Injection molding: one of the most important manufacturing processesIt may sound hyperbolic to state that injection molding is one of the most important manufacturing processes, without which the daily lives of people in industrialized countries would be completely different. But the evidence in terms of our common experience with the products in our homes, offices, restaurants, hospitals and more, suggests very strongly that if injection molding were not available, we would have less varied, less functional, and more expensive products around us. Injection molding is by far the most effective and economical process for high volume complex plastic component production. In this blog you’ll learn how to determine whether it is the appropriate manufacturing process for your application by examining: 3 factors of the product being produced: 1. material, size, and complexity, 2. manufacturing economics, and 3. prototypes and low-volume production.
Injection Molding Factor 1: Material, size, and complexity.
MaterialThe most important parameter when considering injection molding is the material requirement: the application must demand a thermoplastic or thermoset material that can be molded in a closed tool. For the purposes of this blog, we will focus on thermoplastic materials which are much more common than thermosets, except in certain electrical, chemical, and thermal applications. Today the choice of thermoplastic resins is enormous and encompasses a broad range of mechanical, chemical, physical, aesthetic, and economic characteristics that make them suitable for everything from toothbrushes to car bumpers. Moldable polymers can be practically the full spectrum of colors, may be flexible or stiff, extremely strong and tough, and may have excellent chemical resistance. However, it is the incredible combination of these properties that a single polymer can exhibit that makes them the material of choice for so many injection molding applications.
SizeThe next factor to evaluate when considering injection molding is the size of the component to be produced. On the small end of the spectrum there are micro-molded components used in medical devices that weigh a few tenths of a gram and easily fit in a 5x5x5mm cube. On the other end we find components for agricultural equipment weighing more than 100kg with maximum dimensions well over 1500mm. The majority of injection molded components, measured in billions per year, are caps, closures, disposable / single use products, toys, and personal electronics to name a few applications, which generally weigh in the 1-500g range.
ComplexityThe degree of geometric complexity further influences the choice of manufacturing process. Injection molding can produce components with thousands of features, complex textures, combinations of materials in different locations in the same component, and molded-in metallic components in the exact same time it would take to mold a completely plain, featureless, single material, similarly sized component. While there are some geometric limitations based on the physics of a molten liquid polymer filling a metal mold, solidifying, and then cooling pre- and post-ejection from the mold, the geometric possibilities are, for all intents and purposes, limitless.
Injection Molding Factor 2: Manufacturing economics – how to evaluate valueFollowing material properties, component size, and complexity, the design engineer needs to consider the manufacturing economics of the application. In its simplest form, manufacturing economics is the exercise of maximizing the rate of value creation versus invested capital (including direct material and labor expenses, as well as tooling and equipment investments). Or seen another way: minimizing the fully burdened manufacturing cost per component.
For a given capital expenditure, the rate of production of complex polymer components by injection molding can be many multiples of other means of forming the same geometry. This is where injection molding dramatically separates itself from other processes.Doing a bit of back-of-the-envelope math for our telephone handpiece cover, let’s say we have our hypothetical factory floor with a CNC mill, an industrial 3D printer (as distinct from a desktop 3D printer), and a standard production injection molding machine lined up ready to start production. An average CNC mill for a component this size new would cost $100-$150k, the industrial 3D printer $50-75k, and the injection molding press including a four-cavity steel mold $175k ($75k of which is the mold). Ignoring the extra cost of automated fixturing and robotics needed to run production volumes in CNC machining, and eliminating the time required to unload and post-process 3D printed parts, the following table suggests the relative production performance of injection molding:
Manufacturing Process Comparison: CNC Milling vs. 3D Printing vs. Injection Molding Example: manufacturing a plastic office telephone handpiece cover.One could CNC mill an office telephone hand piece cover from a block of the desired ABS, which would take, at best, tens of minutes per part and would not have the required surface texture. Or, this same hand piece could be 3D Printed in an ABS material with similar, but different, properties and have compromises in surface finish and dimensional accuracy at the rate of a few per hour. Lastly, a hand piece could be injection-molded every 20-30 seconds with exactly the desired material properties, surface finish, and geometry. For even higher production rates, multi-cavity injection molding tools would be used to increase the effective production rate.
Injection Molding Factor 3: Injection molding for prototype and low-volume productionThe traditional injection molding tool manufacture and molding process is rarely the correct choice for prototype and low volume production expressly due to the cost and lead time as noted in our example telephone hand piece. However, this is precisely where Xcentric delivers enormous value to our customers by helping them bring their plastic products to market on time, on budget, and with lower risk.
Proprietary Process Engine: From CAD to Production, Faster
 Given the scope of the materials topic, details about thermoplastics will be covered in a separate dedicated article.  This scenario already suggests a production rate that is beyond current 3D printing technologies when post processing and surface finish are factored in.  Some limitations in project type and additional fees apply for expedited projects.