The blow moulding technique
Extrusion blow moulding, also known as blow moulding of hollow bodies, is a plastics processing method for the production of hollow bodies from thermoplastics.
In this process, the molten polymer is pressed through the nozzle by means of an extruder via a conveying screw, resulting in a tubular parison.
This is transferred to a blow mold and adapted to the shape of the mold by internal pressure and the internal contours (see pic. 1).
A uniform wall thickness of the blow molded parts is of decisive importance for the material requirements, a good wall thickness distribution ensures flat surfaces without ribbing.
Cooling time can be significantly reduced by a high wall thickness constancy. For technically demanding blow molded parts, the radial profile is also changed during the blow molding cycle. This is achieved by means of a flexible ring inserted in the nozzle, which can be deformed in a controlled manner with servo cylinders. This process is known as PWDS (Partial Wall Thickness Control).
The wall thickness of the parison is controlled by an adjustable outlet area at the nozzle of the parison head. A servo-hydraulic system moves the two conical parts of the nozzle towards each other and controls the flow rate over the remaining gap width. This allows on the one hand to adjust the wall thickness in the finished product and on the other hand to continuously adjust the wall thickness of the hose during its formation (see picture 2).
In the next step of the blow molding process, the water-cooled blow mold closes around the extruded parison. The parison is separated from the material strand still exiting the nozzle by means of a separating device (knife, glow wire cutter or hose closing flap). During or immediately after the parting process, the clamping unit with the blow mold in it moves under the inflowing material into the inflation station (see Fig. 3).
(Pic. 3) (vgl.https://www.techpilot.de/lexikon/blasformen/)
The "blowing" process
1.) The mould moves diagonally or in an arc down to the inflation station.
2.) During and after the parting process, the extruder including the parison head is lifted in each cycle, after the mould has been moved into the inflation station, the extruder is lowered again. Blow molding machines with this concept thus have a technically simpler option with a linear horizontal movement of the clamping unit.
When the blow mould reaches the blowing station, a likewise water-cooled blow mandrel dips into the parison in the blow mould, forms the neck contour of the later blow moulded part with the tip of the blow mandrel and applies compressed air to the inside of the parison squeezed into the mould.
The compressed air causes the parison to stretch and lie against the cooled wall of the blow mold. The air on the outside of the parison escapes via the mould parting line and via additional venting holes in the blow mould. The hot, plastically deformable plastic, which is pressed against the mold wall by the compressed air, then releases a large proportion of its thermal energy to the cooled blow mold and solidifies. Blow molding of polyolefins is usually carried out at inflation pressures of 6 to 10 bar.
Depending on the process, the thermal energy is released during blow molding via the outside of the blow mold. Since plastic is a poor conductor of heat, the required inflation and thus cooling time increases very significantly in a quadratic function with increasing article wall thickness, which requires a quadrupling of the inflation time if the article wall thickness is doubled.
The long cooling times for thick-walled blow molded parts (and the resulting lower productivity) are attempted to be reduced by additionally cooling the inside of the article by means of purge air, reverse air, water mist, cryogenic CO2 or liquid nitrogen.