Colour Variation
The most frequent surface defect. Causes and controls:
Substrate inconsistency. Specify float glass from one supplier, identical thickness and batch. Reject sheets with mould or water staining.
Enamel batch variation. Mix the full enamel quantity for an order in one batch to reduce colour-matching passes. Pigment differences between batches arise from firing stability, particle-size distribution, raw-material changes and viscosity drift. Also verify that the float-glass substrate itself has no batch-related colour shift.
Colour complexity. Limit blends to three colours. Use a single professional-grade glass enamel brand.
Tin-side printing. The tin side of float glass forms an oxide layer that interferes with colour development. Print on the air side only.
Thickness inconsistency. Transmittance shifts with enamel thickness. Maintain identical wet-film thickness across the batch.
Firing and tempering temperature drift. Fire each enamel at its specified sintering temperature and keep furnace profiles stable. Process a single batch through the same tempering furnace to avoid colour shifts caused by temperature variation.
Atmosphere. Sintering requires a weak oxidising atmosphere. A reducing atmosphere triggers redox reactions that cause fading or discolouration.
Coating Uniformity
Roller marks. Regular patterns are normal in roller-coating; scars or grooves indicate a damaged roller that must be replaced.
Mesh marks. In screen printing, check that mesh tension is even and that air pressure is stable without vibration.
Tramlines. Sharp, continuous lines usually indicate a worn squeegee. Grind or replace the blade and inspect the screen for damage.
General unevenness. Unstable squeegee pressure causes inconsistent deposition. Adjust screen snap-off distance. If enamel viscosity is too high and flow is poor, thin the ink appropriately.
Low Opacity
Thin enamel. Increase wet-film thickness or the number of print passes. Control thinner additions.
Low pigment loading. Highly saturated colours such as deep blue often contain insufficient opaque pigment. Even at greater thickness their hiding power remains low. Blend in a small proportion of high-pigment enamel (for example, white) to improve opacity.
Pinholes
Screen blockage. The usual cause. Check mesh count, enamel viscosity, particulate contamination and environmental cleanliness. Clean the screen immediately when clogging appears.
Substrate contamination. Control airborne dust, ensure thorough glass cleaning and exclude insects from the print room.
Excessive print speed. Ink transfer relies on squeegee pressure forcing enamel through the mesh. Too high a speed leaves incomplete transfer. Reduce to the appropriate rate.
Bubbles
Sources include dirty glass, air entrainment during dilution or mixing, overly fast printing and overly fast stirring. Let mixed enamel stand for several minutes before use. If bubbles persist, investigate the enamel formulation or thinner.
Edge Chipping
Occurs when the enamel layer is uneven and edges are too thick. Improve overall thickness uniformity and scrape excess enamel from the edges after drying.
Poor Adhesion
Ensure the glass surface is free of grease and oil. Verify that the enamel reaches its specified firing temperature; under-fired enamel remains uncured and bonds weakly.
Flaking and Peeling
Excessive enamel thickness, excessive firing temperature, or a coefficient of thermal expansion (CTE) mismatch between enamel and glass substrate. Select an enamel whose expansion coefficient is matched to the base glass.