This causes changes in material viscosity that affect volume accuracy over time. This dispenser has virtually no competition in dispensing short pot life materials, because an expensive dispenser is not lost if the material is not cleaned from the unit in a timely manner. It also does an adequate job dispensing epoxy and adhesives in some surface mount applications where precision and repeatability are not an issue. Metering dispensers are basically a descendant of time-pressure syringe technology with enhancements incorporated into the design to control the amount of material dispensed. There are three basic valve technologies in this category - the diaphragm, spool and needle.
Pressure typically is applied through an electric solenoid or motorized device.
The diaphragm valve is most widely used to dispense lines of solder mask for wave solder applications. The spool valve meters material flow with a spool that is seated in a circular, fluid outlet seal. The spool is attached to a piston and when it is cycled on, the spool pulls out of the valve seal and the pressurized material flows to the dispensing tip. The spool valve is best used for dispensing lines of material and fills. It is not accurate in dispensing material dots.