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What Are the Main Process Factors Controlled in the Extrusion Blow Molding Process?

Ⅰ. The main control process factors

The main control factors in the extrusion blow molding process are: extruder temperature, screw speed, blowing rate, blowing pressure inflation ratio, mold temperature, cooling time and cooling rate, etc.

Ⅱ. The influence of process factors on the molding process

1. Temperature of the Extruder: In the process of extruding the parison, the temperature control of the extruder is pivotal. It directly influences the forming process of the parison and the resultant quality. Properly increasing the temperature of the extruder barrel can diminish the viscosity of the melt, ameliorating the melt's fluidity, and curtail the power consumption. This adjustment also aids in enhancing the product's strength, surface gloss, and transparency. However, excessively high temperatures can prolong cooling, augment shrinkage, and, in materials like PVC, precipitate degradation. Conversely, too low temperatures might impair plasticization, yielding a rough, lackluster parison prone to breakage under stress. Thus, operational temperatures should be optimized to balance surface quality, uniform plasticization, and melt strength without overburdening the transmission system.

2. Screw Speed During Extrusion: The screw speed, particularly in jwell blow molding machines, influences the extrusion rate and, consequently, the parison's stability and surface quality. While higher speeds bolster output and mitigate parison sagging, they might compromise surface integrity or induce melt fractures in materials like PE or thermal degradation in less stable plastics such as PVC. The objective is to maintain screw speeds that yield a smooth, uniform parison without overtaxing the extruder's drive, typically capped below 70r/min. This necessitates employing a robust extrusion unit, potentially like those found in specialized hdpe blow moulding machines or plastic pallet machines, to handle the demand without compromising quality.

3. Blowing Rate: The air injection rate in blow molding must be optimized to hasten inflation, promoting uniform wall thickness and surface finesse. Excessive air speed, however, could induce low-pressure zones or parison rupture at the die, undermining the blow-up process. The aim is to achieve rapid yet controlled inflation, which ensures product consistency and integrity across various applications, whether in standard blow molding contexts or specialized scenarios involving, for instance, plastic pallet production.

4. Blowing pressure: The blowing air should have sufficient pressure, otherwise, it is difficult to inflate the parison or the pattern on the surface of the product is not clear. Generally, the pressure of thick-walled products can be lower, and the pressure of thin-walled products and materials with high melt viscosity needs to be higher. The general blow molding pressure is 0.2~1.0MPa.

5. Inflation ratio: it refers to the ratio of the maximum diameter of the container to the maximum diameter of the parison, which is the multiple of the parison inflation. The size of the parison and the inflation ratio directly affect the size of the container. When the parison size and quality are constant, the larger the inflation ratio of the parison, the larger the container size. The expansion ratio of the parison is large, and the thickness of the container wall becomes thinner. Although raw materials can be saved, inflation becomes difficult, and the strength and rigidity of the container decrease. If the inflation ratio is too small, the consumption of raw materials will increase, the wall thickness of the product will decrease, the effective volume will decrease, the cooling time of the product will be prolonged, and the cost will increase. Molding should generally be determined according to the type and characteristics of the plastic, the shape and size of the product, and the size of the parison. Generally, the inflation of large thin-walled products is relatively small, which is 1.2~1.5 times; the inflation of small thick-walled products is relatively large, which is 2~4 times.

6. Mold temperature: The temperature of the blow mold should usually be determined according to the properties of the material and the wall thickness of the part. For general-purpose plastics, it is generally 20~50°C. For engineering plastics, due to the high glass transition temperature, the mold can be released at a higher mold temperature without affecting the quality of the product. High mold temperature also helps to improve the surface smoothness of the product. Generally, it is advisable to control the temperature of the blow molding mold at about 40°C lower than the softening temperature of the plastic.

When the mold temperature is too low during the blow molding process, the extensibility of the plastic clamped at the nip will become lower, and this part will be thicker after inflation. Too low temperature often causes spots or orange peels on the surface of the product. When the mold temperature is too high, the phenomenon at the nip is just the opposite of that when it is too low, and it will also prolong the molding cycle and increase the shrinkage of the product.

7. Cooling time and cooling rate: After the parison is inflated, it is cooled and shaped. Generally, water is used as the cooling medium. The heat is taken out through the cooling channel of the mold, and the cooling time controls the appearance quality, performance and production efficiency of the product. Increasing the cooling time can prevent the plastic from deforming due to elastic recovery. The product has a regular shape, clear surface patterns, and good quality, but the production cycle is prolonged and the production efficiency is reduced. And reduce the strength and transparency due to the crystallization of the product. If the cooling time is too short, the product will generate stress and porosity will appear.

Usually, on the premise of ensuring that the product is fully cooled and shaped, the cooling rate is accelerated to improve production efficiency; methods to increase the cooling rate include: Expand the cooling area of the mold, use refrigerated water or refrigerated gas to cool in the mold, and use liquid nitrogen or carbon dioxide to inflate and internally cool the parison.

The cooling rate of the mold depends on the cooling method, the choice of cooling medium and the cooling time, and is also related to the temperature and thickness of the parison. Generally, as the wall thickness of the product increases, the cooling time prolongs. However, different plastic varieties have different cooling times due to different thermal conductivity. Under the same thickness, HDPE has a longer cooling time than PP. For PE products of general thickness, after cooling for 1.5s, the temperature difference on both sides of the product wall is nearly equal, and there is no need to extend the cooling time too much.

For products with large size, wall thickness and special configuration, balance cooling is adopted, the cooling medium with high cooling efficiency is used for the neck and cutting parts, and the general cooling medium is used for the thinner part of the main body of the product. For special products, a second cooling is required, that is, air cooling or water cooling is used after the product is demoulded, so that the product can be fully cooled and shaped to prevent shrinkage and deformation.

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