A mold with a view

Ever wish you could see inside a running mold? Plastics educators vault through a window of opportunity with a unique tool.

Resin flows quickly into the cavities until they’re filled. The material shrinks, pulling away from the cavity walls, and the part drops out of the mold.

We all know this happens, but what if you could see it happen? What if you could watch the material flow, observe its transformation from light to dark as it cools and shrinks, and even trace the path of a single colored pellet introduced at the gate?

This opportunity was recently afforded IMM at the MGS Technical Center in Germantown, WI. John Klees, John Bozzelli, and MGS Mfg. Group collaborated to create a mold with a glass window to provide a glimpse into the complex world of plastic behavior.

A Joint Adventure
Bozzelli, principal of Injection Molding Solutions (Midland, MI) and proponent of scientific molding, has pursued a mold with visible resin flow for longer than some processors have been alive. After two failed attempts to secure the illusive tool, in 1991 he asked Klees, president of John Klees Enterprise Inc. (Candler, NC) and a seasoned tool design engineer and consultant, to design it. Both Bozzelli and Klees teach seminars covering various aspects of the injection molding process, often in collaboration, and they anticipated tremendous educational advances through the use of the mold.

The third party in this project, MGS Mfg. Group (Germantown, WI), has played host to numerous Bozzelli/Klees seminars over the past six years. In the spirit of cooperation and mutual benefit, MGS provided a toolmaker, the man-hours, and the materials to build the mold.

Without a customer to foot the bill, which MGS VP-engineering and project leader John Hahn estimates as exceeding $200,000 for the company alone, the project had to be fit around paying jobs. MGS designer/engineer Eric Anderson and toolmaker Brian Hering worked on it as time allowed over the past three years.

In total, MGS invested more than 1100 hours developing the idea into a working tool. It also purchased high-speed digital camera equipment, capable of up to 1000 frames/sec, at a cost of more than $30,000 to record the trials frame by frame. (To enable better viewing inside the tool, a light on a potentiometer was built into the mold.)

Other contributions came from Bozzelli, who provided the pressure transducers from RJG that were installed behind the ejector pins, and D-M-E Co., which donated the hot sprue bushing. Finally, in June 2002, the years of work came to fruition.

“I wanted to get the camera as close as possible to the mold window to capture the clearest image,” says MGS marketing director John Berg, “so we cut a lens-sized hole in the press’s safety window to mount the camera. Before we ran the mold for the first time, I looked around and all the toolmakers and process engineers had taken safe refuge behind a neighboring machine. They didn’t know what would happen with those pressures behind glass.”

Solid Construction
They needn’t have worried. The glass window, which is separated from its steel housing by two Santoprene TPE O-rings and a polycarbonate spacer, is 4 inches thick. The substrate is fused silica produced by the advanced product department of Corning Glass Works, reportedly the same material as that in space shuttle windows. Klees and Bozzelli bought two of these windows for $3500.

Another remarkable feature of this tool is the orientation of the part and, as a result, the way the mold opens to release the part. Rather than a perpendicular positioning to the nozzle, the cavity continues in the same line as resin flow. As the clamp retracts at the end of the cycle, the front and back of the tool release in a perpendicular direction to the platen movement. So, if you were facing the glass window, the side with the window would move toward you while the back of the mold, which contains the automatic ejector mechanism, moved away from you (see rendering above).

But while the construction itself is exceptional, its significance for plastics education is even more important, as demonstrated in the following trials.

Glass Mold in Action
While IMM watched, Klees and Bozzelli ran tests with the two cavity/core inserts that have been built so far, each part roughly the size of a business card—about 2 by 3 inches. A third insert is yet to be built. In the first run, a four-cavity insert was filled with clear acrylic in a 3.38g shot on a Mitsubishi 90MGII-2.5 90-ton molding machine. Bozzelli set the machine’s parameters, starting slowly at 2500 psi.

Or so he thought. After warming up for a few minutes, he realized the cavity pressure had climbed to 7000 psi. This was not outside the testing range; theoretically, the mold could run up to 20,000 psi. But in an earlier test at 24,000 psi, the window cracked near the gate. Although well below that level, 7000 psi was enough to make him nervous, so he backed it down to 2500 psi.

Hering, the toolmaker, was drafted to perform a cavity change. In the second trial, a cavity was used that allowed for multiple gating options simply by rotating two gate inserts. Bozzelli placed an orange color pellet in the nozzle, and we watched as the orange viscous material in the middle of the nozzle moved through the part’s center, insulated from the cavity walls by the less-viscous, white material.

(Regular readers of IMM will remember this concept of melt imbalance and its reported solution through the MeltFlipper, August 2001 IMM, and April 1998 IMM)

Proof of another well-known scientific principle brought great excitement to Bozzelli and Klees: laminar flow. In a previous trial, they introduced a wet pellet into the shot at the nozzle. Because resin flow is not turbulent, but laminar, the wet pellet did not mix and melt.

Arguably, the flashiest show was the white high-impact polystyrene’s (HIPS) visible shrinkage as the part cooled. For the first time, one could see how fast the HIPS pulled away from the mold surfaces. Portions of the part in contact with the mold surface were white, and portions that had pulled away from the mold surface looked dark.

Educational Implications
U.S. institutions and educators in the plastics field had a few tools available to them prior to this point, but none that showed flow under high injection speed and packing pressures. A 1974 film Klees has used in countless seminars exhibits plastic flow out of a melt rheometer into a glass mold secured on the rheometer. Another film from the 1960s shows melt stresses and plastics’ movement around ejector pins and sharp corners through the use of a plunger press.

These films show mold fill in several minutes, whereas the glass-window mold records filling in .1 second. The sequence captured by the high-speed camera can then be played back in slow motion.

Closer to what Klees, Bozzelli, and MGS have achieved are two glass-window molds built within the last decade by the IKV Institute of Plastics Processing in Aachen, Germany. These have been used to illustrate water-assist technology and other phenomena related to plastics injection.

However, with the digital movie taken at the recent mold trials, Bozzelli and Klees can now show actual plastic behavior under injection pressures common to the plant floor.

“We can see the flow and orientation of the polymer, air entrapment, how it shrinks from the wall, how weldlines and knitlines are formed, how moisture in the resin forms splay, how an unmolded particle is transferred if there’s no gate freeze . . . .” Klees could have continued the list, but his point is clear: This hidden world has now become visible.

And what does MGS get out of the deal?

“It’s a good educational tool for customers and employees,” says Hahn. Obviously, this is not the only benefit, he admits. MGS receives exposure, and that expensive camera equipment can also be used to check part ejection on other machines. However, the best benefit is when Bozzelli and Klees hold their seminars at the MGS technical centers; many of the attendees are from OEMs—potential customers.

In truth, everyone wins—especially the green students eager to learn about plastics.