Stretch blow molding process refers to a blow molding process through biaxial directional stretching. The stretch blow molding process is developed on the basis of ordinary extrusion blow molding process and injection blow molding process. The parison is first made by extrusion or injection, and then the parison is processed to a suitable stretching temperature for the plastic, and is stretched longitudinally by internal (using a stretching mandrel) or external (using a stretching clamp) mechanical force, at the same time or later, it is blown by compressed air for transverse stretching, and finally the product is obtained.
Purpose of stretching
In order to improve the physical and mechanical properties of plastics, for amorphous thermoplastics, stretching is performed within the thermoelastic range. For partially crystallized thermoplastics, the stretching process is carried out in a narrow temperature range below the crystalline melting point. During the stretching process, a certain stretching speed must be maintained. Its function is to make the plastic stretch before blow molding. The macromolecular chains are stretched and oriented without relaxing.
At the same time, it is also necessary to consider the nucleation rate of the crystal and the growth rate of the crystal. When the crystal has not yet formed, even if the appropriate stretching temperature is reached, it is meaningless to stretch the mold, so in some cases Under this condition, a nucleating agent can be added to increase the nucleation speed. The container shows excellent performance through axial and radial orientation, and the transparency, impact strength, hardness and rigidity, surface gloss and barrier properties of the product are significantly improved.
Stretch blow molding process
The one-step method refers to the three main processes of parison preparation and stretch blow molding being completed continuously in one piece of equipment. It is also called the hot parison method. The parison is a semi-finished product in the production process.
The combination of equipment is:
- It consists of an extruder and a blow molding machine;
- It consists of an injection molding machine and a blow molding machine.
Two-step production, the first step is to prepare the parison. After cooling, the parison becomes a semi-finished product to be processed, 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, also known as the cold molding method. The output and process condition control of the two-step method are unmatched by the one-step processing method, and it is suitable for mass production, but consumes more energy.
There are four combinations of stretch blow molding:
- One-step extrusion stretch blow molding, used to process PVC;
- Two-step extrusion stretch blow molding, used to process PVC and PP;
- One-step injection stretch blow molding Plastic, used to process PET and RPVC;
- Two-step injection stretch blow molding, used to process PET.
The stretch blow molding process includes: injection molding parison directional stretch blow molding, extrusion parison directional stretch blow molding, multi-layer directional stretch blow molding, compression directional stretch blow molding, etc. Its characteristics are to control the parison temperature below the melting point and use biaxial stretching to improve the strength of the product.
Injection molding parison directional stretch blow molding
The bottomed parison is first injection molded, and is continuously sent to the heating furnace (infrared or electric heating) from the conveyor belt (or rotary belt), heated to the stretching temperature, and then put into the blow mold for axial stretching with the help of a stretching rod. Stretched and finally blown into shape. The technological feature of this method is to add a stretching rod to the usual blow molding machine to axially stretch the parison 1 to 2 times. For this purpose, appropriate stretching temperature needs to be controlled. This method can be carried out with multi-cavity molds (2 to 8 pieces), and the production capacity can reach 250 to 2400 pieces/h (capacity of 340 to 1800mL beverage bottles).
Extruded parison directional stretch blow molding
The plastic is first extruded into pipes and cut into a certain length to serve as cold blanks. Put it into the heating furnace and heat it to the stretching temperature, and then take the heated parison out of the furnace and send it to the forming table through a transport device, so that one end of the parison forms a bottleneck and screw threads, so that it can be stretched 100% along the axial direction~ After 200%, close the blow mold for inflation; another method The method is to take out the heated parison from the furnace, stretch it along the axial direction of the tube in the drawing device, and send it to the blow mold. The mold clamps the stretched parison and then blows it into shape, trimming the waste edges., the production capacity of this method can reach 3000 bottles/h, with a capacity of 1L.
Selection of raw materials for stretch blow molding
Generally speaking, thermoplastics can be stretched and blown through two-way stretching, which can significantly improve the tensile strength, impact toughness, rigidity, transparency and gloss, and improve the barrier properties against oxygen, carbon dioxide and water vapor. From the technical level, the main plastics that can meet the above requirements are: 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 good barrier properties against water vapor. After biaxial stretching, it has greater low-temperature (5~20°C) brittleness. Improvement, starting from the price of raw materials and their physiological harmlessness, people are constantly researching and developing new raw materials for stretch blow molding.
