Design Tips & Considerations

What is injection molding

Injection molding is the technique where molten plastic is injected into a metal mold. The mold is composed of two halves, the “A” side and “B” side. The halves are separated and allow the plastic component to be removed once it has solidified, thus creating plastic parts.

What should we keep in mind when designing for injection molding.

1. Gate position and weld line

Gates are where the molten plastic enters the cavity of the part in the mold. These gates, once the part cools, leave a mark/ indication of where the gate was, even when attempted to be removed by a post process. Gate location is sometimes determined by:

• Where it will be less noticeable;

• Where it will not interfere with the rest of the part;

• How the plastic material will flow evenly through the part;

• Where the weld lines will occur;

• Or a combination of all these.

 2. Wall thickness

Keeping all major wall sections throughout a part as consistent as possible is key to injection molding. Doing so keeps material flow in the cavity consistent and makes for even cooling.  Most injection molding materials do not react well to flowing through large differences in thickness. Inconsistent wall thickness can cause pressure spikes, material degradation, too high of differential pressure across the part, and poor process control. The plastic also cools at different rates when you have varying wall thickness. 

3. Draft Angles

Draft angles allow for removal of the plastic from the mold. Without draft angles, the part would offer significant resistance due to friction during removal. Draft angles should be present on the inside and the outside of the part.

The amount of draft you should apply to the part depends on the application. Below is a list of different design considerations where you want to add to the amount of draft.

Features on the A-side of the Tooling 

The mold opening process should remove the part from these features, allowing the part to stay on the B-side. Putting more draft on the A-side features vs. the B-side promotes proper transfer to the B-side during mold open. 

Amount of Texture 

The more aggressive the texture, the more draft you need. Think of the texture as microscopic undercuts on the part. The higher draft angle will make sure the texture releases on ejection and doesn't drag as it moves off the molding surfaces. 

Deep or Thin Ribs 

Small and deep ribs can be tough to injection mold and even tougher to eject. Adding more draft to these features ensures that they come out clean every time.  

Small Features 

Small features like holes can make a part stick during ejection or cause cracking. Even the smallest features should have some draft applied. 

4. Ejector Pin Locations

It is very important to think about the way that the component is to be ejected from the tool. The design of the ejection system is very important to get right. As problems with the ejector damaging the part, not ejection the part fully or the ejector system jamming up and breaking is a problem you don’t want.

From the ejector system we need all the components to be ejected positively, without causing the parts to twist, distort or hang-back.  Therefore to decide on the best ejector system for your mold you will need to take a number of factors into account:

• The part geometry

• The material

• Gating

• Ejection balance

• Machine specifications

• Part finish requirements

5. Adding Radii

Not only are radii more visually appealing, they also help plastic flow more smoothly and reduce the likelihood of fractures. When designing radii, keep in mind the uniform wall thickness best practice. Adding radii helps make sure plastic flows through the part consistently, keeps the wall sections even, makes the part more visually appealing, and avoids potential cracking/fracture points.

6. Surface Finish

Building injection molds requires several different pieces of equipment. To create molding surfaces, they may be CNC machined, ground, EDM'd, turned, and so on. Each of these manufacturing processes will generate different surface textures. These textures (tooling marks) may be acceptable for non-show surfaces, but in many cases, tooling marks need to be smoothed out or textured. Creating a texture not only makes the surface of the molded part consistent, but it also has implications on part design.