This article will focus on the 5 factors affecting the quality of the extrusion blow molding parison.
The parison is a semi-finished tangible plastic body used in blow molding or other processing processes, generally in a tubular form. Extrusion blow molding is first to manufacture parisons, whose quality has a great influence on the performance and appearance of products.
This article will focus on the factors affecting the quality of the extrusion blow molding parison.
In general, the main factors affecting the quality of the extrusion blow molding parison are raw materials, screw speed, die, and processing temperature.
The raw material is a very important factor affecting the quality of the preform. In addition to the performance requirements of the product, the following characteristics of the raw materials have a significant impact on the quality of the preform:
①The melt index of raw materials
The melt index is low, the parison sags easily, and the parison with a uniform thickness can be easily obtained. However, if the melt index is too low, the probability of unstable flow of the melt increases. Once the unstable flow occurs, the appearance quality of the product will deteriorate, and even unqualified phenomenon will appear.
Therefore, before blow molding, raw materials should be reasonably selected according to specific product properties and processing properties. In general, when blowing large and medium-sized products, the melt index of the raw material should be smaller. On the contrary, when producing small blow-molded products, the melt index should be larger.
②Molecular weight of raw materials
The raw material has a large molecular weight and has a strong ability to prevent cracks when subjected to stress. Therefore, raw materials with large molecular weights are generally used. However, if the molecular weight is too high, the extrusion of the parison and subsequent blow molding will be more difficult, and the molecular weight should be considered comprehensively.
③Molecular weight distribution of raw materials
The wide molecular weight distribution is conducive to obtaining high-quality preforms. However, if the molecular weight distribution is too wide, the strength of the product may decrease.
④The extensional viscosity of the raw material
Raw materials whose tensile viscosity increases with the increase of tensile stress are beneficial to blow molding processing.
Besides, the raw materials should have the characteristics of preventing oxygen, carbon dioxide, nitrogen, and water vapor from penetrating into and out of the container during processing.
(2) Screw speed
The screw speed is another important factor affecting the quality of the preform.
The screw speed is fast, the extrusion output is high, and more parisons can be obtained. However, a faster screw speed may cause unstable flow of the melt, deterioration of the surface quality of the parison, and even melt fracture in severe cases. Moreover, when the screw speed is too fast, a large amount of frictional heat will be generated, which increases the risk of instant degradation of the plastic.
Therefore, under the premise of being able to extrude even and smooth tube blanks without overloading the extrusion transmission system, the screw speed should be as fast as possible, but not too fast. Generally, it should be controlled below 70 rpm. The specific screw speed depends on the specific material.
It is an important device that determines the size and shape of the parison during the die. The die is mainly composed of two pairs of half-female molds, the inner surface of which must have sufficient smoothness, and the size must be processed according to the design requirements. The size of the die setting section is generally about 8 times the gap value of the die mandrel.
(4) Processing temperature
The processing temperature is high, the parison droops severely, and the parison length increases, but the diameter and wall thickness will decrease. In severe cases, it may even cause the parison to break.
When the temperature drops, the surface of the preform will become rough, and shark skin, scars, bamboo joints, etc. may appear. The parison will be poorly plasticized, uneven in size and quality, and ultimately reduce the strength of the product.
The above are the four factors that affect the quality of the extrusion blow molding parison.
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