Stretch blow molding refers to a blow molding process that is biaxially oriented and stretched. Stretch blow molding is developed on the basis of ordinary extrusion blow molding and injection blow molding. First, the parison is made by extrusion or injection, and then the parison is treated to the appropriate stretching temperature of the plastic. It is longitudinally stretched by internal (with a stretching mandrel) or external (with a stretching fixture) mechanical force, and transversely stretched by compressed air inflation at the same time or later, and finally the product is obtained.

The purpose of stretching

It is to improve the physical and mechanical properties of plastics. For non-crystalline thermoplastics, stretching is carried out within the thermoelastic range. For partially crystalline thermoplastics, the stretching process is carried out within a narrow temperature range below the crystalline melting point. During the stretching process, a certain stretching speed must be maintained. Its function is to stretch and orient the macromolecular chains of the plastic without relaxation before blow molding.

At the same time, it is also necessary to consider the rate of crystal nucleation and the growth rate of crystallization. When the crystal has not yet formed, even if the appropriate stretching temperature is reached, it is meaningless for the bad stretching. Therefore, in some cases, a nucleating agent can be added to increase the nucleation rate. The container shows excellent performance through axial and radial directional effects, and the transparency, impact strength, hardness and rigidity, surface gloss and barrier properties of the product are significantly improved.

Process flow

The one-step method refers to the three main processes of preparing the parison and stretching and blow molding in one device, which is also called the hot parison method. The parison is a semi-finished product in the production process. The combination of equipment is: ¢Ù composed of an extruder and a blow molding machine; ¢Ú composed of an injection molding machine and a blow molding machine.

The two-step production method, the first step is to prepare the parison, and the parison becomes a semi-finished product to be processed after cooling, which has the characteristics of specialized production. The second step is to provide the cold parison to another enterprise or another workshop for reheating, stretching and blow molding, which is also called the cold bad method. The output and process condition control of the two-step method are incomparable to the one-step processing method, which is suitable for mass production, but consumes more energy.

There are four combinations of stretch blow molding: ¢ÙOne-step extrusion stretch blow molding, used for processing PVC; ¢ÚTwo-step extrusion stretch blow molding, used for processing PVC and PP; ¢ÛOne-step injection stretch blow molding, used for processing PET and RPVC; ¢ÜTwo-step injection stretch blow molding, used for processing PET.

The stretch blow molding process includes: injection molded preform directional stretch blow molding, extrusion molded preform directional stretch blow molding, multi-layer directional stretch blow molding, compression directional stretch blow molding, etc. Its characteristics are to control the temperature of the preform below the melting point and use biaxial stretching to improve the strength of the product.

Injection molded preform orientation

First, the bottom preform is injection molded, and it is continuously sent to the heating furnace (infrared or electric heating) by a conveyor belt (or a rotary belt), heated to the stretching temperature, and then put into the blow mold for axial stretching with the help of a stretching rod, and finally blown. The process feature of this method is to add a stretching rod to the usual blow molding machine to stretch the parison axially by 1 to 2 times. For this purpose, it is necessary to control the appropriate stretching temperature. This method can be carried out with a multi-cavity mold (2 to 8), and the production capacity can reach 250 to 2400 pieces/h (capacity of 340 ~1800mL beverage bottles).

Extrusion parison orientation

First, extrude the plastic into a tube and cut it into a certain length as a cold parison. Put it into the heating furnace and heat it to the stretching temperature, then take the heated parison out of the furnace and send it to the forming table through the conveying device, so that one end of the parison forms a bottleneck and a screw tooth, so that it is stretched 100% to 200% in the axial direction, and then the blow molding mold is closed for inflation; another method is to take out the heated parison from the furnace, stretch it in the axial direction of the tube in the pulling device, and send it to the blow molding mold. The mold clamps the stretched parison and then blows it into shape, trims the waste edge, and the production capacity of this method can reach 3000 pieces/h, with a capacity of 1L.

Raw material selection

Generally speaking, thermoplastics can be stretched and blow molded. Through biaxial stretching, the tensile strength, impact toughness, rigidity, transparency and gloss can be significantly improved, and the barrier properties to oxygen, carbon dioxide and water vapor can be improved. From the perspective of technical level, the plastics that can meet the above requirements are mainly: polyethylene terephthalate (PET), polyvinyl chloride (PVC), polypropylene (PP), and polyacrylonitrile (PAN). Among them, polyethylene terephthalate is used in the largest amount, and polyacrylonitrile is used in the smallest amount. Polypropylene has better barrier properties to water vapor. After biaxial stretching, its low-temperature (5~20¡æ) brittleness is greatly improved. From the perspective of raw material price and physiological harmlessness, people are constantly researching and developing new raw materials for stretch blow molding.

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