Stretch bladders
The stretch blow molding or injection stretch blow molding is a process for the production of plastic -Hohlkörpern from thermoplastic plastic, such as PET , PVC and PP . This process is used to manufacture PET bottles . The process differs from extrusion blow molding , by means of which a hollow body is produced from a thermoplastic tube that is only drawn in length. In the stretch blow molding process, hollow bodies are produced that are stretched in the circumferential and longitudinal directions. This procedure is based on the biaxial (in two directions) alignment of the macromolecules in the plastic. The hallmark of the hollow bodies produced in the stretch blow molding process is a sprue point that can be seen on the bottom of the container, in contrast to the pinch seams visible in extruded containers.
Procedural bases
The basis for using the stretch blow molding process is the change in the chain mobility of the macromolecules under the influence of temperature. When exposed to temperature, the macromolecules can be processed above the glass transition temperature. The mobility of the macromolecules increases and they realign themselves within the workpiece. This reshaping of the material takes place in a temperature range of 80 to 120 ° C.
The biaxial orientation of the molecular chains has the advantages that a higher strength is achieved with a smaller wall thickness, a permeation reduction and a stronger surface gloss.
Procedure
In the production of hollow bodies in the stretch blow molding process (two-stage process), the preforms supplied are heated to the processing temperature of 90 to 120 ° C in the first process step in the heating furnace. The plastic that has become viscous as a result can be shaped in the preform in the second step, stretch blow molding. In the heating process, IR radiant heaters are used, past which the preforms are guided. Due to the separate adjustability of the radiator tubes, an individual heating profile can be created depending on the preform properties. The areas around the mouth remain protected by cooling units to prevent deformation of the threaded mouth area and the support / transport ring. Individual equalization times follow, during which the prefoms are not heated any further, but temperature equalization takes place over the entire wall thickness.
In the second process step, the actual molding process of the plastic bottle takes place in the blow wheel of the stretch blow molding machine. For this purpose, the tempered preforms are transferred to the blow molds . The aluminum or steel blow molds are designed as negative molds according to the desired design and are water-cooled. The stretching process begins as soon as the blow mold has been fitted with a preform that has been preheated and sealed. For this purpose, a stretching rod is inserted from above into the mouth of the preform so that the preform is mechanically stretched in length. Then the pressure molding takes place, whereby the PET material is pressed against the walls of the blow mold and receives the final contours there. Depending on the bottle size and preform quality, pressure profiles of 7 to 15 bar (pre-blow pressure) and up to 36 bar (final blow pressure) are created. The water cooling of the blow mold ensures that the now formed bottles cool down quickly. After the pressure has been released, the blow station is opened and the finished blown bottle is transported out of the machine via a conveyor system.
Machine designs and performance data
Stretch blow molding machines are designed as linear machines or as rotary machines. Linear machines have a stationary blow station with up to six blow molds (cavities), which are shifted in cycles in the temperature control and molding process. After the preforms have been heated up in the heating furnace, rotary machines use a carousel-style machine as a blowing wheel, which is equipped with the number of blow molds required depending on the output. During the carousel run, the complete molding process takes place after the blow mold has been locked. This enables continuous production for PET bottles. The use of linear or rotary machines depends on the desired output rate of PET bottles / hour and the planned container volume. Large-volume containers with a capacity of 3 liters or more are only produced in linear machines, since the acceleration forces in carousel operation when opening and closing the then very large-sized blow molds are difficult to control. Depending on the container volume, the stretch blow molding machines achieve outputs of between 1,000 bottles per hour and 2,250 bottles per hour and blow molding station. Linear machines are equipped with one to six blowing stations. The size of a stretch blow molding machine in the rotary concept ranges from four to 40 blow molding stations.
Injection blow molding
A simplified variant is injection blow molding, in which the pre-stretching by the mandrel is dispensed with and the preform is simply blown up. This means that no biaxial orientation is achieved, but only one in the circumferential direction.
Web links
literature
- Michael Thielen, Klaus Hartwig, Peter Gust: Blow molding of plastic hollow bodies. Hanser, Munich 2006, ISBN 3-446-22671-0 .
- Walter Michaeli : Introduction to plastics processing. Hanser, Munich 2010, ISBN 978-3-446-42488-3 .
- Ottmar Brandau: Stretch Blow Molding. A hands-on guide. PETplanet, Heidelberg 2003, ISBN 978-1-4377-3527-7 .
Individual evidence
- ↑ Walter Michaeli , Introduction to Plastics Processing, p. 111
- ↑ ibid., P. 166f.
- ↑ ibid., P. 172ff
- ↑ Two-stage stretch blow molding - linear stretch blow molding machines for the production of PET bottles (PDF), article in the beverage industry 6/2003
- ↑ Strech Blow Molding - Contents