The critical dimensions for the press fit around the rims of the retainer mold were scaled to account for expected shrinkage, which is inherent in injection molding processes. In order to estimate the amount of shrinkage, sample injection molded parts of similar or identical dimensions and curvature were obtained, along with the molds from which they were made. Digital calipers were used three times on three different places on multiple copies of the part, and the corresponding measurements were made on the molds as well. The percent shrinkages were then calculated using the averaged three measurements across the part.
For example, the average outer diameters of three sample parts with 0.075” wall thickness were 0.0766”, 0.0777”, and 0.0756”, and the average outer diameters of the mold were 0.0775”, 0.079”, and 0.077”. The percent shrinkages were 1.12%, 1.6%, and 1.48% respectively, yielding an average of 1.4% shrinkage. Since the desired thickness of our part was 0.075”, the molds were therefore designed with a 0.07605 thickness to compensate for expected shrinkage.
The CNC routines for milling the core and turning the cavity were designed through MasterCam, and the final runner for the cavity was filed out by hand to control the size of the gate (Fig. 1).
Ejector pin holes were drilled and reamed in the core, in order to assist in removal of the injection molded part, and a draft angle was added to the outer edges of the border design (Fig. 2).
Due to an error in the toolpath parameters in MasterCam, the center drilling on the first ejector pin hole went too far into the core; the hole was plugged and repaired using some aluminum stock, and the faint outline of the plug can be seen on the bottom ejector pin hole in Figure 3.
The tabs turned out fine, though you can see from the picture below that the center stuck to the core a bit, leading to a concavity in the part. The outline of the plug can also be seen on the interior (Fig. 5).
You can see a tiny bit of dishing in the areas between holes in the border pattern, but we decided it wasn't significant enough to detract from overall appearance and functionality.
We tried fitting it with the updated thermoformed window. Hey presto! It's fit and flush!
Up next: altering the mold for a better press fit with the body.
The CNC routines for milling the core and turning the cavity were designed through MasterCam, and the final runner for the cavity was filed out by hand to control the size of the gate (Fig. 1).
Figure 1: Cavity for the retainer mold. Note the partially milled runner and that the gate was filed by hand.
Ejector pin holes were drilled and reamed in the core, in order to assist in removal of the injection molded part, and a draft angle was added to the outer edges of the border design (Fig. 2).
Figure 2: Completed core
Figure 3: Completed core. Note the plug in the bottom left ejector pin hole.
The first injection molding runs yielded significant amounts of flash (Fig. 4)
Figure 4: Flash! (aaaah! Savior of the universe!)
The tabs turned out fine, though you can see from the picture below that the center stuck to the core a bit, leading to a concavity in the part. The outline of the plug can also be seen on the interior (Fig. 5).
Figure 5: Underside of the part. You can see the tabs, as well as the outline of the plug in the lower right corner.
To prevent the part from sticking to the mold, a mold release was applied between every fourth part. The process was optimized by decreasing shot size, decreasing pressure, and increasing cooling time.
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