Design for Injection Molding

In a recent survey conducted over linkedin, a good 40% of the respondents said that they spend anywhere between 10-20% of their time reviewing design from manufacturability aspects. 20% of the respondents even mentioned that they spend more than 20% of their time doing design reviews related to manufacturability. Young design engineers (less than 35 years of age) spent more time on such reviews than experienced engineers. Clearly, addressing the manufacturability issues right at the design stage is the key to time to market, improved quality and reduced cost. Also, capturing the knowledge from the retiring and experienced workforce and passing it on to the new generation is another crucial area.

There are many design guidelines available on the web which mentions the best practices but there are very few tools which can actually enforce these guidelines through automated guidelines.

Some of the common design guidelines for Injection Molding include:

1. Uniform wall thickness : Consistent wall thickness is required to prevent sinks, warps and distortion of parts.
2. Thin wall sections: Thinner walls result in shrinkage during cooling. Gussets provide additional support to reduce warpage.
3. Thick wall sections: Thicker and non-uniform wall thickness can result in sinks in the material. The use of thinner, uniform wall thickness helps to avoid sink.
4. Sharp corners: Sharp corners should be avoided as it results in high stress concentration. Sharp corners should be rounded to reduce the chance of warp, short shorts, splay and flash. If sharp corner on the outside is critical for the part function, EDM process is required to produce sharp corner on the tool.
5. Sudden change in wall thickness: Wall thickness should vary gradually from thick to thin regions. Sharp change in thickness gradient results in high stress concentration.
6. Nominal boss wall thickness: Thick sections need to be avoided to minimize aesthetic problems such as sink marks.
7. Mold wall thickness: Design of the part governs the design of the mold. Mold wall thickness should be adequate for longer tool life.
8. Minimum radii at the base: Minimum radius at the base of the boss is recommended to reduce stress.
9. Minimum radii of core pins: The core pin should have a radius to reduce material turbulence during filling and to help keep stresses to a minimum.
10. Minimum draft angle: Draft on vertical surfaces of part enables easy removal of the part from the mold. A minimum draft angle is required on the core and cavity side.
11. Recommended rib thickness: Thin ribs on thicker walls may provide stiffness, but they can also result in sink. For thick ribs, “core out” the rib from the back. This creates a hollow space underneath the part and maintains a uniform wall thickness.
12. Depth of blind holes: Blind holes are made by core pins supported on one end only. The pins can be deflected and pushed off center by the pressures of the molten plastic material during the molding process.
13. Blind hole bottom thickness: For blind holes, the thickness of the bottom should be greater than 20% of the hole diameter in order to eliminate surface defects on the opposite surface.
14. Through holes: With through holes the cores can be longer as the opposite side of the mold cavity can support them. An alternative is to use a split core fixed in both halves of the mold that interlock when the mold is closed. In cases where longer cores are required, careful tool design is necessary to ensure balanced pressure distribution on the core during filling to limit deflection.
15. Minimum distance between holes: Minimum distance between holes should be maintained. If these distances are not maintained, the holes will be egg shaped or the part will deform in areas around the holes.

After the two modules of DFMPro, Design for Sheet Metal and Design for Machining, the next release of DFMPro will contain Design for Injection Molding. The first release will incorporate some of the rules mentioned above and some more rules as requested by the beta customers.

Stay tuned to catch the first version of the release in couple of months time from now.