Prevent excessive deflection of the latch area of an injection-molded motorcycle luggage housing without modifying the geometry.
Autodesk© Moldflow Insight (AMI) was used to determine whether converting to a co-injection molding process or adding fillers to the base PPE/PA resin would sufficiently stiffen the plastic housing. By using AMI, several potential solutions were investigated in a short amount of time. The simulations showed that maintaining the mono-injection molding process and converting the resin to a 20% talc-filled PPE/PA resin improved the stiffness more than converting to a co-injection process.
This case study presents a coupled mold filling and structural analysis of a plastic motorcycle luggage housing. The goal of the analysis was to determine how to prevent excessive deflection of the latch area without modifying the geometry of the housing. By eliminating the possibility of modifying the geometry there were two options left - change the resin or change the manufacturing process. Physically testing each option can be a costly process and take a significant amount of time to complete. By using AMI, cost and time were drastically reduced.
The geometry used for the simulation is shown in Figure 1. The housing was manufactured using a mono-injection molding process with an unfilled PPE/PA resin. It was filled using a single center gate on the bottom surface of the housing. Figure 2 shows how the resin fills the cavity.(insert photo here FIGURE 3 will need link to animated photo). The mold filling analysis simulated four different filling scenarios. Three of the simulations maintained the mono-injection molding process, but added different fillers to the base resin. The fourth simulation explored converting to a co-injection molding process using an unfilled PPE/PA resin for the skin and a 30% glass-filled PPE/PA resin for the core.
Distributed load under latch of housing
Housing flow front with single gate located on bottom face.
|Mono-Injection||PPE/PA 30% Glass-Fiber Filled|
|Mono-Injection||PPE/PA, 20% Talc filled|
|Co-Injection||Skin Resin: PPE/PA, Unfilled
Core Resin: PPE/PA, 30% Glass filled
Before the analysis, the customer thought that converting the injection molding process to co-injection would produce the most rigid housing. The customer also wanted to explore adding fillers to the base resin as an inexpensive alternative.
Co-injection molding uses two different resins to fill the housing cavity. First a skin resin is injected to partially fill the cavity, then a second resin is injected into the cavity to finish filling the part. The second resin is often referred to as the core resin. Co-injection allows a part to have a laminate structure where a core resin is sandwiched between two layers of the skin resin. This process is often used to modify the properties of a part. Simulating the co-injection process helps determine when the core resin should be injected in order to reach the area of loading.
The key to successful co-injection is to get as much of the core resin into the part as possible without having it break through the skin resin and become visible on the surface of the part. For this project, The Madison Group used different co-injection scenarios that varied when the core resin was injected. Three scenarios are presented below. The first scenario began injecting the core resin when the housing was 70% filled; second, when the housing was 59% filled; and third, when the housing was 61% filled. Figure 3 shows the distribution of the glass-filled core resin (blue) in the final part for the three scenarios.
Figure 3: The earlier the core resin is injected, the further it penetrates into the part. Blue represents glass-filled core resin; red represents the unfilled skin resin.
In order to sufficiently stiffen the part, the core resin has to reach the area of loading. Figure 4 shows injecting the core resin when the housing is 70% filled does not allow the core resin to reach the area of loading. Injecting the resin when the housing is 59% filled does reach the area of loading, but breaks through the skin resin and is visible on the surface. The final simulation shows that the latest the core resin can be injected and still reach the area of loading without breaking through the skin resin is when the housing is 61% filled.
|Percentage of cavity filled when core resin is injected||Core distribution in area of loading||Resin distribution at the surface of the part|
|70% fill: The core resin (blue) does not reach the area of loading and does not break through the skin resin.|
|59% of fill: The core resin (blue) does reach the area of loading, but breaks through the skin resin and is visible on the surface.|
|At 61% of fill: The core resin (blue) just reaches the area of loading, and does not break through the skin resin.|
Figure 4: Therefore it appears that if the co-injection does sufficiently stiffen the part, the processing window for injecting the core resin would be fairly narrow (60-61% of fill) which is not desirable.
By performing mold-filling simulation first, the actual mechanical properties of the housing can be better approximated. When materials with high aspect ratio fillers such as glass fibers are injected into the mold, the fillers will be oriented by the flow of the resin. This leads to anisotropic (directional) properties. Figure 5 shows the resultant fiber orientation through the thickness of the part. This series of images demonstrates that not only does the fiber orientation change in different areas of the part, but also through the thickness of the part.
Figure 5: A series of pictures showing how fiber orientation varies throughout the part and through the thickness the part.
Structural Analysis (FEA)
With the mold filling analysis complete, the next step was to perform the structural analysis. The mold filling simulation allowed us to calculate the anisotropic properties of the glass-filled resin, in both mono-injection simulation and co-injection simulations. This allowed for the material properties for the housing to be more accurate as opposed to assuming isotropic (uniform) mechanical properties.
Figure 6 shows the boundary conditions used for the simulation. They include constraining the bottom face of the housing in all degrees of freedom at all four mounting posts and applying a 10-pound distributed load just underneath the latch. Figure 7 shows the resultant displacement plot for the housing using the mono-injection process with the unfilled PPE/PA resin. This result was used as the baseline to compare other solutions that were also simulated.
The results from Table 2 show that converting the manufacturing process from mono-injection to co-injection would not yield a significant benefit for improving the stiffness of the housing. However, using either the 30% glass-filled resin or the 20% talc-filled resin would result in a stiffer part. Due to the aesthetic requirements of the part, the 20% talc-filled resin was chosen.
Model with boundary conditions for structural analysis.
Image showing the displacement of the housing as a result of the 10-pound load.
|Mono-Injection||PPE/PA, 30% Glass-fiber filled||0.238"|
|Mono-Injection||PPE/PA, 20% Talc filled||0.250"|
|Co-Injection - 61%||Skin Resin: PPE/PA, Unfilled
Core Resin: PPE/PA, 30% Glass filled