Housing Design

Often the design of the plastic housing is much too trivialized. It is frequently assumed that as long as the strip itself functions according to plan, the housing is just a package for the strip and it doesn't really matter much what it looks like or how it is designed. On the other extreme, Marketing may decide that the housing is the most important element of the product and what matters most is a fancy and unique shape. How the housing relates to the strip or the means by which it will be manufactured are often ignored.

The sizes and shapes of devices currently on the market vary widely. Rectangular, circular, square, oval, triangular, and freeform shapes are all commonplace. There are molded plastic housings, strips laminated between layers of plastic much like a credit card, fancy curved surfaces, and bright colors. It is important to realize that, in addition to simply enclosing the strip, the housing must support the strip properly to allow for proper fluid flow. The strip has to be accurately positioned endwise so that the test and control lines are clearly visible in the read window. The strip must also fit snugly within registration features so that it does not slide around during the assembly process and to ensure that its edges remain hidden under the sides of the read window. Proper housing design should also ensure that the layers of the lamination are pressed together just enough but not so tight as to create a restriction of fluid flow. Additionally, the housing must be designed so that it can be easily and efficiently assembled in an automated process.

In this discussion we will focus on the injection-molded plastic housing, the most common in the industry. In addition, we will focus on housing designs that allow for the simplest, lowest-cost automation processes. These housings are not particularly elegant or fancy; they are simple, symmetric rectangular elements.

To allow for the lowest possible assembly cost, the lower housing should be designed with straight parallel sides and a flat bottom. Figure 3 illustrates three different housing designs.

Housing B in this illustration is the simplest to automate because its sides are straight and parallel. Housings A and C are more difficult to align in automated assembly systems because their sides do not provide a simple, straight reference edge for accurately registering the part.

It is acceptable to have pockets or recesses in the under surface of the housing, as long as they do not prevent the housing from sitting flat. The design should include raised edges to help keep the housing stiff and prevent warpage or twisting. The lower housing should include a strip nest. The purpose of this nest is to properly align the strip in the housing in both the lengthwise and sidewise axes. This nest can be comprised of short sections of raised walls, small pins, or it can be a recess in the floor of the housing. A good rule of thumb is to have the height of the walls of the nest at least two-thirds of the maximum thickness of the strip. The strip should be a press fit in the nest with an interference of approx 0.003 to 0.008 in (0.075 to 0.20 mm). Lengthwise, the strip should have a clearance of approx 0.010 in (0.25 mm). The floor of the nest may require stepped elevations to properly present the surface of the membrane to the underside of the read window in the mating upper housing. The edges of the strip nest should be designed to be parallel to the outer edge of the housing. This allows for simple registration when placing the strip in the hous-

Fig. 3. Three types of housing design.

ing by automated processes. The lower housing should include at least six pins for securing the upper housing. These pins should be at least 0.040 in (1 mm) in diameter and should include a chamfer of approx 0.010 in x 45° (0.25 mm x 45°). The pins should be positioned asymmetrically to prevent the upper housing from being assembled backwards on the lower housing.

One element that is frequently overlooked in the design of the lower housings is a means by which to detect the orientation of the part when employing automated assembly processes. The upper housings are generally easy to orient, as they typically have openings through the part (sample well and read window) that can be easily identified with optical sensors; but the lower housings are usually relatively flat and symmetric parts. For this reason, the lower housings are generally much more difficult to orient automatically and at high speed. It is very important, therefore, to provide a defining feature. This feature must be prominent and it must be located in an asymmetric position on the housing. The feature should be located on the top surface of the part and it should include a height difference from the floor of the housing of at least 0.080 in (2 mm) so that it can be easily detected by an optical sensor even if the part has some warpage.

As discussed previously for the lower housing, the upper housing should also be rectangular. As for the lower housing, the upper housing should also include raised edges to help maintain flatness and provide some stiffness. The housing should be designed such that when placed on a flat surface with its open side down (the side that mates to the lower housing), it is stable and does not rock. As with the pins on the lower housing, the mating sockets on the upper housing should include a lead-in chamfer of approx 0.010 in x 45°

Fig. 3. Three types of housing design.

(0.25 mm x 45°). These chamfers on the pins and sockets will provide easier alignment of the housings during the assembly process. These pins and sockets are the main structures that hold the assembly together. The fit of the pins into the sockets must be worked out very carefully with the housing designer and the molder. If the fit is too tight, the pins will break during the assembly process. If they are too loose, the assembly will not be held together securely enough.

When dimensioning the plastic housings, one edge and one end of the housing should be chosen as the datum edge. This edge should be the same for both the lower and the upper housing (i.e., when the parts are assembled together, the datum edges should be at the same locations on both components).

One very important dimension on the plastic parts—one that it seems is quite often overlooked—is a flatness specification. When plastic parts are molded, they can twist and warp if the mold process is not controlled properly. Warped parts are difficult to process at high speeds in automatic equipment and can cause frequent and annoying machine jams. A reasonable flatness specification for molded housing components should be in the range of 0.03 inch (0.75 mm). This means that when the plastic part is laid on a flat surface, the part should not bow or rock by more than this amount.

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