The Institute for Machine Elements, Design and Manufacturing in Freiberg developed a compression demolding test rig, which enables the measurement of adhesion and friction effects over the demolding path [1]. Since 2017 the test rig has been extended and automated for 24/7 usage. The resulting test rig uses an UR5 robot and several interfaces for an automated testing process.

An additional adaption of the compression demolding test rig allows the measurement of the current mold fouling for each demolding cycle using a graphical approach: After every cycle, a borescope is placed in the oven. The light is introduced via an integrated halogen lamp and a camera attached to the borescope records the contamination of the mold.

Over 50 different recipes were examined for their demolding characteristics and mold fouling. In order to be able to analyze the recorded mold fouling images based on characteristic values, the original files are processed further with Matlab. For this, the Estimator Registration App is used and the resulting images get color-segmented. The next processing step is the adjustment of the RGB thresholder values. Finally, the images are compared with each other in order to determine the additional, overlapping and disappearing deposits (Fig. 1).

During the vulcanization process, some recipe components such as zinc salts, oils and waxes as well as sulfur migrate to the mold surface. [2] The zinc sulfides (ZnS) that are formed in the vulcanization process are viewed as an unwanted by-product, which are often cited as the cause of mold fouling. [3, 4] The experiments showed that an increasing mold fouling does not necessarily correlate with increasing demolding forces. Furthermore, a recipe does not need to contain Zinc oxid (ZnO) to show mold fouling effects. The analysis in figure 1 is the result of 25 demolding cycles with a recipe without ZnS. After the tenth cycle the deposits disappear.

References

[1] R., Nepp Dissertation Experimentelle und theoretische Adhäsionsanalysen an Reifen-Profilklötzen und O-Ring-Dichtungen, 2018.

[2] C., Fraser.; J. Hoover Patent Methode for reducing mold fouling, 2000.

[3] B., van Baarle Rubber World Mold fouling during rubber vulcanization, 2001.

[4] B., van Baarle Rubber World Mold fouling during rubber vulcanization, 2004.