One of the most important characteristics of an engineer is the capability of designing products that can be manufactured with economically convenient processes, exploiting the existing technologies while protecting the environment. DFA is a technique to reduce the total costs of a product.
Jessica Irons
Lots of engineers are probably acquainted with the terms DFM (Design for Manufacturing) and DFA (Design for Assembly).
DFA is a technique to reduce the total costs of a product, by decreasing the total number of components, assembling times and the complexity of assembly operations. DFM is a technique to reduce the component costs through the choice of the best manufacturing process, optimal material and the best equipment to manufacture the product.
One of the most recent trends in the design world consists in coupling these two techniques (DFM and DFA) in order to simplify both the manufacturing and the assembly. The concept that derives from it, called Design for Manufacturing and Assembly or DFMA, is a method applied with higher and higher frequency in the design of products.
DFMA can be used in the whole product development cycle, from the concept choice to the design monitoring, the estimate of costs and the product benchmarking. An OEM of cash registers, for instance, has redesigned one of its main products with the DMFA technique, succeeding in obtaining the reduction of the number of components by 85%, of the assembly times by 75%, of manpower by 44% and of the number of suppliers by 65%. It managed also to diminish the use of mounting tools and fixtures, saving 1.1 million dollars of manpower for the whole duration of the product life and freeing 33% of the surface dedicated to the production.
Analysis of the complete product
DFMA (Design for Manufacturing and Assembly) is centred on the analysis of complete products and related components and sub-assemblies. If a relevant part of the final cost of a product is ascribable to materials, it is reasonable to assume that the fastest way to reduce costs would be to eliminate some components/sub-assemblies. Boothroyd and Dewhurst, the pioneers of the DFMA method, have developed the concept of the minimal theoretical number of components, which represents the target that the designer should set to attain a design providing for the lowest possible number of components/sub-assemblies.
To determine whether a component is a candidate to the elimination, it is necessary to answer a series of simple questions. We should wonder whether the component is necessary to the ends of the product functionality; whether the component has a relative motion with regard to the other components; whether the component must be made with different material from that of the other components; and whether the component is necessary for the service and the disassembly of the product. If you answer “yes” to one of these questions, the component at stake can be necessary. If on the contrary the answer is “no” to all questions, the component is candidate to the elimination. The product design exercise, in such a way as to meet the criterion of minimal theoretical number of components, is left to the imagination and to the creativity of the design team.
The reduction of the component number is a key advantage of the DFMA method application.
The number of components concerns all the departments of an organization: from engineering to sourcing, from production to purchase, from sales to marketing and the customer service quality. The reduction of the total number of components has the clear advantage of a minor number of SKU (Stock Keeping Unit, identification product code) to be managed and of a minor number of raw materials to be used, but it provides also several other hidden benefits. A lower number of components implies an inferior number of drawings to be managed, fewer tolerance problems, fewer assembling lines/equipment (and then less manpower for the assembly), an inferior number of tools, jigs, fixing systems etc. and a simpler global provisioning chain.
Identification of the components to be eliminated
The candidates to the elimination are typically brackets, fasteners and sheet trays, whose only function is to fix other components and sub-assemblies so that the product works. Such components are those more frequently considered for the elimination when questions are made about the minimum theoretical number of components. A way to reduce the total number of screws, fixing elements etc. consists in using a different method to house the internal components, such as for instance the use of an inner chassis made of expanded polypropylene (PPE).
The resilience of the expanded PPE allows the fixing of internal components such as printed circuits, fans and feeders through friction coupling, without the need of further elements. Components are positioned simply making them slide or with snap insertion and they are kept in position by the expanded material. In line with the DFMA principles, we can then eliminate screws, brackets and other fixing devices whose only function is to keep the components in position.
A method to replace the inner structure of a device with PPE consists in adopting a sandwich approach with upper and lower parts made of PPE locking components one another. Instead of fixing a muffin fan or a printed circuit to a wall or to the chassis with screws, components can be snapped into the inferior PPE piece on which it is applied the upper PPE piece as a “sandwich”. This system allows not only eliminating fixing elements and brackets but it also contributes in damping vibrations and/or the fan noise.
Faster snap-fit fixing
During the assembly, the snap insertion of the various components into the expanded polyurethane is much faster than the assembly that requires four or more screws to fix a component to the chassis. The expanded sandwich method allows a sensible reduction of labour times – in some cases by over 50% – and the elimination of the need of tools for the assembly, like for instance the screwdriver. Besides, when a maintenance operator must replace or repair some determinate components, the possibility of removing and replacing components rapidly will make repairs much more efficient. Finally, thanks to the snap coupling, the removal and the separation of components for the disassembly and the recycling at the product life end is much easier.
Besides resilience, the PPE properties confer high resistance and rigidity to a chassis structure made of expanded material. Thanks to this characteristic, the external envelope of a device can be made with a thinner material and, in some cases, it can be even completely eliminated, leaving the function of internal protection, support and external envelope to the expanded material. In the case of the heat pump shown, the expanded substance acts as internal support and external envelope, protecting the appliance during the transport. In case the external envelope must have a particular texture or surface finish, the expanded material can be moulded to confer the desired aspect to the finished product.
A unique advantage of the PPE is the possibility of moulding different types of channels in the component. Channels are useful for directing tubes, wires or the air flow, too. The small compressible ribs moulded at the sides of the channels allow introducing and keeping wires and tubes in position without needing further fixings. In case of channels for the air flow, they can be designed and moulded in such a way that the air is deviated towards the heat sink, moving it away from components. The direction of the air flow inside the chassis towards the point where it is needed allows using smaller fans that are often more silent and efficient from the energy point of view, too.
In a single package, the medical device has an external heat sink with the expanded polyurethane inside that directs the air flow towards the right points, and an external envelope that acts as heat sink.
Shortening the design process, too
The DFMA method allows shortening the design process by 40% and even if the initial design phase of the concept takes more time, afterwards less time will be needed for the possible changes to be made in the design and in the documentation. The use of PPE to implement the internal chassis helps also in reducing the design and prototyping time, because prototypes can be manufactured without the aid of specific equipment.
Once ended the initial design phase, it is possible to make a preliminary prototype by using a CNC milling cutter. After accomplishing the design with a 3D CAD modelling programme, the designer programmes the CNC machine for the cutting of a PPE billet. The prototype can be tested through the assembly of components and if some modifications are necessary, the engineer can execute them with the aid of a shaving cutter. The improvements are then added to the CAD model and the successive prototype iteration can be rapidly carried out on the CNC milling cutter.
The inner PPE chassis provides designers with higher flexibility and freedom in the design choice of the internal chassis in relation to the external sheet or to the plastic envelope. Designers must not care anymore of how to fix the components to the structure but only of designing the expanded material in the most suitable way for the external envelope, in order to be able to insert and to fix the snapped components. In some cases, they can even succeed in eliminating the external envelope. In case the external envelope cannot be replaced, designers are free to modify the inside of the device by making some changes to the external envelope.
A touch of creativity
As the PPE can be moulded also in extremely complex shapes, it leaves a lot of space to the designer’s creativity. Nevertheless, since the use of the expanded PPE as component inside appliances and devices is a relatively new concept in the United States, the most difficult task is often to teach designers to conceive a literally new way of designing their products. And it is here that DFMA comes into play.
To introduce a designer to the concept of DFMA and of reduction of the total number of components will encourage him to think in a new manner. In its turn, this will induce him to think of reducing the biggest possible number of components, making the idea of using the PPE as internal chassis much more feasible. Designers can then share their knowledge with colleagues to contribute in diffusing the concept beyond their design team or work group.
Once the idea has caught on, it will spread through the whole organization, permitting to save time, money, materials and headache. You have just to start thinking inside the box – literally replacing the inner of that box with the expanded material.
