Laser Scanning Helps Solve Injection Molding Shrinkage Problem in 20 Days

Laser scanning helped solve a problem caused by differential shrinkage in multiple injection molding operations in only 20 days. Four different injection molded parts had to fit together with the right amount of interference in order to successfully perform an ultrasonic welding operation. But because it was so difficult to predict how the geometrically complex parts would shrink in the mold, the interference was too high in some areas and nonexistent in others. It would have taken a week or two to measure the complex curvature of the prototype parts with a coordinate measuring machine (CMM) and it’s doubtful that the accuracy would have been sufficient. Instead the injection molder sent the prototype parts to a laser scanning service bureau called GKS Inspection Services, Minneapolis, Minnesota. In just five days, the service bureau created a surface model of each prototype that was accurate to just 0.001 inch and mapped it onto the original part geometry, making it easy to modify the mold so that the final parts shrank to the correction size. In only nine more days after the error map was received, the tools were modified. It took six more days to mold and weld the sample parts, which performed perfectly. The parts are now being used in the receiver for a hidden fence system. “It would have taken months of expensive trial and error to get these parts right using conventional methods,” said the Quality Assurance Manager for the injection molder. “Laser scanning helped us do it in much less time at a fraction of the cost.” With over three decades of experience, the injection molder offers its customers custom injection molding services, ranging from prototype and market entry molding, to low and high volume manufacturing."
 

Challenge of creating precision interference fit

A leading manufacturer of hidden fence systems is one of the injection molder's customers.  The injection molder produces two parts for the system, the case and the partition. Each of these parts is created by welding together two injection molded plastic components. The greatest challenge in producing these parts is the need to create the very precise interference fit required for ultrasonic welding. High frequency vertical vibrations are used to increase the temperature of the plastic by the absorption of the vibrations, the reflection of vibrations in the connecting area, and the friction of the surfaces of the parts. The special challenge in producing these parts was that the geometry was so complex that they shrank in a very nonuniform and difficult to predict manner. The only way to design the mold was to take a guess at the shrinkage, build the mold and measure the parts to see what really happened. The problem is that their complex 3D geometry made them very difficult to accurately measure.

“As the geometrical complexity of injection molded parts increases, it’s becoming increasingly impractical to use a CMM to measure them,” said the Quality Assurance Manager for the injection molder. “The problem is that a CMM measures one point at a time, but when you are dealing with a 3D contour you often need millions or tens of millions of points to get the geometry exactly right. Fortunately, the technology of laser scanning has emerged in recent years to present 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. They collect thousands of points every second at a higher level of accuracy than is possible with contact methods so they are able to accurately digitize much more complicated parts. The elimination of the need to maintain contact with the workpiece also means that the results are independent of the skill of the operator.

With a relatively small number of parts that need scanning every year, the injection molder's management felt that it didn’t make sense to rush into a purchase of a machine.  So the company looked for a service bureau that could provide the high accuracy and fast turnaround the company needed on nearly every project. “We talked to a number of injection molding companies and they were nearly unanimous in recommending GKS Inspection Services,” the Quality Assurance Manager said.

Correcting the part in a few weeks.

The injection molder shipped the four prototype parts to GKS Inspection Services by overnight courier. From the time they were received, only twenty days were left before the company had committed to deliver working samples. GKS Inspection Services scanned the parts and generated a point cloud in which each individual point was accurate within 8 microns and the surfaces generated from the point cloud were accurate to at least 0.001 inch. The company then converted the point cloud into a surface model and imported it into software that compared the original solid geometry to the as-built parts. This comparison provided color-coding that showed exactly where the part differed from its CAD model. When they sent this file to the injection molder, the company worked together with the mold builder to quickly calculate the exact changes that needed to be made to the molds to bring the parts back into specifications.  When the mold was delivered back to the injection molder, the company immediately went to work, first producing a series of sample parts and then performing the ultrasonic welding operations. The new parts provided just the right interference fit so the entire process went smoothly and the company was able to deliver perfect parts to the customer on the exact day it had promised.

“This project provides an excellent example of how laser scanning can substantially improve the process of inspecting injection molded first articles,” the Quality Assurance Manager concluded. “When CMM was our primary inspection tool, we sometimes spent expensive engineering and production time in a lengthy trial and error process to ensure that we delivered the highest possible quality parts to our customers. With laser scanning, we can deliver even higher quality parts in a fraction of the time."