How to control the molding process during large-scale blow molding?

The process flow of large-scale blow molding includes plastic melting and plasticization, extrusion of pipe blanks, blowing of pipe blanks, cooling, mold opening, removal of products, trimming, and inspection. The control of product molding can be carried out from the following four aspects.

1. Inflatable pressure

In the process of large-scale blow molding, the introduction of compressed air has two functions: one is to use the pressure of compressed air to inflate the semi molten tube blank and tightly adhere to the mold cavity wall, forming the desired shape; Secondly, it has a cooling effect on blow molded products.

The air pressure depends on the type of plastic and the temperature of the preform, and is generally controlled between 0.2 and 1.0 MPa. For plastics with low melt viscosity and easy deformation, take a lower value; For plastics with high melt viscosity, the higher value is taken, and the billet wall thickness is also taken as a higher value. The blowing pressure is also related to the volume of the product. For large volume products, a higher blowing pressure should be used, while for small volume products, a lower blowing pressure should be used. The most suitable inflation pressure should be able to make the appearance and pattern of the product clear after molding.

2. Blowing speed

In order to shorten the blowing time, which is beneficial for obtaining a more uniform thickness and better appearance of large blow molded products, it is required to inject a large flow of air at a low airflow speed to ensure that the billet can expand uniformly and quickly in the mold cavity, shorten the cooling time in the mold cavity, and improve the performance of the product. Low airflow velocity can also prevent the occurrence of Wendoulli effect and local vacuum inside the billet, which can cause the billet to collapse. This can be ensured by using a larger blowing tube.

3. Inflation ratio

Blowing ratio refers to the ratio of the maximum diameter of a large blow molded workpiece to the diameter of the billet. When the size and quality of the billet are constant, the larger the size of the product, the greater the blowing ratio of the billet, but the thinner the thickness of the product. The inflation ratio is usually determined based on the type and properties of the plastic, the shape and size of the product, and the size of the preform. As the inflation ratio increases, the thickness of the product becomes thinner, the strength and stiffness decrease, and at the same time, forming becomes difficult.

4. Blow molding mold temperature

The temperature of blow molding molds has a significant impact on the quality (especially appearance quality) of large blow molded products. The temperature distribution of the mold should be uniform, and the product should be cooled as evenly as possible. The temperature of the mold is related to the type of plastic, the thickness and size of the product.

A few plastic molds should be temperature controlled in sections. Production practice has proven that if the mold temperature is too low, the elongation rate of the plastic at the clamp will decrease, making it difficult to inflate and thicken the product in this area. At the same time, it will be difficult to form, and the contour and pattern on the surface of the product will be unclear; If the mold temperature is too high, the cooling time will be prolonged, the production cycle will increase, and the productivity will decrease. If the cooling is not enough at this time, it will also cause product demolding deformation, increase shrinkage rate, and deteriorate surface gloss. Generally, for plastics with high molecular chain rigidity, the mold temperature should be higher; For plastics with high molecular chain flexibility, the mold temperature should be lower.