Navigating Injection Molding Design Elements

Creating a custom injection mold is a great way to guarantee a quality, high performing component that completely meets the requirements of the product.  There are several elements to an injection mold design that can be a challenge to navigate.  Below, let’s take a closer look at the various mold elements to understand how each feature adds to a custom, high performance piece.

Wall Thickness

One of the benefits of using custom injection molding is the ability to fully design and dictate wall thickness.  With a custom design, the mold wall thickness can be fully thought out, eliminating the occurrence of mold defects.  Liquid plastic will always take the path of least resistance, meaning it will fill up thicker parts before flowing into thinner, narrow wall parts.  Keep this in mind when creating a successful mold.


It can be difficult to navigate corners with injection molding.  Liquid plastic has difficulty flowing around sharp corners and geometric shapes in an even, consistent way.  This can lead to compromised corners or defects.  When possible, it is a best practice to used curved corners, with similar interior and exterior radiuses, to allow the liquid plastic to flow smoothly and evenly.

Weld Lines

A weld line occurs when two independent plastic flows meet one another in the same mold.  These typically occur around holes where plastic will have to flow in two different directions, meeting around the obstruction.   Every mold is going to have weld lines, but the goal of a well-designed mold is to minimize the appearance of weld lines, and to place lines in such a way so that the overall strength of the component is not compromised.


An integral part within the walls of a molded component, ribs are used to help reinforce the walls of a custom injection molding.  A good metric to remember when factoring rib thickness is that ribs should never make up more than 70% of the wall thickness, but should not be less than 50%.  Too much or too little rib contribution to wall thickness can lead to the walls of the mold sinking.


Boss design is necessary when the component being molded is just one part of a larger design.  Often molded components will have to be assembled together with other pieces with bolts, screws, clips, or pins.  Bosses within the mold design serve as an anchor for the assembly pieces.  Bosses work hand in hand with ribs to give the custom injection molding added strength.

Gates and Vents

A gate is the point at which the liquid plastic exits the mold runner and enters into the cavity of the part.  The position where the gate is placed can greatly impact the strength and integrity of the component.  Similarly, a vent is the position within the custom mold where air is able to escape as the molten plastic rushes in.  Having vents placed in appropriate places can help reduce the appearance of weld lines on the final product.

Understanding the various terms and elements within a custom injection molding can help the designer create a better, more successful injection molding.  Knowing how the mold works and what makes a successful design will only improve the end product.

What do you think?

Written by Michael Curry

Tips on How to Select the Best Storage Facility  

Starting Up a Catering Business? Invest in These Must Have Items at Your Local Restaurant Supply Sacramento Store