New metal surface treatment for improved elastomer molding.

A common issue in the molding of elastomer compounds is mold fouling. Mold fouling is the inherent tendency of the compound to adhere to the mold surface during curing. The level of mold fouling can be a slight stickiness to a point where actual deposits of compound are left on the mold surface. Mold fouling not only results in reduced production efficiency, but also increased scrap rates. Production is impacted as the parts need to be more carefully removed and the mold periodically cleaned, resulting in lost production time. Scrap is increased from parts tearing in the mold and molded-in defects from the mold deposits.

A variety of approaches are used to improve the mold fouling of elastomer compounds. These techniques include compound internal release aids, external release sprays, mold surface treatments and proper process parameters. The optimum system is usually a combination of all these approaches.

Essentially, all elastomer compounds have at least one internal release aid in the formulation. These internal release aids are usually waxes, fatty acids, phosphates or other low molecular weight chemicals that migrate to the surface during molding and provide a barrier on the mold surface. While effective, adding too much internal release aids can lead to an increase in mold fouling from a build-up of release aid on the mold surface.

External release sprays act similar to internal release aids, but are applied directly onto the mold surface prior to molding the part. They can also be over-applied, resulting in a detrimental effect of mold fouling. External release sprays can be semi-permanent, reducing the frequency of application and potential excessive build-up. Today, most are aqueous solutions with the main ingredient being silicone or they are fluoro-based.

Mold surface treatment helps with abrasive wear from the processing of the materials, provides corrosion resistance from chemical attacks by the various chemical ingredients and by-products from the curing process and provides a smooth, pore-free surface for part quality and reduced mold fouling. Chrome and nickel plating have been widely used. Incorporation of Teflon, silicon carbide and industrial grade diamond in nickel coatings can significantly extend life.

Vamac ethylene acrylic elastomers (AEM) have an inherent tendency to adhere to metals. While this is beneficial for bonding AEM compounds to metal parts, it also results in a greater tendency for mold fouling. Essentially, all Vamac compounds have a recommended internal release aid package consisting of Armeen 18D, stearic acid and Vanfre VAM. Most producers of AEM parts also use an external release spray, along with molds plated with chrome or Teflon impregnated nickel. While this combination results in satisfactory production performance, customers still express a desire for improved mold fouling performance for AEM compounds.

Vickers SA, Switzerland, has developed a surface treatment process called Vickersil especially adapted for metal surfaces in contact with molten plastics and/or elastomers. Vickersil offers significant advantages to the current treatments used in elastomer molding. This new treatment has demonstrated improved wear resistance and reduction of mold fouling with demonstrated success in production. Results are presented demonstrating the improved performance on various processing equipment treated with Vickersil. A mold sticking test was used to show the improved release properties of Vickersil against a non-treated mold surface with a standard Vamac compound.

Experimental

The AEM compound used in the study was a standard Vamac G formulation with the standard release aid package. Compounds were mixed in a 9.2 liters internal mixer and extruded into approximately 3.2 mm diameter rope for testing.

An o-ring sticking test was developed by DuPont Performance Elastomers (ref. 1). All tests were performed per ASTM procedures as applicable.

Results and discussion

Treatment structure

The Vickersil is a thin electrochemical coating applied to a metal surface (steel, copper, brass, bronze; but not aluminum and tungsten carbide) at a thickness of 8 to 15 microns. A micrograph picture of a treated mold surface is shown in figure 1. The morphology of the coating is compact nodules as opposed to plates like conventional chrome plating. The compact nodules contribute to increased corrosion resistance. The hardness of the coating is 1,800 Hv. This is more than twice that of traditional chrome and nickel based platings and half the hardness of diamond. In comparison with hard chrome or nickel phase vapor deposition, the Vickersil is a passive coating avoiding the stickiness of plastics and elastomers. The passivity and increase in hardness contributes to the improved wear performance of Vickersil. Examples of several treatments are shown in figure 2, showing the non-coloration of the treatment.

Performance improvement

The abrasion resistance performance of the treatment was demonstrated on a torpedo in an injection molding process using polyamide with 35% glass content. Figure 3 shows the significant amount of wear after only 15,000 cycles on a hardened steel torpedo without treatment versus virtually no wear on a hardened steel torpedo treated with Vickersil after 600,000 cycles.

Figure 4 shows the molded part surface between a normal hardened steel mold and the mold after treatment. The molded surface from the treated mold is smooth, with no surface defects compared to the untreated mold.

Mold Sticking evaluation

A compression mold was made consisting of side-by-side identical arrays of 60 o-ring cavities, with one set of o-ring cavities treated with the Vickersil, while the other was standard hardened tool steel. The mold is shown in figure 5. A standard Vamac compound was prepared and used in the test. The test consisted of starting with a clean tool and repeatedly molding o-rings with the AEM compound. No mold cleaning or use of external release sprays occurred during the test. A controlled air blast was used after each cycle to remove the o-rings. The number of o-rings remaining after the air blast were counted and used as an indication of the level of mold fouling. The results were plotted as number of stuck o-rings vs. number of cycles.

The results are given in figure 6. The untreated cavities immediately exhibited a high level of sticking (approximately 50%) with no general trend indicated in sticking level changing over time. The treated cavities started with a very low level of o-rings stuck, with an eventual trend of increasing numbers after 11-12 cycles, but still lower than the untreated cavities. It was also noted during the experiment that the flash on the treated side was easier to remove than on the untreated side. These results indicate the ability to go for longer cycles before external release sprays would need to be used and before the mold would need to be cleaned.

Improved productivity

Long term productivity improvement was demonstrated by running a side-by-side comparison of a treated mold vs. a standard hardened steel mold over a one-year time period. A production flourosilicone molded part was used for the comparison. Figure 7 shows the improved productivity resulting in 94,500 additional parts with the Vickersil treated mold, or the equivalent of 31 days of production. The improvement with the Vickersil treatment was obtained through a combination of less downtime for mold cleanings, reduced scrap rates and faster cycles from easier part removal.

Treatment limitations

There are current limitations to parts that can be treated, mainly due to equipment limitations. Most metals can be treated, except aluminum and tungsten carbide. The maximum diameter of screws (extrusion or injection) that can be treated is 65 mm, with L/D ratio of 25. Molds can not be larger than 600 mm x 600 mm and 70 kg in weight. For tool dies, the minimum inside diameter size is 4 mm. There is only one treatment facility located in Cornaux, Switzerland.

Conclusion

A new metal surface treatment has been developed offering improvements in abrasion resistance, part surface quality and mold fouling. All these improvements contribute to improved productivity vs. traditional metal treatments for the processing of elastomeric parts.

References

(1.) S. Bowers, "Advanced polymer architecture peroxide curable fluoroelastomers," KGK Kautschuck Gummi Kunststoffe 55. Jahrgang, Nr 6/2002.