“Common Dispensing Mistakes Made When Using Light-Curable Adhesives”
There are several mistakes that can be made when implementing dispensing systems for light-curable materials. Understanding these common pitfalls and taking the suggested precautions will help reduce problems when dispensing.
- Incompatible Materials – Dispensing system materials of construction should be compatible with your light-curable adhesive. Metal parts and fittings should only be 300-series stainless steel. Other metals can cause the adhesive to polymerize. Plastic materials that are compatible with Dymax light-curable adhesives include polyethylene, polypropylene, teflon, nylon, and acetals. Other plastics may be attacked by the resin.
- Transparent Fluid Lines – Black or opaque plastic fluid lines should be used to ensure that the adhesive is not exposed to ambient light, which may cause the adhesive to polymerize.
- Air Bubbles – Air bubbles may become trapped in fluid lines when an empty adhesive container is removed for replacement in a dispensing system. To avoid this problem, purge the fluid line after refilling or replacing the empty container. Maintaining only the line length necessary to transport the fluid from the reservoir to dispense point will facilitate the purging process.
- Pour-In Pressure Pot versus Drop-in Pressure Pot – For lower viscosity fluids (<500 cP) that naturally release air bubbles, either pour-in or drop-in pressure pots can be used. For fluids that do not release air bubbles naturally, drop-in pressure pots are recommended. Dymax recommends the use of a 10 gallon drop-in pressure pot for adhesives with viscosities up to 25,000 cP. For resins with viscosity >25,000, or where pressure exceeding 30 psi (0.2 MPa) is required for dispensing, ram-style pail pumps are recommended.
- Excessive Air Pressure – The application of excessive air pressure >30 psi (0.2 MPa) to pressure pots may cause air to dissolve into the adhesive. When this pressure is alleviated (either when the pressure pot is opened or the fluid is dispensed) this dissolved air may come out of the solution in the form of air bubbles that become trapped in the adhesive. To maintain appropriate pressure and prevent the formation of air bubbles in the adhesive, use larger ID fittings and tubing, minimize tubing length, fully open the dispense valve, and use a shorter and larger ID dispensing needle. If none of this is effective and pressure of 30 psi (0.2 MPa) or greater is still needed, a “ram-style pail pump” is recommended. This involves force being applied directly to the adhesive in the pail via a follower plate allowing for very high pressures without air. Ram-style pail pumps are recommended for resins with a viscosity of 25,000 cP or greater.
- Narrow and Long Fluid Lines – Generally, the shorter and wider the fluid line, the better. A fluid line diameter of 3/8″ (10 mm) is desirable. The longer and narrower the line, the more air pressure is required to transfer the fluid to the dispense valve. This can result in a slow flow rate and the need for high pressure to move the material with the unfortunate result of air-bubble formation.
- High-Shear Pumps and Valves – The use of pumps that produce shear, such as gear pumps, is not recommended with light-curable materials. Shear occurs when the adhesive is caught between two tightly fitting, moving metal parts, which can cause the adhesive to polymerize and clog the system. Simple pressure pots with pneumatic and ram-style pail pump systems are recommended.
- Positive Displacement Valves – Positive displacement valves should be tested for compatibility with light-curable materials prior to their incorporation into a dispense system. Contact Dymax for further guidance in selecting an appropriate valve for dispensing a particular adhesive.
- Using Vacuum to Remove Air Bubbles – A vacuum should not be used to remove air bubbles from a light-curable material. The use of a vacuum may remove constituents from the adhesive, altering performance and/or reducing its shelf life.
- Excessive Vacuum Suck-Back on Syringe Dispensers – Caution should be taken to apply only the amount of suck-back or vacuum pressure needed to prevent adhesive drip following dispensing. Excessive vacuum pressure may pull the plunger out of the syringe barrel or suck air into the syringe, creating bubbles.