Feature article: PIM International, Vol.2 No. 2 June 2008, pages 58-61, 1463 words

Author: Frederic Jadoul, Philips Advanced Metal Solutions, Belgium

Development Engineer, Philips Advanced Metal Solutions, Steenweg op Gierle 417, 2300 Turnhout, Belgium

Philips Advanced Metal Solutions (Philips AMS) with headquarters in Turnhout, Belgium, is specialised in the production of tungsten and molybdenum components for a wide range of applications and industries. It has historically developed various components for many lamp types, and its research into refractory metal properties has opened new avenues in automotive, medical, electronics, music and chemical industries amongst others. Frédéric Jadoul explains the successful adoption of MIM technology for tungsten HID lamp electrodes.

High intensity discharge (HID) lamps have proven to be a very effective means to produce light. The luminous efficacy is in the range between 45 and 95 lm/W depending on the application. This is substantially higher than the traditional incandescent lamps (9-20 lm/W) or LED (around 40 lm/W).
The discharge between the electrodes ionises the gas and excites atoms in the arc as shown in Fig. 1. When returning to their equilibrium state the atoms emit light. Fig. 2 shows the glowing of the electrodes in a sport lamp application.
Within the discharge lamps market, two very different families co-exist:

- low pressure lamps, also known as fluorescent lamps (TL, CFL) are used as cheap, efficient diffuse light sources

- HID lamps are more compact, can give much more light and can have better light properties. Due to the compactness of the light source, the light can easily be projected on screens, scenes or just as accent lighting.

In HID lamps, the arc between the electrodes is short, whereas the pressure, current density and temperature are high. Raising the temperature of the electrodes facilitates the emission of electrons and increases the efficiency of the lamp. Therefore, in most HID lamps, the thermal load on the electrodes is high and tungsten is the material of choice. Even then, it is not uncommon for lamps to have designs which result in the tips of the tungsten electrodes melting (at around 3400°C) .......

Further sections of this article include:

- Manufacturing options for HID lamp electrodes

- Feedstock for metal injection moulding

- Summary

Figures and Tables:

Fig. 1 Discharge between two UHP electrodes

Fig. 2 HID electrodes used in a sports lamp application

Fig. 3 Production of assembled HID electrodes with assembly one of the final steps

Fig. 4 Classical tungsten electrodes made by assembling two components

Fig. 5(a) Simple re-design is achieved by laser shaping by locally melting the component, which results in a spherical or elliptical shape

Fig. 5(b) Complex laser shaped forms by re-shaping a rod and coil electrode

Fig. 6 Further example of complex shape possible by laser re- shaping

Fig. 7 (a) to (c) Laser assisted butt welding is an interesting option for small ratios between the electrode head diameter and electrode rod diameter

Fig. 8 (a) and (b) Laser cutting can be used to trim electrode head to required length

Fig. 9 Grinding can achieve the desired shape but at higher cost compared to MIM

Fig. 10 MIM HID electrodes offer unprecedented design freedom for customers

Table 1 Comparison of the various manufacturing processes