TABLE OF CONTENTS A schematic of the film blowing process is given in Figure 6.18. The plastic material is fed through an extruder to an annular die opening.
The cylindrical molten tube is inflated from the inside by blowing air, creating a bubble of material that can be fed and collected onto rollers. Cooling is achieved by blowing air through a cooling ring situated above the die.
Process waste is not generated in great quantities once the machine is running and material can be reprocessed providing that it stays free from contamination. Like blow moulding, large occlusions or contaminants in the process can cause bursting and inflation problems. Agglomeration may be necessary to aid feeding in some cases, due to the low bulk densities of shredded films.
Large amounts of film scrap are available, because of the short lifetime associated with packaging and industrial film materials such as carrier bags, dustbin liners and plastic sacks. A typical lifetime of products of this type is only two years.
100% recyclate material may be used in low-grade applications such as bin liners. Other products such as carrier bags may incorporate scrap in with the virgin material to reduce cost.
Again, like blow moulding, film blowing has specific material requirements in terms of melt strength, viscosity and inflation characteristics. Generally film blowing is limited to polyolefin materials, the majority of usage being of LDPE, LLDPE and HDPE.
