Comer undercut

FIGURE 7.47. Schematic illustration of comer undercut based on the (111) step etching model. 

TPEs in comparison to thermoset are also suited for thermoforming, which is a fast, low-cost production. The maximum draw ratio decreases with decrease in hardness. Draw ratio up to 0.1 can be attained with TPEs in the hardness range of 40-50 Shore D. The major limitations are to avoid undercut sharp comers, and adequate taper angle for rapid ejection of parts. 

Waiting for the natural etch stop means that L , T or U -shaped V-troughs necessary for microreactors cannot be produced. To produce these troughs, therefore, etching is interrupted when the desired depth has been reached. These V-cross-sectional troughs have convex 90° comers at any bends, and at these points there is considerable undercutting of the mask. The remedy is the corner compensation. Sacrificial surfaces are created at the convex corners, calculated in such a way that the etch time in the direction of the convex 90° comer will produce the correct shape. 

Indentations in the surface are not undercuts if they are perpendicular to the parting line because they are in the direction of the draw. Therefore, it is desirable to set the parting line such that indentations are in that plane. In Fig. 8.36, the comer Z and the indentation at 17 are imdercuts because they are in the direction parallel to the parting line. The imdercut at U could be stripped from the mold however, the one formed by the comer Z would require a removable section in the tool. Parts which are rotationally molded are often quite large and the materials used (polyethylenes) have high shrinkage rates. It is often possible to strip the undercut if the part will shrink more than the... 

Your molds should be given slight sloping sides with no undercuts. This is so the pattern can easily be removed from the formed plastic when the job is complete. It s best to slightly round all outside and inside corners. On wooden molds, I use auto body putty to fillet inside corners and I also use it to fill imperfections in the mold. Then I sand the mold smooth and round outside comers with the sandpaper at the same time. 

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