Performance Aspect 4: Aesthetics and Ergonomics
While both aesthetics and ergonomics are subjective concepts that
cannot be universally measured, we can define several quantifiable
performance aspects that give us some relative measure of these
qualities. For this model, aesthetics is defined as a measure of how
attractive a product appears, and ergonomics is how well a product
feels when used/held.
Although there are potentially many arbitrary ways to define PA's for
quantifying aesthetics and ergonomics, this model uses two simple and
measurable characteristics unique to injection molding processes.
Specifically, SMM quality evaluation models were extended to MMM to
correctly predict defects encountered in the MS industry.
Two important types of defects, part flash and crush, will be used to
define ergonomic and aesthetic PA's, respectively. These phenomena will
be defined and their measurement procedures will be detailed below.
PA 4a: Flash
The phenomenon of part flash will be used as the PA for measuring the relative ergonomic quality of molded assemblies.
Definition of Flash
Flash is defined as the undesirable formation of superfluous strips of
material on a molded part's surfaces and/or edges. This is a relevant
measure of ergonomics because flash is generally thin and sharp,
causing discomfort or even scratches/cuts when held or otherwise
interfaced with by a human. In general, the possibility of flash
occurrence should be minimized by the product's design, or it will have
to be manually removed after molding (i.e. through grinding or filing).
Figure 1 shows a photograph of a MS part where the second material
shot has flashed over top of the first:
Source: [MGS02]
Formation of Flash
Flash occurs when some of the molten resin escapes the intended mold
cavity through thin gaps in the mold, usually where the core and cavity
halves meet. The resin then solidifies in the shape of the gap, and the
part is ejected with these unwanted thin plastic strips attached. An
example of where flash can form in a single-material mold is
illustrated in Figure 2 below:
Figure 2 shows how flash can form on surfaces where the mold halves
meet, including the parting surface and any core-cavity shutoff
surfaces. In this specific example, the flash occurs at the bottom
outside edge of the part ("parting flash") and at the top of the
through hole on the top surface of the part ("shutoff flash").
In addition to flash formed at metal-on-metal contact surfaces, MSM
allows for the unique occurrence of flash at metal-on-plastic contact
surfaces. As with metal shutoff flash, plastic shutoff (or "MS") flash
can occur anywhere the mold metal meets ("shuts off") on plastic, as
illustrated in Figure 3 below:
In the specific example above, the flash could form along the flat,
slanted outer surfaces of shot A and at the bottom of the shutoff hole
on the top of the assembly. Depending on the type of surface on which
the flash forms, three distinct types of flash can be identified: 1)
parting surface flash, 2) shutoff flash, and 3) MS flash.
Note that although both single-material flashes and MS flash form via
very similar mechanisms, the end results are slightly different. That
is, single-material flash usually forms on outside edges of the part
whereas MS flash typically forms along the surface of previous
materials. For example, the parting flash in Figure 2 forms on the
outside edge of the part directed away from the surface, whereas the MS
flash (material B) of Figure 3 forms along the surface of material A.
This distinction between single-material and MS flash can cause
significant differences in the appearance and/or quality of the final
assembly. For example, it would be much easier to remove exterior
parting line flash rather than MS flash adhered to a surface. The
former might require only simple cutting or grinding whereas the latter
might require careful peeling.
For this model, it is assumed that MS flash only forms when there is an
actual gap between the mold metal and the previous plastic shot. In
other words, if the mold metal penetrates the plastic surface during
shutoff, it is assumed there will be no flash between the mold and the
plastic due to a tight seal being formed. This simplification allows
the two phenomenon of MS flash and crush to be clearly distinguished and measured as separate PA's.
The occurrence of flash depends on a variety of factors such as mold
accuracy/tolerances, resin properties, and processing parameters. It is
difficult to predict when and to what extent flash will happen.
Furthermore, for a particular molding run, flash can form on some parts
and not on others (due to variations in the processing variables).
Although it is nearly impossible to predict when/where flash will
happen on individual parts in a batch, locations of probable flashing
can at least be identified. Therefore, all locations where the
formation of flash is possible will be used to measure PA 4a as
discussed below.
Measurement of Flash
As described above, flash can form at three different types of
locations (e.g. at parting surfaces). All three types of flash are
undesirable to some degree, depending on their exact location on the
parts. Therefore, it becomes important to form a PA that measures the
effect of flash, weighted based on the relative importance of its
location. The most obvious way to measure flash is based on its size.
Because flash is a geometric phenomenon, it can be uniquely defined by
its dimensions, which include its thickness, width, and length. More
specifically, only the total perimetric length of flash will be
measured. This is because while width and height of flash may also be
considered important, it is impossible to predict their values as they
can vary from shot to shot. Therefore, only the flash length will be
considered for PA 4a. Specifically, PA 4a (measured in inches) is
defined as the total length of flash on both parts A and B.
The length of a particular piece of flash can be defined as the
perimeter of the mold gap in which it could potentially form. A simple
example of a MS assembly with two types of flash is illustrated in
Figure 4 below. The length of the parting flash, , is the arc length of
the
semi-circular gap that forms the parting line between mold halves. The
length of the MS flash, , is the straight length of the edge formed
where cavity B contacts the surface of material A.
Using reasoning similar to that of the specific example above, the
lengths of the various flash pieces can be calculated for any generic
SMM&A or MMM product. A simple analysis of the part and mold CAD
models will yield the perimetric length of each mold gap (parting line
or shutoff edge) which equals the flash length. Once the individual
lengths of each flash piece for both parts are calculated, PA 4a can be
formed as a weighted sum of these lengths.
It is recommended that some sort of appropriate weighting scheme be
used to assign relative importance to each pieces of flash based on
their location. For example, if a certain piece of flash is somewhere
on the part where it won't be seen or come into contact with, it
wouldn't have much effect on the ergonomics, and could be weighted as
unimportant by multiplying it by a small number (or even zero). On the
other hand, flash occurring at a crucial location such as a handle
would seriously affect the ergonomics, so it should be weighted by some
appropriate factor (at least greater than unity). The exact weighting
scheme would depend on how important the designer deems the effect of
each flash piece.
PA 4b: Crush
The phenomenon of part crush will be used as the PA for measuring the relative aesthetic quality of molded assemblies.
Definition of Crush
Crush is a phenomenon unique to MMM caused by mold metal impinging upon
("crushing") a plastic surface formed in a previous shot. The mechanism
is very similar to MS flash except that there is no gap between the
metal and plastic. Rather, the mold partially penetrates the plastic
during shutoff, causing a visible mark on the part surface. A photo of
a MS product with crush is shown in Figure 5 below:
Source: [MGS02]
Figure 5 shows a 2-material product (power tool housing) where the
mold for the second shot has formed an indentation on the surface of
the first shot. This discontinuity is visually unappealing and can
cause discomfort when gripped at the handle. Figure 6 below
schematically illustrates MS crush:
From Figure 6 two potentially undesirable effects of crush are
identifiable on the surface of part A: 1) a flattening of the texture
or altering/marring the finish, and 2) an indentation or recess. In
many cases such discontinuities caused by crush are considered visual
defects that decrease the aesthetic quality of the product. Therefore,
quantifying crush is one applicable PA related to aesthetics.
Formation of Crush
Crush is formed any time the metal-on-plastic shutoff is designed such
that the mold metal partially penetrates the plastic surface. This is
opposite the situation of MS flash, where there is a slight gap between
the mold and plastic. Obviously the extent of the crush depends on how
deep the shutoff penetrates the plastic.
Wherever possible, crush should be minimized. There are several unique
DFM rules that can be used to reduce or hide the occurrence of crush,
but they will not be discussed here (see [MGS02] for some crush design
tips). Regardless, the mold-plastic shutoff interface specified in the
product's design completely determines the location and extent of
crush.
Like flash, crush is another phenomenon that can be easily measured
based on the mold geometry. Crush is characterized by the depth and
surface area over which it occurs. The depth of crush can vary from
shot to shot based on process variations. Because of this, and the fact
that the exact depth is relatively unimportant in terms of visual
appearance, only the surface area of the crush will be used in the
measurement of the aesthetic PA. Therefore, PA 4b is defined as the
total surface area over which crush occurs (measured in square inches).
Measurement of Crush
Crush only needs to be measured for the MMM variant because, by
definition, the crush on SMM&A variants is zero. The total surface
area where crush will occur is simple to compute from the CAD models of
the parts and cores/cavities. All metal-on-plastic shutoff features
must be identified, and their associated surface area is equal to the
crush area. The sum of these crushed surface areas is then the value
for PA 4b.
Validity of Performance Aspect 4
The two aesthetic/ergonomic measurement models described above
represent valid ways to quantify relative performance as they address
two of the most common and important types of defects associated with
producing plastic parts. These issues have been identified by experts
in the field to be two of the major problems in manufacturing quality
plastic parts. The measurement techniques described are objective
methods for classifying these performance aspects.
