JEC COMPOSITES MAGAZINE - Issue #70 - January/February 2012 - 51

Tab.2: Test data for open-ended, dual-rib tray arms made with PPS and PEI compounds

Peak load (lbf) Material With 8-mm side support 59 55 81 82
With 20 mm side support

% improvement (20-mm vs. 8-mm side support) 12 16 12 10

OC006XXQ OC008XXQ EC006PXQ EC008PXQ

66 64 91 90

FEA indicated that with the 20-mm pin support and 120 lbs 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. Several possible reasons behind this premature failure have been identified. 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. FEA iterations were performed to determine a design that would minimize the stress at the critical rib/core intersection of the tray arm. Several configurations were considered. Vaupell assisted with design input for the spider-rib configuration. Table 3 shows the maximum stress and the stress in the rib/core intersection area for different scenarios. With the spider rib and entirely filled design, the maximum stress was localized 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 were more aligned in one direction in that region compared to the circular bottom part of the tray arm. The data in Table 3 indicates that additional filling or full filling 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 moulded parts and a slight increase in part weight; therefore, the closed-end, spider-rib design was considered for translation to tooling. Figure 7(a) shows the stress distribution from the FEA of the closed-end, spider rib tray arm with 8-mm side support and 120Table 3: Maximum stress and stress in the rib/core intersection for different tray arm designs

Fig. 7: (a) Stress distribution in closed-end, spider rib tray arm; (b and b’) fibre orientation and weld line location with end gate; and (c and c’) fibre orientation and weld line location with side gate. Weld line locations are marked with arrows.

lbs vertical load. Before translating this design to tooling, MFA was performed to predict the influence of gate location (side gate vs. end gate). In an end gate configuration (Figure 7b), MFA indicated a weld line close to the rib that was located opposite to the gate (Figure 7b’). On the other hand, with the side gate (Figure 7c), the weld line moved to a location where the stress level was lower (Figure 7c’). 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. Also, with the end gate, the filler loading will be better controlled in the critical area, i.e. the bottom part of the tray arm. Therefore, both the end gate and side gate designs were evaluated in the actual tray arms. Figure 8(a) shows the peak load/break load for the end-gate, spider-rib tray arms made with EC006PXQ and EC008PXQ materials. Both the tray arms were tested at this stage with 8-mm and 20-mm side supports. The data was very encouraging. The performance of the tray arms improved significantly with the open-ended, dual-rib design moved to the close-end, spider-rib design. For example, for the EC008PXQ material, the peak load for the tray arm with open-end, dual-rib design and 8-mm side support was 82 lbf, whereas for the closed-end, spider-rib tray arm, the peak load was 149 lbf, an ~80% improvement. During the testing of the end-gated, spider-ribbed tray arms, a tiny deflection was observed in the stress strain curves that appeared much earlier than the peak load. The deflection was barely noticeable in the stress-strain curve, but was accompanied by 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 accom-

Design
Closed end, dual rib Closed end, multiple (spider) rib, cored out Closed end, fully filled, cored out

Maximum stress 227 204

Stress in the rib/core intersection

145 -165 92 -114

273

61 - 91

No70 January - February 2012 /

jec composites magazine 51
JEC COMPOSITES MAGAZINE - Issue #70 - January/February 2012

Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #70 - January/February 2012
