What is blow molding?
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Blow molding is common for the manufacture of hollow objects weighing more than 350 grams from the most common polymer materials, such as: LDPE, HDPE, PP, PS, PC. The parison (continuous sleeve), which is produced by an extruder, a continuous-action unit, serves as a blank for blowing.
Due to the continuous process of extrusion of the parison, the structure of the melt in it is uniform along its entire length. It has a beneficial effect on the physical properties of the molded part.
What is the blowing process?
However, the blow molding process itself is cyclical, which introduces certain difficulties in the practical aspect of the process implementation - during the formation of the next parison of the required length, a whole sequence of phases must be comes out: closing, blowing, cooling the product and removing it from the mold. I had to solve this problem in different ways: continuous extrusion and cyclic process.
The parison (continuous sleeve), which comes by an extruder - a continuous unit, serves as a blank for blowing. Due to the continuous process of extrusion of the parison, the structure of the melt in it is uniform along its entire length. It has a beneficial effect on the physical properties of the molded part.
Shuttle type machines
In machines of this type, the processes of parison extrusion and blow molding are parallel. This comes due to the reciprocating movement of the carriage with the mold from the position of gripping the parison and cutting it from above to the position of directly calibrating the neck and blowing it out.
Thus, at the moment when the parison comes out to the required length from the extrusion head, the carriage is in a position under the parison with open mold plates on both sides of the parison, the plates close, pinching the parison at both ends.
Why vacuum is provided?
Vacuum is also provided in the mold. Simultaneously with the closing of the mold, the parison trimswith a guillotine or rotary knife. After that, the carriage with the mold moves to the blowing position, where the blowing mandrel enters the neck. The blow molding blowing mandrel blows the parison, as a result of which the product is molded.
Why moveable gates are common?
The method makes it possible to obtain containers with a displaced neck that are asymmetric about the vertical axis. It is also possible in blow molding to mold regular and recessed handles, obtaining characteristic canisters of the most varied shapes and capacities. To obtain recessed handles, molds with movable gates are common, which squeeze the parison under the molded handle.
What must be the thickness of the parison?
Taking into account that the drawing of the parison will be different along its entire length. Moreover, it depends on the design of the container it is necessary to ensure the corresponding difference in thickness of the parison in certain segments. - Where the final position of the wall of the finished product is farther from the axis of the parison.
This will ensure the uniformity of the wall thickness of the product in its most diverse areas of blow molding. Correction of the parison thickness comes out in the extrusion head itself at the last section of the sleeve calibration. It is due to the relative movement of the outer ring and the calibrating mandrel, which forms the inner surface of the parison.
How the thickness can be controlled?
The inner surface of the outer ring of the extrusion head has a conical shape. Moreover, it makes it possible, by changing the relative position of the calibrating mandrel and the outer ring. It is to change the thickness of the parison ring.
How the productivity is increased in molding process?
To increase the productivity of the molding process blow molding, multi-strand dies are common. Here it divides into several strands and passes through a block of extrusion dies, in each of which a separate parison comes.
What is the disadvantage of multi-strand system?
Serial machines can have from 4 to 16 strands. The number of cavities in the mold, like blowing mandrels, corresponds to the number of strands in the extrusion unit. Obviously, the main disadvantage of multi-strand systems is their high initial cost (including tools) and high operating costs.
Therefore, with relatively small batches and the need to periodically switch to another design, it is most reasonable to stop at four-slot machines. In addition, even with a small number of cavities, the process of blow molding productivity is easy to increase by using a machine with two blowing stations.
How we can decrease cost and increase production?
In this case, a more efficient extruder serves two carriages with molds located on both sides of the extrusion head at once. They seize the parison one by one and send it to the blowing station. This design provides an 80% increase in productivity. But at the same time it is only 30% more expensive than single-station models.
Advantages
Disadvantages:
Impossibility of PET recycling
- A lot of rubbing moving parts
- Machines can have rather complex drives and automation that controls blow molding
- Insufficient performance of one machine for large volumes
- A certain complexity of multi-cavity systems
- Nevertheless, for conditions shuttle machines are still more preferable.
Blowing in machines with a reciprocating
In equipment of this type, the extruder not only plasticizes the melt, but is also responsible for pumping the melt into the extrusion head. Its screw works in the same way as in an injection molding machine in blow molding, in addition to rotating it also reciprocates to shift the melt into the extrusion head.
Introduction of the Working Principle of Blow Molding Machine
Blow molding machine is a plastic processing method that involves inflating softened thermoplastic parisons within a closed mold surface. The inflated parison solidifies to form a hollow plastic product. What is the detailed principle and working process of blow molding machine?
What is blow molding machine?
Blow molding, also known as hollow blow molding, is a rapidly developing plastic processing method. A tubular plastic parison made of thermoplastic resin is molded by extrusion or injection. While still hot or softened by heat, it is placed in an open mold and immediately inflated with compressed air inside the parison, causing the plastic to expand and tightly adhere to the inner walls of the mold. After cooling and demolding, various hollow products are obtained. The manufacturing process of blow-molded films is similar in principle to that of hollow products, but molds are not used. Here, we recommend you our hdpe blow molding machine.
From the perspective of plastic processing technology classification, the forming process of blow-molded films is usually considered part of the extrusion production line. The blow molding process was first used to produce low-density polyethylene bottles during World War II. In the late s, with the birth of high-density polyethylene and the development of blow molding machines, blow molding technology was widely used. The volume of hollow containers produced by blow molding can reach thousands of liters, and some productions are computer-controlled. Blow molding machines are suitable for blowing plastics such as polyethylene, PVC, polypropylene, polyester, and the resulting hollow containers are extensively used as industrial packaging containers.
Working principle of blow molding machine
Simply put, the working principle of a blow molding machine produced by the blow molding machine factory is similar to blowing bubbles. After the plastic and the hopper enter the screw barrel of the extruder, they are pushed toward the head by the rotation of the screw thread. Due to the resistance of the filter, splitter plate, and head die at the head of the machine, as well as the gradually decreasing capacity of the screw thread, the advancing material encounters great resistance, while also being heated by the heat transmitted from the barrel. On the other hand, when the plastic is subjected to forces such as compression, shear, stirring, etc. during movement, friction between the plastic and the barrel, between the screw threads, and between plastic molecules will produce a large amount of heat. Since the temperature of the plastic inside the barrel keeps rising, its physical state gradually changes from the glassy state to the high elastic state, and finally becomes the molten state, further completing plasticization. As the screw continues to rotate, the molten plastic is extruded from the head die in a pressure-balanced and constant amount, forming a plastic product with a certain shape. After cooling and shaping, the extrusion molding work is completed.
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