Materials Innovation

This test part shows the failure location of this open-end single rib tray arm.

The airline industry is continually striving for weight reduction to improve fuel economy. A company such as American Airlines, which operates a fleet of approximately 600 planes, could save up to 11,000 gallons of fuel annually by removing just 1 pound from each of its aircraft.1

Lower fuel consumption also means a lower carbon footprint; lowering fuel consumption by just one gallon translates into a reduction in CO2 emission of 19.4 lb.2

A smaller emissions impact not only benefits the environment but also provides a marketing benefit to the airline industry.

The Innovative Plastics strategic business unit of SABIC has developed several high-performance engineering thermoplastic compounds that, along with smart (optimized) plastic design, have the potential to replace metals to achieve weight savings. These high-modulus, high-strength materials are part of LNP’s Thermocomp product line.

Today, productions of several aircraft seating components are in die-cast or machined aluminum. The objective of this study was to validate the replacement of aluminum with LNP Thermocomp polyetherimide (PEI)-carbon fiber compounds in a food tray table arm. The use of a modified tray arm design was to translate, efficiently, the plastic material properties.

Open-end, single-rib tray arm designed by Vaupell Holdings Inc.

Vaupell Holdings Inc., a global contract manufacturer of custom injection-molded components, originally designed the thermoplastic composite tray arm and assemblies focused on the aerospace, medical and defense marketplaces. Intention of the Vaupell design was for 1:1 replacement of aluminum. The tray arm was side-gated and the bottom of the arm was open-ended and contained a rib (see photos to the right). Vaupell used different thermoplastic composite materials with varying tensile modulus and strength properties to fabricate the tray arms. Vaupell tested the load-withstanding capability of these tray arms using a custom-built testing system and found that almost all the parts failed in a narrow range of load (60 lb/ft to 80 lb/ft) irrespective of the strength of the materials, indicating that the material properties were not efficiently transferring to the tray arms.

Vaupell approached SABIC Innovative Plastics to develop a tray arm capable of withstanding a peak load of 120 lb/ft or higher utilizing SABIC Innovative Plastics resins.
 

Experimental Materials
Two compounds were developed from SABIC Innovative Plastics Ultem* PEI resin and aerospace-grade carbon fibers. They contained 30wt% and 40wt% of fiber reinforcement (Thermocomp* EC006PXQ and EC008PXQ, respectively). The compounds also contained proprietary flow modifiers that not only improved the flow but also boosted the mechanical properties. Both materials have excellent FST (Flame-Smoke-Toxicity) properties and are FAR 25.853 compliant. The FST test data (tested by Herb Curry Inc.) for EC008PXQ material is in Table 1.

In addition to the PEI-carbon fiber compounds, use of 30wt% and 40wt% carbon fiber-filled PPS compounds (Thermocomp OC006XXQ and OC008XXQ, respectively) in this study were to compare their performance in the tray arms with the EC006PXQ and EC008PXQ materials.

Table 2 compares the tensile properties of Thermocomp EC006PXQ, EC008PXQ, OC006XXQ, and OC008XXQ materials with die-cast aluminum (7075-O) and machined aluminum (7075-T6).

Data in Table 2 indicates that EC006PXQ and EC008PXQ materials are approximately 50% lighter than aluminum. Although the tensile modulus of aluminum is higher than EC006PXQ and EC008PXQ materials, the stiffness-to-weight ratio of the EC008PXQ material is comparable to aluminum and the strength-to-weight ratio of EC006PXQ and EC008PXQ materials are better than die-cast aluminum (7075-O) and comparable to machined aluminum (7075-T6).

Tray Arm Test Set-up
Use of a modified MTS system (Instron) was to test the materials (Figure 2). The tray arm attached to the MTS system with a bottom and a top fixture and pins. The top fixture also attached to the load cell of the MTS system. Support of the tray arm was by an 8mm side support, similar to the Vaupell test set-up. The orientations of the right tray arm and left tray arm in the MTS system are in Figure 2(a) and 2(b), respectively. The decision was to test only the right tray arms until arriving at an optimum design.
 

Finite Element Analysis
The use of Abacus finite element analysis (FEA) software was for simulating the loads, boundary conditions, and resulting stresses and deflections on the design geometry of the tray arm. Consideration was for non-linear geometric effects and non-linear material properties. Modeling of the Thermocomp EC008PXQ compound was as an isotropic elastic material using the data from the stress-strain curve of this material. For boundary conditions (how the part is supported and constrained), the tray table arm rested on the pin at the bottom and a vertical load of 120 lb/ft was applied on the top end.

The benefit of FEA is that it shows the locations of stress in the part, especially around pin-loaded holes, ribs, or areas in contact with support structures. In this study, use of structural FEA, in combination with mold flow analysis (MFA), was to understand the fiber orientation and weld line locations in the tray arm and predict their impact on the mechanical performance of the tray arms.

Utilization of the stress distribution information obtained from the FEA for the design improvements. The goal was to decrease stress in critical areas and better accommodate the as-molded material properties (e.g., fiber orientation and weld line) in the part. Once we were confident in the analysis results, these design changes translated to tooling, and then molding of the parts for testing and validation.

Results, Discussion
As mentioned in the introduction section and as depicted in the photo on page 73, the bottom part of the tray arm designed by Vaupell was open-ended with a single rib. The failure location of this open-end single rib tray arm is in the photo, single-rib part failure, page 72. As a starting point, SABIC Innovative Plastics suggested the addition of a rib to the location where the failure was. Before modifying the tray arm tool, performing FEA on this open-end, dual-rib tray arm helped predict the stress levels at different locations of the arm. FEA indicated that with the 8mm side support the maximum stress concentration was close to the new rib and the side support location, and the maximum stress value (273Mpa) was very close the tensile strength of the PEI compounds. In order to reduce the stress on the rib location, further performance of FEA was with increased side supports. As the side support width increased from 8mm to 20mm, the maximum stress was still close to the rib and the side support location, but the maximum stress reduced from 273Mpa to 261Mpa.

Modification of the tray arm tool added the second rib. The location of the gate remained unchanged for the dual-rib design. Molding of tray arms were with the PEI and PPS materials (EC006PXQ, EC008PXQ, OC006XXQ, and OC008XXQ) and testing of the right tray arms were with the modified MTS system as described in the experimental section. Testing of the tray arms were with 8mm and 20mm stainless steel side supports. All the samples failed along the new rib (see photo page 86), which was in line with the prediction from FEA.

A summary of the open-end, dual-rib tray arm test data is in Table 3, which shows the maximum loads that the tray arms were able to withstand before they failed. As predicted by the FEA, the tray arms performed better and were able to withstand approximately 10% higher load with the 20mm side support. The PEI-based compounds survived higher loads than the PPS-based compounds, but in both cases the peak load/ break load was not close to the target peak load of 120 lb/ft. Although the FEA indicated that with the 20mm pin support and 120 lb of applied load, the maximum stress would be below the break stress (tensile strength) of the PEI materials, the tray arms actually failed earlier than FEA suggested. There could be several reasons behind this premature failure:

1. Modeling of the composite in FEA was as isotropic (uniform) for simplicity of analysis purposes; whereas the test materials were actually anisotropic, (strength and stiffness vary in flow vs. cross-flow fiber orientation).
    
2. Measuring of the tensile strength of the materials was in the flow direction; most of the fillers (carbon fibers) aligned in the tensile bars in this flow direction. In the actual part, not all the fibers aligned in the same direction.
    
3. The gate location could affect the filler size distribution and loading of the fillers at different locations of the tray arm. With the side gate, the carbon fibers hit the sidewall of the tray arms, perpendicularly, which could significantly reduce the carbon fiber length and ultimately affect the strength of the materials.
    
4. The gate location will also affect the weldline location in the tray arm. A weldline around a hole will typically only be as strong as the base resin since composite fibers turn sideways where flow fronts intersect and do not bridge the weldline in order to reinforce it.

Given the fact that the tray arm’s load-withstanding capability would be lower than the tensile strength of the constituting material – due to the orientation of the fillers in different directions at the bottom of the tray arm – a redesign of the part was necessary. Performance of FEA iterations was to determine a design that would minimize the stress at the critical rib/core intersection of the tray arm.

There was consideration of several configurations: closed-end, dual-rib design, closed-end, multiple-rib (spider rib) core-out design, and entirely filled core-out design. Vaupell assisted with design input for the spider-rib configuration. Table 4 shows the maximum stress and the stress in the rib/core intersection area for different scenarios. With the spider rib and entirely filled design, localization of the maximum stress was at the side support location, not at the rib/core intersection. Moving the maximum stress to the side support location was advantageous as there was an additional support at that location and the fillers aligned more in one direction in that region compared to the circular bottom part of the tray arm. The data in Table 4 indicates that more filling or filling entirely helps to reduce the stress at the rib/core intersection. The cored-out, fully filled design exhibited lowest stress near the core among all the designs evaluated by FEA in this study, but there was concern about sink in the molded parts and a slight increase in part weight; therefore, consideration of the closed-end, spider-rib design was for translation to tooling.

Stress distribution from the FEA of the closed-end, spider rib tray arm with 8mm side support and 120 lb vertical load was seen. Before translating this design to tooling, performing MFA predicted the influence of gate location (side gate vs. end gate) with this design. In an end gate situation, MFA indicated a weld line close to the rib that was located opposite to the gate. However, on the other hand, with the side gate, the weld line moved to a location where the stress level was lower. As the weld line is a weak point in the structure, it would be advantageous to move it to a location where the stress level is lower. However, having the gate at the end also has its advantages: the fillers will experience a curved path at the entrance and therefore they are less likely to crush into smaller lengths. In addition, with the end gate, control of the filler loading is better at the critical area, i.e., the bottom part of the tray arm. Therefore, both the end gate and side gate designs received evaluation in the actual tray arms.
 

Chart 1: (a-left) Peak load for the tray arms made with EC006PXQ (30%wt CF filled) and EC008PXQ (40%wt CF filled); (b-right) Initial crack load for the tray arms made with EC006PXQ and EC008PXQ compounds

Chart 1(a) shows the peak load/break load for the end-gate, spider-rib tray arms made with EC006PXQ and EC008PXQ materials. Testing of the right and the left tray arms, at this stage, were with 8mm and 20mm side support. The data was very encouraging, although there was still the need for a solution to address the issue explained below. The performance of the tray arms improved significantly with the open-end, dual-rib design moved to the close-end, spider-rib design. For example, for the EC008PXQ material, the peak load for the right tray arm with open-end, dual-rib design and 8mm side support was 82 lb/ft, whereas for the closed-end, spider-rib right tray arm, the peak load was 149 lb/ft, an approximately 80% improvement.

For the left tray arm, the peak load was lower for both the EC006PXQ and EC008PXQ materials. This may have been due to the difference in test set-up between the right and left tray arm (see photos to the left). The flat part of the right tray arm received a back support from the fixture during the testing, whereas only the rims of the left tray arm got the support from the fixture. This difference in test set-up is likely to affect the peak load between the right and left tray arms. The peak loads for both the EC006PXQ and EC008PXQ materials were above the initial target of 120 lb/ft for both the right arm and left arm. Moreover, the tray arms showed 8% to 10% improvement in peak load when tested with a 20mm side

As noted in the previous paragraph, the outstanding improvement in the load-withstanding capability of the tray arm with the end-gated, spider-rib design did need further refinement. During the testing of these tray arms, observation was of a tiny deflection in the stress strain curves that appeared much earlier than the peak load. The deflection was barely noticeable in the stress strain curve, but accompanying this was a soft cracking sound during the testing. After this tiny deflection point, the tray arm survived for a long time and the peak load was much higher than this deflection point.

To investigate the origin of this deflection point and the accompanying soft cracking sound, the test stopped when hearing this sound and the test part examined minutely. Observance was of a tiny crack at the rib/core intersection just opposite the gate location. Mold flow analysis indicated that the weld line would be located close to this rib/core intersection with the end gate design (Figure 7b). Therefore, the weld line, the weak point of the structure, was generating an initial crack. Although the tray arm survived increasing amount of load for a long period after the initial crack, the concern was that this initial crack would affect the fatigue properties of the tray arm, i.e., after repeated use, the tray arm would fail at a load much lower than the peak load. To achieve robust performance from the tray arms, this initial crack needed prevention or delays up to a certain load – in this case, a minimum of 120 lb/ft.

Chart 1(b) shows observation of the load where the initial crack was for the right and left tray arms. The tray arms, especially the left tray arms, showed initial crack much lower than the target 120 lb/ft.

Chart 2: Tensile properties of LNP Thermocomp* EC006PXQ, EC008PXQ, OC006XXQ and OC008XXQ compounds and different aluminums (Al)

At this point, modification of the tray arm tool was to change the gate location from the end to the side. The MFA suggested that the weldline location should be in a low stress zone with the side gate molding, so theoretically, this configuration should delay or prevent the initial crack. The actual test data beautifully supported this prediction (Chart 2). The closed-end, spider-rib, side-gated tray arms (both right and left) made with the EC006PXQ material did not show any initial crack (with 8mm and 20mm side support) and survived up to a maximum load of 213 lb/ft (right arm, 20mm support). The right and left tray arms made with EC008PXQ material also did not show any initial crack while tested with 20mm side support and survived up to a maximum load of 192 lb/ft. With 8mm side support, the right tray arms did not show any initial crack. Some of the left tray arms (made with EC008PXQ compound) did show an initial crack while tested with 8mm side support, but the crack occurred at very high load, 130 lb/ft to150 lb/ft, so it was not of much concern.

Besides the peak load/break load of the tray arm, it required consideration of two other factors while designing the tray arms: the stiffness of the arms and ductility of the arms. A stiffer arm will be able to withstand higher load before it deflects to a certain distance. In addition, the tray arms should not be too ductile; high elongation will cause a large deflection of the arm that will shake the food tray, which, of course, is not desirable. Table 5 shows the stiffness and deflection of the tray arms made with EC006PXQ and EC008PXQ materials. It should be noted that the stiffness values reported in Table 5 do not indicate the stiffness of the EC006PXQ and EC008PXQ materials, rather, the term stiffness indicates how much load will be required to deflect the tray arm each inch (up to the linear part of the stress strain curve).

Depending upon the performance requirements of the tray arm, EC006PXQ or EC008PXQ could be the material of choice; tray arms made with EC008PXQ material will provide better stiffness and lower deflection with great load-bearing capacity, while tray arms made with EC006PXQ material will provide even greater load-bearing capacity with some compromise in stiffness and deflection.
 

Testing of the tray arms were with 8mm and 20mm stainless steel side supports. All the samples failed along the new rib, which was in line with the prediction from FEA.

Weight Savings, Cost Reduction
The thermoplastic composite tray arms made with the EC006PXQ and EC008PXQ compounds will not only help deliver excellent performance, but also provide significant weight savings and cost benefits. Table 6 shows the individual weight of the closed-end, spider-rib tray arms made with the EC006PXQ and EC008PXQ materials, as well as the weight of a dual-rib, open-end aluminum tray arm of similar size and shape. The data indicates that the tray arms made with the EC006PXQ and EC008EXQ materials result in weight savings of 46% and 44%, respectively, as compared to the machined aluminum tray arms. In a 200-seat passenger plane, this will potentially save approximately 45 lb and will translate into estimated savings of 0.5 million gallons of fuel savings and a reduction of 9.9 million lb in CO2 emissions yearly for an airline the size of American Airlines (approximately 600 planes).

Table 7 shows the relative cost per tray arm made with EC008PXQ material and different grades of aluminum for release quantities of 5,000 and 10,000 parts. These are normalized costs based on estimates provided by Vaupell. EC008PXQ compound will result in a cost saving of 77% compared to aluminum 7075, 73% compared to aluminum 6061 and 19% compared to die-cast aluminum for a release quantity of 10,000 parts.
 

Conclusions
In this study, LNP* Thermocomp* materials based on Ultem PEI resin and containing 40% and 30% carbon fiber reinforcement were evaluated in a food tray arm application proof of concept to validate that engineering thermoplastic composites can successfully replace metals in various applications. To maximize part performance, it might be necessary to redesign the part to accommodate the difference in mechanical behavior between metals and plastic compounds. If designed properly, parts made with high-modulus, high-strength engineering thermoplastic compounds could be excellent alternatives to metal parts, providing the benefits of weight reduction (and, consequently, carbon emissions reduction), cost savings and design flexibility.

* Trademarks of SABIC Innovative Plastics IP B.V.

1. American Airlines Website: http://www.aa.com/i18n/amrcorp/newsroom/fuel-smart.jsp

2. U.S. Environmental Protection Agency Website: http://www.epa.gov/otaq/climate/420f05001.htm#calculating

SABIC
Innovative Plastics
Pittsfield, MA
sabic-ip.com