Direct Fire vs. Preheat

The advantages and disadvantages of direct fire metallizing versus those of preheat/flash metallizing.

Direct Fire

Direct fire immediately brings filaments to firing temperature and holds until evaporation is complete. This can be done via rheostat or with a single switch, manually or using a controller to monitor and regulate firing time and temperature. When cycle is complete, the controller automatically shuts the power off and vents the chamber.

Advantages

Disadvantages

Preheat/Flash

Preheating brings the filaments to the temperature where the evaporant just begins to flow and wet the fila- ments and then smoothly and steadily brings the heat up to firing temperature.

Advantages

Disadvantages

Many chambers now use computer controlled firing. This allows the use of preheat for proper wetting while also eliminating variations in firing technique.

There are different methods to control automated firing. One method uses a quartz crystal to measure the thickness of the coating applied. The desired thickness is set on a digital controller. During coating, the crys- tal communicates with either the voltage or current controller. As the thickness nears the preset value, the voltage or current is reduced. Once the desired thickness is reached, the evaporation cycle is cut off.

Feedback is easily obtainable with this method. Digital readouts can show deposition rate and also condition of the crystal. This is important because if the crystal goes bad, the system can cycle without firing. (VIEW CHART) The quartz crystal method is ideal for applications such as optics where coating thickness is an issue. Al- though this method provides us the ability to measure the exact thickness deposited and the deposition rate, it carries a higher initial cost and does not optimize filament life. Comparisons of automated vs. manual firing:

Manual Firing Advantages

Manual firing allows flexibility in firing techniques. Manual firing allows greater flexibility concerning part size. This makes this method good for job shops where the parts being fired can change from one day to the next. Filaments are used to their optimum potential.

Disadvantages

Automated firing Advantages

Disadvantages