Blow Mould: Process and Factors

Blow mould technology is commonly used for the mass production of hollow packaging containers. It is an efficient method to rapidly produce a large quantity of thin-walled parts.

There are three different methods: Extrusion Blow, Injection Blow, and Injection Stretch Blow. Each process has its specific techniques and is suitable for different industries.

Material Selection for Blow Mould

Material selection plays a crucial role. First, the material's properties must meet the product's functional requirements. Secondly, the material must be compatible with the forming process. For example, high-density polyethylene (HDPE) is typically chosen with a melt index range of 0.94–0.96 g/cm³. Low melt index resins help prevent parison sagging and facilitate uniform wall thickness. However, when the screw speed increases, low melt index resins tend to produce rough surfaces. Therefore, for products made with a blow mould of medium to large size, a lower melt index is preferred to avoid parison sagging, while smaller products usually require a higher melt index.

Temperature Control of Blowing Mold

Precise temperature control during the extrusion of the parison greatly affects its quality. If the temperature is too low, the parison surface becomes rough; if too high, the surface appears glossy but sagging worsens. When extruding heat-sensitive resins like PVC, it is critical to control the temperature carefully to avoid degradation.

Effect of Screw Speed on Parison Extrusion

Screw speed is an important factor affecting parison quality. Higher extrusion speeds increase output and reduce parison sagging, but the surface quality may decline. For some plastics like HDPE, high shear rates can cause melt fracture. Additionally, increased frictional heat at high screw speeds can cause instantaneous degradation of plastics like PVC. Therefore, machines generally use larger extrusion equipment to keep screw speeds below 70 rpm.

Effect of the Blow Mould's Die on Parison

The die is a key component that determines the parison's size and shape. Its inner surface must be polished to a smoothness level of 10, and dimensions must strictly adhere to design specifications. The die's sizing section is typically designed with a gap equal to about eight times the clearance between the mandrel and the die.

In summary, producing high-quality products depends heavily on good material selection and precise control by engineers over the machine's temperature, screw speed, and die design. 

The above information introduces essential knowledge about forming techniques and blowing mold design principles. Mastering these concepts is very helpful for improving process performance and is particularly important for beginners learning mould technology.