Occasionally customers can encounter a problem with the polyurethane partially setting in the 4 mm pipe attached to the end of the mixer tube. The blockage causes pressure to build up in the liquid inside of the cartridge. This pressure eventually slightly expands the sides of the cartridge chamber, causing the pressurised liquid to escape around the piston at the back of the cartridge.
There are various causes of this problem as follows:
Working in a hot environment
This causes the resin to set more quickly. The recommended room temperature is about 22 C with a relative humidity of no more than 35
Alternatively, you can slow down the curing time by endeavouring to reduce the room temperature to as low a figure as can be achieved with conventional air conditioning. This will extend the gel (cure) time by several minutes and reduce the risk of blockages occurring. Or consider using a slower curing material, as suggested below.
Having the 4mm pipe longer than is absolutely necessary
We suggest that you cut off any excess 4mm pipe beyond that needed to reach all the labels on the sheets of labels you are doming. This is because the longer the mixed polyurethane is in the narrow pipe the more likely it is to become partly set and so block the flow of liquid from the cartridge.
Slow Resin Travel
Doming small labels
If you have many small labels to dome this can mean that the resin is moving along the mixer and the 4mm pipe very slowly. Particularly if the room is too warm (as mentioned above) this also can cause obstructions in the system. If you are doing a lot of small label doming it may be best to order a slower curing product such as Unisect 11LV-25 which has a 25 minute gel time as opposed to the 8 or 10 minute gel time of the standard product.
Switching the mixer and tube from the last cartridge to the new one
Unless this is done very quickly there is a chance that the resin in the mixer and pipe will have begun to set giving rise to the problems. Packs of spare mixer tubes are available from Mockridge Labels and Nameplates Ltd.
Pausing of Operator
Finally (and this is the most common cause of problems with polyurethane from cartridges) is the tendency for the machine operators to stop work or slow down the speed at which they are dispensing, with the result that the resin starts to gel in the pipe causing the pressure to rise in the cartridges.
If you leave the machine standing with a partly used cartridge in place it almost certain that the result will be leakage from the back of the cartridge. If you have to leave the machine standing for even a few minutes then it is necessary to replace the mixer and 4mm dispense tube on the cartridge before proceeding. Failure to do so will inevitably result in leakage of pressurised material from the back of the cartridge. Packs of spare mixer tubes are available from Mockridge Labels and Nameplates Ltd.