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Blow molding
process
Gas tanks
are typically produced by a two-stage blow molding process. In the first
stage the parison, a cylindrical tube of polymer, is extruded. Gravity
and die head motion shape the parison thickness profile, which is
usually nonuniform. After extrusion, the parison is positioned between
the two halves of a mold. Then, in the second stage, the parison is
closed by the two mold halves and air is pumped in to expand it to fit
the shape of the mold. When the two halves of the mold are opened, a
completely formed tank is removed that has the precise shape of the
mold.
It’s critical to try out the mold, accurately measure the geometry of
the tanks that it produces, and make corrections when needed to the mold
to ensure that the final tank geometry matches the customer’s design.
CMM and laser
scanning inspection methods
In the past,
first article inspection was done with a CMM. The operator moved the
contact probe around the surface of the tank and the machine recorded
the position of each point. The use of CMMs to inspect parts is
time-consuming because of the need to manually move the machine probe
into position for each individual point to be measured. As the
geometrical complexity of the part increases, the number of points
needed to fully characterize the geometry skyrockets. It takes many
thousands of points to fully define a surface as complicated as a fuel
tank so it could take several days to measure a complex tank. Even if
operators spend several days generating thousands of points, they can
never be sure that they haven’t missed a critical feature. It’s always
possible to miss a point that would have otherwise called out a
difference between the original computer aided design (CAD) model and
the prototype.
Laser scanning
systems work by projecting a line of laser light onto surfaces while
cameras continuously triangulate the changing distance and profile of
the laser line as it sweeps along, enabling the object to be accurately
replicated. The laser probe computer translates the video image of the
line into 3D coordinates, providing real-time data renderings that give
the operator immediate feedback on areas that might have been missed.
Laser scanners are able to quickly measure large parts while generating
far greater numbers of data points than probes without the need for
templates or fixtures. Since there is no contact tip on a laser scanner
that must physically touch the object, the problems of depressing soft
objects, measuring small details, and capturing complex free form
surfaces are eliminated.
Instead of
collecting points one by one, the laser scanner picks up tens of
thousands of points every second. This means that reverse engineering of
the most complicated parts can often be accomplished in an hour or two.
The software provided with the scanner greatly simplifies the process of
moving from point cloud to CAD model, making it possible in minimal time
to generate a CAD Model of the scanned part that faithfully duplicates
the original part. Special software can be used to compare original
design geometry to the actual physical part, generating an overall
graduated color error plot that shows at a glance where and by how much
surfaces deviate from the original design. This goes far beyond the
dimensional checks that can be performed with touch probes on CMMs.
Working with a
service bureau
“We considered
purchasing a laser scanner but decided that both the initial investment
and the cost of maintaining a trained operator on staff would be
difficult to justify, considering our relatively small volume of work,”
said a Validation Coordinator for the manufacturer. “Then we heard that
GKS Inspection Services was operating a service bureau in the Detroit
area that provides laser scanning services on a fast-turnaround basis at
an economical cost. This seemed like a good opportunity to evaluate
laser scanning without having to make a major investment.
The next time we
produced a first article we sent it to GKS. They scanned the tank and
then processed the resulting point cloud to provide a surface model of
the as-built part. They sent the model back to us along with free viewer
software. We were very impressed with how the model represented the
complete geometry of the part rather than just the points that the
operator had decided to measure. It took less than a week to get the
model to us and since then GKS has demonstrated the ability to provide
even faster turnaround when necessary.”
As a next step,
the company sent the CAD model representing the design intent to GKS.
GKS then used Geomagic Qualify software to superimpose the CAD
and as-built models and call out the differences between them. The error
plot highlighted several areas in the initial prototype that were too
far from the target dimensions. Being able to view the design intent and
as-built models on top of each other made it easy to determine what
changes to the tooling and cooling fixtures were needed to correct the
parts. Based on the success of the first project, the company now sends
first articles to GKS on a regular basis.
“In the past all
we had was dimensions of different points so it was difficult to
determine exactly what changes needed to be made,” said the Validation
Coordinator. “Often we had to change the tool several times because the
first change did not completely fix the problem. Now we can see the
complete surfaces rather than just a few points. This makes it possible
to visualize the entire extent of the discrepancy. So we can be much
more precise in our tooling changes. We also save money on first article
inspection because the cost of the service bureau is considerably less
than what it cost to do the job internally on a CMM. The experience in
working with GKS has been very good. They have demonstrated their
commitment to meeting our requirements and they are very nice people to
work with.” |
