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.