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Will Cross Linked Shrink Film also Encounter Crystal Point Problems?

Mar. 03, 2020

As we all know, crystal point is a common problem for Cross Linked Film Manufacturers. For Cross Linked Shrink Film, the crystal point obviously affects the transparent appearance. For plastic bags that require higher printing quality, especially for large-area plastic bag printing, the "white dot" waste products caused by crystal points are a large percentage of the loss in the industrial production process of plastic bags. Therefore, controlling the generation of crystal points and eliminating the causes of crystal points are important process links to improve the technological level of the plastic bag industry.

Cross Linked Printed Film

1. What is the crystal point

The crystal point is actually an "excess polymer", that is, the molecular weight of the polymer at the "crystal point" is higher than the molecular weight of the surrounding polymers. Due to the high molecular weight, the polymer at the crystal point has a higher melting point. When melted, the melt has a higher viscosity. When the polymer at the crystal point is blown into the film, it cannot be uniformly dispersed and mixed with the surrounding polymers of the same kind, and after the melt is blown into a film, it solidifies before the surrounding polymers of the same kind. Therefore, solidified bodies that form "arrow-shaped" or "spherical" hyperpolymers are conventionally referred to as "crystal points".

2. Causes of Crystal Point

Material reason

(1) There is residual catalyst in the polymer;

(2) Polymer melt sticks to the metal surface of production equipment (including polymerization equipment and film blowing equipment), and remains at high temperature;

(3) The catalyst continues to catalyze the polymerization of the polymer, thus forming an excessive polymer;

The oxygen contained in the polymer will also cause the polymer melt to generate crystal points (adding a certain amount of antioxidants can prevent the generation of oxygen on the crystal point).

Process reason

(1) The crystal point caused by the polymerization process. Different polymerization equipment and processes have different effects on the formation of crystal points. For example, some well-known raw materials have low crystal residues due to high production technology, low residual catalyst content in the polymer, and good equipment structure. However, some raw materials of poor quality have more crystal points due to production technology and equipment problems.

(2) The crystal point caused by the process. For example: thin films prepared by good molding equipment have fewer crystal points. Thin films made by poor equipment have many crystal points.

3. Methods to reduce or substantially eliminate crystal points

In summary, there are two main ways to eliminate the crystal point in the film blowing process:

(1) Improve the screen fineness of processing equipment.

(2) The surface lubricant is added during the production process, so that the processed polymer melt cannot stick to the surface of the film blowing equipment.

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