Ink Base
Gel inks are water based products; water has a boiling point lower than the solvents used into a ball pen ink. For this reason refills need a rubber cap to protect them from the air, and a silicon follower to protect their back from drying. Moreover the high temperatures of the storage may cause the formation of air bubbles which may cause inconsistency on writing performances.
This formation of bubbles follows Henry law: The solubility of a gas into water is inversely proportional to the temperature and directly proportional to the pressure. When water comes out of the bottle or from the water tap, the pressure on it decrease, while the temperature increase. The solubility at this point decrease and gases condense into small bubbles which will clutch to small harshnesses on the container wall.

Microbiological Contamination
Every ink is protected with special anti bacteria and, during the production process, every product to be used must be preliminarily sterilised. Nevertheless water still remains the ideal carries for the development of fungus and mildews which could cause ink degradation.
Before using containers, tubes and injection pumps should be washed with deionised water, rinsed with ethylic alcohol and washed again with deionised water (be careful that alcohol can cause ink degradation).
This process is compulsory every time a machine is started after long stops.
Refills tubes should be kept into a dry stock and for their handling it is better to use disposable gloves.
Ink tanks should be kept covered during the entire production process.

Viscosity
Gel inks are fluids with a peculiar rheology so that they can work also at a very low viscosity. For this reason if the tip is not sealed all the ink will flow back from the refills; the silicon follower in fact act only has a non-drying agent and has no resistance to the ink flow.
Another problem due to this peculiar rheology is the push back effect into pens not well designed. In this case the cap is producing an air tight force against the wall of the tube and push the air into the tube through the tip itself causing irregular writing. Due to this pressure the ink englobe the air which block the flow. For this reason it is important to design the cap so that the tip enter into the rubber stopper before clicking into the body. In this case the air exit from the cap and is not pushed into the tip.