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Design Guidelines
Part Radiusing and Draft
Proper use of draft and reinforcing fillets will aid in ejection, add rigidity to part ribs and strengthen the mold.
Avoid unnecessary stresses by rounding out corners
It is also a good idea to avoid the creation of stress risers in sharp corners. This can be done through the use of a radius to distribute the stresses and also streamline the flow of the molten plastic.
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Sharp corners have high stress concentrations and plastic flow is hindered. |
Rounded corners have reduced stress concentrations and plastic flow is enhanced. |
Include sufficient draft
Draft facilitates the removal of the part from the mold and is particularly important in Rapid Injection Molding where the molds are straight pull only (i.e. no side actions).
The guidelines associated with the number of degree of draft required will vary with geometry and other part characteristics (e.g. surface texture requirements), but in general the more the better.
Here are some rough guidelines to follow:
- We strongly advise using at least 0.5 degrees on all "vertical" faces.
- 2 degrees works very well in most situations.
- 3 degrees is minimum for a shutoff (metal sliding on metal).
- 3 degrees is required for light texture (PM-T1).
- 5 or more degrees is required for heavy texture (PM-T2).
Helps with part ejection
Minimizes tool wear and potential flash with telescoping shutoffs
Use drafted shutoff surfaces
A “telescoping shutoff” is when the shutoff surfaces slide with respect to each other as the mold closes and opens. The more sliding that is done, the greater the wear and the more potential for mold problems (e.g. breakage) and part problems (e.g. flash).
So the “steepness” of a telescoping shutoff surface is ideally a few degrees off vertical to avoid these problems.
This example shows a part feature that is much more easily manufactured with drafted walls than without them.
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No draft results in sliding parallel mold surfaces |
Draft results in improved mold shutoff surfaces |
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Sliding Shutoffs Video
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