JEC COMPOSITES MAGAZINE - Issue #77 - December 2012 - 46

IR welding
in lower strengths. Using no extra sheets of PPS has an even worse effect on the failure forces of the bond, as these are the lowest of all experiments discussed here. An optimum choice for this welding procedure appears to be two layers of PPS. By the time the initial pressure was applied, the temperature had already dropped to around 240°C as a result of convective cooling to the ambient air. This, however, is an important temperature for PPS, as it is approximately the temperature where crystallization occurs during cooling. The cooling rate during the pressure step of the welding process has a great influence on the strength of the bond and can be examined with differential scanning calorimetry (DSC) [6,7,8]. If the temperature must be controlled more accurately, it is recommended that both plunger and anvil have a temperaturecontrolled heating and cooling system implemented. Two-sided welding The principle referred to as ‘one-sided welding’ worked but yielded poor quality and poor reproducibility of the bonds, which are not acceptable in any production process. Therefore, it was decided to add the PPS in a separate phase prior to welding, as illustrated in Figure 4. In the initial preparation phase, referred to as Welding Phase I, layers of PPS are placed exactly at the location where the bond is expected (Step 1). The remaining area of the specimen is then shielded with a heat-resistant film, like in the previous welding setup. The specimens are heated until the melting temperature of the PPS material is reached (Step 2). In Step 3, the PPS is pushed on the surface with a polished aluminium plate, using just enough pressure to ensure a flat surface. In the actual welding phase, referred to as Welding Phase II, the specimens are first placed according to the desired geometry (Step 4). A PTFE-coated support is used to ensure a correct positioning of the top adherend of the lap shear specimen. In Step 5, the top specimen is lifted with the plunger and the vacuum setup, and the temperature sensor is attached to the bottom sample. Both specimens are heated to the desired temperature and/or until enough PPS has melted. For Step 6, the infrared lights are removed and the plunger applies the necessary consolidation pressure. During the melting phase, the heat-resistant film still functions as a shield to prevent melting of this area. The paragraph above describes the two-sided welding process, since PPS is pre-consolidated on both adherends prior to welding. If no pressure is applied with the aluminium stamp in Step 3, the PPS forms air pockets and a very rough surface, making accurate positioning for the welding phase impossible. The main purpose of welding phase I is to have extra layers of PPS attached to the surface of the adherends. This step could also be implemented in the production process of the Cetex® composite laminates to ensure a PPS-rich area on the surfaces of the plates. Figure 5 shows the results of the lap shear experiments, again with a horizontal offset between the cycles for clarity.

Fig. 5: Force-displacement response of the two-sided welding process

Fig. 4: Sequential processing steps for two-sided welding

Two remarks can be made: 1) the reproducibility within a single welding cycle is very high, especially when compared to the results from one-sided welding, and 2) even between batches with different settings, which in some cases had a significant influence for the one-sided welding procedure, the reproducibility is still remarkable. As such, a fairly large process window is available to produce qualitative results. However, the last welding cycle, LS-25, shows that there

46 jec composites magazine / No77 December 2012

JEC COMPOSITES MAGAZINE - Issue #77 - December 2012

Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #77 - December 2012