1) Drafting 

This is the most known DFM concept for injection molding. But it really is the most important feature to allow the part to successfully release every single time. For requirements that need to be virtually vertical, a 0.5° draft is possible. However, the most common situation will see a 2° draft and changes in the surface finish will cause the mold to stick. Therefore any light to medium textures need from 3°- 5°+ 

2) Material Selection & Wall Thickness 

Even wall thickness allows for even cooling of the part. Which is important for structural integrity of the part. BY FAR, cooling time is the biggest portion of the cycle time, anywhere from 40%-60%. And cycle time is directly proportional to the overall cost for injection molded parts. If structural support is a concern, gussets can be added for reinforcement to maintain a uniform wall thickness. Matweb.com is a fantastic source to be able to look through possible materials and compare and contrast their properties. 

3) Unnecessary Undercuts 

Undercuts can vary from product to product but by majority, holes and snap fits tend to be common issues when designing. A design feature, like a hole, will require an extra tool, like a core pin, in the mold to help release it. This adds cost to the unit price. Undercuts plus a complex mold will always be expensive. Through-core design features are required to release these types of molds and will not raise the price. 

4) Adding Radii 

Sharp corners are the cause of stress concentration on a part. These areas are usually the first to break or fracture. Therefore, adding a radius to a corner, or fillet, will help disperse that stress onto a larger area. Depending on your wall thickness, your outer radius is recommended to be 1.5 times your thickness and your inner radius should be half of your thickness. 

5) Cycle Time 

As stated before, cycle time is directionally proportional to the overall cost of manufacturing injection molded parts. The cycle time includes opening and closing the mold, filling, packing, and cooling. Earlier, we discussed how cooling accounts for about 60% of cycle time, however, optimizing any other of these areas will decrease that time which in turn decreases cost. 

6) Surface Finish 

The SPI finish chart is the industry standard to help standardize surface finishes. But the rule of thumb is, the higher the luster, the higher the price. Both textured finishes and high polish finishes will be the most expensive. Also having a piece with two different types of surface finishes will be an additional cost. So design to avoid those pitfalls. 

7) Post-molding Operations 

Post-molding operations include any operation done on the part, after the injection molding process. Items like: additional machining, packaging, threaded, and quality testing are just a few post-mold operations that will carry and additional cost to your part. 

8) Threads 

Threads can either be designed in the mold or inserted as a post-molding operation. Think of all the water bottle heads that have this feature. If you are manufacturing less than 100 parts, you can heat stake the thread inserts as a post-molding operation, as this may be cheaper than machining a mold with embedded threads. But once you surpass 100 units, the cost balance shifts, and designing a mold with threads becomes more cost-effective. 

Resources:

https://www.xometry.com/injection-molding-service

http://matweb.com/