Case Study II: Vent



Vent Product Description

The second case study was performed on an assembly with considerably more complexity than the concept part of Case Study I. The product, as illustrated in Figures 1 through 3, is a two-material vent device. The relevant characteristics of the vent are discussed below.

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Functionality

The vent is a two-material assembly designed to control the amount of airflow through an orifice. This device is a simplified version of those found commonly in automotive applications (e.g. dashboard vents). The vent allows minimal airflow when closed (Figure 1a) and maximum airflow when opened by ninety degrees (Figure 1b).

The vent consists of a prismatic base and a flat, hinged slat. The base provides support and would be attached to the outlet or opening through which air escapes. The slat controls the amount of airflow out of the base by rotating to cover/uncover the outlet hole in the base.

The slat has one rotational degree of freedom relative to the base. Once the product is assembled, the slat should be free to rotate along its hinge axis with minimal human effort. This means the hinge action should be free enough to allow the vent to be easily opened/closed, but not so loose as to allow free spinning of the slat due to small disturbances like vibrations.

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Materials

The base ("part A") is molded out of a generic white polycarbonate. This material was chosen because it is quite rigid. The sleeve ("part B") is molded out of a generic white high-impact polystyrene, which is also rigid enough for this application. These materials were chosen because they exhibit no adhesive bonding when used for MSM, allowing relative movement between components in the MMM variant.


Comparison of Variants

The major difference between the variants is how the slat is attached to the base. In the SMM&A variant, both the slat and the base have holes through which are aligned with hinge pins. On either side of the assembly, a cylindrical metal fastener is inserted, forming clearance fits with the base, and press fits with the slat. This allows the slat to freely rotate, while the pins stay joined with the rest of the assembly. The MMM variant incorporates a built-in hinge on the slat, which is formed during molding by shot B filling in the holes in the base. As shot B cools, it shrinks away from shot B, forming enough clearance between the formed pins and their corresponding holes. This difference, similar to the hinge box discussed previously, is most readily noticed in Figure 2 above.


Molding Considerations

The molds required for the vent are considerably larger and more complex than those for the knob. Furthermore, the core/cavity geometry required for shot B differs greatly for both variants. This needs to be taked into account when performing machining time estimations. Compared to the smaller knob, the vent's relatively large mold size and extensive machining required will drive up the tooling costs. Furthermore, mold cavity A for both variants will require two external side action mechanisms to produce the undercuts formed by the hinge. Other than the mold base, core/cavity machining, cooling channel drilling, and side actions, the mold will not require and other costly operations such as finishing or polishing. This case study assumes the same mold material will be used regardless of the production volume.


Vent Cost Estimation

The plots of total cost and unit cost against production quantity are reproduced below in Figures 4 and 5:

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As seen from Plot IIa, the SMM&A variant is initially less costly due to the lower capital investment and tooling costs, but is caught by the MMM variant (at 125,161 units and $133,222), which has a lower per-piece costs (slope) due to reduced processing and material costs. These results were expected.

Plot IIb is a log-log plot of per piece cost verse production quantity. It shows how c = C/Qd eventually flattens as Qd increases and settles at the slope of the cost curves displayed in Plot IIb. Both curves start to flatten out at around 120,000 units. This is where the fixed costs such as tooling and capital equipment start to become amortized over a large enough volume of production to become economically feasible.


Vent Performance Evaluation

The relevant performance aspects of the vent were analyzed. The results are summarized below.


PA1:Weight

The weights for both assemblies were calculated and summed. It can be seen that the MMM variant is lighter than the SMM&A variant by only just over .004 lbs (less than 2 grams). Although this isn't much of a difference, it gives the MMM variant a slight advantage in terms of weight. Of course it is not enough to be much of a deciding factor in selecting the better variant.


PA2:Interface Strength

Although the vent is not intended to be a high load-bearing structure, interface strength is still an important issue and valid basis for comparison of variants. Here, "interface strength" will refer to the load required to break the assembly at the hinge so that the slat becomes detached from the base. From analysis of the vent assembly, the most likely interface failure mode appears to be from shear fracture of the hinge pins undergoing simple beam bending. This type of failure would occur when a large enough resultant load is applied in any direction perpendicular to the axis of the hinge. Because there is no bonding in either variant, interface strength is only achieved through mechanical locking. The most accurate way to estimate the interface strengths of both variants would be to conduct a detailed FEA analysis on the entire assemblies. This would result in the final failure loads and fracture locations for both variants. Unfortunately, due to time limitations, and inability to access SolidWorks' full-featured FEA package, COSMOSWorks, only simplified FEA analysis was conducted on the individual components using the limited package, COSMOSExpress. This limited the analysis to specifying simple loads and restraints on single components in order to estimate the resulting stress distributions. However, this analysis, combined with simple reasoning and intuition was enough to identify the stronger variant, in a qualitative sense.

From simple stress considerations, it was initially hypothesized that the SMM&A variant was stronger due to the hinge pins being composed of steel rather than the polystyrene used in the MMM variant. Furthermore, it was assumed that both assemblies would separate through fracture of the slat somewhere near or at the hinge pins. This reasoning was used to set up the FEA analysis with simple loads and restraints and yielded the stress distributions shown below in Figure 6.

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The results of Figure 6 coincide with the initial hypotheses regarding the fracture locations and the higher interface strength of the SMM&A variant. The maximum Von-Mises stresses computed were 1.745E6 N/m2 and 1.618E7 N/m2 for the SMM&A and MMM variants, respectively. This implies that under identical loading situations, the stresses experienced by the MMM variant are almost an order of magnitude greater than those experienced by the SMM&A variant. Although exact force values for interface strength were unable to be found through this simplified analysis, the results seem to agree with the initial hypothesis that the SMM&A variant is better in terms of interface strength.


PA3: Assembly Clearances

Because the slat is designed to have a rotational degree of freedom with respect to the base, assembly clearances become an important performance aspect for comparative purposes. An accurate quantitative assembly clearance analysis would require a detailed mold flow and cooling simulation in order to predict the shrinkage of all four shots (two for each variant). Due to a lack of material information and molding/cooling simulation software, a detailed quantitative analysis of this performance aspect is outside the scope of this thesis. However, careful analysis of the fitting requirements can yield results suitable enough for qualitative comparison of the variants. Figure 7 below will be used to explain the qualitative analysis used to compare the vent variants:

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SMM&A Variant Clerance Analysis

As shown in Figure 7, the SMM&A variant requires four separate fits between the components: two clearance fits between the slat's hinge pins and the holes in the base, and two press fits between the same pins and the holes in the slat. The clearance fits allow the pins and the connected slat to freely rotate, while the press fits keep the pins secure inside the slat. This causes the pins to rotate with the slat and prevents them from separating axially from the entire assembly. Assuming the dimensions of the hinge pins are within tight enough tolerances to assume constant, these four fitting requirements mean that four separate dimensions must be adequately controlled so that the assembly functions as desired. This involves controlling the amount of shrinkage that the holes diameters can experience. Because all four holes are formed by protrusions in the mold (i.e. the side action pins), their tendency to shrink will be limited, and their variance minimal. However, it is still possible for the holes to become smaller if the side actions are disengaged while the part is still cooling.
MMM Variant Clearance Analysis
As shown in Figure 7, the SMM&A variant requires only two separate fits between the components: clearance fits between the slat's hinge pins and the holes in the base. Even though it is important for enough clearance to exist between the pins and the holes, an adequate amount is almost always assured by the way the assembly is molded. This is because no matter what the diameters of the base's holes are, the pins will always be smaller and form a clearance. This is because the pins are molded inside the holes, so they will tend to shrink away from the cavities formed by the holes.
Conclusions
From a qualitative standpoint, the MMM variant is better in terms of assembly clearances because it only posses two clearance dimensions which are automatically controlled, whereas the SMM&A variant possesses four clearance dimensions that need to be controlled though maintaining proper processing conditions.


PA4a: Flash

The amount of potential flash was measured for both variants through analysis of the part and mold model files. The results for each part are summarized below:
SMM&A Variant, Part A
For part A, there are two parting surfaces at which flash could form. This is illustrated in Figure 8 below:

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From measuring and summing the length of the two flash locations in Figure 8, we get a total flash length for the SMM&A vent, Part A is 20.803 inches.
SMM&A Variant, Part B
Part B of the SMM&A variant only has one parting surface and corresponding parting line, located on the top outer edge of the part. This is the only possible location for flash, as shown in Figure 9 below:

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As shown in Figure 9, the total flash length for the SMM&A vent, Part B is 6.650 inches.
MMM Variant, Part A
The MMM variant of part A is molded in the same manner as the SMM&A variant, and hence has the same flash length of 20.803 inches.
MMM Variant, Part B
Assuming that cavity B comes down on top of Shot A resting on the common core, there is no parting surface through which flash can occur. Hence, the total flash length for the MMM vent, Part B is 0 inches. Because cavity B presses against Shot A, the issue of crush will be addressed later.
Summary
From the results of the analysis for PA4a it is clear that the MMM variant has less flash, and hence the advantage over SMM&A in terms of PA4a. For this analysis, no weighting scheme will be used to emphasize (or de-emphasize) the relative importance of flash at different locations. That is, all flash is measurement was weighted by a factor or unity.


PA4b: Crush

The total surface area of crushed surfaces for both variants was measured through analysis of the part and mold model files. The results are summarized below:
SMM&A Variant
By definition, the total amount of crushed surface area in the SMM&A variant is zero. This is because at no point does mold B contact part A to crush it.
MMM Variant
Because it is assumed that cavity B forms a tight seal on Part A, it will tend to crush the top horizontal surface of Part A. Although cavity B actually comes in contact with all other external surfaces on Part A, they are drafted or tapered enough to not be affected by this crushing action. Furthermore, because a tight seal between these surfaces is not required, proper mold design will ensure they are not crushed by cavity B. The affected surface, and its measured area is illustrated in Figure 10 below:

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As seen from Figure 10, the total crushed surface area for the MMM vent variant is 1.624 square inches.


Summary and Conclusions

As with the first case study, it is apparent that neither variant is better in terms of all the cost or performance categories. However, the MMM variant becomes less expensive to produce after 125,161 units, which is a relatively small production quantity for injection molding. Furthermore, the MMM variant has an advantage over the SMM&A variant in three out of five PA's. Based on these results it is likely that many designers would choose the MMM variant.