Reasons and solutions for silver stripe defects in injection molded products
Silver pattern, also known as material flower, is a common phenomenon in injection molded products. It behaves like layered plastic shavings that need to be peeled off. Mainly due to the evaporation of air or moisture in the plastic, or the decomposition of other types of plastic mixed in and burnt, splashing like scars are formed on the surface of the product, usually radiating outward from the gate in a fan-shaped manner. Plastic generally absorbs moisture from the air during storage. If it is not sufficiently dried before injection molding, the residual moisture will be converted into water vapor during the injection process, resulting in splashing marks on the surface of the product. During the plasticization stage of the injection molding material, a certain amount of air will be trapped in the melt. If this air cannot be smoothly discharged during injection, silver lines will also appear on the surface of the product. In addition, cracked or burnt plastics can produce silver lines on the surface of the product.
Silver lines are mainly caused by the following reasons:
1. Improper storage of raw materials may result in the mixing of moisture or other plastics.
2. Insufficient drying of raw materials before processing. For example, before injection molding ABS products, a hot air drying hopper is usually used to dry the material at a temperature of 80 ℃ for about 2 hours to remove moisture from the raw materials.
3. The plasticizing ability of the machine is insufficient.
4. There is moisture on the surface of the mold, and the heat generated when the material is filled into the mold cavity evaporates it, merging with the molten plastic to form silver lines.
5. Poor exhaust of the mold.
6. The cold material well of the mold is too small, and the cooled raw materials during injection are brought into the mold cavity. Some of them will quickly cool and solidify into a thin layer. Due to the low temperature of the mold in the early stage of production, this problem often occurs.
7. The gate and runner are too small or there are foreign objects blocking the nozzle. Due to the fast filling speed, the instantaneous frictional heat generated will cause the temperature of the plastic to rise sharply and decompose.
8. The breakage of the screw head or the presence of foreign objects in the material tube can cause plastic decomposition.
9. The temperature setting of the material pipe is too high or the temperature control system is abnormal, causing the plastic to decompose.
10. The injection speed is too fast and the pressure is too high.
11. The injection pressure is low or the holding pressure is insufficient, resulting in the product not being compacted and forming silver lines on the surface.
12. The screw speed is too fast, and the shear rate drops too much during plasticization.
13. The material stays in the barrel for too long, causing overheating and decomposition.
14. The mold temperature is too low, and the surface of the plastic injected into the mold cavity solidifies into a layer in the early stage, and the later molten material is extruded with it to form silver lines.
15. Insufficient back pressure, too fast screw retraction speed, or too long rewinding distance can cause air to enter the material barrel along with the material.
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