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Design For Manufacturing & Assembly Introduction

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Whether you’re looking to develop a new product or are trying to reduce the cost of an existing product, optimizing your design for manufacturing and assembly (DFMA) is a critical component in making your product and business successful. If you haven’t yet designed your product for manufacturing then you’re likely wasting large sums of money on production costs.

Iterative Process To Design for Manufacturing

Design for manufacturing is an iterative process that is constantly improved upon during the lifespan of a product. As volumes increase, using new higher volume methods reduce the cost of manufacturing components and assembling your product. To implement high volume methods, you may need to implement product design revisions. As your product continues to scale, it may also be possible to reduce its total part count and material which can significantly reduce costs especially at higher volumes.

Minimize Your Part Count

While there are some rare exceptions, reducing your part count is a great way to reduce your manufacturing cost and improve your products performance. Reducing part counts simplifies assembly, reduces the amount of fasteners or adhesive required, removes connection weak points, reduces material, simplifies the supply chain, reduces warehousing costs, reduces the amount of high-cost tooling required, and more.

Design criteria to determine wither a part is necessary or should be considered for removal from the product to reduce part count.

  1. During operation of the product, does the part move relative to all other parts already assembled?
  2. Must the part be of a different material or be isolated from all other parts already assembled.?
  3. Must the part be separate from all other parts already assembled because otherwise necessary assembly or disassembly of other parts would be impossible.

Source: “Introduction.” Product Design for Manufacture and Assembly, by G. Boothroyd et al., third ed., CRC Press, 2013, pp. 10–11.

Using the above guidelines you can determine the theoretical minimum part count needed for your product. In practice it is near impossible to get a product’s real part count equal to it’s theoretical minimum part count. Manufacturing methods, maintenance requirements, and economics will limit your ability to reach the theoretical minimum part count, however a designer should take the time to justify the need for every single part that is theoretically not needed. Determining the optimal design is an iterative process as it requires quoting different parts using various manufacturing methods, determining assembly cost, and product lifespan cost.

Determine Your Production Volume

If you’re producing a low volume product then your ideal manufacturing methods are going to be largely different than a high volume product. Whether or not your production volume is considered low, medium, or large will also depend on the manufacturer, method, and product.

Low Volume Products: Range between 1 annual unit to 100s of annual units. Low volume products are typically large and expensive products, custom made products, or prototypes.

Low volume products rely on manufacturing and assembly methods which don’t use tooling or automation as the return on investment (ROI) for tooling and automation is very poor for low volume manufacturing.

Medium Volume Products: Range between 100s of annual units to a few thousand annual units. This may be done for a niche product with smaller demand or as an initial production run to test the market prior to scaling.

Large Volume Products: Typically over 10,000 annual units, but may be as low as 5,000 annual units. This is typically done for large market products that are either launching with retailer support or significant brand awareness or are already on the market and have passed initial market testing.

Extra Large Volume Products: Many consumable products need to produce over 1,000,000 annual units. These products are typically food, other consumables, or materials used in other products such as aluminum cans or packaging.

Select Your Product Materials

Selecting the best material for a component depends upon the component’s performance requirements, cost, compatible manufacturing processes, and required component geometry.

Cast Iron

Ductile Iron

Carbon Steel

Alloy Steel

Stainless Steel

Aluminum and Alloys

Copper and Alloys

Zinc and Alloys

Alloys contains useful properties such as affordably adding hardness to copper to make brass or to make solder which has a lower melting temperature. Zinc is commonly used to provide corrosion protection through galvanizing. Zinc may also be used for higher volume die casting.

Magnesium and Alloys

Magnesium alloy was made to be a lightweight material for the aerospace industry with 2/3 the density of aluminum. Magnesium alloys also have good high-temperature properties and good to excellent corrosion resistance.

Titanium and Alloys

Nickel and Alloys

Nickel provides excellent heat and corrosion resistance. NIckel alloys also offer substantial strength and durability.

Refractory Metals

Refractory metals are ideal for demanding applications due to their resistance to high heat, corrosion, and wear. Refractory metals include Tungsten, molybdenum, niobium, tantalum, and rhenium.

Thermoplastics

Select Your Manufacturing Process

You manufacturing/production method depends on a lot of variables. Volume, performance demands, availability, finances, and more.

Additive Manufacturing (Aka 3D Printing)

Fused Deposition Modeling (FDM)

If you found an introductory or low cost 3D printer, then this is likely the type you came across. FDM works by depositing plastic filament. The filament melts and binds to the layer beneath it and then hardens. While being more affordable, these printers typically require support material, have lower resolution and have a tendency to have layers delaminate when they don’t bind properly.

Stereolithography (SLA)

SLA is more expensive than FDM but provides a much higher quality part. SLA uses a liquid resin which it then solidifies using a laser. SLA doesn’t require any structure for its parts and prints in excellent resolution.

Digital Light Processing (DLP)

DLP is very similar to SLA with the only difference being a light bulb is used in place of a UV laser to cure the photopolymer resin.

Selective Laser Sintering (SLS)

SLS is similar to SLA, however instead of using liquid resin it uses a laser to sinter powdered plastic material.

Selective Laser Melding (SLM)

SLM is an additive manufacturing method that uses a high power-density laser to melt and fuse metallic powders together. Unlike other additive manufacturing methods which primarily print using plastics, SLM prints using metals including copper, tool steel, inconel, and stainless steel.

Laminated Object Manufacturing (LOM)

LOM uses layers of adhesive-coated paper, plastic, or metal laminates which get glued together and then a cut to shape with a knife or laser cutter.

Electron Beam Melting (EBM)

EBM is similar to SLM, however it uses an electron beam to fuse metal particles instead of a high power-density laser.

Material Removal

Machining

Machining consists of using a large range of machines including CNCs, mills, lathes, drill presses, shapers, and more. Machines use a very hard and sharpened cutting edge to precisely remove material. The cutting edge may be stationary, sliding or on a spindle depending on the machine type.

Electrochemical Machining (ECM)

ECM removes material by use of an electrochemical process. Most commonly used in mass production. ECM works by using a cathode (tool) to form an anode (workpiece) using pressurized electrolyte which is injected at a set temperature to dissolve material from the workpiece. ECM delivers a good surface finish in parts that are difficult to manufacturing with machining due to complex shapes and/or material.

Electrical Discharge Machining (EDM)

EDM has similar advantages as ECM and may be used on any conductive material. EDM uses thermal energy to remove material through the use of a high-frequency electrical spark discharge from a graphite or soft metal electrode (tool) which disintegrates conductive materials. This works well for forming hardened steel, carbide, titanium, and other hard materials that aren’t easily machined.

Sheet Forming

Laser Cutting

Laser cutting uses a high-power laser to cut material by either melting, burning, vaporizing the material which is then typically blown away by a jet of gas leaving a high-quality surface finish. Laser cutting is typically guided by a CNC and can obtain tight tolerances similar to machining. Today’s highest energy laser cutters can cut up to 2 Inch thick steel plate, though typically thinner material is preferred as the cut rate reduces dramatically as material thickness increases. Laser cutting is only effected by a materials thermal and optical properties and therefore may be used to cut high hardness materials such as hardened tool steel, spring steel, titanium, and others which would be difficult to stamp or machine.

Waterjet Cutting

Water jet cutting offers similar capabilities to laser cutting through utilizing a high pressure stream of water which contains cutting aggregate to erode a narrow line into plate or sheet. Waterjet cutting is slower than laser cutting, however it may cut thicker plate steel (up to 4″ titanium plate) or greater. The surface finish is also not as clean as laser cutting, however the machines are typically more affordable and therefore it is more cost effective for parts with looser tolerances.

Thermoforming

Thermoforming is a process where a plastic sheet is heated to pliable forming temperature and then formed around a mold using a vacuum pump and trimmed.

Metal Spinning

Metal spinning cold forms a metal work piece using a roller tool as the work piece is spun using a spindle. Through cold forming the material instead of cutting it, there isn’t any material waste and fatigue strength is increased. The material also undergoes work hardening which may be desired or may be removed through annealing.

Stamping

Stamping is used to form sheet metal with a tool and die surface. Depending upon the part complexity, either a single stage or multiple stages (progressive) are used. Stamping can form, cut, and with some stamping presses tap parts. Stamping is limited to use with ductile steel and doesn’t work well with higher strength and higher hardness metals. Servo presses deliver increased control to the rate of pressing which helps with being able to press higher strength metals, but is still limited in comparison to laser cutting and other methods. Stamping’s biggest advantage is speed and volume, at high volumes (>10,000 units) stamping can be a fraction of the cost of laser cutting especially after tooling is made.

Bulk Deformation

Impact Extrusion

Impact extrusion is done by pressing a metal slug at high speed with high force into a die or mold using a punch. This provides a low cost product through a very high production rate and by eliminating excess material.

Hot Extrusion

Hot extrusion is similar to impact extrusion, however it uses material heated to the point where work hardening doesn’t occur in order to make the material more malleable and reduce wear on tooling.

Cold Forming

Cold forming works by using a succession of tools and dies to form metal by stressing it beyond its elastic limit but not going beyond its tensile limit to permanently form it while preventing the part from fracturing. This reduces material waste, is able to deliver a very high production rate, and increases a parts fatigue life.

Rotary Swaging

Rotary swaging uses cold working to form a round workpiece by reducing the diameter, producing a taper, or adding a point to a round workpiece.

Closed Die Forging

Closed die forging is a process where raw material is heated to prevent work hardening and is then placed on the bottom die of a form or tool which has the negative of the final component. The top die then impacts to heated raw material to form it into the negative volume of the tooling and create the component. Small components may even be formed cold depending on their size and required amount of forming.

Powder Metal

Powder metal generates components with very little wasted material by taking metal powder and pressing it with a form or tool and sintering it to form a solid part.

Solidification

Sand Casting

Sand casting use molds made out of sand with a bonding agent which is then moistened to give it the strength and plasticity needed for molding. The mold cavities and gate system are then formed directly into the sand. 3D printing has also made forming a negative using sand casting easier.

Investment Casting

Investment casting or lost wax casting works by making an original pattern out of wax, clay wood, plastic, or 3D printing. A mold, aka the master die, is then made to fit the pattern. A mold may also be machined directly without the use of an original pattern. Wax patterns are then created using the mold by creating a uniform coating on the inner surface. This is repeated until the desired thickness is reached. The mold may also be filled completely to form a solid wax object. The wax mold is then removed. The ceramic mold, aka the investment, is then produced by using the wax mold by coating, stuccoing, and hardening until a desired thick. To do this, the wax mold is dipped into a slurry of fine refractory material and then drained to a uniform thickness. The ceramic molds are then cured at which point the wax mold are melted out in a furnace or autoclave. The mold is then burn out to remove any moisture and residual wax and sinter the mold. Molten metal is then poured into the ceramic mold. Once the metal hardens, the shell is hammered off or removed by other means to reveal the casting. The casting is then cleaned up and finished to bring it within specifications.

Die Casting

Die casting delivers low cost components after the initial investment in expensive tooling. Die casting is performed by injecting molten metal under high pressure into a mold cavity created by two hardened tool steel dies which have been machined into shape. Die casting materials are typically non-ferrous metals including zinc, copper, aluminum, magnesium, lead, pewter, and tin alloys. Die casting is primarily used for high volume components that need added strength beyond what injection molding can deliver. Die casting however does not provide the same high strength capabilities of other manufacturing methods.

Injection Molding

Injection molding produces parts by injecting molten material, most commonly thermoplastics, into a mold while under high pressure similarly to die casting.

Structural Foam Molding

Structural foam molding is uses two components along with an inert gas to form a resin which is injected into a mold. The inert gas changes the nature of the chemical reaction and causes the resin to not completely fill the mold which creates a honeycomb structure. As the inert gas is activated by the polymer reaction which causes it to expand and fill the empty space of the mold with foam. This works with lower pressures compared to injection molding and allows for thicker wall sections and stronger parts.

Blow Molding

Blow molding is used to make hollow plastic and glass components including light bulbs and other parts. Blow molding works by taking hollow stock, and then clamping on it with a mold after which air is blown in to apply pressure on the inside to push out and form the hollow stock to the internal walls of the mold.

Rotational Molding

Rotational molding uses a slowly heating hollow mold which is filled with material that softens and is evenly dispersed to stick to the walls of the mold while the mold is slowly rotated.

Design for Assembly

Your assembly method will depend greatly on demand for your product. Whether you need to product 20 units/week or 60,000 units/week will decide whether manual assembly or a more automated assembly process makes sense for your product. If you’re working to develop a mass market product than it is common for the production costs to exceed the selling cost while at low volumes if you are competing with suppliers who have large volumes. To scale a product from low volume to large volume typically requires a large investment in both marketing/sales and manufacturing so that you can generate the demand and supply needed to compete in the market.

There are many options to assemble a product. You can use welds, rivets, fasteners, adhesives, solder, clips, press fitting, or interference fitting. It is always usually best to minimize the number of parts and material used to make a product so eliminating adhesive, or fasteners and replacing them with press fitting or interference connections can reduce cost. However, during assembly design is important to ensure that a product can be disassembled as desired especially if your product is expected to be able to be repaired.

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