(Munich, September 2nd, 2015) Process stability is a concept that embodies the idea of uniform quality with minimal scrap. The new adaptive process control (APC) machine function from KraussMaffei provides this stability even though with continuous changes. The customer KH Foliotec utilizes APC in the production of premium quality interior components with in-mold labeling (IML).
At first glance, it seems like an injection molding process should simply function with exceptional reliability and stability once optimal machine parameters have been found and no changes should be required from that point on. But a multitude of factors affect the quality of the parts produced. Temperatures and movement affect molds and machines. Material can be affected by anything from fluctuations in viscosity between individual batches to different recycled material admixtures to the moisture content of the pellets. Interruptions to the production cycle have a particularly serious effect. In the event of an interruption, the melt stays in the machine longer than usual, normally decreasing in viscosity as a result. The configured machine parameters are no longer correct because it is easier to fill—and ultimately overfill—the cavity. This results in excessive part weight and deviations in dimensional stability.
The APC machine function from KraussMaffei evens out these fluctuations in the injection molding process by adapting and continuously modifying the changeover point between pressing and holding pressure for each individual shot by utilizing online measurement of the melt viscosity.
Optimum product quality after restarting
KH Foliotec GmbH is headquartered in Sparneck in the German region of Upper Franconia and manufacturers high-quality surface parts for the automotive industry using the in-mold labeling process and has retrofitted all the necessary KraussMaffei machines with APC. APC can be installed quickly and easily without impairing production in any way. A KraussMaffei CXZ 500-2000/750 is now used to create a two-component radio dial panel in a 1+1 injection mold. First the transparent gray PC component for the display is injected. Then the mold rotates and black PC-ABS for the case body follows in the same cavity in the second step while the other cavity is already being filled for the next display. This all takes place on the back of a PC film with scratch-resistant coating and decorated in black that is inserted into the mold. Its glass-like surface makes it necessary to clean the mold several times per shift in order to remove particulates. These interruptions in the production process, and the accompanying need to compensate for fluctuations, provide an ideal application space for APC for achieving perfect product quality again immediately after restarting the process. Compared to operation using a fixed changeover point, standard deviations in molded part masses (over a 24 hour run time) have been reduced from 0.16 grams to 0.03 grams with APC enabled. According to Daniel Müller, Divisional Manager for the Molding Department at KH Foliotec, it also significantly reduces the rejection rate in series production.
He also points out another noteworthy aspect of IML production that the process temperature control system has brought to light. If the company ran the mold at an operating temperature of 60 degrees and inserted a piece of film, the film would act as an insulator, meaning the back of the film would be colder. If the hot plastic hit that location, it would harden faster than desired, which could increase the tendency of the finished plastic part to warp. One option for addressing this problem would be to increase the mold temperature. But this is best avoided in IML applications, because the more the film is heated in the mold, the easier it is for the screen-printed color coating applied on the back to run. In the injection area, this can cause the incoming plastic to wash off a portion of color, leading to circular spots. APC makes it possible to control the part's tendency to warp without having to increase the mold temperature.
Significant reduction in scrap rates
Due to the complex value-adding chain, investing in APC truly pays for itself in IML production. Screen printing as well as forming and punching combined together each constitute approximately 25 percent of the value-adding portion of IML parts. The typical scrap rates for both production steps is in the low single-digit range. The subsequent injection molding process provides the other 50 percent of added value, but also entails the highest risks for the complete part. The scrap rate for black, high-gloss visual parts can reach up to 30 percent. For every defective IML component—some of which can be worth more than ten euros a piece—this means the value added from imprinting and forming in earlier steps is also ruined As a result, manufacturers of premium quality IML components have a keen interest in maintaining the stability of their injection molding process.
APC does not replace the mold setter on a machine; instead it provides the setter with valuable support. Sebastian Huss, Process Optimization Officer at KH Foliotec explains, "APC helps to reduce the sources of errors and to achieve better part quality as a result."
More about KH Foliotec
KH Foliotec is headquartered in Sparneck in northern Bavaria and is an in-mold labeling specialist. Employing approximately 350 people, the company manufactures high-quality components for automotive interiors. The company is part of KH Group, which has 1700 employees in five production locations spread over Germany, the Czech Republic, China and Mexico.