Fig.1. A rear light cluster - typical hot plate welding application
Hot plate welding, also known as mirror, platen, or butt welding, is used for welding injection-moulded components, for example in automotive applications, and extensively in joining plastic pipes for gas and water distribution. This article deals only with welding injection-moulded components. A typical hot plate welding application is shown in Fig.1.
The process uses a heated, metal plate to heat and melt the interface surfaces of the thermoplastic components. Once the component interfaces are sufficiently heated, the hot plate is removed and the components are brought into contact to form the weld. An axial load is applied to the components during both the heating and joining phases of the welding process.
Hot plate welding equipment
An example of a hot plate welding machine used for welding injection-moulding components is shown in Fig.2. The machine consists of four main parts; the hot plate, the machine slides, holding fixtures, and machine control system.
Fig.2. An example of a hot plate welding machine
The hot plate is typically flat, but can be shaped to match the profile of the components being welded. If necessary this could be three-dimensional. Hot plates are usually manufactured in aluminium or aluminium bronze. The latter has greater dimensional stability at the high temperatures required for the hot plate welding process.
To prevent the molten thermoplastic materials sticking to the hot plate during the heating phase of the process, the plates are often coated with a non-stick surface. Typically, a PTFE (polytetrafluoroethylene) based material is used for the coating. This could be a permanent coating or a semi-permanent, adhesive backed PTFE fabric. It is important when using PTFE-coated hot plates, not to set the temperature above 270°C, since toxic fumes are produced, which can lead to fluoropolymer fume fever.
The component-holding fixtures are mounted on to the machine slides. The slides are designed to support the holding fixtures and the components being welded, as well as applying the axial load without distorting the machine frame. The load is typically applied by a pneumatic system. The components can either be restrained in the holding fixture using mechanical clamps or using a vacuum system. It is important, where practical, that the holding fixtures support the component directly behind the welding surfaces. This will prevent distortion of the components during the heating and joining phases.
The machine controller controls the heating and joining phases of the welding process. On modern equipment, this would typically be microprocessor control led.
Hot plate welding process
In order to achieve good quality welds, it is important to understand how the process works. The process begins when the components are loaded into the holding fixtures. Components should be firmly positioned in the fixtures to ensure that the surfaces to be heated are flat and free from distortion. When the welding cycle is initiated, the components are brought into contact with the hot plate surface and the heating phase begins.
The heating process is the most important part of hot plate welding. Heating takes place in two phases. In the first phase, often known as the 'bead up' phase, the components are pushed against the hot plate, under an axial load and melting begins to occur. Once the entire joint surface of the component is melted as shown by a small continuous bead of molten material (approximately 1 to 2mm in height) around the perimeter of the component, the axial load is reduced. This reduction can be achieved either by reducing the pressure in the pneumatic system, or by using melt depth stops. The melt depth stops are positioned on the machine, between the hot plate and the machine slides to give a predetermined amount of melt or 'bead up'.
When the load is reduced, the 'heat soak' phase of the heating cycle begins. Heat is allowed to soak into the material to give a 'thermal mass' of molten material behind the surface in contact with the hot plate. This ensures that there is sufficient heat present in the material to prevent the welding surfaces cooling below the melt temperature when the components are removed from the hot plate and brought together for welding. It also ensures that there is molten material behind the weld interface when the interface material is squeezed out as the components are forced together to form the weld. If the heat soak phase is ignored, a brittle cold weld is formed.
The final phase of the welding process is the joining phase, when the components are brought together and allowed to cool under axial load. Again, melt depth stops can be used to control the amount of material displacement during the joining phase. When the joining phase is complete, the welded components are removed from the holding fixtures.
Hot plate welding parameters
There are six welding parameters that govern the hot plate welding process:
- Bead up time or bead size
- Heat soak time
- Dwell time
- Cooling time
- Heating pressure and cooling pressure
The bead up time is the time taken to achieve a minimum size bead around the perimeter of the component. This time will depend upon the wall thickness of the component being welded. The thicker the wall, the longer it will take to achieve the minimum bead size. A typical minimum bead size will be between 1 to 2mm in height.
The heat soak time is the length of time the component remains in contact with the hot plate under reduced axial load. This is again dependent upon the wall thickness of the component. The thicker the wall of the component, the longer the required heat soak time. Longer heat soak times will be needed for materials with higher melting temperature.
The dwell time is that taken for the components to be removed from the surface of the hot plate and then brought into contact with each other. Once the components are removed from the hot plate their surfaces begin to lose heat. It is, therefore, essential that the dwell time is kept as short as possible to prevent the surfaces cooling to below the temperature required for welding before being brought into contact.
The final welding parameter relating to time is the cooling time. This is the amount of time that the components remain under an axial load after they have been brought together before they are removed from the holding fixture. During the early stages of this phase, the thermoplastic molecular chains at the weld interface can diffuse to form the weld.
The heating and cooling pressure is the pressure at the component interface during the welding process. In the welding of injection-moulded components, it is recommended that melt stops are used to control the displacement of material during the heating and cooling phases. In this case, the heating and cooling pressure is not critical to the welding process, providing it is greater than the value required to push the parts against the melt stops.
The welding temperature is the temperature at which the hot plate is set in order to carry out the welding process. This is typically 60 to 100°C above the melting temperature of the material. If the temperature is set too high, it can lead to degradation of the material at the weld interface, which will result in a poor quality weld.
Typically, the butt joint shown in Fig.3 is joined by the hot plate welding process. Unlike in the majority of the plastic welding processes, the welding flash (excess material) produced around the weld line during the process is not normally hidden or removed. However, flash traps (shown in Fig.4) can be added to the weld area if the weld flash is required to be hidden.
Fig.3. A butt joint of this kind is typically joined by the hot plate welding process
Fig.4. A flash trap may be included in the welding process to hide the weld flash