Understanding the plastic blow molding process Blow molding is a specific manufacturing process by which hollow plastic parts are formed and can be joined (greased). It is widely used for making Blow Molded Furniture. In general, there are two main types of blow molding: extrusion blowing and injection blow molding. The extrusion blow molding process starts with melting the plastic and transforming it into a molten and hollow plastic, which is known in the industrial environment as parison or which is nothing more than a piece of plastic similar to a tube or hose with a hole in it. an end through which compressed air can pass. The formation of the parison is very important for the development of the product, the plastic hose is extruded in the trafila and in the male (bushing) of the head of the blower, the manufacture of both sets is dimensioned respecting how the formation of the parison will take place, so that the blown body has a thickness and diameter according to the design Production starts as follows: the parison is then “grabbed” in the blow mold and air is blown into it. The strong pressure of the compressed air pushes the plastic into the cavities of the mold for plastic blowing, and through the cooling system of cold water that passes inside the mold, the plastic cools and hardens, transforming into a product, and then the mold opens and the part is extracted through the blowing pin. Figure (a) demonstratively illustrates the blowing cycle of extrusion molding.
Extrusion blow molding is usually done with polymers such as high or low density polyethylene (HDPE/LDPE), as well as polypropylene (PP). There are two types of conventional extrusion molding blow molding machines, the continuous blower or the accumulator head. Machines with continuous extrusion: In this type of equipment, the extruder screw works uninterruptedly, the mold then closes when the programmed length is reached. Normally the plastic blow mold moves to the die which is cut after closing the mold and then moves to the blowing position. This type of machine is used to manufacture smaller bottles and gallons of up to a maximum of 5 liters. Machines with accumulator head: In this type of equipment, the plasticized material is stored inside the accumulator head that works as a reservoir. In this equipment concept, the mold does not need to be moved to another position during the blowing process, since the material that will form the trafil of the next cycle is being accumulated in the head, being released at the programmed moment. This last type of machine is widely used for parts larger than 5 liters, such as cans, drums, etc. In the case of injection blow molding, the formation of the vial or bottle takes place through the previously injected plastic part, which is called a preform , the preform. is an injected tube with already