Best Practices for Injection Molding Tool Design

Many factors affect the quality of a finished injection molded product, such as process repeatability, raw material properties, the molding environment, complex part geometries and tool design. Even with every variable meeting the most desirable characteristics, a poor tool design will result in poor quality parts every time. It may not even be a poor design – there just may be a better way to achieve the same results with best practices for injection molding tool design.

Contact QT Manufacturing to Learn More

Quality Tool is the Key to Quality Parts

An experienced tool designer will know the most important considerations when designing molds and the best way to achieve repeatability with complex parts. Designing tools for injection molding requires careful consideration of many factors to avoid the most common part defects. The tool designer must follow strict guidelines with meticulous scrutinization of the best way to achieve common design features.

What looks good on a blueprint may not be feasible to mold with consistent quality or to perform with real-world part functionality. An experienced tool designer can offer suggestions when considering new tool builds to improve part quality, process consistency, mold performance and maintenance. 

Common Injection Molding Part Defects

The most common part defects that occur with injection molding are due to the flow of material or to a non-uniform cooling rate when raw resins solidify.  


Warping occurs when sections of a finished part cool faster than others, which causes shrinking and may cause a permanent bend in the part or warping from internal stresses. Parts designed with inconsistent wall thicknesses are most prone to warping.

Sink Marks

Sink marks occur when the interior of a part cools and hardens before the surface, which creates a small recess on the surface referred to as a sink mark. Parts with poorly designed ribs or thick walls are most prone to sinking.

Drag Marks

When the walls of a plastic part slide or scrape against the mold cavity during ejection, drag marks can occur. This is more common on parts with vertical walls rather than parts designed with a draft angle.

Knit Lines

Knit lines, also called parting lines, are any lines where two resin flows meet and may or may not be visible. A knit line may only be a cosmetic issue and affect the aesthetics of a part, or it could cause a functional issue which usually decreases the part strength. 

Short Shots

Short shots occur when the flow of resin is inhibited during injection and results in an incomplete part. Trapped air in a mold may cause a short shot. A good tool designer can improve the flowability of resins. Short shots occur most often in parts with poorly designed ribs or parts with very thin walls.

While resin choice and molding conditions contribute to these various part defects, a tool designer can ward off many of these issues with a good tool design. Tool design engineers will be aware of the possibility of these defects occurring and can make adjustments.

Considerations for Injection Molding Tool Design

Mold makers are very meticulous in their design checklist with experienced tool makers knowing exactly what to look for to avoid molding problems later. 

Part Design

Of course, the first consideration for tool makers is the part design. The part design will include the part size, shape, weight, and all critical dimensions. This will help a tool designer to locate thin walls that may succumb to underfill or dimensions that are critical to keep an assembly line moving. This will also let the tool designer know what surface finish is required on the part and if there are any restrictions on parting lines to properly design gating.

Cam Action

If the tool requires cam action, designers take extra care to make sure there is enough room for all of the required movements. Cam action is used to create an undercut feature and refers to sliding plates which are needed to create part features and then slide out of the way for proper ejection. Cam action and undercut features add complexity and maintenance requirements. If possible, avoiding undercuts altogether with shut-offs can be useful for undercuts on internal areas like for a snap-assembly or on the sides like holes or handles.

Lifters and Ejection System

A lifter is a mechanism in the ejection system, designed at an angle in order for other features to mold the geometry of the part. Any required lifters must be apparent prior to cutting steel.

The Molding Environment

The molding environment includes the type of press the mold will run including the tonnage, the clamping pressure, whether it is hydraulic or electric, barrel capacity and mounting system.  Additional specifications that the mold designer should know include the location of water fittings for cooling systems, electrical connectors and wiring preferences. 

QT Manufacturing: Complex Tool Design for Injection Molds 

QT Manufacturing is an injection molding and mold making company with the capabilities to design and build the complex tools that other molders shy away from. We specialize in precision molds with master injection mold makers experienced in all types of resins and familiar with modern tool design technology. QT Manufacturing adds value as a supplier to many industries including automotive, aerospace, optoelectronics, telecommunications, firearms, defense, medical and many other demanding industries requiring critical components you can rely on.

Mold Making an Injection Molding under One Roof

QT Manufacturing is your one-stop for quality tool builds along with reliable injection molding to meet all your production demands. Our tool designers and mold makers work closely with our process engineers for collaboration which benefits our customers. This improves lead times and allows us to meet your production requirements with mold maintenance, service or repair under one roof.

QT Manufacturing is an ISO9001:2015 and AS9100 rev D supplier and ITAR compliant, following a strict quality policy. We are committed to continual improvement through investment in our staff and equipment and R&D for product and process improvements. Contact us to speak with an engineer about your injection molded sourcing needs and our experienced injection molding tool design and mold making capabilities – 972-221-0537. 

Request a Quote Now