Introduction：Molding is the first and foremost process in the cosmetic package, hollow blow molding is one of the most commonly used plastic molding methods, but also the development of a faster plastic molding methods. Plastic products are widely used in various industries, it can be said that hollow blow molding products have been integrated into our lives. This article describes the knowledge of hollow blow molding, the content of the UPC system for the collection of friends for reference:

Blow Molding

Hollow blow molding is the extrusion or injection molding of the resulting billet placed in the mold, in the billet through the compressed air will be blown up, so that it is tightly adhered to the wall of the mold cavity, and then cooled off the mold to get the molding method of hollow products. This molding method can produce bottles, pots, buckets and other packaging containers, daily necessities and children's toys.

The plastics used for blow molding are polyethylene, polyvinyl chloride, polypropylene, polystyrene, linear polyester, polycarbonate, polyamide, cellulose acetate and polyacetal resins. High density polyethylene (HDPE) is the most consumed and is widely used for packaging of food, chemicals and handling liquids. High molecular weight polyethylene is suitable for the manufacture of large fuel tanks and drums. Polyvinyl chloride is commonly used in the packaging of cosmetics and detergents because of its better transparency and air tightness. With the development of non-toxic PVC resins and additives, as well as the development of stretch blow molding technology, the use of PVC containers in food packaging is rapidly increasing, and has begun to be used in the packaging of beer and other beverages containing carbon dioxide gas. Linear polyester is a new material that has entered the field of hollow blow molding in recent years. Because of its products have a glossy appearance, excellent transparency, high mechanical strength and containers of good preservation of goods, waste incineration does not pollute the environment and other aspects of the advantages of incineration, so the development of packaging bottles very quickly, especially in the pressure-resistant plastic food containers in the most widely used.

The use of polypropylene is also increasing year by year due to the modification of its resin and the advancement of processing technology.

02. Process classification and characteristics
Extrusion - blow molding: simple production method, high output, low precision, more applications

Injection - blow molding: high precision, good quality, high price, suitable for large batch products.

Stretch blow molding: the product is stretched, high strength and good air tightness. Including extrusion - stretch - blow molding (referred to as extrusion - pull - blow) injection - stretch - blow molding (referred to as injection - pull - blow) two methods. Blow) two methods.

blowing pressure

Blow molding, compressed air has two roles, one is to make the billet expansion and close to the wall of the mold cavity, the formation of the desired shape; the second is to play a cooling effect.

According to the different plastic varieties and type blank temperature, the air pressure is not the same, generally controlled between 0.2-0.7 MPa, the most appropriate is to make the products in the molding of the shape, pattern and so on the surface of the pressure of the clear.

For the lower viscosity, easy to deform take the lower value; for the viscosity and modulus of the higher plastic take the higher value; large volume and thin-walled products should be used with higher pressure; small volume and thick-walled products use lower pressure.

2) Inflation speed

In order to shorten the blowing time, in order to facilitate the products to obtain a more uniform thickness and better surface, the inflation rate (the volume of air flowing through the unit time) should be as large as possible, but not too large, otherwise it will bring adverse effects to the products: First, it will be in the air inlet to create a vacuum, so that the part of the parison inward, and when the parison is completely blown up, the inward part of the parison will be formed diaphragm sheet; Second, the mouth of the mold part of the parison may be pulled by the very fast air flow, resulting in scrap. Secondly, the mold part of the mouth may be pulled off by the very fast airflow, resulting in scrap.

For this purpose, it is necessary to increase the blowpipe caliber or reduce the inflation rate appropriately.

3) Blow-up ratio

The ratio of the size of the product to the size of the blank is usually called the blow-up ratio. When the size and weight of the parison are certain, the larger the size of the product, the larger the blow-up ratio of the parison. According to the plastic species, nature, shape and size of the product and the size of the blank to determine the size of the blowing ratio. Usually, the blowing ratio is controlled at 2 or 4 times.

4) Mold temperature and cooling time

In order to ensure product quality, the temperature of the mold should be evenly distributed, the mold temperature is generally maintained at 20-50 ℃. Mold temperature is too low, it will make the elongation of the plastic at the nip reduced, not easy to blow up, and make the products in this part of the thickening, and at the same time make the molding due to the difficulty of the products, such as the contour and pattern is not clear. The mold temperature is too high, the cooling time is prolonged, and the production cycle is lengthened. The products are deformed and shrinkage increases.

The mold temperature depends on the variety of plastic, when the glass transition temperature of the plastic is high, you can use a higher mold temperature; on the contrary, as far as possible to reduce the mold temperature.

Usually with the increase of the wall thickness of the product, the cooling time is extended. Sometimes in addition to the cooling of the mold, can also be molded products for internal cooling, that is, to the products inside the cooling medium (such as liquid nitrogen, carbon dioxide, etc.) for direct cooling.

5) Molding cycle

The cycle of blow molding includes extruding the parison, intercepting the parison, closing the mold, blowing, cooling, deflating, opening the cross, and taking out the product (the subsequent trimming, matching, and packaging are counted separately).