Precision Lens Molding of Glass: A Process Perspective by Jayson J. Nelson

Author:Jayson J. Nelson
Language: eng
Format: epub
ISBN: 9789811542381
Publisher: Springer Singapore

4.3 Release Coatings

4.3.1 Release Coatings—General Information

A partial list of necessary attributes for release coatings includes:Good adhesion to the underlying material

Resist interaction with the heated glass (oxidation, leeching, etc.)

Low friction between surface and heated glass

Low wear from sliding forces

Sufficient hardness to resist scratches

Good resistance to both compressive and shear forces

Good resistance to degradation due to thermal cycling.

Release coatings for molded glass articles are typically applied to the mold tool surface; however, in certain cases they may be applied to the glass preform, or to both the molding surface and the preform. The properties of release coatings may be similar to those of protective coatings, but this is not always true. For example, graphite may be a good choice for both protecting the molding tool surface and as a release agent for many oxide glasses, but glasses having a high lead content have been known to interact with the graphite and actually leech lead to the glass surface during molding to create a lead rich region that may not be acceptable for some applications. The fact that tool coatings are a necessity has been shown in literature for both inert and vacuum environments, and careful control of the molding environment will provide an effective means for preventing the oxidation of surfaces and suppressing volatile elements from diffusing from the glass and onto the mold tool surface [2].

Coatings should not appreciably increase surface roughness of the molding surface or else mechanical adhesion will become a problem. As we discuss mechanisms that promote the adhesion and release of glass to substrates and surfaces, we assume that surfaces are free from micron or larger scale surface defects that provide a mechanical means of adhesion. Surfaces must also be produced with optical quality smoothness, since rougher surfaces will enable the softened glass to gain a mechanical grab in any apparent or latent defects. When molding processes are designed for low glass viscosity at pressing, the molding pressure forces glass into voids and surface asperities, which promotes adhesion, often with catastrophic results. This condition can be ameliorated by pressing the glass at a lower temperature (higher viscosity), but this is not always possible and may in some cases lead to premature tool failure. For best results, surfaces should be polished with a maximum roughness of 2 nm RMS prior to coating.

One predictor of surface reactivity is a measure of their surface energy. This property is often determined by placing a small amount of liquid (water is commonly used) on the surface and measuring the contact angle of the droplet. The liquid is said to “wet” the surface interface when low contact angles are measured, and wetting is evidence of low surface tension between materials, low interfacial stress, and low adhesion. High energy surfaces normally form low energy interfaces with water, while low energy surfaces may be evidenced by high energy interfaces with water.

Caution should be exercised when characterizing molding surfaces with surface tension measurements, especially when they are made at temperatures much lower than those experienced during molding.

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