High-speed digitising breaks the mould

The mould maker as a provider of end-product design support may seem like a violation of the natural order, but US-based Duplicraft Inc., has established precisely that role for itself.

In its search for ways to add value to the mould making process, the company hit on the idea of applying the digitising technology it purchased for toolpath generation to help its customers modify, analyse and refine their designs. "The expanded range of services strengthens our bond with customers," says Duplicraft's Darold Faurot. "Now, our involvement in customer processes begins long before there's a final mould to be machined."

Faurot says Finite Element Analysis (FEA) explains much of the need on which his company capitalised. Duplicraft makes moulds and secondary tooling for extrusion blow moulding, meaning its customers are almost exclusively makers of plastic bottles and similar containers. For these customers, bottle design used to be a hit-or-miss process that left the strength and functionality of end product indeterminate. Questions related to the behavior of the liquid when poured, the bottle's resistance to impact, and its ability to bear the weight of crates stacked on top of it during shipping - all had to wait until a finished mould produced a finished bottle that could be tested. FEA changed that, says Faurot. Now, Duplicraft's customers answer most of these questions using only a "virtual" container derived from the CAD model.

There's just one problem: The use of CAD in the container industry is relatively new, and far from universal. What about bottle designs that aren't created in the computer, like hand-made models, or semi-new designs resulting from direct alterations to moulds created years ago?

In these cases, says Faurot, customers now turn to Duplicraft. The company uses high-speed, high-resolution digitising to measure and map out physical models and cavities, and translate them into digital equivalents that can be evaluated through FEA, often after modification in CAD. Sometimes, the company even digitises actual bottles, allowing it to make proposed changes to the design entirely in the computer, and submit a digital prototype of the new bottle to the customer for evaluation.

"The technology allows us to grow beyond just our metalworking specialty, a change we see as key to the continued success of this business," he says. "We're not just mould machinists anymore; we are our customers' partner throughout the development and production of moulded products."

Analogue scanning

Duplicraft carries out its digitising using Renishaw's continuous-path Cyclone scanning machine. Much faster than point-to-point touch-trigger digitisers, the Cyclone scans the model by sweeping its analogue probe in a series of rapid passes, sometimes in excess of 2.5m/min. Users select the data capture technique that best matches the characteristics of the model, choosing from among a series of 2D traces along the model profile, or a 3D digitizing pattern comprised of parallel or radial moves.

Users also select the format to which the data output is structured. Choices here include outputting an NC part program customised to the user-selected machine tool controller brand, or a CAD/CAM-compatible data file formatted to DXF, VDA, ASCII or IGES. Virtual models in one of the CAD/CAM formats can then be modified directly in the computer using CAM software, with toolpaths generated to match the revised part. The Tracecut software supplied by Renishaw that controls the Cyclone can do this job, as can third-party software like the CAMAX system Duplicraft uses.

Faurot says that Duplicraft's design and engineering applications for the Cyclone came later. The company originally purchased the unit because it was an efficient means of toolpath generation that was less expensive than laser digitising.

"Before the Cyclone let us digitise efficiently, our toolpaths came at the discretion of whomever measured the model. Even skilled technicians interpret fine radius gauge measurements differently, but when those measurements went into the CAM system, the range of interpretation became the margin of error between the machined cavity and the model it was meant to reproduce."

Now, says Faurot, Duplicraft has effectively eliminated this discrepancy. "With the Cyclone, what you see is what you get."

DNC link

Digitising on the Cyclone is now a standard part of the mould making process for any design that doesn't come from the customer as a CAD file. Though the unit can send data directly to an individual CNC, Duplicraft's Cyclone shares the company's DNC network with 16 CNC machines, generating surface data models that can be used to program any of them via workstations running CAMAX software.

Once the part is set up, says Faurot, the Cyclone runs unattended. Digitising a typical part or model takes between one and two hours, up to a maximum of six hours for the most complex bottle designs. Says Faurot, "The unit lets you achieve as much speed as the design will allow, by choosing the 'pick' between surface coordinate measurements to match the surface complexity." For example, a recent model for a hand-held spray bottle included a series of 0.38mm radius ribs, which could only be accurately captured by digitizing at 0.1mm pick. At the other extreme, Faurot cites a model for a motor oil bottle, whose smooth and unbroken surface permitted a pick of 2.5mm. The Cyclone's TRACECUT software also lets users define multiple meshes for a single model, with each mesh defined by a different combination of pitch and stepover. This lets users establish high data density only where it is needed, which is key when the model combines regions of complex detail with an otherwise smooth surface.

A "chordal tolerance" default parameter lets the Cyclone minimise computer processing time, by reducing the data file to the lowest number of points necessary to define the model. Chordal tolerance dictates how far off of a chord a data point must lie to merit inclusion in the model. The Cyclone's TRACECUT software stitches chords together on-the-fly while the digitising progresses, and uses this parameter to determine which data points are redundant and therefore unnecessary. Setting the chordal tolerance low and pick high, say, 0.05mm and 1.3mm, respectively, lets the Cyclone capture the fine detail of a model's complex areas, without wasting samples or data points on the areas that are smooth.

These are software features, but Faurot says Duplicraft takes advantage of a special hardware feature of the Cyclone, too. Its stylus pressure is so low, he says, it lets the company digitise not just bottle models, but actual bottles, too.

Reverse engineering

The ability to digitise plastic bottles makes it easy for Duplicraft to serve customers wanting to optimise or modify designs for bottles that are already on store shelves.

"We can make most changes ourselves without the customer having to reinvent the design, whether in CAD or in model form," Faurot says. "After digitising the bottle, we can make localised modifications to distinct features, or global changes like scaling and mirroring. We can even create an entire family of moulds for different-sized bottles using the data from just one bottle 'grandfather.' "From there," he says, "the new designs can go to FEA, or straight to toolpath generation and machining."

Duplicraft also uses the Cyclone to evaluate proposed rebuilds of existing moulds. "It's a quick way to see what's possible with a mould that may have been out of service for three or four years," Faurot says. "Maybe the customer wants to change the handle angle, or open up the cavity to increase the bottle's volume. Based on the digitised model of the old cavity, we can show where inserts will be needed, and massage the new design in CAMAX until we get a cavity that's metalsafe."

Unique opportunities

"Machining, rebuilding, reverse engineering, product evaluation - we are still finding new ways to turn digitising on the Cyclone into a service for our customers," Faurot says.

Sometimes, unique opportunities present themselves. Faurot recounts one example, involving a customer who had to go overseas to give a presentation related to a line of bottles that didn't yet exist. The customer gave Duplicraft a rough sketch of his idea for one representative of the line, but needed something more formal for his speech, and needed it soon.

Creating the bottle in CAD would have taken too long, says Faurot. Instead, a Duplicraft patternmaker quickly created a male model from the sketch. After this was digitised on the Cyclone, engineers scaled and modified the resulting computer model to produce the other members of the line.

"From one hand sketch, we were able to give the customer precise 2D article drawings detailing the entire line," says Faurot. He adds that Duplicraft got the order to begin machining those moulds soon after the customer returned from overseas.