Taking a product from prototype to production

Moving from prototype to production means turning a design that works into one that suppliers can make repeatedly, to a consistent standard and at the target unit cost. A prototype is usually made by whatever process gives a good part quickly. Production uses the process that suits the volume, and that difference drives most of what changes: the parts themselves, how they are tooled, how they are toleranced and inspected, and how they come together on an assembly line.

The work between the two is easy to underestimate. A product can be fully functional as a prototype and still need significant redesign before a mould can be ordered, and decisions that were easy to defer during development, such as cosmetic standards and spares, all have to be settled before the first saleable unit ships.

Process changes and what they do to the design

The most common change is from parts that were printed or machined to parts that are moulded, cast, pressed or extruded. Each production process brings its own rules, which our guide to design for manufacture covers, and a prototype part seldom follows them.

• Printed to moulded. A printed housing can have uneven walls, sharp corners, undercuts and no draft. The moulded version needs uniform walls, draft on every face in the direction the tool opens, ribs in place of solid sections, and undercuts removed or designed for side actions. Snap fits that worked in SLS nylon may break in a stiffer moulded material, or creep in a softer one.
• Machined to cast. A cast part needs draft, generous radii and machining allowance on the faces that locate or seal. Casting alloys are usually less ductile than the wrought billet the prototype was cut from, so stresses need rechecking against the cast properties.
• Machined to sheet metal. A machined bracket often becomes a folded part, welded or riveted, with bend radii, reliefs and new fixings.
• Hand-built to production fabrication. Jigs replace measuring and clamping by eye, nested cutting replaces cutting to suit, and weld sizes and sequences are fixed so every frame comes out the same.

Tooling

Production tooling, whether an injection mould, a die-casting die, press tools or welding jigs, is often the largest single investment in a launch, and it is slow and costly to change. A few points shape the decisions around it.

• Settle the design first. Every change after the tool is cut means modifying the tool. Some modifications are cheap, such as thickening a rib or lengthening a boss, because they remove steel. Others mean welding the tool, a new insert or a new tool.
• Design steel-safe. Removing steel from a tool is much easier than adding it. Where a critical dimension may need adjusting after the first parts, set it so the correction removes steel: leave a rib slightly thin or a snap-fit catch slightly shallow, then cut the tool to bring the part on size.
• Cavities and tool life. A multi-cavity tool makes more parts per cycle but costs more and needs more work to balance. Choose the number of cavities and the tool steel from the annual volume and the expected life of the product.
• Ownership and storage. Agree in writing who owns the tool, where it is kept, who maintains it and what happens to it if you change supplier.

Design changes for volume

At volume, assembly time and part count matter as much as the cost of each part. Common changes at this point include:

• Combining several parts into one moulding or casting, so there are fewer parts to buy, stock and assemble.
• Replacing screws with snap fits, or cutting the number of fastener sizes so one driver does the whole assembly.
• Making parts symmetrical, or clearly asymmetrical, so they cannot be fitted the wrong way round.
• Adding features that locate parts for assembly, such as pins, slots and lead-in chamfers, so an operator does not have to hold alignment by hand.
• Matching tolerances to the capability of the chosen process. A process capability index (C_pk) of 1.33, which places the nearest tolerance limit four standard deviations from the process mean, is a commonly quoted minimum for important dimensions. A tolerance the process cannot hold at that level leads to scrap or sorting.

Getting supplier quotes from a drawing pack

Quotes can only be compared if every supplier is quoting for the same thing. Send each one the same pack: drawings and 3D models at a stated revision, the bill of materials, material and finish specifications, annual volume, batch sizes and expected product life, packaging and delivery requirements, and the documents you expect with each delivery, such as dimensional reports or material certificates.

Ask each supplier to state their assumptions, including tool steel and number of cavities, expected cycle time, which operations they subcontract, and any changes they would suggest to make the part easier to produce. That last item is design-for-manufacture advice from the people who will make the part, and good suppliers offer it readily. Quotes that differ widely usually differ in their assumptions, and seeing those assumptions is what makes the comparison fair.

Pilot builds

A pilot build is a small batch made with production tooling, production processes and, ideally, the people and line that will build the product. It sits between the last prototype and full production, and it is where problems appear that no prototype can reveal: parts from different cavities or batches that fit differently, an assembly sequence that is awkward at line speed, a screw that strips its boss when driven by a production tool, a cosmetic flaw that only shows on moulded parts.

The first parts off a new mould, often called T1 samples, are measured against the drawing and the tool adjusted, sometimes over several rounds, before the pilot build begins. Units from the pilot build then go to the testing that needs production-representative units, such as environmental testing and customer trials. Problems found at this point cost a tool adjustment or a drawing revision. Found after launch, the same problems also cost returns and rework.

What to settle before releasing to production

Release is the point at which suppliers start making saleable parts in quantity. Before it, the following should be decided and written down.

• The design, at a single revision for every drawing and model, with every supplier working from that revision.
• Critical dimensions and characteristics marked on the drawings, along with how and how often they are measured.
• Cosmetic standards for visible surfaces, ideally with physical boundary samples showing the worst acceptable flow line, sink mark or scratch.
• Second sources, or agreed stock levels, for bought-in parts whose supply is uncertain.
• Packaging, labelling, assembly instructions and the end-of-line test.
• Spares, and whether service parts will be stocked for the life of the product.

Changes after launch

Products change after launch for many reasons: a cost reduction, a change of supplier, a bought-in component going obsolete, a problem found in service. The drawing change is usually the easy part. Each change also has to account for four other things.

• Interchangeability. If old and new parts can be mixed freely, a new revision of the same part number is usually enough. If they cannot, the new part needs its own number so that stores, service teams and spares stock keep the two apart.
• Stock and work in progress. Decide whether existing parts are used up, reworked or scrapped, and from which batch or serial number the new part applies.
• Tooling. Find out whether the tool can be modified, and whether the change removes steel or adds it.
• Testing and regulation. Work out whether the change affects anything that was tested against the specification or a standard, and whether that test has to be repeated.

Some problems only appear once a part is being made and finished in earnest. The thin aluminium disc case study is one example: a machined disc with a 1.2 mm web that warped after anodising, most likely from residual stress released unevenly plus heat from sealing, addressed through changes to material, machining, fixturing and finishing.

How SixGate™ can help

SixGate™’s Production stage takes products into manufacture by CNC machining, 3D printing, injection moulding or sheet metal fabrication, from short runs to volume. Where the parts were designed and the engineering pack prepared in the earlier stages, production starts from drawings that suppliers can quote and build from, and F-CAD® stays involved when changes are needed once the product is in production. You can also bring a product that is already in production and has a problem. On the bulk material handling project, the result was a product line that scales, supported by full drawing packages and bills of materials for fabrication.