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.
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.
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.
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:
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
Vent Performance Evaluation
The relevant performance aspects of the vent were analyzed. The results are summarized below.
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.
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.
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
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:
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
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.
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.
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:
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
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
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.
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:
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.
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:
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.