Choosing Between 3D Printing and Injection Molding Processes
June 6, 2018
The advancement in manufacturing processes is creating an opportunity for the design engineer to manage projects more effectively and produce higher quality parts. It is also true that with more processes to select from, there are more questions as to which process should be used and for what purpose. Engineers are building their own roadmaps to create viable paths so they can systematically produce their designs and deliver products to market in a timely manner. Based on what the engineer has been exposed to, their path may not always be an optimal one. The purpose of this article is to provide the engineer with a grounded approach to determining which process type should be considered and why.
Although some of the points provided below may be quite fundamental, it is exactly for this purpose that they are brought back to the forefront as reminders, because no matter what new processes and technologies are introduced there is always room for recalling fundamentals while applying the latest that technologies have to offer.
Know Your Part
No matter which manufacturing process you select for your project, the most important question you can ask yourself is “How will the part be used?”. Consider the following quadrant as a means to create scope around your project and part.
• Concept or Final Production – one end of the continuum demands that you are in pilot mode manufacturing multiple prototypes to determine the best design, while the other end of the continuum demands efficient manufacturability alignment with the intent to flawlessly go-to-market. Materials testing, form-fit-function, tolerances, design iterations, production part quantity all take on a different meaning based on which end of the continuum defines your scope.
• Part Simplicity or Complexity – regardless of which end of this continuum defines your part there are a multitude of manufacturability compromises that could derail your intended design. For simple parts build a list of non-negotiables that the manufacturer must adhere to, as not all parts can be manufactured as you intended even though the design is “simpler”. For complex parts involve your manufacturer early to improve your design’s manufacturability and potential approaches that could remove complexity and cost out of your design.
Optimize Your Design Cycle
Applying yourself to the quadrant that best defines your project and part allows you to manage the design cycle with less error and more confidence toward a predictable outcome. Considerations for optimizing your design cycle should consist of the following.
• Time-to-Market planning – understand where you have the most leniency and where there is little to no compromise. Once you know your strategy, you can leverage key constituents such as the right manufacturing process and manufacturer to meet your expectations. As an example from the software industry, Microsoft developed a strategy to introduce products to market knowing that “version 1.0” will be swiftly iterated and followed by the launch of “version 1.1”. The question you need to answer is “What’s my project’s strategy?”.
• Process integration – it is becoming more and more clear that reliance on one process type may not always be the best strategy. As an example, reaching an aggressive time-to-market with a new product introduction containing complex parts requiring iterative testing as to form-fit-function is a reality for many engineers. This requires inexpensive prototype testing providing the most speed prior to moving into a mid-volume production environment. Beginning with a 3D printed part to validate your design while you are tooling-up for mid-volume injection molding runs may be the best time-to-market solution to reduce your risk and predict your outcome. This approach also suggests that you involve yourself earlier in the design process with a manufacturer who will understand your strategy and align processes to fit.
3D Printing Highlights
Consider the most popular 3D printing technologies and how they could apply to your project. While this is not the full comprehensive list of technologies available it does represent the majority. The following table provides considerations when selecting some of the more prominent 3D Printing technologies as a process.
Each technology provides unique capabilities and benefits that you can apply to your project.
• Stereolithography (SLA) – SLA has been available since 1989 using an ultraviolet laser that cures parts one layer at a time in photo-reactive epoxy resin. It is one of the most accurate 3D technologies and ideal for fine detailed, small featured parts as fine as .002” layer thickness. And capable of producing large parts as well.
• Fused Deposition Modeling (FDM) – FDM extrudes thermoplastics layer by layer, with a variety of thicknesses as fine as .005” per layer. FDM uses real engineering grade thermoplastics, functional parts that can withstand rigorous testing, and create end-use production parts with a variety of color options. It has excellent tensile strength, flexibility, high melting points and chemical resistance, and UV resistance.
• Selective Laser Sintering (SLS) – SLS utilizes engineering and high performance powder based materials, activated by thermal energy of a laser in the Z axis to build one layer at a time. SLS uses real thermoplastic base materials producing robust parts, end-use aerospace applications. Accurate fine features and complex geometries, and fire retardant plastic materials, UL 94 V0 are standard. Popular uses also include living hinges and high-flex snaps.
• PolyJet – This technology is similar to inkjet printers where jets layer a liquid photopolymer that is instantly cured with UV lights attached to print heads. It produces fine layer high resolution parts. PolyJet is high speed, fine detail and smooth surfaces directly off the machine, and can print at 16 microns. Uses include living hinges, overhangs, and complicated geometries without needing to be assembled. There are multiple color options, multiple materials, and durometers in one print.
Injection Molding Highlights
Injection molding offers a predictable and scalable process for both rapid prototyping and production project needs. Its ability to produce parts from multiple materials with a high degree of consistency and tight tolerance makes it a proven approach to manufacture parts. The following table provides prototype and production considerations when selecting Injection Molding as a process.
There are a number of best practices to consider including resin selection, wall thickness, draft, runners and gates, ribs, bosses, corners and transitions. Beyond best practices, injection molding offers several key features that can transform your design including overmolding, insert molding, and undercuts. Tolerances are ± .005” and can reach ± .001” with tooling. Consider these key features as a reason why injection molding should be applied to your project.
• Overmolding – Overmolding plastic parts can help in wide range of functional and structural uses. A wide range of materials are capable of being overmolded, including both hard and soft plastic resins. One of the more common reasons to utilize the overmolding process is to create a soft grip. Overmolding can also be cleverly used to add rubber-like grips to clips designed to grab inanimate objects, and it can be utilized in more than one area of the same part to achieve even more functionality including cosmetics and color contrasts.
• Insert Molding – Insert molding is the process of injection molding molten thermoplastic around pieces placed in the injection molding cavity resulting in a strong bond between integral pieces of your final part. Accurate mold design and construction is essential to insert molding to not only maintain part tolerances but also assure the tooling reliability.
• Undercuts – An undercut is any indentation or protrusion that prohibits the ejection of a part from a mold. Undercuts can be used to carry out complex forms of molding such as the overmolding process and insert molding process. Undercuts are used to create interlocking or snap and latch features, allowing for clamshell or housing designs to come together for quick and easy assembly, or capturing holes or ports for wiring, button features or assembly, and vertical threads and barb fittings typically used in medical device products.
Remember to rely on fundamentals prior to thinking of any manufacturing process type. Thinking through how your part will be used and what non-negotiables drive your design cycle will lead to the most successful project outcome. Also, consider whether you are best served by a single process type, integrating processes, or integrating manufacturers to reach a viable conclusion to your project. Regardless one fact remains – thinking about manufacturability earlier in the design cycle to properly select and leverage 3D Printing and Injection Molding processes is essential to your success.
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