Custom-made

Custom-made devices implement an electric press to apply the force. But they go one step further.

The new device or mold does not rely in an existing platform. As shown in the last section, the attachment to traditional equipment carries the original machine limitations. Essentially, researchers untie their designs from existing platforms to gain more control over the experiment.

In fact, even with the collaboration of big manufactures, the existing platform is too close to the scope of the new custom-made devices.

2.4.2.1 PVT

A force piston with a displacement sensor and a heated mold cavity setup a Pressure – volume – temperature machine. Figure 2.34 shows a TMA device diagram. It is in fact a PVT machine; the only difference relies in the amount of force applied during the test. TMA test applies a negligible force to measure the volumetric changes as function of the temperature. Constant, ramps or modulated temperatures measure the material properties by means of the change in the free volume.

This device uses small samples, generally to prevent thermal gradients. Researchers characterize materials with thickness of 0.5 to 2.5 mm with heating rates ranging from 1 to 5 °C/min to decrease the thermal effect [68].

Figure 2.34: TMA instrument [68]

In comparison to TMA, PVT devices have the capability to apply massive pressures of up to 250 MPa [99]. Researchers could also call them high pressure dilatometers. Their pressure capacity makes them ideal to test thermoplastic polymers properties. Injection machines mold this type of material under high pressure due to their high viscosity. Wang [100] review these devices, he found custom machines as old as from 1976. All of them attach a hydraulic pump to a piston. In other words, they put a small sample inside a mold and compress it with a hydraulic piston. In general, researchers classify them into two main categories: piston die and confining fluid. Their main difference is that in the confining-fluid mold, the liquid applies a hydrostatic pressure in all directions, but the fluid used is mercury which makes this experiment non-viable according to modern environmental restrictions. In the case contrary, the piston die is easier to setup, but the sample could stick to the walls, introducing errors in the measure. Either way, researchers found a 4% of difference between measures from one device to the other [100].

Naturally, the specific volume depends of the temperature and the pressure. Figure 2.36 shows the PVT tests on a polymer at different pressure conditions. The section denoted by A-B-C represents the Tg and how the pressure displaces and distorts the linear behavior of the volume and the glass transition.

Figure 2.35: Mold categories. (a) Piston-die. (b) Confining-fluid [99]

Figure 2.36: PVT tests on polystyrene under isobaric conditions [101]

Finally, Ramos et al. [74] study the influence of the pressure over the cure degree on an epoxy-amine system. They obtained the cure degree from the specific volume variation in an isothermic and isobaric PVT test. Figure 2.37 shows that the pressure has very low influence over the resin cure kinetics. Fortunately, in the manufacturing of thermoset composite materials the consolidation pressure is near 600 kPa [41] which compared to the 20 MPa from the test makes the influence of pressure in the cure of PMCM molding process negligible.

Figure 2.37: Cure degree vs time at different isobaric pressures [74]

2.4.2.2 PVTα and PTV-HADDOC

2.4.2.2.1 PVTα

The PVTα device originally developed from the Nantes University in France from 2003 to 2013 [33-35, 83] is the most active and reported in the literature reviewed about custom-made devices.

They made a variation of the PVT piston die device. A mold equipped with a heat flux sensor and an electric press allows the characterization of thermoset polymers with a diameter of 40 mm and variable thickness. The PVTα allows the characterization of simultaneous properties as the material cures. The heat flux sensor obtains the properties as function of the cure degree in a single experiment. Figure 2.38 shows the schematic diagram of the mold. A displacement sensor provides the feedback to measure the volumetric changes during the test. They reported the CTE, cure shrinkage, bulk modulus and cure degree properties.

The device presents the following challenges:

• Heat losses in the mold and thermal expansion of the LVDT sensor.

• The applied dynamic force was not completely sinusoidal, less than 0.5 Hz which makes not possible to obtain the mechanical properties directly.

• A thermocouple embedded inside the sample allows to measure the thermal gradients inside the sample. The data represent the thermal conductivity of the thick sample.

• A silicone capsule covers the sample to prevent the resin to stick to the mold cavity. For this reason, the sample injection and the setup of the middle thermocouple is the challenging part of the test.

Figure 2.38: PVT-α sketch 2.4.2.2.2 PTV-HADDOC

The PTV-HADDOC is the latest improvement of the Nantes University team with alliances with Airbus Group. This device was recently presented in 2017 [37]. Figure 2.39 shows the new device.

It is a confining fluid PVT type. They put a sample of 105 mm x 105 mm and a variable width inside a mold cavity surrounded by silicon oil. Both liquid and a specialized Instron E10000 electrical press maintain a constant pressure inside the mold. The displacement sensor in the piston with the profilometer measures the sample volumetric changes. Researchers measure the volumetric changes in all directions and the cure degree in the same experiments. They test two composite materials, a Glass/Polyester with short glass fibers randomly oriented and a Carbon/Epoxy with plies unidirectional oriented. In both cases the volumetric CTE and volumetric cure shrinkage measure was very close as the one measure with the through the thickness on the PVTα. In the case of the composite with oriented fibers, the researchers observed no influence of the reinforce over the chemical shrinkage.

Figure 2.39: PTV-HADDOC Windowed mold design [37]

The device presents the following challenges:

• Synchronize the piston and fluid pressure. This requirement makes possible the hydrostatic pressure.

• The mold and sensors require of a special fine-tune and corrections.

• No significant improvement over the piston die technique. In fact, the challenges on the confining fluid PVT imposes too much work with no beneficial results.

• Despite the addition of a high-end Instron press, researchers have not presented dynamic mechanical tests.