Design Tip - 8

This design tip is the first one about the friction design. Parts that have to move one over the other need a good design in order to get a smooth movement.
If the use of PTFE or PTFE filled material is not possible for cost reasons, and the use of POM is not possible because the parts have high aesthetics requirements (as explained on Design Tip - 7), the design of the friction areas is even more important.
This first tip is about to avoid 'surface to surface' friction areas. The attached sketch is simplified to a flat surface but it also applies to not flat surfaces (even more critical); the movement of the part is normal to the computer screen.
It is impossible to get a perfect flat surface (tolerances exist!), so it is also impossible to get two flat surfaces, if we want to move an not perfect flat surface over another not perfect flat surface we don't know what will be the friction area, so the friction area is out of control. 3D shaped surfaces are more difficult. An out of control friction area means that the movement could be smooth or not, the friction force could be high or low, etc.).
In order to have a controlled friction area, a 'surface to point' or 'surface to line' area has to be defined. The second sketch shows this kind of friction combination. It is a section cut so it could be any of the 2 friction systems. The friction "ares" will be points on the first system and lines on the second system. They are controlled surface areas with low friction force.
The second sketch also has a another tip included in the design. The "flat" surface has 2 slots, so the friction area is in the slot and not on the main surface of the part. The part slot will be a "bump" on the mold. This king of "bump" is very easy to polish. Friction surfaces have to be polished but it is not necessary to polish the entire part, so with this small tip we are helping to have a more robust mold. Apart of the polishing, the defined friction surface is very easy to modify. If after first Tool Try Out we realize the movement is not correct, the surface could be adjusted easily. My suggestion is to leave a small gap on the design in order to be able to modify the mold in a cheaper way (steel safe, no weld required).