Blow Molding is a method that involves pre-forming through the parison by extrusion or injection,

blowing air inside to make it swell, cooling and solidifying it, and forming a solid matter of a specific shape.

■ Blow Molded Products

Blow molded products using HDPE

Blow molded products using PP

■ Process

Stage 1

Raw material resin is provided to the extruder. It is then transferred to the Die/Head of the extruder as it is melted by the frictional heat of the screw and the surrounding temperature.

Stage 2

The Die/Head exit of the extruder is a cylindrical shape from which cylindrical extrusion is issued. This cylindrical extrusion is called a parison.

Stage 3

Outside the Die/Head, generally a two-part mold is on standby. In the divided left and right molds is an empty space with the shape of the product to be molded, indicating that a cavity (mold) is formed.

Stage 4

In a state where the parison is extruded into the proper length at the Die exit, insert the parison between the two divided cavities.

Stage 5

Blow compressed air into the parison after insertion between the cavities so it fills the cavity.

Stage 6

When the parison is fully enlarged to in the shape of the cavity, it is cooled in that state. For our products, the temperature of the parison and its temperature after the cooling vary slightly by grade. The parison temperature of our company’s product is approximately 180-240 °C, whereas the appropriate temperature for the cavity coolant is about 5-20 °C.

Stage 7

Once the parison inside the cavity becomes solidified from cooling at the room temperature, open the cavity and take out the product.

Stage 8

Product molding is completed by removing unnecessary parts attached to the product.

■ Key points for selecting resin

When selecting resin for blow molding, MI, density, and molecular weight distribution of resin should be taken into account.

Normally, MI should be less than 2 to ensure good blow molding. Wider molecular weight distribution is known to have better moldability.

Since blow molded products are mainly used for containers, the exterior of a molding must have enough rigidity to withstand certain amounts of pressure and have ESCR properties to ensure that the inner contents do not leak in the case of prolonged use in cold or hot weather.

■ Mold specification needed to select the resin according to the mold design below

Density is the most influential factor in the adjustment of rigidity and ESCR properties in resin. Changes in density cause, rigidity and ESCR properties to exhibit the opposite characteristics.