In the production of polypropylene webbing and luggage straps, burr formation is often related to various factors, including raw material characteristics, mold design, molding process, and equipment condition. To effectively reduce burrs, it is necessary to optimize parameters at each stage from a process improvement perspective, ensuring the stability and accuracy of the production process.
Raw material selection and pretreatment are the primary steps in reducing burrs. The flowability, particle size uniformity, and water absorption of polypropylene raw materials directly affect molding quality. If the raw material has excessive flowability and low melt viscosity, it easily seeps into mold gaps, forming burrs; while uneven particle size leads to unstable feeding, resulting in localized underfilling or overfilling. Therefore, raw materials with uniform particle size and moderate flowability should be selected, and they should be thoroughly dried before production to prevent the decomposition of hygroscopic materials at high temperatures, reducing melt viscosity and thus minimizing the risk of burrs.
Mold design and maintenance are crucial for burr control. Poor mold parting surface accuracy, unreasonable cavity design, or poor sliding core fit can all lead to molten material overflow. For example, foreign objects adhering to the parting surface, mold deformation, or fatigue collapse around the cavity can directly cause burrs. Therefore, it is necessary to improve mold machining accuracy, ensure a smooth parting surface and cavity alignment, and regularly clean the mold surface and repair worn areas. In addition, optimizing the gate design to avoid unilateral tension or uneven filling force can also effectively reduce burrs.
Optimizing molding process parameters is key to reducing burrs. Excessive injection pressure or speed increases mold opening force, leading to molten material overflow; while excessive feed rate, excessively high barrel temperature, or uneven mold temperature reduces melt viscosity, exacerbating burr problems. Therefore, process parameters need to be adjusted according to raw material characteristics, such as reducing injection pressure and speed, controlling barrel and mold temperatures, and reducing holding time. Simultaneously, adopting a "slow-fast-slow" injection principle to avoid excessively high end-stage speed causing a sudden increase in mold cavity pressure is also an effective way to reduce burrs.
Equipment condition and operating procedures also affect burr control. Insufficient clamping force is a common cause of burrs, especially when the projected area of the molded part is large. If the clamping force is less than the injection pressure, gaps will appear at the mold parting surface. Therefore, it is necessary to select a suitable injection molding machine according to the part size to ensure sufficient clamping force. In addition, improper equipment maintenance, such as wear on the check ring, barrel or screw, or failure of the inlet cooling system, can also lead to insufficient material feeding or molten material overflow. Regular equipment inspection and replacement of worn parts are necessary.
Improving the cooling and shaping process also significantly reduces burrs. The design of the cooling tank must ensure stable passage of the strip, avoiding uneven crystallization due to excessive time or depth in the water. The distance from the mold opening to the tank surface needs to be set reasonably; too long will lead to increased crystal phase in the product, making stretching difficult; too short will cause wrinkles and deformation on the strip surface due to rapid cooling without a transition section. Therefore, it is necessary to determine the optimal cooling distance and water temperature through experiments to ensure uniform crystallization of the strip and reduce the risk of burrs during subsequent stretching.
Optimizing the luggage straps stretching process is also a key aspect of reducing burrs. The stretching ratio, stretching temperature, and stretching speed must be set appropriately based on the characteristics of the strip. If the stretching ratio is too high or the stretching temperature is too low, the strip is prone to breakage during stretching, forming burrs. Unstable stretching speeds lead to uneven stress on the strip, resulting in localized burrs. Therefore, it is necessary to adjust the speed difference of the drawing rollers, control the temperature of the stretching tank, and ensure the smooth surface of the stretching rollers to achieve uniform stretching of the strip and reduce burr formation.
The standardization of the production environment and operational management is equally important. Low factory temperatures can cause the flat yarn to harden, increasing the burrs scraped off when passing through the guide holes and raising the yarn breakage rate. Therefore, it is necessary to maintain a suitable production environment temperature, especially in winter when insulation measures are required. Furthermore, operators must strictly adhere to process specifications to avoid damage to the molds or fluctuations in process parameters due to improper operation, which could lead to burr problems.
