End product defines best route to useful prototypes

Written by: Tom Shelley | Published:

There is nothing like a physical product to appreciate how something is going to look and work. Apple, famed for innovative product ­designs such as the iPod and iPhone, owes much its global success to the design tools it uses.

Jonathan Ive, Apple's vp of design, says: "I can't begin to imagine designing without making. We don't do renderings, we do prototyping. Our ideas go straight into three dimensional objects.

"The team spends a lot of time handling the models and talking round them; something you don't get in front of a display."

Although major publicity is given to the growing number of additive rapid prototyping methodologies, more traditional methods – such as CNC machining and clay hand modelling – still have a place. But what is the best way of making a 3D model?

Royston based Prototype Projects offers stereolithography (SLA), fused deposition modelling (FDM) and selective laser sintering (SLS).

Managing director Justin Pringle says: "There are strengths and weaknesses in each method, depending on whether you want to use it, test it or just look at it. If it's a basic shape, it may be quicker to CNC machine it. But CNC usually requires some manual input to turn the CAD data into a machine file, whereas rapid prototyping machines normally accept STL files, which do not require modification. You shouldn't CNC something if it's going to be a moulded part at the end of the process."

Some parts cannot be CNC machined because of internal cavities or undercuts. But these can be rapid prototyped as a single part relatively quickly. Pringle says that FDM and SLS produce parts that are 'strong and durable, but require a lot of hand finishing if they are to be smooth enough to function as masters for silicone tooling'. SLA, on the other hand, produces very smooth parts and Pringle recommends it as the preferred method of making real­ly low cost tooling for reaction injection moulding and vacuum casting.

"The processes are now pretty stable," he says. "It is the materials that are making advances," says Pringle, who warns: "Rapid prototyped parts should not be made any stronger than the finished parts, even though it is possible to do so."

Wisconsin based Advanced Design Concepts (ADC) recently used a combination of CAD, rapid prototyping, clay modelling and scanning to halve the time to develop a hand controller for US municipal snow ploughs.

Force America wanted a controller that could accommodate three different joysticks for controlling snow plough blades, three switches for salt distribution plus electronics and lights. The company gave ADC a set of internal components and a rough clay model. ADC president Mark Schaefer produced Pro/Engineer CAD models of the buttons and circuit boards areas, based on drawings and 3D scans. He then created a volume model around the components to provide a clearance guideline for the clay work.

An Objet 3D printer then produced a rapid prototype of the volumetric clearance model for the controller's internal components. This allowed ADC's senior engineer Chris Mulhall to add clay to the RP model and guarantee that the final offset – the wall thickness of the part – would not interfere with the internal components. "Making a part that feels and looks good is very difficult", Schaefer says. "Using clay to get the shape right saved about a month in CAD and physical prototyping iterations."

The geometry of the clay model was captured using a GOM ATOS white light scanner, saving the data as an STL file which could be imported into Geomagic Studio software. Inconsistencies in the clay surface were smoothed and models created for the top and bottom shells. Parting line surfaces and curves were then generated. The Pro/Engineer models were brought back into Geomagic Studio, where the parts were modified to meet tooling draft requirements. Offset surfaces maintained the wall thicknesses of the parts and created final part surfaces and defined offset internal geometry.

From Geomagic, the surfaces were returned to Pro/Engineer, where structural ribs, holes and assembly features were added. The whole process was completed in three weeks.

Stereolithography (SLA)
Speed: Average
Accuracy: Very good
Surface finish: very good
Strengths: large part size, accurate
Weaknesses: Post processing, messy liquids, cost
Process: an additive manufacturing process using a vat of liquid UV-curable photopolymer "resin" and a UV laser to build parts a layer at a time. On each layer, the laser beam traces a part cross-section pattern on the surface of the liquid resin. Exposure to the UV laser light cures, or, solidifies the pattern traced on the resin and adheres it to the layer below.
Uses: Prototypes made by SLA can be very beneficial as they are strong enough to be machined and can be used as master patterns for injection moulding, thermoforming, blow moulding, and also in various metal casting processes. Although there are almost no limitations when it comes to the shapes of the parts that can be created. However, the cost of SLA machines and resins are expensive.

Selective Laser Sintering (SLS)
Speed: Good
Accuracy: good
Surface finish: very good
Strengths: accurate, good finish
Weaknesses: Size of machine, cost
Process: A high power laser fuses small particles of plastic, metal, ceramic, or glass powders into a 3D object. The laser selectively fuses powdered material by scanning cross-sections generated from a 3D digital description of the part (for example from a CAD file or scan data) on the surface of a powder bed. After each cross-section is scanned, the powder bed is lowered by one layer thickness, a new layer of material is applied on top, and the process is repeated until the part is completed.
Uses: Can produce parts from a relatively wide range of commercially available powder materials, including polymers (nylon, also glass-filled or with other fillers, and polystyrene), metals (steel, titanium, alloy mixtures, and composites) and green sand. In many cases large numbers of parts can be packed within the powder bed, allowing very high productivity. It is increasingly being used in limited-run manufacturing to produce end-use parts.

Laminated Object Manufacturing (LOM)
Speed: Good
Accuracy: fair
Surface finish: fair
Strengths: Price and size
Weaknesses: limited materials and accuracy
Process: A sheet is adhered to a substrate with a heated roller, a laser then traces the desired dimensions and cross hatches non-part area to facilitate waste removal. A platform with completed layers moves down out of the way, when a fresh sheet of material is rolled into position, then the platform moves up into position to receive next layer. The process is then repeated.
Uses: This is a relatively low cost method of rapid prototyping due to readily available raw material. The paper models have wood like characteristics and may be worked and finished to a decent standard. Although the dimensional accuracy is slightly less than that of SLA and SLS, milling is generally not necessary. Additionally, no chemical reaction is necessary.

Fused Deposition Modelling (FDM)
Speed: Poor to average
Accuracy: fair
Surface finish: fair
Strengths: Price and available materials
Weaknesses: speed
Process: FDM works on an additive principle by laying down material in layers. A plastic filament or metal wire is unwound from a coil and supplies material to an extrusion nozzle which can turn on and off the flow. The nozzle is heated to melt the material and can be moved in both horizontal and vertical directions by a numerically controlled mechanism, directly controlled by a computer-aided manufacturing (CAM) software package. The model or part is produced by extruding small beads of thermoplastic material to form layers as the material hardens immediately after extrusion from the nozzle.
Uses: Machines and materials vary in price, and have corresponding performance. Various materials are available for use with FDM with different trade-offs between strength and temperature properties. Most available commercial printers using FDM technology utilize positioning systems employing either stepper motor or servo motors to move the extrusion


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