The vacuum cups used generally have a short service life that is dependent on the exposure temperature and time of the exposure. The material is an ESD safe silicone rubber and has a heat resistance of up to 180°C, however during use, the vacuum cups can be exposed to temperatures in excess of 400°C which will cause the material to break down.
When the vacuum cup becomes soft and/or sticky, it is an indication that the vacuum cup is now worn out and should be replaced.