5 Tips to Make Injection Molding Less Daunting


You’ve spent months (probably years) developing a product concept, your crowdfunding campaign was fully funded, and it’s time to deliver a product to your customers. Now what?

Making the leap into production can be intimidating. Not only does it require large financial commitments, but many manufacturers can be, to put it lightly, frustrating to work with. But with the right knowledge beforehand, you can ramp up production on the right foot.

While 3D printing is a popular manufacturing solution, it’s usually not economical at production volumes—the same goes for CNC machining. At Proto Labs, in addition to 3D printing and CNC machining services, we offer rapid injection molding, which is often the most effective way to scale manufacturing operations for plastic components. Since injection molding is such a large part of our business, Make: asked us to share some tips to help you improve your experience, no matter who you use.

In short, the process works by injecting hot plastic resin into a mold where it then cools and solidifies into a plastic part. There are many factors that go into injection molding, so to make the process more digestible, we’ve broken it down into these five considerations.

Pick the right tool for the job

When you hear the term injection molding, your first thought is probably high-volume manufacturing. For part quantities in the millions, molds are typically machined from steel. It usually takes months to create a steel tool and the financial investment is substantial—at least $20,000, but often much more. The advantages of steel tooling go beyond high volumes, though. Steel tooling can support more cavities—meaning more parts per shot—and handle more complex geometries and surface finishes.

3D printing has become another method for producing molds. 3D-printed molds are low in cost and fast to create compared to molds machined from metal. But keep in mind that the rough surface finishes inherent in 3D-printed parts will require some secondary processing to smooth out. The idea of low-cost molds is alluring but will only save money in extremely limited runs. After 100 shots or so, the mold will degrade and need to be replaced.

Aluminum tooling is a solid middle ground between 3D-printed and steel molds. Even though aluminum molds cannot handle the same volumes as steel, they can be suitable for anywhere between 10,000 and 50,000 shots, and even up to 100,000 or more in some instances. And, by optimizing your injection molding design, you can improve the longevity of the aluminum tool.

Select the right material

Another key factor to injection molding design is material selection. It’s important to first determine which mechanical properties are going to be critical to your part’s function. Start by asking yourself: Will my part have to support weight? Does it need to endure high temperatures? Is transparency a requirement? Answering questions like these will help narrow your options—of which there are thousands—and help guide you through the material selection process.

For our purposes, let’s touch on a few of the most common thermoplastics:

  • Acrylonitrile butadiene styrene, commonly referred to as ABS, will work for many plastic parts. Its cost is not too steep, it’s pretty strong and tough, and it also looks nice with its glossy finishes—Legos are a great example of ABS plastic components. Another benefit is that it can easily be painted. Some of its limitations include poor UV and chemical resistance.
  • Polypropylene, or PP, is an affordable plastic. It’s used in many consumer products such as automobile interiors, patio furniture, and food storage containers. PP is useful for designs that incorporate living hinges. It also has decent stiffness, heat resistance, and clarity.
  • Acrylic is the go-to material for applications that require a transparent plastic part like an enclosure for an LED. It’s also useful for products that will be outdoors due to its UV stability—meaning its appearance won’t degrade from long-term sun exposure.

This is just a brief overview of thermoplastic materials. There are many others that you will routinely encounter like nylon, polyethylene, polystyrene, and liquid silicone rubber, so additional Googling is highly recommended.

Design for Moldability

If your design was first created with the intent to be manufactured via 3D printing, you might need to make some adjustments to ensure it can be molded. Whereas 3D printing is capable of handling complex geometries, injection molding has a few requirements to ensure consistent, quality results.

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Adjusting features to be suitable for molding will help resin flow well into the mold, allow it to cool evenly, and reduce the chance of cosmetic defects such as sink, knit lines, flash, and warp.

This is a quick rundown on how to handle common features in injection molding designs:

  • Wall Thickness: Consistent wall thickness will reduce the potential for warped parts, prevent sink, and help the material cool uniformly so unsightly knit lines do not form or are less prominent. A good rule of thumb is to design walls that are no less thick than 40 to 60 percent of adjacent walls.
  • Rounded Features: Avoid sharp edges and corners to help resin flow through the mold, reduce stress, and add strength to your part. If sharp corners are critical to your part’s function, there are materials that support sharp features better than others.
  • Draft: On a similar note, you should apply draft to vertical walls. Draft is the angle applied to the surfaces of a part that run parallel to the motion of the mold opening.  Our guidance is usually 1 degree of draft per 1 inch of cavity depth. Draft will ensure that your part cleanly ejects from the mold.
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Find a Manufacturing Partner

The type of tooling your product requires will be the primary factor in determining your ideal manufacturing partner. Just like how you might approach buying a new car, it’s good to explore many suppliers.

Selecting an injection molding manufacturer is a substantial business decision, so there are more factors that come into play than just price. Some of the most important items to consider are lead-time, minimum order quantity, and the total cost of tooling and parts.

It’s also important to note the manufacturer’s level of experience since injection molding is a complex process. There are numerous aspects that affect manufacturing quality such as: material handling, quality controls, machining capabilities, and more. Customer service is another element that should not be overlooked. It’s invaluable to have a manufacturing partner that is responsive, actively communicates, and is quick to act on the behalf of your requests.

Use Injection Molding for Prototyping

One last thing before we go. Injection molding is often thought of as a process reserved for production. Many believe 3D printing or, in some instances, machining are the only way to cost-effectively prototype plastic parts. But there is a substantial business case for injection-molded prototypes.

For example, if you need to conduct market testing, prototype volumes can often creep into the hundreds and 3D printing or machining at these volumes can get expensive. With low-cost, rapid injection molding that uses aluminum tooling, you can prototype with production-grade materials. Additionally, sometimes aluminum tooling pays for itself, even for a few prototype parts, by providing certainty that your molded design will function as intended once in production.

Discuss this article with the rest of the community on our Discord server!
Becky Cater

Global Product Manager at Proto Labs, and on the injection molding team.

View more articles by Becky Cater


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