Weld Lines

Whenever molten flows come together a weld line will be produced. Weld lines are common to almost all processing operations as often on passing through extruders or moulds (extruder dies, injection moulding tools, blow moulding tools), molten flows will meet obstructions which split material into different streams, this is shown in Figure 6.15. How well the materials can 'knit' back together, will affect the weld line strength.

Figure 6.15 Molten material flowing around obstruction

Figure 6.15 Molten material flowing around obstruction

In extrusion blow moulding of bottles, for the bottle to be sealed the parison must weld at the seam. The parison is pressed together by the blow moulding tool, creating a weld line as shown in Figure 6.16.

Figure 6.16 Cross-section of a parison as it is welded to form the base seal

Figure 6.16 Cross-section of a parison as it is welded to form the base seal

Chain Entanglement
Figure 6.17 Weld lines showing (i) no entanglements and (ii) chain entanglements

The strength at the weld will depend on how well the material can knit across the interface. In Figure 6.17, two possible outcomes are shown. On the right, polymer chains (indicated in bold), are able to cross over the interface forming a bond between both sides of the blown parison. To do this the polymer needs to be in the molten state: if the material solidifies, chains cannot pass from one side of the interface to the other. The left-hand diagram shows no chain entanglements at the interface and therefore there is no bond between these two plastic sections. The number of entanglements will be related to the weld line strength, the more entanglements there are the higher the strength will be. The weld line strength depends on: the contact time of the molten material, the material temperature, the cooling rate and the properties of particular materials. Materials vary in their ability to form strong weld lines. Changing the properties of a material by reprocessing may also affect weld line characteristics.

6.6 Film Blowing

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.

Plastic Film Blowing Machine Diagram
Figure 6.18 The film blowing process (Courtesy of Alpha Marathon Technologies Inc.)

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.

Was this article helpful?

0 0

Post a comment