Rather than traditional CMMs, designers of molds for bottles and other beverage containers are turning to laser scanning.
In the past, Uniloy Milacron (Tecumseh, MI, USA; www.uniloy.com), a provider of blow-molding systems and services, found that the point-by-point approach used by coordinate-measuring machines (CMMs) limited the accuracy that could be achieved in reproducing an existing bottle or model. So the company identified a laser-scanning service bureau that can, in as little as a single day, generate the millions of points required to define the complex contours of a modern beverage container.
"While the majority of our customers request that we create new designs, we still occasionally receive requests to duplicate older, non-Uniloy models," says Jim Janeczek, product design engineer for Uniloy Milacron. "Laser scanning has substantially improved the accuracy with which we can duplicate model geometry, and the cost and lead time of this new technology is substantially less than the contact-probe technology that it has replaced."
NEED FOR REVERSE ENGINEERING
The first step in the mold-design process is to define the geometry of the container being produced. The traditional method is to create a 2-D drawing that is then used to create a 3-D wood model. The wood model is extremely complex, with many areas of the model being hand worked to blend the different contoured surfaces together. The wood model is cast in plaster to create a stone, and the stone is digitized to create the tool paths.
Today most blow-mold manufacturers use solid modeling instead of the stone as the master geometry to guide the blow-mold-manufacturing process. But even many of the manufacturers that have switched to solid modeling have discovered that it's sometimes easier to obtain just the right look for an older container by adding and removing material to a stone model than it is by manipulating a solid model on the computer screen. In addition, there are many situations where manufacturers want to produce a new mold for an existing bottle that was created without the use of computer-aided design (CAD). In all of these situations, the ability to convert the physical model to CAD geometry is critical to achieving just the right look in the finished bottle.
The CMMs that Uniloy used in the past did not provide the ideal solution to this problem. A CMM's key advantages include the ability to measure individual points to a high level of accuracy and to move from sample location to location under computer control. But as the blow-mold geometry has grown increasingly complicated, the number of points required for accurate measurements is increasing at an exponential rate. Millions of points are required to accurately model geometrically complicated bottles. The result is that the time needed to capture points one by one has grown to days for complicated parts.
The technology of laser scanning presents a viable alternative. Laser scanners work by projecting a line of laser light onto surfaces, while cameras continuously triangulate the changing distance and profile of the laser as it sweeps along, enabling the object to be accurately replicated. Scanners collect thousands of points every second at a high level of accuracy so they can accurately digitize complicated parts. The elimination of the need to maintain contact with the work piece also means that the results are independent of the skill of the operator.
HOW IT WORKS
The SURVEYOR DS Series 3D Laser Scanning System is used for the Uniloy bottle application. This system scans at more than 75,000 points/s to an accuracy of ±0.0005" (0.012 mm) and can scan a part automatically. The laser probe projects a line of laser light about 2 in. long onto the part and captures coordinates along this line at scanning rates up to 75,000 points/s. The laser probe is moved back and forth and up and down in three-axis motions to capture the shape of the part using the system's scanning motion control software, developed by Laser Design (Minneapolis, MN, USA; www.laserdesign.com). The part is also rotated on the four-axis part-holding fixture that has an integrated rotary stage. This programmable rotation allows for surfaces not viewable from any one angle to be exposed for scanning at a more suitable orientation. These various scanning orientations are all combined into a single coordinate system for easy part reverse engineering.
The point cloud is then processed into a NURBS Surface CAD Model using the bundled Geomagic STUDIO software for a turnkey solution system for reverse engineering. The CAD model is output via IGES or STEP formats into the customer's CAD/CAM system for further modification or manufacturing into a blow mold with no changes.
DIGITIZING A DAIRY CONTAINER
With a relatively small number of parts that need scanning every year, Uniloy Milacron engineers felt that the company did not have to purchase a machine. Instead, they looked for a service bureau that could provide the high accuracy and fast turnaround the company needed on nearly every project. Uniloy Milacron chose GKS Inspection Services (Plymouth, MI, USA; www.gks.com), a division of Laser Design.
The first part that Janeczek sent to GKS Inspection Services was a plastic container for which the customer had asked Uniloy Milacron to build a new mold. The plastic bottle was a 16-oz single-serve dairy container that was too soft to accurately digitize with a CMM. He shipped the bottle to GKS Inspection Services, whose engineers then scanned the part, generating a point cloud with the millions of points needed to accurately define the complex surface geometry of the part.
Each individual point was accurate within 20 µm, and the surfaces generated from the point cloud were accurate to at least 0.004 in. GKS Inspection Services engineers used Raindrop Geomagic software to convert the point cloud to a surface model. They then exported the surfaced model as an IGES file and imported it into CATIA, one of the CAD software packages used by Uniloy Milacron.
Since that initial project, Janeczek has sent about a dozen other stones or containers to GKS Inspection Services for digitizing. "Another example is a 2-l dairy container that was created by a modeler using hand tools," Janeczek says. "The model had many details that were so subtle that you couldn't see them with the naked eye, although you could feel them by running your finger over them. We were able to verify that GKS Inspection Services reproduced each of these details by comparing the CAD model to cross sections of the stone. All in all, the GKS Inspection Services service bureau has met our needs for fast and accurate reverse engineering of existing products and models," Janeczek adds.