ABS Blow Molding
In the automotive industry, ABS plastic blow molding technology is used to manufacture automotive dashboards, headlights, body shells, and more. In addition, this technology is also applied to the manufacturing of medical device casings, surgical instruments, and other related materials.
Stretch Blow Moulding: Process Parameter Selection
Stretch blow moulding is a biaxial directional stretching blow molding method that combines ordinary extrusion blow molding and injection blow molding processes. Its basic principle includes: first, the billet is longitudinally stretched, and then compressed air is introduced for transverse expansion. This process can achieve the effect of biaxial stretching, change the physical properties of the formed part, and make the formed part have better flexibility, high-temperature resistance, tensile resistance, and deformation resistance while saving costs and reducing energy consumption.
Selection of Stretch Blow Moulding Temperature
The selection of temperature for stretch blow moulding should be considered from multiple aspects, such as the crystallinity of the material and the selection of stretching process methods. The control of stretching and blow molding temperature requires ensuring that the polymer can be reoriented to optimize molding and performance, so stretching temperature is also known as orientation temperature. For special materials, such as ABS blow molding, it is necessary to select a suitable drying temperature to maintain it at a suitable processing temperature.
In the process of stretch blow moulding, it is very important to control the stretching temperature, which is a key condition for products to be reoriented and arranged. Each thermoplastic suitable for stretch blow moulding has its specific optimal stretching temperature. Therefore, manufacturers need to control the stretching temperature of the preform during molding to ensure that the product can achieve optimal performance. Setting the stretching temperature too high can easily lead to insufficient orientation; If the stretching temperature is too low, it will affect the transparency and aesthetics of the product. Generally speaking, manufacturers should set a lower stretching temperature as much as possible. This orientation effect is better, but it can easily lead to lower heat resistance of the final product. After being heated, it will undergo significant shrinkage, which is significantly different from the sample. The most suitable temperature for crystalline plastic preforms is between the glass transition temperature and the melting point, within which large molecules can move more freely and fully. The crystallization process of the billet occurs between the glass transition temperature and the melting point, and the stretching blow molding process should not be carried out near the maximum crystallization rate temperature. For example, if the maximum crystallization rate temperature of polypropylene (PP) material is between 120~125 ℃, the manufacturer generally controls the stretching temperature between 135~150 ℃.
Selection of Stretching Ratio
The setting of the stretch ratio is a crucial parameter in stretch blow moulding. Under the action of tensile force, the large molecules in the blown mold are pulled apart or straightened along the biaxial direction, and the molecules move between each other. When the blow-molded blank is stretched several times its original length along the biaxial direction, its area will correspondingly increase and its thickness will correspondingly decrease.
The magnitude of the stretching effect is related to the stretching ratio and blowing ratio. The larger the stretching ratio and expansion ratio, the more complete the stretching orientation, the higher the axial and radial impact strength of the container, and the better the physical and mechanical properties. After the stretching orientation is complete, the orderliness of the macromolecules increases. When exposed to light of a certain wavelength, the scattering and reflection amount will correspondingly decrease, thereby increasing the transparency and transmittance of the formed part, resulting in a higher aesthetic appearance of the container. Therefore, manufacturers should choose a larger stretching ratio as much as possible while ensuring process conditions and smooth processing. However, in practical applications of stretch blow moulding, it is necessary to ensure that the container has excellent practical stiffness and sufficient practical wall thickness. Therefore, the stretch ratio cannot be set too high.
The bottom of the container formed by stretch blow moulding has a relatively small stretch, so the impact resistance of the bottom is generally low. An effective method for manufacturers to increase the stretching ratio at the bottom of containers is to set movable bottom inserts in the stretching blow moulding mold, which can move inward to form the bottom of the container and seal the clamping mouth at higher temperatures.
The guiding significance of stretch ratio in actual stretch blow moulding production mainly includes:
- Manufacturers can determine the cycle of stretch blow moulding based on the stretching ratio. When the stretching ratio is large, the billet needs to have a larger wall thickness, so the forming cycle will be longer.
- Based on the stretching ratio, product height, and radial dimensions, manufacturers can roughly estimate the size of the corresponding blank.
- The larger the stretching ratio, the higher the tensile strength and impact resistance, which can effectively improve the ability of the product to prevent gas penetration.
The Influence of Stretching and Cooling Rates
The influence of the stretching element and cooling rate is crucial in the process of stretch blow moulding.
The blank should maintain a certain stretching speed during the stretch blow moulding process to ensure a certain degree of orientation. The crystallinity of crystalline materials is not 100%, and the internal structure of the material exists in both crystalline and amorphous regions, namely the amorphous region. The deformation of the crystalline and amorphous regions during the orientation process of stretch blow moulding can occur simultaneously, but the speed and time of the process are not the same. The orientation development in the crystalline region is faster, while the orientation development in the amorphous region is slower. When the amorphous region reaches a moderate degree of orientation, the orientation of the crystalline region has reached its maximum. Therefore, the setting of stretching speed should be able to adapt to the changes in different speeds in the two zones. After heating and plasticizing, the billet should have sufficient stretching speed to prevent directional reduction during the stretching process. This is also an effective measure for manufacturers to prevent a sharp drop in the stretching temperature of the billet. However, excessive stretching speed can damage the internal structure of the polymer and cause small cracks to appear in the product.
Maintaining a certain cooling rate during stretch blow moulding is also important for the orientation of the product. The cooling rate of the product is related to factors such as mold temperature, structure, material, and product wall thickness. The cooling rate of stretch blow moulding affects the performance, appearance, and production efficiency of the product. Setting a reasonable cooling rate by manufacturers can optimize the quality of plastic products while shortening the molding cycle.
The relationship between the stretching ratio, stretching speed, stretching inflation temperature, cooling speed, and the orientation of the product is usually as follows:
- When the stretching ratio and stretching inflation temperature are determined, the higher the stretching speed, the higher the degree of molecular orientation of the stretched blow-molded product.
- When the stretching ratio and stretching speed are determined, the principle of stretching expansion is that the lower the temperature, the better.
- When all other conditions are determined, the faster the cooling rate of the product in the mold, the higher the degree to which it can maintain orientation.
- Under the given stretching speed and stretching inflation temperature, the higher the set stretching ratio, the higher the degree of molecular orientation.
Selection of Inflation Ratio and Inflation Pressure
The inflation ratio is one of the most critical parameters in the stretch blow moulding process conditions. Therefore, manufacturers need to strictly control the inflation ratio.
The expansion ratio of stretch blow moulding should be appropriately selected based on the actual processing situation. The preform undergoes stretching and blowing to achieve stretching orientation between macromolecules, reducing the gaps between molecules, and making the arrangement more compact, resulting in higher density and surface orientation. This significantly reduces the gas permeability of the stretching and blowing molded parts, which is beneficial for better preservation of food and even carbonated beverages.
The stretched billet enters the inflation stage, where compressed air is introduced to make the billet expand and make close contact with the inner wall of the mold. When the blank is inflated, it is necessary to maintain sufficient inflation pressure for some time, otherwise, it is easy to cause inconsistency between the product and the sample or blurred surface patterns. The inflation pressure is related to the wall thickness and volume of the container. Generally, the inflation pressure of thick-walled containers is lower, while that of thin-walled containers is higher; Larger containers require higher pressure, while smaller containers require lower pressure. The inflation pressure is generally set at around 2MPa.
Stretch blow moulding is one of the important components of blow molding technology, which is a new process combining extrusion blow moulding and injection blow molding. Due to its special technology and outstanding advantages, it can quickly enter the blow molding market and is accepted by most manufacturers.
Related News
Applied’s expertise in modifying materials at atomic levels and on an ihdustrial scaleenables our customers to transform possibilities into reality.
