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8 mistakes to avoid when designing for injection molding

Designs for injection molding should be as simple as possible while still meeting the requirements of your application.

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Image: Unsplash

Injection molding is the process of heating and injecting liquid plastic into a hard, two-piece metal container. The heated plastic cools quickly as it is injected inside the cavity of the mold under high pressure. As it cools, it takes on the shape of that cavity before being ejected from the mold.

Injection molding has been used for decades to manufacture plastic products and with the market size expected to grow to $345 billion, it’s never been more popular. And it’s no surprise; these machines can produce hundreds or even thousands of parts per day, depending on size and complexity.

Here are some common mistakes designers make when designing for injection molding that you should avoid.

Using Texture

Textures are created by small grooves or patterns pressed into the surface during manufacturing. The molds for these designs have detailed textures built right into them, and they can be costly to make and time-consuming to change.

Adding texture as a design element increases its complexity and makes it more difficult to manufacture. You should avoid textured designs unless they are essential for a particular product.

Using Rounded Corners

Rounded corners can create issues in injection molding because they take more time and energy to fill with the material than sharp-cornered parts. As a result, manufacturers typically charge a premium for rounded corners.

Designers should avoid using rounded corners in areas subject to high pressure or irregular in shape. Sharp edges will ensure that the plastic fills every nook and cranny of the mold and will ultimately make your product look better and last longer.

Designs That Incorporate Flex or Are Thin in Places

Parts with an angled surface pose particular issues for injection molding because they increase the manufacturing time and energy needed to fill a cavity. This makes them more costly, especially given that long slopes must be angled both ways to ensure that the plastic flows right.

Using too many colors can lead to visible seams between molded parts because each color requires its own mold. However, thin designs are more expensive to manufacture and are prone to breaking, decreasing both the quality of your product and its marketability.

Designs That Require Paint to Be Added

After a part is made, it can be painted or dipped for an attractive finish. However, this also increases the cost of the product due to the time and materials needed for the application.

For example, a polished look from a glossy dip requires one step while painting requires two. The more steps your product requires, the higher its cost will be.

Using Designs That Require Assembly

Designs with many individual parts can be challenging to assemble, and each additional part makes the product more expensive.

So, designers should avoid designing products that need to be assembled by buyers after they are molded, especially if the assembly is complex.

Thick or High-Relief Designs

Parts with a sizable amount of plastic are difficult to mold and typically take longer to produce. In addition, if a design is thick in an area where it doesn’t need to be, you may see cracks or variances in the plastic finish.

If a design is high-relief, it can also create issues in molding because the elevated surface area cannot be molded as quickly. Designers should avoid using high-relief designs or make sure they are necessary for a particular product.

Using Too Many Colors

fitbit versa lite in various colors
Image: Fitbit

Color choices are limited to the CMYK spectrum, so it may not be possible to avoid using multiple colors. It is recommended that your design has a “neutral” color or one similar in shade and tone to the injection-molding plastics available on the market. 

While dark colors can look great in solid areas of design, you should avoid using a dark color on areas that will be subject to the high pressures of molding.

Using Hidden Lines

Hidden lines refer to areas of design that are not visible when the product is assembled. For example, a plastic part with an opening for a screw might require an additional wall around the edge of the hole so that it is strong enough to hold screws and won’t crack from pressure after it is molded.

This creates an invisible line in your design that you must remove from the mold before production occurs. If the part is then painted, two steps are needed to ensure a finished product of the desired quality: paint it and remove that wall from the mold.

Final Thoughts

Designs for injection molding should be as simple as possible while still meeting the requirements of your application.

Manufacturers will not reshape or add details for you, so it is crucial to understand how they work and what designs are appropriate.

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