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September 1, 2003

4 Min Read
Smoothing the surface finish of mircrofoamed parts

Adding the gas counterpressure process to MuCell molding appears to dramatically improve part surface finish and raise impact resistance as well.

Figure 1. The difference in the surfaces of the MuCell samples molded with (top) and without (bottom) gas counterpressure is even more dramatic than what you can see in this photo. The gas version (top) appears to be a white material with a clear coating, yet is only one shot of PC. The “look” may offer interesting design possibilities.

Figure 2. The difference in surface roughness is not just appearance, as a scanning microscope reveals.

Figure 3. Notched impact resistance of PC test strips made by MuCell foaming or with gas counterpressure is considerably greater than a PC strip that is not foamed.

Since MuCell’s introduction by Trexel Inc. (Woburn, MA), one of the most frequently heard concerns has been the surface quality of the parts. As small as MuCell gas voids are, they are not small enough to permit a smooth surface. That has not stopped a growing number of applications in nonappearance parts, and now a process developed in Germany and Austria could make surface quality a nonissue.

At machinery maker Engel’s recent technology symposium in Austria, an injection machine equipped with MuCell technology was molding polycarbonate tensile test bars with a very high-quality surface—glassy smooth, in fact. The gas counterpressure technology, known as GGD for its German initials, has been developed by the Institut für Werkstofftechnik (materials technology) at the University of Kassel, Germany, working cooperatively with Engel.

The visual difference in the surface quality is striking (see Figure 1). The test bars molded without gas counterpressure have the surface appearance normally associated with MuCell—reasonably smooth to the touch, but not to the eye, and certainly not to the microscope. The gas-assisted parts are very smooth and glossy (see Figure 2).Gas as Control Mechanism

We were cautioned that the process is still in development and so far only polycarbonate has been tested. However, results to date have been positive enough that the Institute will be testing other materials and variations. It should be noted that the innovation here is not the gas counterpressure itself. That has been around for some time. The breakthrough is in using it with MuCell technology.

The process sequence begins with nitrogen gas injected into the mold under precisely controlled pressure prior to the injection of any material. The gas enters through a port located at the farthest point from the material gate and builds a pressure pad. Material is then injected against that pad. The pressure inside the mold keeps the gas in solution on the flow front. That prevents the creation of surface swirls caused by dissolved gas between melt and mold without the gas counterpressure process.

The gas maintains control by pressure as it is pushed back by the melt. It can exit at either end of the cavity through film gates that block any outflow of material. The resulting surface of the molded test bars is visually smooth. It is also smooth under the microscope. Surface roughness without the gas pressure process is 23.11 mm (Rz); with the gas counterpressure, surface roughness drops to .85 mm (Rz).

The Engel machine that was molding the bars during the company’s technical symposium was a 120-tonner equipped with the MuCell system. A special gas counterpressure unit from Engel supplied the gas pressure. Gas recovery is also possible. The material was an unreinforced grade of Bayer’s Makrolon PC. The two-cavity mold was running in 53-second cycles.

While we concentrated on the high-quality surface finish, we almost missed another significant result of the Institute’s study. The notched impact resistance of PC test strips made either by normal MuCell foaming or with the addition of gas counterpressure was considerably greater than a nonfoamed version of the same PC.

Contact information
Institut für Werkstofftechnik
Kassel, Germany
Hendrik Kirschling
+49 (561) 804 3674
[email protected]e

Engel Austria GmbH
Schwertberg, Austria
Peter Egger; +43 (7262) 620 4184
www.engel.at; [email protected]

imm_10th_anniv_logo50x50tra.gifProblem solving, electrics, and moldfilling

Editor’s note: IMM’s Troubleshooter Bob Hatch shares his thoughts on processing trends since 1993.

My area of troubleshooting molding problems has pretty much stayed the same over the last 10 years. It’s still difficult for molders to get the nozzles, sprues, runners, gates, and vents right. On the other hand, we have been and still are in a transition period. Toolmakers and molders are closing their doors at an alarming rate. They blame China, but it’s just our collective lack of efficiencies and failure to get molds delivered more quickly that is pulling the rug out from under us.

We are going to electric molding machines at a much faster rate than I anticipated, but for good reason. The electrics are very repeatable, clean, and the prices are becoming more in line with hydraulics. I also see more reliance on moldfilling results than ever before. I think that every mold built in this country should be required to have a moldfilling study done on it prior to the build, like in Australia.

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