Injection Molding Design Guidelines

Designing parts for injection molding requires a good understanding of the process, materials and design guidelines that must be followed to ensure that the details can be successfully produced. Here are some injection molding design guidelines to keep in mind!

Wall Thickness

Walls in any plastic-molded part should be no less than 40 to 60 percent that of adjacent walls, and all should fit within recommended thickness ranges for the selected material.

Proper wall thickness will reduce the risk of cosmetic defects in plastic parts. One of the biggest considerations with wall thickness is which material to use for your project. This helps reduce the cooling and cycle time and minimizes the risk of warping and sink marks. Start by looking for which attributes are most important to the finished product:

• Does it need chemical or ultraviolet light (UV) resistance?
• Will the plastic be subjected to direct flame, or extreme temperatures?
• How strong must the part be, and will it need to flex under load?
• If color is important, can the plastic be painted, or colorant added to the resin before injection?
• What about opacity? Some plastics have good optical properties, others not so much
• Will the product be used in an electromagnetic environment?

Once you have decided on certain parameters, the right material can be selected.

Ribs and Bosses

Ribs and bosses can be used to increase the strength and rigidity of the part. Ribs should be no more than 60% of the nominal wall thickness, and bosses should be 1.5 times the nominal wall thickness.

Ribs should be designed with a thickness that is ½ the wall thickness to avoid a thick section at the wall base. To minimize sink marks, design ribs that are approximately 60% of the joining wall thickness for minimum risk. Best practices include spacing at a distance of at least twice the wall thickness. Glossy materials require a thinner rib (40% of wall thickness).

When it comes to bosses, if your design requires more strength than this guideline would provide, you should consider ways to strengthen the boss without thickening its walls. The most common of these is to surround the boss with gussets to support and strengthen its walls. Depending on the height of the boss, this draft can be anywhere from a half degree to 3 degrees. If the draft of the inside diameter of the boss is not acceptable, the boss can be molded as a solid and the inside diameter drilled, by you, as a secondary operation. Also, because a boss is formed by a blind hole in the mold body, we may have to add vent pins to the rim of the boss to allow the escape of trapped gas during mold filling. Otherwise shorts or burns may form on the rim.

Draft Angle

If you don’t design your parts with the right amount of drafting, you’ll be compromising your part’s functionality, fit and cosmetic quality. 

A general rule of thumb during injection molding design is to add between one and two degrees of draft for every 25 mm of cavity depth. However, other factors such as molding material, wall thickness, and mold surface texture can affect the required draft angle.

Scratch marks often occur on parts when you design inadequate draft angles. Without any taper, not only will the part come out scratched, but also the mold will endure unwanted wear and tear. This will in turn shorten the life of the tool. On top of all of this, you may not be able to eject the part in this case, meaning you’ll have to grind production to a halt to manually peel the part out of the mold. 

What’s also important is to overcome the vacuum forces by letting air in between the metal and the plastic for the part to release properly. Adding an adequate draft angle ensures that warpage upon ejection is prevented while maintaining a decent cosmetic finish.

Corner radii and fillets

Proper placement of corner radii and fillets in injection molding design creates strong, high-quality, cost-effective plastic parts. There are several other factors that can impact even a well-radiused part design. These include:

• Material selection. Some plastics are more forgiving of sharp-cornered parts. Choosing the right one for your application is a necessary step towards accurate, functional parts.
• Wall thickness. Beefing up adjacent walls may absorb some of the stress associated with sharp internal corners, but can, in turn, create other design challenges.
• Part geometry. Some parts are simply more “moldable” than others. Achieving proper form, fit and function depends on sound part design, a large piece of which is appropriate corner radii.

Despite its widespread use across nearly every industry, injection molding is a complex process. The considerations listed above impact the amount of corner radius required in any given part design, and whether it will be effective. This design tip will help you achieve a Goldilocks balance of each.

Gates and Runners

The location and design of the gates and runners are critical for achieving good part quality. Entrances should be as small as possible in the thickest section of the part. Runners should be designed to minimize pressure drop and maximize flow.

The runner is the channel that feeds directly into the gate of each part. If the Injection Mold only has one cavity then there will only be one branch to the runner.  If there are multiple cavities, then multiple branches will have to be engineered to ensure proper balance of flow.  This way each cavity gets the proper amount of material.  When designing a runner it is important to include a cold slug at every point there is a hard transition between branches.  The reason for this because as the molten plastic flows through the runner system, it begins to cool.  The cold slug is considered an area where this cold plastic is dumped in order to prevent it from entering the gate.

Then, the plastic enters into the gate.  This is where a lot of problems occur in the injection molding process.  Gates can be tricky, but understanding their purpose can help in designing a proper gate for a plastic injection molded part.  First, many think the larger the gate the better.  Although this may assist in getting the material in the cavity, it often leads to undesirable vestige that has to be hand worked as a secondary operation.  This is called gate trimming. Gate size and gate position in plastic injection molding are the key to successful product design.

Surface Finish

The part’s surface finish should be specified based on the application requirements. A higher surface finish may be required for visible pieces, while a lower surface finish may be acceptable for hidden parts.

Choose injection molding surface finishes by considering part function, the material used, and visual requirements. Most of the typical plastic injection molded material can have a variety of surface finishes.

The surface finish selection must be established in the early embodiment design stage of the product design because the surface dictates material selection and the draft angle, influencing the tooling cost. For example, a course or textured finish needs a more significant draft angle so that the part can be ejected from the mold.

Material Selection

The material selected for the part should be suitable for injection molding and meet the required mechanical, thermal, and chemical properties.

More than 85,000 commercial options for plastic materials are listed in materials databases, and the real number is probably over 90,000. This extensive set of options can be sorted into approximately 45 polymer families or blends, and these 45 families can be further divided into two broad categories: thermosets and thermoplastics. While thermosets were the first commercial polymers, their use has diminished to the point where they constitute only about 15% of all the material processed in a given year. Therefore, this paper focuses on thermoplastics.