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February 2, 2023
5 min read time

The Basics of Injection Mold Design and Custom Tooling



Custom designed and tooled injection mold created by Houston Plastic Products


The Proper Design and Careful Craftsmanship Your Parts Require

We all know that a well-manufactured injection molded plastic component doesn't just happen. It takes a team of designers, technicians, and engineers. They must be experts in DFM, materials, machining, and more to conceptualize the product and determine the best possible way to manufacture it long before it hits the market. Injection mold design and custom tooling are just two of the highly specialized processes that can make or break the success of a finished plastic product. Let's look at the basics of injection mold design and custom tooling — what they are, how they work, and the benefits of incorporating them into your manufacturing process.


What is Injection Mold Design?

Injection mold design is, well, just how it sounds. It's the design process for the custom mold that will have molten plastic injected into it to create a plastic component. Like the DFM (Design for Manufacturing) process, the main goal of injection mold design is to create a cost-saving product with high repeatability and resiliency. 

Mold design picks up (and overlaps) where the DFM process leaves off. Think of DFM as the bridge between product developers and injection mold manufacturers, with mold design as the next crucial step. If your mold lacks precision, there's no way your finished part will meet quality standards. 

Depending on the product design, molds can range from simple to highly complex. Each mold consists of a cavity (the front), a core (the back), and a runner system (the channel guiding melted plastic into the cavity). Injection mold design considers the type of plastic, cooling time, and ejection of the completed part. Mold flow analysis, a software simulation showing how resin will fill the mold during the injection process, is critical to mold design. It allows designers to understand material fill to better position gates, locate difficult-to-fill spots in the mold, and anticipate where knit lines will appear. Moreover, it gives the processor good probable processing data and reduces time to market.


What is Custom Tooling?

Custom tooling and injection mold design go hand in hand. A custom tool is any tool that's not readily available to perform a required task. Custom tooling is the creation of that specialty tool. In our case, the device is a custom-made injection mold. Well-designed, expertly crafted custom molds produce higher-quality finished plastic products than off-the-shelf molds — and that's critical in industries where tight tolerances and high standards are non-negotiable. Our in-house injection mold design and custom tooling capabilities allow us to build, maintain, and store molds at Houston Plastic Products. This on-site service saves time and money while guaranteeing quality.

Rules of Thumb for Design and Tooling

There are many factors to consider during the design and tooling processes. These include material choice, wall thickness, knit lines, draft, sharp corners, the surface of the finished product, and more.


Some molten plastics may resist flowing properly into your mold. Always ensure the mold and the material are compatible. 

Wall Thickness

Consistent wall thickness is vital. Inconsistent thickness can result in sink marks, weaknesses, and other imperfections. Molds should be designed with this in mind. 

Knit Lines

When two separate plastic flows meet within the mold and "resolidify" along their interface, it creates a knit line. These areas may be more prone to breakage, so it's essential to understand why they happen and how to reduce their impact on your design. 


Designers add draft angles or tapers to the mold cavity so that the product is easy to remove once it's cooled. If you can't remove your product from the mold, what's the point?

Sharp Corners

Sharp corners can be less stable and more challenging to manufacture than radiused corners.

Surface of the Part

Molded parts will pick up even the slightest imperfection on the surface of the mold. Bead blasting or polishing the mold might be necessary to remove machining marks left by the end mills.



Injection mold design and custom tooling require specialized skills, knowledge, and machinery. At Houston Plastic Products, we have all three, plus in-house fabrication capabilities. When your injection molds are designed and tooled in one convenient location, it saves time and money with the added benefit of in-house experts. Contact us if you're looking for a reliable partner in end-to-end plastics manufacturing. We'll create a custom solution that meets your bottom line and deadline in mind.



Start molding your vision today