Each manufacturing process has pros and cons which determine their suitability for a given application. Aspects like service characteristics, appearance, production volume, cost, and material compatibility are among the most important factors to consider.
Manufacturing processes can be broadly categorized as either formative, subtractive, or additive. Formative processes use force and pressure to reshape material without notable changes to its mass, like rolling out a log of cookie dough into a sheet. Subtractive processes remove or separate material from an existing base, like using cookie cutters to cut shapes from the dough sheet. Additive processes operate by adding material to an existing base, like frosting the baked cookies. Most products are made through a combination of all types.
Content List
Formative Processes
Formative processes change the shape of a material without adding or removing mass. Common operations like injection molding, stamping, forging, and extruding are all formative.
Pros
- Material efficient
- Can produce variety of output from stock
- Large throughput
Cons
- Usually requires a post-processing operation
- Significant setup costs
- Simulation crucial to avoid defects and preserve tool life
The following are details on just a few of the many formative processes available.
Injection Molding
Injection molding is a subset of molding, where molten plastic is allowed to cool in a shaped mold. The injection aspect is due to the delivery of the plastic, where a screw forces liquid into the mold through a channel called a sprue. The central sprue splits into runners connected to the part(s) through one or more gates. Molten plastic may cool naturally, or coolant liquids circulating through the mold halves may draw heat out quicker for greater throughput. The mold halves separate and the part is removed from the mold using ejector pins. Post processing to remove part bodies from the sprue and connecting runners is necessary.
Designing for injection molding requires consideration of many factors, such as:
- Parting line and separation direction: In the simplest injection molds, the two mold halves separate in a single direction and form a parting line artifact where they meet. Features not aligned with the parting axis that don’t also lie on the parting line will prevent
- Overhangs: To avoid the cost and complexity of side pulls, overhanging features should be avoided. If unavoidable, it is worth employing some tricks to stick to a two-mold method.
- Draft: To avoid fighting vacuum forces when remove parts from the mold, a draft angle between 0.5°-2° in the parting direction is advisable.
- Part thickness: Uniform wall thickness allows for even cooling, reduced internal stress, and more consistent plastic flow. If walls are overly thick the outer faces will cool before the internals.
- Small features: Overly deep or small sized features require thin, protruding metal counterparts in the mold that are fragile. Changing the feature design accordingly will extend mold life and make mold production easier.
- Best practices: Most if not all features should have a fillet radius to remove sharp corners as points of stress concentration. Bosses and other features should be located near walls and connected with ribs; if standalone bosses are unavoidable then they should be supported with ribs spaced in a regular radial pattern (ex. 3 ribs 120° apart).
Injection molding really shines for production volumes in the hundreds of thousands, where its high tooling and startup costs are outweighed by its rapid operation and versatility. Familiar applications of injection molding include LEGO bricks, pens/pencils, storage tubs, and plastic model kits (which often leave the runners intact).
Forging
Forging involves shaping metal through compressing, with tools like hammers or dies. The material being forged may be at room temperature or heated beforehand. Forging is one of the oldest manufacturing processes.
The temperature of the worked material determines how the internal crystal grain structure is affected by the forming process. Room temperature forging introduces dislocations and other defects into the regular structure, adding strength through work-hardening. The addition of heat up to or near the material’s recrystallization temperature allows some of the internal stresses and defects of the forged part to be released as grains reform.
Forged parts are among the most mechanically strong and are desirable in demanding applications like engines, turbine blades, and industrial piping.
Extruding
Extruding is a family of processes where a material is pushed through or into a die, as opposed to drawing where the material is pulled instead. The process is commonly performed with softer materials such as aluminum or plastics, though harder metals are possible.
Similar to forging, extrusion may be done at room temperature or elevated temperature depending on the desired properties of the output. The dies experience extremely high pressures through the extrusion process and may be lubricated to ease some load.
Extrusion is best suited for long parts of uniform cross section such as bleacher seats, structural members, or gutters. Of course, the full extruded length can be processed into shorter pieces for applications like heatsinks and straws.
Stamping
Stamping encompasses many operations performed on sheet material. Presses that bend, curl, hem, and emboss are performing formative work. While largely intuitive to understand, there is much depth in simulating the deformation and friction processes to avoid defects like tearing, wrinkling, and scratches.
Bending has the potential to rapidly produce outputs from very simple starting stock, making it appealing for high volume hardware products like hinges and brackets. Larger parts with complex surface shapes can readily be produced in presses with matching dies.
Subtractive Processes
Subtractive processes include many well-known operations like milling and lathe turning, as well as some newer ones such as wire EDM and abrasive waterjet cutting.
Pros
- Long precedent of established knowledge
- Compatibility with several material types (plastic, metal, wood, etc.)
- High quality surface finishes
- Readily accepts stock sized pieces
Cons
- Produces more waste in the form of chips
- Typically takes longer
- Space required for storing/clamping stock exceeds that of final part
- Toolhead access may limit on internal features
The following are details on just a few of the many subtractive processes available.
Milling
Single axis CNC mills have a vertically translating cutting tool operate on a workpiece clamped to a computer-controlled table. Modern advanced milling machines with more axes of motion may have tables which can rotate as well, reducing tool changes.
End mills have flutes similar to drill bits for chip removal. The tips of end mills vary in design from typical flat, ball, and bullnose shapes to specialty shapes for cutting gear teeth, t-slot profiles, and undercuts. Cuts are made in many passes and with frequent application of coolant to remove material safely and preserve the life of the tool.
Parts made through milling are suitable for end-use and have greater service characteristics. Successful milling operation requires expertise in accounting for tool changes, incorporating path offsets due to varying tool diameters, maintaining the machine, and converting a final part design into programming steps for the machine.
Turning
Turning involves a static cutting tool translating along the axis of a spinning workpiece. Similar to milling, the cutting tool is advanced radially deeper into the stock in gradual steps.
Turning is ideal for parts that have radial symmetry like flanges, shafts, and bushings. Components in assemblies that spin such as motors, axles, and fasteners are best made through turning to preserve even mass distribution and thus balance.
Operating lathes to turn stock material requires expertise similar to milling.
Wire EDM
Wire EDM (Electrical Discharge Machining) uses sparks to remove material from a workpiece. A current carrying length of wire and a workpiece submerged in an electrolyte solution create the conditions for the discharge to occur. As a consequence, wire EDM is only compatible with electrically conductive materials.
Wire EDM is great for very precise parts that have a uniform cross section like gears. The immersion in fluid and localized sparking make wire EDM appealing for parts where minimization of residual stresses is important.
Abrasive/Waterjet Cutting
Waterjet cutting uses a high pressure steam of water expelled from a nozzle to carve through a soft workpiece. By adding abrasives to the water stream, harder materials can be processed as well. The nozzle is kept close to the workpiece to minimize the effects of air resistance, preserving the stream thickness and kinetic energy of the discharged water. Adjustment of the nozzle orientation allows for angled cuts as well.
Waterjet cutters can be used on an extremely wide range of materials, including non-conductive metals that are not suitable for wire EDM. The narrow kerf achievable from waterjets allows denser packing of parts on stock.
Additive Processes
Additive processes are generally all considered forms of 3D printing, in that they operate by slicing a 3D model into layers which are placed in sequence. The type of material and adhesion method determine the class of process.
Pros
- Reduced waste, most material added is part of final product
- Well suited for complex internal geometry
- Easier to manage for small batch production
Cons
- Material options generally more limited
- May need post-processing to achieve required surface finish
The following are details on just a few of the many additive processes available. Additive manufacturing is an active field of research with new and exciting processes being invented frequently!
Fused Deposition Modeling (FDM)
& Fused Filament Fabrication (FFF)
FDM is a trademarked term by the brand Stratasys Inc. while FFF is a generic term used by the community. Both FDM and FFF refer to the same process in which a moving printhead with heated nozzle melts a feed of material onto a printbed, though FDM machines generally hold tighter tolerances and are more precise.
Typical FFF printers are take an input spool of plastic and extrude it through a single nozzle. However, there are many different designs that incorporate multiple nozzles on the same printhead, swappable printheads, automated filament switching tools, and more. There are plastic varieties to suit every use case, each requiring particular heating temperatures and with varying tolerance to moisture.
Rectilinear printers cover a volume using translation along three perpendicular axes, with the printhead traveling along two and the bed along the third. Printers with a bed that moves forward and back are called bedslingers, while ones with a vertically-moving bed are called core XY printers. While bedslingers are simpler, easier to service, and perform well with small models, core XY printers’ allow for better print adhesion, smaller form factor, and faster speeds.
Separate from rectilinear printers are delta printers, in which a very light printhead is supported by three arms connected to rails. The printbeds of delta printers are circular and do not move, with all motion controlled by the arms. Delta printers are able to achieve much greater print speeds than rectilinear models, but their cylindrical build volumes limit model size in directions other than their main axis. Their added complexity generally reserves their use for demanding applications in the aerospace and medical sectors.
FFF printers are ideal for quick iteration and rough prototyping due to their ease of use and minimal required post-processing. The detail level of FFF prints is limited by the nozzle diameter, with smaller nozzles able to produce smaller features but more prone to clogging. Typical diameter is 0.40mm, with a range of 0.10mm to larger than 1.0mm. FFF parts may be appropriate for end use depending on service requirements, but the layer lines’ effect on mechanical properties and appearance can make other methods appealing.
Stereolithography (SLA)
SLA uses ultraviolet light to selectively cure resin in layers. In the more common bottom-up method a printbed gradually ascends out of a resin vat, with new layers added to the existing model where resin meets a clear window. In between layers the printbed clears away from the window to allow resin to fill the space most recently cured. Once the part body has been printed, it is washed with isopropyl alcohol to remove any uncured liquid resin stuck to the surface. Lastly, the whole part gets prolonged UV exposure to ensure a complete cure.
Parts produced using SLA are ideal when excellent surface finish, watertightness, very intricate detail, or optical clarity are desired. Resins are available in a wide range of appearances and mechanical/chemical properties, though most have a finite shelf life. Parts created using SLA exhibit isotropic properties due to layers being chemically bonded, in comparison to FFF whose mechanically bonded layer seams are distinctly weaker than individual layers.
Fumes produced by the resin and the amount of innate post-processing raise the barrier of entry of SLA above FFF. In a ventilated space and with a dedicated post-processing setup, SLA is a solid choice for higher-fidelity prototypes that can still be made in a few days.
Selective Laser Sintering (SLS)
SLS uses the heat and precision of a laser to bind together fine plastic powder particles in a bed. After a layer has been fused, the bed drops down by a layer height, new powder is distributed across the top, and the process is repeated. Once the part has been formed, it is excavated from the powder bed and excess powder is removed through tumbling or blowing.
A major advantage of SLS over FFF and SLA is avoiding dedicated support structures due to the innate support of unsintered powder all around the part. This means SLS has no limits on overhangs, islands, wall thickness, etc.
The energy usage of the laser and safety training keep SLS primarily as a tool for industrial use, where it is popular for early and mechanically functional prototypes.
