Materials and methods

The manufacturing steps could vary from one injection to the other and represent a general view of the manufacturing process. As described, the production of composite materials is labor intense and involves many manual operations. Choosing the pressure direction, open and closing valves, adding and removing pipes require of skilled personnel. Even so, it is prone to human error that can spoil hours of preparation. Figure 3.3 shows the difference between a bad and a successful process. An aged seal or malfunctioning quick pipe fitting could promote air pockets that lead to problems during the injection.

During the experiments we carried out, some tests were a total failure and the next injection was postponed one day due to the three hours of time required for cleaning and the application of the release agent coating. Also, some well-cured parts broke during the de-molding step.

Figure 3.3: Left, Failure test. Right, good cured part

The injected polymer resin was an epoxy DGEBA (Diglycidyl Ether Bisphenol A) or D.E.R. 383 mixed with Methyl Tetrahydrophthalic Anhydride (MTHPA). A catalyst in the form of 1,2-Dimethylimidazol+2-Ethyl-4-Methylimidazol started the chemical reaction. Figure 3.4 shows the polymer viscosity from the data sheet. Also the fully cure Tg for the resin mixed with MTHPA state by the manufacturer is 148 °C [103].

Figure 3.4: Resin viscosity at 85 °C and MTHPA curing agent [103]

From the literature review section 2.3.6, we found that the cure temperature should be lower than the fully cure Tg value to ensure proper gelation. For this reason, the process temperature was selected to be 140 °C. Also, to maintain the resin at low viscosity, the injection time must be between 20 - 40 minutes at 85 °C. For a real-world setting we limit the information of the material

to the manufacturer datasheet, one academic publication and the staff experience. Limit information and operator experience is the challenge faced by composite parts manufactures. In this respect we choose the work of Ivankovic et al.2003 [73]. They studied an Epoxy/Anhydride similar to the resin using during this research. Their results presented in the Figure 3.5 shows that with a heating rate of 5 °C/min, the cure degree remains almost zero for about 18 minutes, confirming the datasheet information.

Figure 3.5: Resin kinetics. From right to left: 3, 5 and 10 °C/min [73]

The maximum heating rate measured in the device was approximately 6 °C/min. With the resin data and the device characteristics, a corrected cure cycle was performed: First, the mold was heated to 140 °C. At the same time, the injection pot was maintained at 70 °C. Then, the polymer mixture was put inside the container where it was heated as fast as possible to the temperature of the mold. When the temperature inside the resin reached 100 °C, around 5 to 7 minutes, vacuum de-gas it. Finally, the injection was made upon reaching 140 °C, considering a 15-minute restriction inside the heating ramp to maintain the cure degree negligible.

The collected data from the video camera and the temperature recorded from the data loggers were analyzed from two successful tests. The video data is helpful in the synchronization of the temperature measurements with the process steps. In fact, it also allows the identification of bubbles or air pockets presented in bad injections. Although the video information was only qualitative, we found this information complementary to the data from the temperature sensors.

Figure 3.6 shows a special characteristic of the chosen epoxy. The resin change colour during the gelation which permits to identify the gelation time from the video recording.

Figure 3.6: Color change representative of the liquid to gel transformation

Figure 3.7 and Figure 3.8 shows two cure cycles with similar temperature profiles. Although the device was maintained at a uniform temperature, at the time of the injection, the resin creates a disturbance in the mold. At that time the control increases the power of the heaters to compensate, but because the chemical reaction releases heat, it cannot compensate for the temperature. As a result, the temperature exceeds the predetermined value. To correct it, manual adjustments by trial and error were made. This peak coincides with the gelation of the material. From that time, the molding continues until complete 80-90% degree of cure, or a total 30 minutes, according to the measurements of Ivankovic et al. [73]. Finally, it is safe to de-mold the piece missing less than 20% of cure since the remaining chemical reaction continues at room temperature in the solid state.

By doing so, we save around 45 minutes, but in massive PMCM parts, this could represent hours.

This is a procedure routinely used in the industry because does not affect the final properties of the material.

Figure 3.7: Mold temperature and video interpretation from experiment 1

Figure 3.8: Mold temperature and video interpretation from experiment 2

From the video and the change of colour in the resin, a gelation time of 4:15 and 3:06 minutes was obtained, values very close to the 3:30 minutes obtained by Ivankovic et al. [73]. The difference of one minute between the two injections is because they were made a day apart. And although we follow the protocol of storing the resin at -15 °C, the chemical reaction continued in the time interval required to cool and thaw the material.