Designers can improve sheet utilization of sheet metal components at design stage

Major consumers of sheet metal include automotive, aerospace, furniture, white and brown goods, electrical and body building. Parts produced here are in high volume which uses hard tooling. Raw material accounts for approximately 60-75% of the total cost. Research says that 80% of the lifecycle costs of a product are determined during design stage. Wastage of raw material is due to cracking, deformation, failure during manufacturing or due to improper sheet metal utilization.
To reduce wastage of raw material, defects should be reduced along with improving the sheet utilization.
Sheet utilization can be improved during design stage by checking the layout of the nested parts on the given sheet. As a simple example, we take a box which needs to be manufactured. There are various design alternatives through which a box can be manufactured. Given below are two design alternatives that will result in the final component (box).

As a designer, it is not very intuitive which one of these will lead to better sheet utilization and hence lower cost and higher production rate. Using nesting algorithm, designers can check the utilization resulting from each of the above design alternatives.




















From the layout it is very evident that the second design alternative is better as it can nest more number of parts on a given sheet.
As a second interesting case, a slight change in the design of the “C” as shown below, leads to significant jump in the sheet utilization.










The above examples clearly show that if the designer is empowered to visualize the layout of his design, he can take better decisions to improve sheet utilization without impacting the quality of the product. In future, we may see such design alternatives automatically being offered by the software tools.

DFMPro for Injection Molding release plan

DFMPro version 2.0 will address the Design for Manufacturing issues related to Injection Molding. The beta version will be out in couple of months. Following features have been planned for the beta release:

• Minimum Radius at Base of Boss : This rule will check if the radius at root of a boss is greater than specified value.
• Minimum Radius at Tip of Boss : This rule will check if the radius at tip of a boss is greater than specified value.
• Mold wall thickness : This rule will check if the clearance between surfaces in the model is greater than a specified value; thus ensuring that the mold wall thickness is above a specified minimum.
• Maximum cutout area : This rule will check whether the cross sectional of area of a through cutout in the part is greater than a user specified value.
• Rib Reinforcement Check : This rule will check whether the rib in the part needs to be reinforced or not.
• Recommended Rib Parameters : This rule will check whether height, width and thickness of rib are as per specified values.
• Minimum Draft Angle for Core & Cavity : This rule will check whether the draft angle on the part surfaces matches or is greater than the specified draft angle.

The final release will contain some more rules requested by beta users. The initial plan is to provide these rules on Pro/ENGINEER. Later, these will be added to SolidWorks as well.

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.