To a certain degree, 3D printing, injection molding and plastic machining technology overlap in their capabilities and offerings. But which fabrication process is most appropriate for your project? What are their key differentiators? The following article assesses the utility of each technology with regard to precision, efficiency, materials and more.
3D printing is an additive manufacturing process that prints layers of plastic substance in three dimensions onto a single point. The popularity of 3D has grown significantly over the past few years, and it can easily be used for rapid prototyping, DIY production and manufacturing by small-scale engineers or designers. Many of the advances in 3D printing have been focused on bringing down prices, rather than technological advancement, so that it can be more accessible to “mom-and-pop” users who are otherwise not eligible for the more expensive and advanced manufacturing processes.
Another additive technology, injection molding is a manufacturing process in which parts are produced by injecting materials in liquid form into a mold. Commonly performed with thermoplastic polymers, it can be used with a variety of other metal and glass materials. Small pellets of material are heated into a molten liquid which is then injected into a mold at high pressures and rapidly cooled—causing it to harden—and then ejected from the mold, exposing the part to air. Two of its key benefits include a fully automated process—permitting very high rates of production—and also that its molds allow for highly detailed and intricate parts.
Finally, plastic machining, or CNC (Computer Numerical Control) plastic machining or CNC milling, involves the precise removal of layers from a material. Typically beginning with a block of the desired plastic material, plastic machining involves removing material layers at a time and allows for many different materials to be used. Introduced by MIT during the 1950s, this technique utilizing drilling tools to carve plastics—and because it’s computer-controlled, extremely precise and tight tolerance products are achieved. Machining is considered “Subtractive Manufacturing.”
Comparing the Options
|3D Printing||Injection Molding||Plastic Machining|
|Cost||Very low cost when you need 1-10 rapid design iterations on a prototype part.||Because creating (or updating) a mold can end up being much more expensive than the actual order itself, this is only cost-effective with very large-scale projects.||Significantly cost-effective when you need a few parts up to a few thousand.|
|Design||Designs can be highly personalized and unique—this technology is often used by artists and creative professionals. Allows for reduced assembly part count and hidden cavities unable to be produced by other methods.||Allows for deep features, square holes and living hinges to be produced easily. Due to high mold costs and extensive manufacturing times, design change freedom is limited. Injection molded parts contain higher rates of residual stress resulting in part distortion and limited wall thicknesses due to voids and sinks.||High design flexibility at lower costs are common. Allows for mid run design changes and inexpensive cost to revise parts. The most precise and tightest tolerances are available in any polymer or composite material, with any size and any wall thickness available.|
|Material||Very limited in comparison to other plastic fabrication methods and unable to achieve the same smooth surfaces as the other technologies.||Injection molding has a much wider range of materials available for use compared to 3D printing. Broadest range of materials are available but limited due to mold design and different material shrinkage rates.||Broadest range of materials are allowed—CNC plastic machining also achieves the smoothest finish with the fewest imperfections.|
|Production||Low setup costs and adaptable for many product development “show and tell” needs. Often unusable for actual life cycle testing.||Requires time to make the mold but once the mold is finalized, production moves quickly. However, revision changes can be expensive.||One of the fastest forms of fabrication and can easily scale between small and large outputs, with the freedom to make changes on the fly.|
|Tolerance||3D printed prototypes are not typically finished parts and are much more likely to delaminate. They are not typically used for “precision” components.||Parts are molded at tighter tolerances than 3D printed parts but are not as tolerant as plastic machined parts.||Highest tolerances available—parts are able to reach +/- 0.002 mm and are best for demanding industries like aerospace and defense, medical and life sciences as well as power generation.|
Why Plastics Machining Ranks Best
From the above chart, you can see the differentiators of each fabrication process. However, we’ve found that CNC plastic machining is by far the most precise and beneficial process for custom applications.
Cost-wise, plastics machining is the most effective for any project that requires unique, strong designs. 3D printing is the cheapest form of fabrication but you get what you pay for with that medium… and the machining level of precision typically just isn’t there. Injection molding, meanwhile, offers much stronger products than those that are 3D printed, but modifications to a mold are extremely expensive and injection molded products are not as precise as plastic machined parts. Injection molded parts contain higher levels of residual stress than machined parts resulting in the potential for time delayed distortion.
Looking at design abilities, each medium offers a wide range of capabilities but again, if it’s precision, design freedom and speed that your project needs, CNC machining is the way to go—3D printed materials are really only suited to limited “real part” applications, personal or home use and injection molded products are more typically found in “mass production” industries that produce, for instance, automotive or disposable consumer products. Further, machining designs are crafted on the same computer applications that 3D printing uses so there’s no cost associated with changing the design of the product (unlike injection molding, where a new mold would then need to be created).
It is important to note that CNC plastic machining is a specialized craft… not just anyone interested in 3D modeling can create the design for a plastic machined product. Here at AIP Precision Machining, we have over 35 years of technical design expertise and we work diligently with our clients to craft products that exactly meet their project requirements.
If you’re looking to decide on a fabrication process via the material selection available to you, plastics machining offers the widest range of diverse thermoplastics for your project. 3D printing has not advanced its material range in many years and because injection molding requires liquidating its materials and rapidly cooling them, you won’t be able to use them for thick cross-sectioned parts or ultra-precision components. CNC machining is the only process that allows you to choose from the full assortment of material selections.
3D printing and CNC machining tie for the fastest production times, as both take minutes to complete. Injection molding, however, requires more time in order to make or adjust the mold, which can take weeks to finalize.
Finally, if your project requires the highest levels of tolerance, precision machining produces the most precise and extremely tolerant products. It’s the only fabrication process suited for critical pieces with mirror-like surface finishes.
Want More Information?
Of 3D printing, injection molding and CNC plastic machining, one process is not necessarily better than the other—it’s dependent on your project requirements. However, plastics machining is by far the most specialized and precise option for high-quality, critical parts needed in industries like aerospace and defense, medical and life sciences and power and energy.
Here at AIP Precision Machining, we commonly craft pieces that are able to survive the rigors of air and space travel, the inside of a human body or the corrosive chemicals used on an oil rig. We offer state-of-the-art technology and extensive expertise to provide you with the highest quality products. Contact us to learn more about our process and what CNC plastics machining could do for you.