What are the common problems with rotational molding introduced by rotational molding manufacturers
There are many factors that can harm the quality of rotary molded products, among which the most crucial ones are the appearance of the mold, the rotation rate of the rotary molded mold, the heating temperature and time of the mold, and the physical characteristics of plastic powder.
Rotational molding process
1. The mold used for rotational molding of products is a single wall closed mold, which obtains the appearance design contour of the product, but cannot accurately capture the details of the internal contour of the product. The raw materials for making molds for rotary molding are generally selected from general carbon steel plates for welding. The cost of welding molds is relatively low, especially when applied to large rotary plastic products. The rotational molding process has the following basic regulations for its molds: the molds are not prone to deformation and tensile strain during continuous heating and cooling. The mold should be equipped with a clamping structure that is easy to loosen, so that it can be tightly closed during the heating and cooling processes, and can be easily opened when the product must be unloaded and discharged. The design and installation of the mold should facilitate the relative motion of the cyclone around the surface of the mold, facilitating uniform heating and cooling of the mold. The mold should be equipped with ventilation pipes to prevent the steam inside the mold from being deformed during the heating process and squeezing the melted plastic along the mold joint, and to avoid causing negative pressure inside the mold and product deformation during the cooling process. The overall design of the mold should try to avoid deep cavities and holes, as well as structures such as rib plates and solid core protrusions, as powdered materials are difficult to cover the surface of the structure. The sum of the net weight of the mold and the net weight of the material cannot exceed the bearing capacity of the rotary molding machine.
2. The rotation rate of the rotational molding machine usually uses two adjustable speed motors to provide driving force for the rotation and swing motion of the mold around the main and secondary axes, so that the rotation speed ratio of the mold around the main axis and the swing around the secondary axis can be adjusted. The operator can obtain a relatively uniform wall thickness of all products by adjusting the speed ratio of the two speed regulating motors. The basic function of mold rotation and swing is to evenly mix powdered materials and spread them throughout the mold surface.
3. The particle size of polyethylene powder has a critical impact on its melting process. The smaller the particle, the greater the ratio of surface area to volume, and the higher the melting rate. Usually, the size of spin molded powder particles should not be less than 30 mesh, but the powder particle size is also not suitable for being too small. Due to the high moisture absorption and agglomeration of too fine powder, it is difficult to air dry, making it difficult to improve the process. When too fine powdery materials roll over in the mold, they may clump, causing uneven powder when heated, resulting in uneven product wall thickness or insufficient melting of plastic in the product. The experiment shows that the particle size of polyethylene powder added to the rotary molding mold at once does not need to be consistent or uniform. Ordered powder particles of different sizes are heated in the mold because the time and speed of heat absorption and melting of the powder of different sizes cause the molten plastic to slowly deposit and gradually form. The fine powder mixed in the coarse particles has the effect of reducing friction on the movement of the coarse particles, which is beneficial for the rolling and fluidity of the material in the mold. In addition, the presence of fine powder is also beneficial for optimizing the surface of the product, reducing the cross-sectional porosity of the product, and improving the quality of the product.
4. Heating time and cooling time: If the heating time is too short, it will cause the powder to not basically melt, and the plastic properties cannot meet the appropriate indicators. If the heating time is too long, it will cause plastic oxidation, which will damage its plastic molecular structure, making it easy to embrittle and endangering the quality of rotary molded products. Cooling time is also very important. If the cooling time is too fast, it can easily cause deformation of the product, endanger its applicability and aesthetics.