two metal molds that represent one half of a plastic bottle.

4 Essential Plastic Mold Design Tips

Plastic mold injection mistakes can result in lower-quality components with sink marks, weld lines, flash (protruding material at the edges), and other defects. While some of these problems may be caused by incorrect temperatures, pressure, or injection speed, many are due to poor plastic mold design.

Designing the right mold (tooling) from the beginning facilitates faster, mass-produced components, greatly enhances product quality, reduces material waste, and lowers manufacturing costs. 

In this article, we’ll cover some of the best practices you can follow when designing your injection molding tooling.

Tip 1: Include Draft

Many mistakes that occur during the plastic injection molding process are due to incorrect draft in the mold design. A draft, sometimes called an undercut, is a taper or angle added to intersecting walls or surfaces. Including draft at interior joints reduces the chance of failure while effectively increasing the strength and toughness of the component.

When designing a mold, the general rule is to allow at least one degree of draft for every inch of depth. It’s also best practice to eliminate 90-degree corners, which makes it easier for the final product to be removed from the mold.

Tip 2: Wall Uniformity

Plastic products can be susceptible to cracking if the mold design doesn’t allow for wall uniformity. Walls that are too thin can cause breakage, warping, and shrinkage, while walls that are too thick can cause sinkholes. Furthermore, inconsistencies in the wall thickness can result in unpredictable cooling, pressure fluctuations, and surface defects.

Keeping walls uniform can be challenging with complex components, but there are design compromises that can be made to maximize wall uniformity.

When determining wall thickness, you must consider the length-width ratio, the required strength of the final component, and the amount of plastic resin needed for the project. Determining the required wall thickness beforehand will help you ensure wall uniformity in the mold. 

Tip 3: Keep It Clean & Avoid Delamination

The process of injection molding is similar to the 3D printing process, where layers of a polymer are bonded together. Debonding or delamination can occur when the mold is dirty or contaminated with a previous plastic resin. Delamination can be made more problematic during the final curing and cooling process when the product experiences the stresses of shrinkage. Delamination also results in the plastic layers of an injection mold component becoming more prone to separation or fracture.

You can avoid this common issue by cleaning foreign materials, excessive moisture, and other contaminants from the tooling and equipment. Additionally, you must avoid using incompatible plastics that don’t bond properly, which will cause surface delamination in the final product. 

Tip 4: Avoid Surface Inconsistencies 

Lastly, you must avoid surface inconsistency in the tooling. Weld lines, flow lines, sink marks, warping, and surface delamination can sometimes be traced back to the plastic mold. The tooling you use in the plastic injection molding process is designed to replicate what the final component will look like. If there are any surface inconsistencies in that mold, you can be sure that they will transfer to the final product as well.

Create Your Tooling With Help From Silver Basis Engineering

The quality of your final product is always determined by the tooling you design. If you need help creating high-quality molds for your project, reach out to Silver Basis Engineering. We’re a global injection mold manufacturer that helps businesses in many industries, from automotive to medical. Contact us today to learn more about our capabilities or to request a plastic mold design consultation.