Injection Part and Mold Design: Everything You Should Know (Well, Almost Everything)

Injection Part and Mold Design: Everything You Should Know (Well, Almost Everything)

What is Injection Molding?

Injection molding is the process by which molds are made of two halves hollowed with a negative cavity of what a part will look like.

After a design is made, a molding company will then inject hot plastic into the tool to fill the cavity till cooling. Once the part is cooled, the two halves of the tool are separated, and the part is ejected from one half of the tool.

That is the short description of injection molding (and you probably already knew this), but there is so much more to consider when designing for the part that you intend to mold.


What Should You Consider?

Are you designing a product for injection molding? If so, we have some guidelines that will be helpful as you work through the part and mold design process.

When developing a design for injection molding, there is a standard set of considerations that any Creative or CAD Designer will need to consider, whether it’s for creation of part design or molds for injection molding of successful parts.

Beyond the basics, we’ll outline how to design for more complex parts, and let you know when is the right time to interact with Anderson Technologies.

At a minimum, the following key points need to be considered when developing a tool for injection molding:

  1. Gate Location

A gate is where the hot liquid is pouring into the cavity of the mold. This will need to be removed after the molding process. The location of the gate is crucial: It needs to be located on a surface that will not be visible once the part is completed. When considering the location of the gate, designers will want to position the gate at a thicker, intersectional area of the part where it can be removed without concern for the visual and structural appropriateness of the design. Plastic flows best from a thick section to a thin section. The gate will normally be located on the cavity or “A” side surface. When a gate is removed, it will leave a mark, so again, it is important to choose a strategic location so as not to disrupt the appearance.

  1. Adding Thickness for Part Shrinkage

Depending on the material used, the part may have some shrinkage during cooling. There are shrinkage rates that can be considered for the material choice, and Anderson Technologies can assist you with these choices.

  1. Parting Lines

When the two halves of the mold separate, there will be what is called a parting line at the point of separation. As with most of these steps, the part line’s location is important to the design’s appearance.

If you have undercuts that require an insert or lifter in the tool, then part lines will most definitely be visual to the exterior of the part. This will likely require special design attention to either hide or utilize the line.

  1. Wall Thickness

Some tool shops and injection molders will tell you they can only make mold parts with a uniform wall thickness. While this can make their manufacturing process easier, the reality is that some parts simply require variable wall thicknesses. At Anderson Technologies, we do what it takes to get the job done.

It is true that contrasting wall thicknesses can make for a more difficult injection mold process, with longer cooling times. But managing this issue is achieved by designing your part with manufacturability in mind. Consulting Anderson Technologies early in the tool design process is key to making sure that variable wall thicknesses can be achieved in an efficient way.

  1. Part Draft

Adding a draft in your molded part design requires special attention to make it easy to eject the part from the mold. Adding a few degrees of draft means that parts will leave the mold much more smoothly, with minimum friction and scraping between the finished, cooled product and the walls of the mold.

There are minimum draft requirements needed for any part based on the depth of the part, the material chosen, and the amount of texture applied to the mold.

  1. Texture

From spark finishes to heavy leather, nearly any type of texture can be added to the inner surface of a tool by etching or milling the mold to create the desired finish. A greater degree of control and uniformity over the look and feel of your part is achieved with this application. It also saves time and money by incorporating two processes into one.

You should know that any texture will require additional draft consideration, so consult the Anderson Technologies technical team on proper drafting when you know what kind of texture you desire for the part’s Class A surface.

  1. Material Choices

Material selection is one of the most critical choices in designing your part. It factors into many aspects of the process, including shrinkage factor, cooling time, flexibility, and more.

Different materials have different minimum/maximum wall thicknesses and require different degrees of draft. Some materials will be more flexible than others for pulling out of the tool. It’s important to keep these things in mind during the design process.

Typical material choices are ABS, POM, HDPE, Polycarbonate, and polypropylene. There are many more engineered polymers, so ­– as always – please allow Anderson Technologies to assist. We can help you choose the right material for your part while incorporating those material properties into your design.


Beyond the Basics

So, those are the basics. The 7 points listed above are just the minimums to consider when designing parts and tools for the injection molding of those parts.

But what if you intend to have a complex form for your part? What if the parting line is not flat, but runs around the edge of the part like a potato chip? What if you want to mold the part in a translucent material and your ejector pin marks are now a visual issue? And it has variable wall thicknesses? And it has inner openings in the part that create more part line and flash issues?

Then it’s time to call Anderson Technologies.

Anderson Technologies has solved such problems in complex part design for the likes of Haworth Inc. in Holland, MI.

Haworth needed a lumbar back support pad that had these characteristics and more. Anderson Technologies took on this challenge successfully, and currently makes over 250k of these parts for Haworth’s Very and Zody chairs. Due to the complexity of the part, and the fact that the part had to meet pressure map standards for lumbar support, Anderson Technologies was integral in helping Haworth’s designers understand the issues that made the part so difficult to manufacture.

The Anderson Technologies team worked with the client and solved those problems during the creation of the tool so that the manufacturing process could be consistent.

“This is a very picky part,” said John Rickfelder, Business Development Director at Anderson Technologies. “Because of the difficulty of the part and it critical processing criteria we have included RJG Sensors in the tool to track the consistency of molding.”

With RJG Sensors, Anderson Technology can collect running data for the life of the tool, adjust the tool for pressure, and control the consistency of the parts. “The trick to making this or any part, it to involve Anderson Technologies up front as you are finalizing the CAD” said Nate Robinson, Director of Engineering.

Anderson Technologies can be of great assistance in guiding you to successful conclusions in solving problems on all the points listed above and direct your designers to creating reliable, manufacturable parts that have repeatability for the life of the tool.

Anderson Technologies is ready to offer the technical assistance needed for success and optimization. From the points listed above, to working directly with your designers, to all the steps in between, we are ready to help.