SMART PRODUCTION OF MOLDS

Based in Hertfordshire, just north of London, England, BK Tooling is a thriving sub-contractor specializing in the design and production of mold tools and prototype/low volume plastic injection moldings. The company has developed a 'smart' production method that covers all aspects of tool design and manufacture, and is applied to every job, regardless of complexity.

Understanding that the fewer times a part is moved, the more accurately it can be made, the shop has formulated a standardized approach to tool manufacture, including the use of Renishaw OMP probes, a System 3R Refix table and tooling plus a bespoke pallet system for electrodes and inserts.

"?Right first time? is a quote often given, but rarely true" claims Bob Tunks, owner of BK Tooling. "However, our strike rate is above 99 percent and that's down to our production methods, quality levels and continual investment in the latest technology?

Having originally started with manual Bridgeports and XYZ Protrak hybrid mills, BK Tooling has installed an XYZ 1060 high speed machining center to cope with increasing demands. The 1060 HS has a 1020 mm (X) by 610 mm (Y) by 620 mm (Z) machining envelope and is equipped with a Siemens 840D ShopMill control that looks ahead to convert a multitude of straight lines into a smooth curve.

"The machine is capable of running a 6 mm cutter at 2,500 mm/min using a 0.1 mm step-over across parting faces, and the two halves of a tool fit perfectly together. Much of the mold tool work involves the high-speed machining of complex 3D data and inaccuracies of just 20 microns would result in part flashing (plastic flowing into the wrong places) and ultimately part failure" explains Tunks.

Important investments made by the company are not only machine tools but also the latest CAD/CAM technology. BK is a heavy user of CAD/CAM and has been using VISI products from Vero Software (Cheltenham, England) for over 10 years. However, it is the use of VISI Flow that has recently proved to be hugely beneficial during a project for the rear passenger arm rest for a prestige automotive supplier renowned for comfort and style.

The part material was a polycarbonate (PC) and acrylonitrile butadiene styrene polymer (ABS) blend that combines a unique set of performances commonly required for the automotive industry, including easy flow, dimensional stability, low shrinkage and heat resistance.

The client CAD data was provided as a STEP file and imported directly into VISI Modelling as a single solid body. After the model had been translated from car position into line of draw, the split line and core and cavity surface sets were created using VISI Analysis.

"Before starting the tool design," notes Tunks, "I decided to run a plastic filling simulation, as the component has a thick and thin end, so I was a little nervous about filling from one gate position. I was surprised to see how well it filled ? in fact, much better than I expected. Most importantly, however, the system highlighted areas where sink marks would occur due to internal voids. Historically, the predictive results from VISI Flow have accurately matched the actual molding conditions so after further analysis, the tool design was modified to core out the problem areas."

The 3D tool design was fully complete within two weeks based around a Meusburger die set and standard component library. "The entire mold was designed, cut and built within five weeks" continues Tunks. "Our 'smart' production method is about project simplicity and standardisation. The 1060 HS mill has a 24-tool carousel, but our complete tooling library is only 100 cutters."

"The advantage is that we always know the neck length for each tool and we always have spares available," he adds. "Mold machining is all about toolpath quality and not necessarily about removing a few seconds from the overall cycle time. As an example, the core plate alone contains over 25 hours of machining, so it is imperative that we produce reliable toolpaths and VISI has never let us down."

TOOL MODIFICATION
After the first batch of prototype components had been molded, a stability issue was detected during part testing. Under normal stress loading the part was flexing slightly so modifications were made to the model; extending the rib length to increase part rigidity. It was initially anticipated that the extra rib depth could simply be spark eroded into the original core block, but further VISI Flow tests on the model revision showed the presence of gas traps.

"The depth of the ribs caused concerns about gas traps and potential ejection issues" recalls Tunks. "We opted for floating inserts that are fixed to the core plate using stripper bolts. This allows the inserts to move freely for approximately 2 mm during the ejection process. The inserts will move away with the part and immediately break the contraction grip and reduce the possibility of surface deterioration through dragging up the wall. This is particularly important when using materials with fillers (glass filled) as they tend to have a reduced shrinkage value."

Concluding, Tunks explains that BK Tooling has found its niche position as a supplier of prototype/low volume molds before companies inevitably move to lower cost economies for higher volume moldings.

"We often deal with complex products and without the use of intelligent software, we could never guarantee project completion and the supply of batch moldings within such short time scales,? he says. ?Vero is highly regarded in our organization and has become an integral part of our operations. Their industry knowledge translates into an excellent product with unrivalled customer support."

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B K Tooling, Pleasant View, Gaston Green, Bishop?s Stortford, Herts CM22 7QS United Kingdom, 01279 600 970, Mobile: 07778 857171, [email protected], www.bktooling.co.uk.

Vero UK Limited, +44 (0) 1242 542040, Fax: +44 (0) 1242 542099, Marc Freebrey at [email protected], www.vero-software.com.