SolidWorks 2011 Assemblies Bible - Matt Lombard [184]
You will now run through two examples of manually intervening in the Mold Tools process. In the first, you will learn how to create a passing shut-off (a shut-off with a stepped parting line), and in the second, you will learn how to create the parting surface.
Passing shut-offs
Snap features are often achieved in molds by using passing shut-offs rather than some sort of lifter or horn pin slide. Eliminating actions from a mold can be economical, as long as the passing shut-off does not introduce wear or alignment problems. When creating parts that require this sort of feature in the mold, it is a good idea to consult your mold builder.
Passing shut-offs can be difficult to visualize, even for seasoned professionals. It might be a good idea to open up the part and study the geometry.
On the DVD
The file passing shut off start.sldprt from the material for this chapter on the DVD is a clip that holds a CD in place in a plastic case. The draft analysis colors have been left on it to help you see which faces belong to which side of the mold. There are no undercuts on this part, as shown in Figure 21.21.
Two pair of passing shut-offs are modeled in this part.
Using the rollback bar is probably the best way to see what is going on with this part. The surfacing involved here may be confusing to you if you are not well versed with surfacing, but looking at the part and understanding the steps will help you learn. The basic steps to create the surface body called Shut-off 1 are as follows:
1. Create a ruled surface for the planar edges.
2. Loft surfaces between the parting line edges and the ruled surface.
3. Extrude a flat shut-off face at the parting line of the Snap feature.
4. Use the cavity or core knitted body to trim the extruded surface.
5. Use the extruded surface to trim the ruled and lofted surfaces.
6. Knit the surface bodies together.
FIGURE 21.21
A part that requires passing shut-offs
The hardest part of creating this passing shut-off is visualizing what the interface between the steel from opposite sides is going to look like. It is best to keep it as simple as possible. Tool builders request a wide range of angles for the passing shut-off (mold steel touching at steeply angled faces). The minimum draft they can possibly stand ranges from 5 to 15 degrees of draft. You should try to give at least 8 degrees and more if you can. The tool builder will also look for a minimum land (flat steel making contact from either side) on the top of the shut-off boss, generally not less than 1mm, or approx 0.050 inch, to work with round numbers.
Don't be discouraged if you don't completely understand this the first time around. The concept itself is difficult, and visualizing the geometry is even more difficult.
Creating non-planar parting surfaces
The method SolidWorks uses to create the parting surface is adequate for simple tasks, such as molding a range of Frisbees or dinner plates, but it will not work well for more complex projects such as handheld medical devices. Figure 21.22 shows the part on the DVD named frame parting surface.sldprt. The result is entirely unacceptable for several reasons, including big gaps in the parting surface, and unnecessarily complex parting surface.
FIGURE 21.22
An automatically created parting surface for the handheld medical device
From this, you can learn that the SolidWorks Mold Tools are not reliable for concave parting lines or non-planar parting lines. Flat parting line disks and boxes work well. Beyond that, you should expect to do some manual surface modeling.
Note
If you want software that will create automatic parting surfaces for you, consider MoldWorks and SplitWorks from R&B Mold and Die Design