Sistema de Contabilidad Características a) Aspectos Conceptuales

After producing working 3D plastic products with electroluminescent capabilities using pre-prepared airbrushed inserts in the injection moulding process, the next step was to introduce the layers into the mould directly.

The airbrush was used to spray directly in-mould the EL layer structure that is shown in Figure 7-13 on page 119; each of the thin layers was dried before the next was applied; the drying times varied slightly due to variation in layer thickness and mould temperature (the drying regime can be seen in Table 7-7).

Table 7-7: The drying regime when applying the modified materials in-mould using an airbrush.

Material:MEK Mix

In Mould Drying Regime Mould Tool

Temperature(°C) Time (min) Clevios™ 65-70 2-2.5 Phosphor 65-70 1.5-2 Dielectric 65-70 2-2.5 Silver 65-70 1-1.5

The same adhesion and release problems encountered in the initial in-mould trials occurred here but not to such a great extent (see Figure 7-30). Again, there was a burning of the Clevios™ material at the electrical contact point so a silver busbar will have to be incorporated into the structure to avoid this.

Figure 7-30: EL layer materials on a plastic part made directly in-mould

The next step was to apply individual layer materials sequentially to determine at which boundary the problems occur.

7.5.1 Adhesion between Layer Materials and the Substrate

The EL materials were applied in-mould starting with a single layer and increasing the number of layers each time to determine which interfaces had adhesion problems.

Figure 7-31: The adhesion between layers is investigated. Silver layer only (left), the silver and dielectric layers (centre), and finally the silver, dielectric and phosphor layers (right).

The mould tool was heated to 65 C to dry the layers; firstly a silver layer was applied and then moulded. The silver material adhered to the PP and released from the mould. Next the dielectric followed by silver was moulded and then phosphor, dielectric and silver. All had no adhesion problems or any material left in the tool

80mm 80mm

Figure 7-32: Testing the adhesion between layers; silver, dielectric, phosphor and Clevios layers.

Finally, all layers were applied in-mould; Clevios™ was applied followed by the phosphor, dielectric and silver and then moulded. The Clevios™ layer remained in the mould after ejection and separated from the phosphor; some of the other layer materials also separated from the part at the injection point (see Figure 7-32).

Two different options can be tried to overcome the adhesion problem; a different injected polymer could be trialled and a mould release can be used on the tool.

7.5.2 Changing the Injection Substrate Material

Clevios™ PEDOT:SS is recommended to be applied to a PC substrate (as stated in the H C Starck processing guide [148]) so PC was also trialled as the injection material and compared to parts produced using PP. From the samples produced it was apparent that there was no improvement in the adhesion between the layer materials and a PC substrate compared to the PP substrate.

In fact, the higher the number of layers that were moulded over, the less material transferred onto the part (see Figure 7-33).

Figure 7-33: Testing the adhesion of layer materials to PC.

However, the material directly adjacent to the substrate is the silver electrode so it may be irrelevant that Clevios™ bonds better to PC; in addition to that, the materials are already in their cured/dried states when the substrate is injected over them. In their solid state they would be less likely to bond to the substrate compared to been applied wet and dried or cured on the surface.

Since PP is cheaper and easier to mould than PC and there appears to be no adhesion benefits to using PC, PP will remain as the substrate materials to be injected.

7.5.3 Mould Release

A number of different mould releases were available in the workshop and were trialled as release options; they were judged by how well the Clevios™ PEDOT:PSS released for the tool (see Table 7-8). Each time the mould release was applied as per the manufacturer instructions and a layer of Clevios™ was sprayed in-mould,

Silver

Silver, Dielectric, Phosphor & Clevios™ Silver & Dielectric

Silver, Dielectric & Phosphor

was injected at 220 C. Finally the mould tool was cleaned in between each test to avoid contamination.

Table 7-8: The results of the mould release trial.

Name Type Release Effectiveness Effect on Surface Finish Finish Kare #1165 Wax About 60% of the

Clevios™ released

Lines were visible due to uneven coverage of

the wax from the sponge applicator Ambersil PTFE Spray No Clevios™ released N/A

Ambersil Formula

20 Spray No Clevios™ released N/A ACMOS 82-2405 Spray

About 85% of the Clevios™ released (see

Figure 7-34)

The surface finish was slightly duller ACMOS 82-7007 Liquid No Clevios™ released N/A

Figure 7-34: The release of Clevios™ from the mould when using ACMOS 82-2405 release spray.

The result of this set of experiments is that ACMOS 82-2405 release spray is the most suitable for preventing Clevios™ from sticking inside the mould tool during the injection moulding process. The ACMOS 82-2405 mould release is a film forming release and is designed for the release of Poly(methylmethacrylate) (PMMA) [151], but it is not immediately obvious why this particular release works the best with