1. UV CURING – A Guide
Jon Anderson

2. Outline Background to UV technology UV lamp technology types
Temperature effect on UV curing
UV curing troubleshooting case studies

3. Background to UV curing – medium pressure mercury lamps
Energy is applied to mercury causing it to vaporise
The vapour becomes a plasma and emits UV light
Visible wavelengths 200 to 400 nm are the most useful range for curing.
Curing is polymerization of monomers and oligomers - polymer cross linking and a phase change from liquid to solid state
Fusion UV

4. Background to UV curing
Dose (J/cm2):
Total energy delivered
Related to total energy emitted time spent underneath UV light i.e. conveyer speed
Irradiance (W/cm2):
Intensity of light delivered
Related to lamp type and geometry of reflector

5. Bulb types Mercury used in bulbs to produce the UV energy
The metal can be doped with another metal (Iron or Gallium)to alter the UV output – used to cure different coating technologies such as thickfilm or pigmented systems
Different doping metals produce different spectral outputs
Standard H bulb
Iron doped bulb
H bulb: This is the general purpose UV curing lamp with strong output in the UVC ( nm) and the UVB ( nm). It is typically used for curing litho inks and overvarnishes
D bulb: With a much higher percentage of its output in the UVA ( nm), this lamp is used where deeper penetration is required. Applications include thick pigmented coatings and very thick clear coats
Courtesy UVDoctors, Inc

6. Cost – initial and running
UV Lamp technology
Arc or Microwave?
Cost – initial and running
Irradiance
Lamp length
Lamp life

7. Microwave – electrode-less
UV light generated by microwaves irradiating the mercury
300 to 600 watts/in power
10 inch maximum length bulb
10 sec start-up time
3000 hour typical lifetime
Consistent doped lamp output
In the early 1970s, a lamp that used microwave energy instead of electricity to energize mercury was introduced to the market. A simplified design of this type of lamp is illustrated in Figure 4. Without electrodes, microwave lamps do not suffer some of the problems inherent to arc lamps. There is no aging of the lamp due to decomposition of the electrode, or reduction in output due to blackening of the lamp. Microwave lamps have more consistent UV output and generally last longer than arc lamp counterparts. However, the physics behind the microwave coupling in the lamp restricts the bulb length to certain discrete sizes. Today, microwave lamps that are 3”, 4.5”, 6” and 10” in length are commercially available. Other curing lengths require a number of shorter lamps be placed end-on-end in a modular fashion. This makes microwave lamps ideal for smaller-footprint applications, but a bit more complex and cumbersome for larger footprints, with redundancy in components such as power supplies, or cooling ductwork (Table 2).
 Since microwave lamps do not require “restriking” an electric arc within the lamp, they can be refired in a matter of seconds, rather than minutes. Due to the more complex nature of the microwave lamp, its sophistication and various patent related issues, there are fewer (though well qualified) suppliers of microwave UV systems
Figure from

8. Arc – with electrode 200 to 750 watt/in power 2 to 3 min start-up time
Lengths up to 72 inches
2 to 3 min start-up time
1000 hour typical lifetime
Metal halide (MH) lamps consist of an arc tube (also called a discharge tube or "burner") within an outer envelope, or bulb. The arc tube may be made of either quartz or ceramic and contains a starting gas (usually argon), mercury, and MH salts. Traditional quartz MH arc tubes are similar in shape to mercury vapor (MV) arc tubes, but they operate at higher temperatures and pressures.
MH lamps start when their ballast supplies a high starting voltage higher than those normally supplied to the lamp electrodes through a gas mixture in the arc tube. The gas in the MH arc tube must be ionized before current can flow and start the lamp. In addition to supplying the correct starting voltage, the ballast also regulates the lamp starting current and lamp operating current. (See "What types of ballasts are available to use with metal halide lamps?")
As pressure and temperature increase, the materials within the arc tube vaporize and emit light and ultraviolet (UV) radiation. A bulb (also called "outer jacket" or "outer envelope"), usually made of borosilicate glass, provides a stable thermal environment for the arc tube, contains an inert atmosphere that keeps the components of the arc tube from oxidizing at high temperatures, and reduces the amount of UV radiation that the lamp emits. Some MH lamps have a coated finish on the inside of the bulb that diffuses the light. Often a phosphor coat is used to both diffuse the light and change the lamp's color properties.
Figure from

9. Curing UV product range
Cures with arc or microwave technology but…..
Sufficient dose and irradiance required
Bulb type important – UV range requires UVA, B and C so ‘H’ bulb must be used
Correct power but wrong bulb = uncured material
UV range = UV40 and 1C63 (UV curable silicone)

10. Dose and Irradiance values
These are the minimum values that will cure UV40 range and result in tack-free surface
But….don’t take them as absolute. Always check the cure of the material!!!
There may be other factors involved:
Temperature from excess IR can lower requirement for dose and irradiance
Reflectance – too much/too little can lead to surface defects
UV C is hardest to reflect.

11. Arc vs. Microwave? Irradiance Microwave Arc
Lamp technologies deliver UV energy in different way Microwave delivers a higher peak irradiance than arc
Microwave
Irradiance
Arc

12. Temperature Effect on UV curing
Experiment conducted to observe increased temperature effect on the UV curing of UV40-250:
1. After coating application test coupons were heated to a set temperature
2. Material was tested for tack-free surface after UV cure
Microwave
Arc

13. Temperature Effect on UV curing
Both lamp technologies will generate IR during operation. Increased IR can reduce the threshold required to provide a full cure i.e. tack-free surface
Microwave threshold
Arc threshold
Lower arc threshold with increased IR

14. Case Study 1: Tacky coating
Coating surface sticky after passing under UV light – surface should be tack-free
Due to insufficient UV C irradiance
- check bulb type – should be mercury ‘H’ bulb
- check lamp height. It may be out of focus = insufficient energy reaching surface
- check reflector – may be dirty/oxidised so not reflecting effectively

15. Lamp Focus
UV Process Supply Inc.

16. Lamp Focus Irradiance In focus Out of focus
Out of focus lamp does not deliver as high intensity UV energy to the coating
In focus
Irradiance
Out of focus

17. Case Study 2: Wrinkling
Coating surface is not smooth after UV curing

18. Causes: Excessive ventilation disturbing the coating surface
excessive temperatures – not enough cooling
Excessive reflection – too much UV light reflected is stressing the coating surface

19. Case Study 3: Curing 3D device
Curing on flat devices (low standoff heights) is standard for most UV curing processes
What about sides of components or housings? – these can be difficult to cure fully

20. Solution – additional reflector
Standard ½ ellipse reflector for ‘flat’ surfaces – some energy is lost out the sides of the reflector
Additional reflector to add depth to UV cure results in increased energy reflected
UV Process Supply Inc.

21. Summary The lamp technology is irrelevant (UNLESS ITS LED!)
What matters is the wavelength, amount and intensity of UV energy reaching the coating
Determined by:
Bulb type, lamp focus, reflectance, conveyer speed

22. Useful links for further reading
for-selecting-a-uv-curing-system

23. Any questions?