In blown film, clarity is often considered synonymous with quality. Haze is a defect. Haze can be caused either by melt fracture on the film’s surface or—in coextrusions—by instability inside the film where layers meet. The solutions to the two problems are very different, so confusing them only compounds the difficulty.
Melt fracture in its mildest form reduces surface gloss, though the roughness itself may not be visible to the naked eye. More severe cases show clearly visible transverse-direction zigzag lines. Interfacial instability occurs between layers of coex film and assumes short-wave or long-wave patterns. Short waves are like melt fracture but occur inside the film and may appear as uniform haze or as visible waves. Long waves appear as clearly defined TD striations (see illustration).
Interfacial instability is often confused with melt fracture in transparent film. But a simple test with a glass of water and a paper clip can easily distinguish the two. Put a film sample in the glass of water, using a paper clip to weigh the sample down. Melt fracture, which is on the surface, disappears in water (see photo). Water cannot affect the bumpy interface between layers, so interfacial instability doesn’t disappear in water, and the paper clip, seen through the film, appears blurry.
In blown film, clarity is often considered synonymous with quality. Haze is a defect. Haze can be caused either by melt fracture on the film’s surface or—in coextrusions—by instability inside the film where layers meet. The solutions to the two problems are very different, so confusing them only compounds the difficulty.Melt fracture in its mildest form reduces surface gloss, though the roughness itself may not be visible to the naked eye. More severe cases show clearly visible transverse-direction zigzag lines. Interfacial instability occurs between layers of coex film and assumes short-wave or long-wave patterns. Short waves are like melt fracture but occur inside the film and may appear as uniform haze or as visible waves. Long waves appear as clearly defined TD striations (see illustration).Interfacial instability is often confused with melt fracture in transparent film. But a simple test with a glass of water and a paper clip can easily distinguish the two. Put a film sample in the glass of water, using a paper clip to weigh the sample down. Melt fracture, which is on the surface, disappears in water (see photo). Water cannot affect the bumpy interface between layers, so interfacial instability doesn’t disappear in water, and the paper clip, seen through the film, appears blurry.
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