Technological challenges of the injection moulding implementation for aeronautical parts

1 Introducción

In the aeronautical sector, injection moulding has been already identified as a manufacturing process for low structural responsibility parts due to low stiffness properties of the injection plastic materials. Some examples of parts manufactured with this technology that are nowadays flying are the A380 Horizontal Tail Plane (HTP) manhole cover.

Actual simulation tools could enable a potential cost reduction in the development for an injection part. A reliable simulation strategy should identify the most relevant parameters of the process allowing a significant reduction of injection trials and mechanical testing among other elements.

This work presents the different activities performed from engineering and manufacturing point of view in order to assess the feasibility of this manufacturing process to be applied on the fairings within horizontal and vertical stabilizers which have been selected as targets.

From enginneering point of view, part identification of the candidates for the potential technology application, strenght validation strategy and mechanical testing at coupon level of the most significant details of the potential applications have been covered. Concerning manufacturing, two different technology demonstrators in terms of size have been developed.

2 Materials

High performance reinforced plastics materials, top of the semi-crystalline pyramid, have beed selected for the different studies. Matrix of the material is a thermoplastic polymer. Both short carbon or glass fibers, have been considered. Materials from different suppliers have been tested.

Concerning reinforements, short fibers have a length of aproximately ~200-250µm and a diameter of ~5.10 µm.

3 Part identification

Horizontal tail plane (HTP) and vertical tail plane (VTP) fairings have been identified as the potential cantidates for the technology applications due to their low structural responsibility and relative small size. Figure 1 and figure 2 present the list of parts selected for HTP and VTP respectively.

From structural point of view the main requirement to be accomplish on these elements consist on mantaining the external shape inside aerodinamic tolerances during flight so that main driver is the stiffness of the element. As could be observed different sized and shapes can be identify on the target elements. These geometrical details play an important role in order to maintain the same level of stiffness, going from the original material to the one propose for this technology.

Althought injection plastics present low stiffness properties in comparison to the current materials of this fairings (CFRP sandwich structure mainly and some alluminum elements), versatility of the technology allows the implementation of more optimized solution allowing to maintain the same level of stiffness with the same weight or a small weight penalty.

Figure 1. HTP potential candidates for injection.

Figure 2. VTP potential candidates for injection.

4 Strength Validation Strategy

One of the main challenge from engineering point of view for injection parts developement is the strenght validation of these elements. Depending on the injection strategy of the part, short fibers will have different orientation in the different areas and a great anisotropy effect need to be consider. This leads to a high interdependency in the design fase between engineering and manufacturinf disciplines.

Due to the complexity on the inner structure it is quite difficult to predict the behaviour with confidence and noawadays validation/certification of injection moulding parts in the aeronautical sector are made by means of a battery of full scale testing of each part separatelly.

Tools have been developed in order to simulated both, injection moulding manufacturing process and material property mapping out of the fiber orientation resulting of the injection strategy proposed.

Figure 3. Digital E2E strength validation Flow Chart.

Figure 3 present a scheme of the strategy to be follow for the strength validation. First step of the stress analysis is to establish material as isotropic, considering mechanical properties obtained at coupon level as a starting point. Injection moulding mindset need to be consider in the design initially, i.e., part need to be demold in a easy way in order to simplify the mold design as much as possible

Standalone finite element model (FEM) for each of the previous parts identified have been put in place with different solutions regarding stiffener pattern appropiated for the specific implementation of this technology. Figure 4 present an overview of these FE models. For parts with a “nose shape”, such as HTP leading edge extention (LEX), HTP Tip or VTP Dorsal fin, the most effective way to achieve similar stiffeness as the original part using reinforce plastic is joining both surfaces with internal elements such as ribs instead of using stiffeners elements. Regarding longitudinal elements, such as S19 Front and Rear Fairings, thanks to the versatility of the manufacturing process, an optimized stiffener pattern as a result of a topological optimization can be implemented in order to have the most effective structural solution in terms of weight.

Figure 4. Fairing FEM examples for Injection candidates.

Out of this isotropic FEM, the first preliminary sizing is obtained with the minimum thickness values from stress point of view.

Once the first model is obtained the digital E2E design process can be started. Using flow simulation tools, design and development of the different injection strategics is preformed. Main aspects to be consider in this phase are the number of injection poins to be included in the mould and the desired fiber direction to be achieved.

Flow simulation tools are able to provide an estimation of the fiber orientation. This data can be interpretated in order to obtain a material property mapping for each area of the part under study, representing then the anisotropic effect of the material in the part. Finally a detailed finite element model considering realistic fiber orientation is built and a proper stress analysis shall be performed.

Figure 5 present the preliminary study of the first digital E2E loop for the case of the HTP LEX. A first thickness maping was obtained using isotropic assumption.(Skin 3.1 mm, internal Ribs 2.1mm) considering boundary conditions presented. A comparison in terms of stress values, buckling behavior and displacements values are presented using both isotropic analysis and anisotropic model considering injection simulation outputs. For both models same boundary conditions under linear analysis have been considered. Two different load levels have been considered: Critical load to assess stresses and buckling analysis and cruise load to obtain displacements for a normal operation cases. Model is loaded with pressure in both cases.

It can be noticed that in general lines same global behaviour is obtained in the sense that critical areas remains the same in both approches,i.e., similar plots could be observed however:

• There is a 12% stress increase on the anisotropic model.

• Lower eigenvalue (~40%) is obtained with anisotropic FEM.

• Concerning displacements, anisotropic model gives 2 times more than the values obtained with isotropic one.

Figure 5. Digital E2E strength validation – HTP LEX: 1st loop results.

5 Coupon test campaign

A preliminary characterization of different high performance thermoplastics materials, in order to assess their potential application in structural parts from research perspective, was performed.

The test specimens were coupons extracted from panels manufactured by injection process as per Figure 6. Two different types of coupons are defined depending on the orientation of the short fibers reinforcing the polymeric matrix:

• Longitudinal: fibers oriented in the coupon longitudinal axis.

• Transverse: fibers oriented perpendicular to the coupon longitudinal axis.

Figure 6. Injection moulding coupons extracted from panels.

A first phase on the coupon test campaign was dedicated to obtain a first assessment on the joint behavior of this material mainly bearing and pull-trough. In addition fatigue un-notched coupons have being tested under different types of loading and stress profile.

A second phase of this test campaig was dedicated to study in more detail the joint area with single lap shear test both in static and fatigue loading. In addition to that shear and compression after impact tests were performed to complete the assessment of the material.

Figure 7 presents a recopilation of the different types of coupons studied during the 2 phases of the test campaign. As a general comment it can be stated that all materials tested are quite fragile not being the type of failure expected as more plastic behavior is always desired. This can be appreciated in the bearing and pull-trough specimens.

Results for the different test are highly dependant on the position in which the coupon is extracted from the panel. As a clear example, better mechanical properties are extracted from the coupons close to the edges as in this position the fibers are oriented in this direction. For fatigue testing this effect has to be added on top of the normal scatter. As a result, a significant dispersion of the figures for a giving load level is obtained.

Regarding the influence of the main fiber orientation in the mechanical properties, going from longitudinal to transversal a drop of 25-35% could be observed in both static and fatigue properties

Figure 7. Different Coupon tested.

6 Manufacturing Trials

High performance plastics (PEEK, PAI, PEI, PPS, etc.) present challenging requirements for the injection moulding. The process window for these materials are quite narrow always associated to high temperatures of service. Due to these elements specific molds need to be design for the use of these materials:

• Mould need to be heated to reach a high range of operational temperatures, i.e., around 200-220ºC depending on the specific plastic used

• Injection system needs to resist high temperatures in order to mantain the molten plastic (at 400ºC) all along the injection channels,

• Mold need to be design so that it allows to absorb the thermal variations of the plates modeling the part.

Figure 8. Plastic Material Pyramid.

To be able to operate with high performance plastics, injections units need to be adapted as well to support high temperatures. One of the important aspect is the temperatue of the noodle of the injection cilinder which need to be at 400ºC being the temperatute on the feeding point, the hooper, 365ºC

Out of the potential parts for the aplication of this technology presented in Figure 1 and Figure 2, the HTP leading edge rib (LER) of the elevator and the Leading edge extention (LEX) of the HTP box were selected,

6.1 LER Mold Design and injection trials

LER mold, Figure 9, due to the small size of the part to be manufactured, was designed with a direct injection system from the injection unit, i,e., no hot runners were used.

Figure 9. HTP LER Mold.

Separation of the part from the mold was performed by using a normal ejection system. This ejection system push the part out making contact with the oposite side of the inyection channel.

As presented in Figure 10, HTP LER is extracted from the inyection unit including the final part of the injection channel. Process for each part took around 25 seconds so that high rate of this process is demonstrated

Figure 10. LER inyection trials.

Two different materilas were used in the injection trials. Quite similar results were obtained for both of them. Small geometrical non-linear distorsions were obtained due to material anisotropy as presented in Figure11.

Figure 11. LER Geometrical distorsion.

6.2 LEX Mold Design and injection trials

Due to the size of the part, dimensions of the LEX mold are quite significant in order to fill the part. Several injection points are needed. The injection was performed in a sequential way so that welding lines are avoided during the process. Figures 12 and 13 give a general overview of the HTP LEX mold

Figure 12. HTP LEX Mold Overview.

Figure 13. HTP LEX Mold Overview.

Different injection simulations were performed to study how to fill the mold following a sequential approach. In addition a spanwise fiber orientation was pursued as this was identified as the best configuration based on the structural performance of the part.

One of the key aspects of the injection process for such a big part is the maxium length which the plastic is able to travel before becoming solid. Several proposals were studied as presented in Figure 14. Solution selected was a total of 8 gates, in a configuration of 5 + 3 with lateral injection, having gates relatively close in order to have a good filling of the mold and the best scenario for the fiber orientation desired.(Figure 15).

Figure 14. Injection proposals.

Figure 15. HTP LEX fiber orientation detail.

To validate the mold, conventional plastics were used. Mechanical behavior of the ejection system and the complete injection system was verified. These trials were performd using firstly Polypropylene (PP) followed by Acrylonitrile butadiene styrene (ABS). This plastic (ABS) requires a heated mold although operational temperatures are signifcantly lower than the ones used for the high performance plastics

After the previous validations, injection trilas with the material objective were performed. On these trials, the difficulty of the injection of the high performing plastics was confirmed:

• Temperarure control of all elements, hot runner and the mold surface in contact with the plastic, is key in order to obtain a good part out of the process.

• Ejection of the part from the mold was another topic to adress. Extra hydraulic cylinders to help ejection plates to start movement were installed in the mold.

Figure 16. HTP LEX injection trial result.

7 Conclusions

Several challenges were identified during the injection moulding development for aeronautical parts. From engineering point, it is fundamental to develop a robust calculation methodology to size these elements considering the anisotropic behavior of the material. The success on this task will avoid an enormous amount of validation by testing which is one of the drawbacks of this technology nowadays for the aeronautical application.

Concerning manufacturing trials, for small parts such as LER good results were obtained achieving desired production rate. Small deviations due to the effect of the anisotropic effect were found. For bigger parts such as LEX, the complexity of the process is critical with very low margin of variation of the different parameters of the process. Further trials need to be performed in order to obtain a part with good quality.

Acknowledgments

The previous project was included in the project FACTORIA, funded by the Ministerio de Economía y Competitividad, by means of the Centro Tecnológico Industrial (CDTI), in the Strategic Programme CIEN 2018