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Process parameters for hot plate welding

The welding pressure is relatively low, 0.1-0.5 MPa (15-73 psi). Part size is unlimited.

DSM’s thermoplastic materials can be hot plate welded and the process is even suitable for very flexible materials such as Arnitel TPE, but the process is less suited for Akulon PA66 and Stanyl . It is difficult to achieve satisfactory weld strength with PA66, due to oxidative degradation of the molten polymer. Stanyl has a very low melt viscosity, which causes dripping.

One of the strong points of hot-plate welding is that different materials can be assembled, e.g. amorphous and semi-crystalline polymers, or polymers with a big difference in melting point.

The recommended plate temperature depends largely on the specific thermoplastic. Amorphous plastics require a temperature 100-160°C (212-320°F) above the glass transition temperature (Tg). Semi-crystalline materials are best welded at 40-100°C (100-210°F) above the melting temperature (Tm).  

A PTFE coating on the hot plate is often used to prevent parts sticking to  the hot plate. In that case, the temperature of the hot plate should be limited to 260ºC , as the PTFE will begin to fume off at a temperature of 270-275ºC. The maximum temperature for a hot plate without PTFE is 450ºC.  PTFE coated hot plates have the following features:

- less sticking of the parts to the hot plate
- lower temperature results in less degradation of the polymer
- lower temperature results in longer cycle times
- limited life of the PTFE coating.

Sticking of the parts to the hot plate can sometimes be avoided by choosing a higher hot plate temperature. High temperature hot plate welding of PC should be done above 300ºC.

Non-contact welding is another way to prevent sticking. In that case, the plastic parts and the hot plate are separated by a small gap and the heat is transferred by radiation instead of conduction. The hot plate temperature must therefore be considerably higher. Temperatures up to 450ºC can be used. Oxidative degradation of the molten surfaces, which leads to a reduced weld strength, can easily occur in non-contact welding.

Recommended hot plate temperatures for DSM polymers are listed below.



Hot plate welding parameters for DSM Engineering Plastics

(*) Higher temperatures, even up to 450ºC, might be used for non-contact welding, or to reduce cycle times, but oxidation of the molten surfaces can easily lead to a reduced weld strength.

(**) When welding without a PTFE coating on the hot plate, use temperatures above 300ºC to prevent parts sticking to the hot plate.

It is important that the relative displacement of the parts and the welding pressure are carefully controlled. If the welding pressure is too high, the molten polymer will be completely squeezed from the weld seam and the weld strength may be too low. There are two basic principles to control the weld penetration: "welding by pressure" and "welding by distance". In the first type, the pressure is controlled throughout the process.

The most frequently used principle however is welding by distance. In that case rigid mechanical stops are used to control the dimensions.

The change-over time between melting of the polymer and assembling of the parts should be as short as possible to avoid premature cooling and to limit oxidation of the molten film.

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