Demolding angle of plastic parts
The draft angle of a plastic part is a crucial parameter in the plastic molding process, directly impacting part quality and production efficiency. The draft angle refers to the angle between the sidewall of the plastic part and the mold cavity wall. Its primary function is to ensure smooth removal of the part from the mold, avoiding deformation, scratches, or even damage during the demolding process. When designing a plastic part, properly determining the draft angle is a key consideration. Different plastic part materials, structures, and sizes require different draft angles.
The required draft angle varies significantly for different plastic materials. Generally speaking, plastics with good flowability, such as polyethylene and polypropylene, can require a smaller draft angle due to their excellent moldability and relatively low adhesion to the mold cavity walls. However, plastics with poor flowability and high shrinkage, such as polycarbonate and polyoxymethylene, require a larger draft angle to prevent the part from clinging to the mold cavity walls after cooling and shrinkage. For example, the draft angle for polycarbonate parts is typically between 1° and 2°, while the draft angle for polyethylene parts can be controlled within a range of 0.5° to 1°.
The structural complexity of a plastic part also plays a significant role in influencing the draft angle. Parts with deep cavities, complex patterns, or raised structures create a larger contact area with the mold, leading to stronger adhesion. Therefore, a larger draft angle is required to ensure smooth demolding. Conversely, for parts with simpler structures and smoother surfaces, the draft angle can be appropriately reduced. For example, a patterned plastic bottle cap, whose internal threads and pattern increase adhesion to the mold, requires a slightly larger draft angle during design—typically between 1.5° and 3°—to ensure easy removal from the mold while preventing scratches on the pattern.
The draft angle also affects the dimensional accuracy of the plastic part. If the draft angle is too small, the plastic part may be subjected to significant tensile forces during the demolding process, resulting in deformation or dimensional deviation. Excessive draft angles can reduce the dimensional accuracy of the part, especially for demanding mating surfaces. Excessive draft angles can impair the part’s assembly performance. Therefore, when determining the draft angle, it is important to minimize the impact on the part’s dimensional accuracy while ensuring smooth demolding. For example, for precision gear parts, the draft angle is typically controlled between 0.1° and 0.5° to ensure that the gear tooth profile accuracy and dimensional tolerances meet requirements.
In actual production, the draft angle must be determined in conjunction with the mold structure and production process. The mold’s surface roughness also has a certain impact on the draft angle. The smoother the mold surface, the less friction there is between the part and the mold, and the required draft angle can be appropriately reduced. Furthermore, injection molding process parameters such as injection temperature and cooling time can indirectly affect the demolding effect, which in turn influences the choice of draft angle. Designers and process engineers need to continuously experiment and adjust to determine the most appropriate draft angle for a specific part to improve production efficiency and ensure consistent part quality.
