Prevention of Blisters in Steam heated autoclaves
Background
The formation of blisters under hot vulcanised lagging is a serious problem that is more common in thick and stiff lagging types. In particular lagging thicknesses of 20mm and above are more prone to the formation of blisters when cured in a steam heated autoclave.
This technical bulletin will outline the causes of blister formation and will detail a series of simple steps that can be implemented to prevent this problem occurring.
How do blisters form
When applying lagging strips to a pulley it is possible for small gaps to occur at the base of the join. This is more likely to occur on thick and/or stiff lagging types as the edge sipes are less flexible and stitching along these joins is unable to compress the rubber to fill all the space below the join.
Steam moves into the gap under pressure and in contact with cold pulley shell and lagging condenses to form water which is then pushed along the join by more steam.
Once the pulley is placed into the autoclave and steam is introduced the higher pressure forces the steam into any gaps along the joins. As steam enters these gaps it is cooled by the pulley shell and lagging and condenses to form water. Steam pressure forces the water along the gap and the condensation process and formation of water continues.
Eventually as the pulley and lagging heat up the temperature reaches 100°C and at this point the water turns back into steam with a resulting 600% increase in volume. The pressure created by the expansion of the steam causes it to migrate to the weakest point and a blister forms.
As the pulley and lagging heat up they reach 100°C and the water trapped in the gap at the base of the join turns to steam with a 600% increase in volume a blister forms at the weakest point.
Elimination of Blisters
The elimination of blisters caused by steam/water ingress
can be achieved in either of two ways:
1. Remove the possibility of gaps between joins.
There is an aspect of workmanship that can contribute to having gaps between strips and for 12mm thick lagging with good workmanship gaps and the resulting blisters should not occur as the edge sipes on the sides of lagging strips are flexible enough to stitched down with no gaps. However as lagging thickness and stiffness increases even with the best workmanship there is the risk of gaps at the base of the joins.
The solution is to trim the excess rubber from the ends of the pulley shell before the pulley is cured. When the lagging finishes flush with the pulley shell end, the steel and rubber can be prepared for bonding and a 2mm thick layer of uncured rubber is applied over the steel shell end and the lagging end. This seals the end of the lagging joins preventing steam ingress.
Additionally coat the joins between each strip with two coats of rubber cement and apply a 1-2mm thick layer of uncured rubber over the length of the joins between each strip along the pulley face and is wrapped over the end of the lagging. This prevents steam ingress along the joins.
2. Use a hot air autoclave as there is no steam present.
For lagging with recessed sipes on the joins install silicone infill strips over the uncured rubber strips before wrapping with wet nylon tape.
