How to judge the quality of foam glue by the cross section after the foam glue is cured

 

Key Observation Point: Cell Structure
Cell Size and Uniformity:

High-Quality: Cells are small and uniformly distributed across the entire cross-section. They resemble a very fine, uniform honeycomb structure.

Poor-Quality:

Coarse and Uneven Cells: Excessively large pores are present, or cell sizes vary significantly (a mix of large and small). This indicates an unstable foaming reaction, excessive gas release, or uneven mixing (e.g., poorly mixed gun foam, expired raw materials, or low temperatures).

Obvious Voids or Tunnels: Visible large voids or tunnel-like defects are present across the cross-section. This is typically caused by localized foaming failure, excessive gas release, improper substrate surface preparation (oil, dust, excessive dryness or moisture), or improper application techniques (e.g., rapid nozzle movement, incorrect angle, or excessively thick and rapid filling).

Local Collapse or Depression: Significant collapse of the surface or interior, disrupting the cell structure.

Cell Shape:

High-Quality: Cells are regular polygons (usually pentagons or hexagons) with thin, intact cell walls. This indicates structural stability and good strength.

Poor-Quality: Cells are irregular, elongated, collapsed, or cracked. This indicates insufficient cell wall strength and can deform or break due to stress (such as internal gas pressure changes, external pressure, or excessive shrinkage) during the foaming process or after curing.

Cell Wall Integrity (Closed Porosity):

High-Quality (High Closed Porosity): The majority of cells are closed, with intact cell walls. A high closed porosity indicates excellent thermal insulation, low water absorption, and good mechanical strength (especially compressive strength).

Poor-Quality (Low Closed Porosity/High Open Porosity): A large number of cell walls are broken or missing, forming interconnected open channels. This significantly reduces thermal insulation (increasing heat loss through air convection), increases water absorption (prone to water absorption and weight gain, reducing thermal insulation, and potentially causing freeze-thaw damage), and reduces mechanical strength.

Density and Density Distribution:

High-quality characteristics: The entire cross-section appears uniform and dense (relative to its nominal density). Cell size and density should be roughly consistent across different areas. Density should meet the product’s nominal value (which can be verified by weighing and calculating the bulk density).

Poor-quality characteristics:

Overall looseness: Excessively large cells, thin walls, or a high open porosity result in a very loose overall appearance and a significantly lower density. This leads to poor strength and thermal insulation.

Uneven density distribution: A noticeable “layering” or “gradient” pattern appears across the cross-section. For example:

High density (fine cells) near the substrate or bottom, and low density (coarse cells) upwards or in the center. This is usually caused by gravity causing uncured material to flow during foaming, or by a curing speed that is too fast or too slow.

Abnormal density (too high or too low) in a localized area.

Color and Impurities:

High-quality characteristics: Uniform color, typically milky white, light yellow, or gray (depending on the product type), with a clean interior free of visible impurities.

Signs of Poor Quality:

Uneven Color: Dark streaks, spots, or areas appear (may indicate localized uneven mixing, incomplete curing, contamination, or overheating).

Visible Impurities: Dust, particles, unmixed raw material lumps, or other foreign matter. This is usually caused by impure raw materials, unclean production conditions, impurities introduced during application, or improperly cleaned guns.