What to Consider When Designing a Custom Injection Molded Part

Designing and producing custom injected molded parts is a fairly complicated process. Though there are some fairly obvious components to this endeavor, there are also some less obvious aspects that must be considered and perfected. Let’s take a look at the most important items to consider before diving into the injection mold design process.

Define the Physical Properties

The physical properties of the custom injection molded part must be defined at the outset of the process. Of critical importance is the part’s caliber of strength, amount of stiffness, impact resistance level, electrical concerns, wear/scratch mar and whether it needs to be flame retardant.


Take some time to think about the types of chemical exposure the custom injection molded part will be subjected to. If it can’t withstand the rigor of such exposure, the design must be revised. Consider the level of warmth and humidity the part will be subjected to. If it fails to function when the temperature and humidity rise, the part’s design must be altered. Perform extensive temperature-failure analyses prior to the production of the injection mold design. Additional environment considerations include stress cracking, UV-define failure-color and property/material degradation due to wear and tear.


Establish a reasonable cost target. You must figure out exactly how much the product will cost to produce. Furthermore, the price of the product for sale should also be defined.

Aesthetic Concerns

Though utility is the utmost concern when it comes to injection mold design, the part’s look also matters. Consider what you would like the surface appearance to look like. The part’s color and decorations should be selected prior to the start of production.

Compliance Issues

Consider compliance concerns before commencing production. The part must be in compliance with regulatory and agency standards.

Consider the Project Origin

Take some time to define the project origin. The subtleties of the injection mold design process hinge on whether it is a new project design, a new product, new application, a metal-to-plastic conversion or a new product that is meant to replace an alternative material.

Mold Ability

Consider whether the part fits injection molding capabilities. Determine if the identified resin is appropriate for the part’s idiosyncratic design. Perform a mold flow analysis.

The Majority of the Work is Done in the Injection Mold Design Stage

The considerations outlined above all play important roles when deciding on injection mold design. The most successful projects are launched without flaw due to a completion of research during the design and conception stage. Everyone from engineers to resin suppliers, molding vendors and beyond will play an important role in working through the considerations outlined above.

Designers, engineers and other relevant team members will prove essential to the successful development and design of plastic injection molded components. The main areas of focus during the design stage should be settling on the proper design for the plastic part, the proper selection of material for the part’s design and the processing conditions necessary for plastic injection molding.

How to Keep Mold Costs Under Control

You can control costs by designing a part that is molded with an up and down motion or a straight pull motion. The straight pull mold allows for the mold’s two halves to separate from one another without plastic impeding the metal’s path in the direction of the pull. Such a path blockage can be caused by undercuts on the part. A corresponding mold action such as core pulls or cams are necessary. Action in the mold will have a monumental affect on the mold’s total cost.

Combat Defects

A uniform wall thickness guards against defects. This is a basic and essential requirement for effective injection mold design. Consider the fact that plastic shrinks during the cooling process. Such shrinkage can cause defects like warping, sink marks stresses, voids and so on. Plastic resin solidifies in mold near the outside portion of the part. This is the section closest to the mold surface. Part sections with considerable girth often pull inward and generate voids, sink marks or stresses. Narrow sections tend to cool faster than thick sections. This can cause stress to build up between the thick and thin sections and ultimately warp the part.

Draft Lets Parts to Successfully Release from Molds

Draft is necessary on every part. Drafts in the direction of mold movement permit parts to eject from the mold in a fluid manner. Draft is best defined as the angle at which a part is tapered to permit its release. As a part cools, it naturally shrinks toward the mold’s core side. Implementing draft of a half degree to two degrees helps the part release with ease.

The Use of Bosses

Bosses will help for assembly and mounting. They are typically added to the part design for locating as well as assembly and mounting. Failing to properly place a boss can cause varying wall thickness that compromises aesthetics, weakens the part or shrinks it. A wall thickness of 55 to 65 percent of the nominal wall thickness for walls less than 1/8” thick is ideal around boss features. If the walls are greater than 1/8” thick, the wall thickness around the boss feature should be about 40 percent of the nominal wall thickness. Boss height should never exceed two and a half times the diameter of a boss hole.

Holes Minimize Weight and Boost Part Functionality

Holes can be implemented to decrease weight (coring) and improve functionality. Holes are formed with core pins so the molten plastic does not fill that specific space. Blind holes do not go all the way to the opposite end. Core pins for blind holes are supported by a single end so it is more difficult to create them without significant defects. Yet forming holes also has the potential to lead to defects or diminish part aesthetics. It is possible for a visible weld line to form as molten plastic moves around the core pin. Such a tarnished area can also prove to be weaker than the rest of the part.

Reduce Corner Stress with a Radius

Add radii to angles to prevent the formation of sharp corners. Such sharp corners can cause unnecessary stress, decrease part strength and even limit material flow. The outside radius of a corner must be equal to the thickness of the part plus the interior radius. The interior radius of the corner should be half that of the wall thickness.

Ribs Boost Part Stability and Strength

Add ribs to parts to enhance stiffness. Ribs strengthen the part so it can withstand a heavier load. There are some guidelines pertaining to rib application that are meant to enhance functionality while reducing the odds of defects. In general, rib thickness should be less than the thickness of the walls. A thickness of 60 to 80 percent of the nominal wall is ideal.

It is prudent to add additional ribs to bolster strength/stiffness rather than making ribs larger or thicker. Ribs should be spaced out form one another, a minimum of two times the nominal wall thickness. Rib height must be less than three times that of the nominal wall thickness. If a thick rib is necessary, the center portion should be cut out to ensure wall thickness is uniform.