Design for Moldability

Best injection molding designers always foresee the mold making and injection processes while designing a part for injection molding. This approach will save you money and trouble in the future. Even small design changes to part and tool design can have a major effect on the part performance and the cost.

Minimize the Tool Change Activity

The smaller the tool diameter the longer it takes to finish the mold. Understanding some basic mold machining concepts would help you design a cost effective plastic part. For example, a standard 1/4” end mill can effectively mill down to 1-1.5” deep if there is little or no draft. When there is large draft, this can be extended to 2-2.5”. A smaller tool, 1/8” end mill, will be able to half of what 1/4” can do. If you can finish the mold milling process by using only one cutting tool endmill, the cost of the mold will be the lowest.

Eliminate the Deep and Narrow Pockets

Machining gets really complicated when there are deep pockets to mill on the mold. The narrower, the steeper and the deeper it gets, the required cutting tool changes and in some cases it makes it impossible to mill. In those cases, the EDM process may need to be used. That option is one of the highest costs in mold making.

Use Uniform Wall Thickness

Varying wall thickness creates uneven fill throughout the cavity. Thinner sections will cool faster than thicker areas causing highly stressed areas where distortion and warping will occur. It is recommended to core out the thick areas to create uniform walls, which will reduce cycle time and plastic usage and improve part cosmetics. Depending on the type of material being used, nominal wall thickness should not vary more than 15%.

Use the Highest Draft Possible

Each part should have at least 1% or ideally 3% minimum or more draft angle. If the part design allows, the part designer should use the highest draft he/she can. Always think about the "ice cube tray" concept.

Have Radii on All Corners

There should be .030" radius minimum. The deeper the cut the larger the radius. 

Keep It Simple

Eliminate Undercuts

If the material is not flexible, it can not be ejected from the mold. Side action, inserts or secondary operations can be applied to mold parts with undercut. Those actions make the mold more expensive. 

Eliminate Ribs

Ribs should be avoided if it is not necessary for the part structure. The height of the rib should be less than 3 times the wall thickness and its base should be less than 60% of the wall thickness.

Eliminate Unfunctional Features

Be prepared to pay premium for texture, numbers and logos. If you have to use numbers and logos, have mill friendly fonts larger than 20pt size and less than .015" deep. You may also need to increase the draft. Designing as extrusion also helps and makes it faster to machine.