shape; the other is to cool.
Depending on the type of plastic and the temperature of the parison, the air pressure is not the same, generally controlled between 0.2-0.7 MPa, and the most suitable is to make the product show clear pressure under the shape and pattern after molding.
For lower viscosity, easy to deform lower values; higher values ​​for plastics with higher viscosity and modulus; higher pressure for large volume and thin wall products; lower pressure for small volume and thick wall products .
2, inflation speed
In order to shorten the blowing time, in order to facilitate the product to obtain a more uniform thickness and a better surface, the inflation speed (the volume of air flowing per unit time) should be as large as possible, but it should not be too large, otherwise it will bring the product Adverse effects: First, a vacuum will be created at the air inlet to trap the parison in this part. When the parison is fully inflated, the invagination part will form a diaphragm; nextly, the parison of the die part may be Extremely fast airflow breaks, causing waste.
To do this, it is necessary to increase the diameter of the blow pipe or to appropriately reduce the inflation speed.
3, the inflation ratio
The ratio of the size of the article to the size of the parison is generally referred to as the inflation ratio. When the size and weight of the parison are constant, the larger the size of the article, the larger the blow ratio of the parison. The size of the inflation ratio is determined according to the type and nature of the plastic, the shape and size of the product, and the size of the parison. The blow ratio is usually controlled at 2 or 4 times.
4, mold temperature and cooling time
In order to ensure the quality of the product, the temperature of the mold should be evenly distributed, and the mold temperature is generally maintained at 20-50 °C. If the mold temperature is too low, the elongation of the plastic at the jaws is lowered, the swelling is not easy, and the product is thickened in this portion, and the molding is difficult, and the outline and pattern of the product are not clear. The mold temperature is too high, the cooling time is extended, and the production cycle is lengthened. The product is deformed and the shrinkage is increased.
The temperature of the mold depends on the type of plastic. When the glass transition temperature of the plastic is high, a higher mold temperature can be used; otherwise, the mold temperature is lowered as much as possible.
The cooling time is generally extended as the wall thickness of the product increases. In addition to cooling the mold, the molded article may be internally cooled, that is, directly cooled into the interior of the product by introducing various cooling media (such as liquid nitrogen, carbon dioxide, etc.).
5, molding cycle
The cycle of blow molding includes the steps of extruding the parison, cutting the parison, clamping, blowing, cooling, deflation, opening, and taking out the product (subsequent finishing, matching, packaging, etc.).
Fourth, product features and applications
LDPE: food packaging container
HDPE: commodity container
Ultra high molecular weight PE: large container, melt tank
PVC: mineral water, detergent bottles
PP: thin walled bottle
PET: beverage bottle
Polycarbonate bottles can be used for filling packages at high temperatures, but if polycarbonate bottles are used to hold carbonated beverages that require good barrier properties (to prevent carbon dioxide from escaping from the beverage) or cooking oil (to prevent oxygen from entering the bottle, In order to avoid oxidation and rancidity of the edible oil, the product cannot be well protected and the desired packaging effect is not obtained.
The use of ordinary polyester stretch bottles to package carbonated beverages can effectively prevent carbon dioxide from escaping in beverages (excellent barrier properties of polyester stretch bottles), and its use in the packaging of edible oils can delay the oxidative deterioration (polymerization)é…Ž Stretch bottle barrier performance is good, can effectively prevent atmospheric oxygen from entering the bottle through the container wall), but it is used to package high-temperature filled juice products, polyester tensile bottles will be serious when filled at high temperature Degenerate and lose the value of use.
The polyethylene bottle can hold substances such as acid and alkali, and is not suitable for organic solvents such as benzene and toluene (the swelling strength of the polyethylene bottle is obviously decreased or the organic solvent escapes through the container wall).
Although nylon containers are not suitable for storing substances such as acids and bases, they are suitable for holding organic solvents such as benzene and xylene.
Multi-layer composite containers PE/Pa/PE and PE single-layer containers are almost identical in appearance. Even in the plastics industry, it is difficult to separate them in terms of appearance, but the performance of the two containers is different. The difference is very large, especially in terms of the barrier properties of oxygen, carbon dioxide, nitrogen and organic solvents.
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