In the production of medical plastic packaging, controlling wall thickness uniformity in positive and negative pressure molding machines requires a systematic solution encompassing multiple dimensions, including mold design, process parameters, equipment coordination, and material properties. Mold design is the foundation of wall thickness control, with its core focus on optimizing mold structure to balance material flow. The fillet radius of the mold bottom and sidewalls must be adjusted based on material flow. Larger fillets reduce material flow resistance and prevent localized accumulation or excessive stretching. The mold wall draft must match the material shrinkage rate. Too small a draft can cause the wall thickness to be stretched and thinned during demolding, while too large a draft can lead to insufficient material filling. Furthermore, the mold cooling system must utilize zoned temperature control, using independent water or oil circuits to precisely control the cooling rate in different areas to prevent differential shrinkage caused by uneven cooling.
Precise control of process parameters is crucial for achieving wall thickness uniformity. During the positive and negative pressure molding process, the timing of the switching between positive and negative pressure directly influences material distribution. If positive pressure is applied too early, the material may be overstretched, resulting in a thinning bottom; if negative pressure is applied too late, the sidewalls may be thickened due to material accumulation. Simulation analysis can determine the optimal delay time. For example, after the sheet is initially fitted to the mold, low negative pressure is applied to guide the material to spread evenly, and then positive pressure is gradually increased to complete the final molding process. Controlling the heating temperature is equally important. Excessively high temperatures can lead to excessive material flow, causing accumulation at the mold edges; excessively low temperatures can result in insufficient material flow, making it difficult to fill mold details. Therefore, a gradient heating curve should be set based on the material's characteristics to ensure the sheet maintains optimal flow throughout the molding process.
Equipment coordination and dynamic monitoring are essential to ensure wall thickness stability. Positive and negative pressure molding machines must be equipped with high-precision pressure sensors and temperature control systems to monitor and adjust the pressure and temperature distribution within the molding chamber in real time. For example, if the sensors detect an abnormally high pressure in a specific area during the molding process, the system can automatically reduce the positive pressure or increase the negative pressure in that area to balance material flow. Furthermore, a multi-stage pressure control strategy is employed: initially, low pressure is used to guide the material to fill slowly, preventing uneven wall thickness caused by turbulence; later, pressure is gradually increased to ensure sufficient material compaction. Furthermore, the mechanical accuracy of the equipment requires regular calibration, such as the parallelism of the mold mounting surfaces and the synchronization of the stretching mechanism. Any slight deviation can accumulate into variations in wall thickness.
Material properties and pretreatment significantly impact wall thickness uniformity. Common materials used in medical plastic packaging, such as PVC, PP, and PE, vary in fluidity, shrinkage, and thermal stability. For example, PVC easily decomposes and produces impurities at high temperatures, requiring strict control of heating temperature and time.
PP has a higher shrinkage rate, requiring mold compensation design or post-processing to reduce deformation. Material pretreatment is also critical. Insufficient drying can cause moisture to evaporate during the molding process, forming bubbles or voids, disrupting wall thickness continuity. Overdrying can make the material brittle and reduce molding quality. Therefore, standardized pretreatment processes should be established based on material type, such as using a hot air circulation dryer at a specific temperature and time, and performing moisture content testing to ensure material stability.
Optimizing the molding sequence and stretching rate can further improve wall thickness uniformity. In positive and negative pressure composite molding, negative pressure is first used to initially fit the sheet into the mold cavity, followed by positive pressure for final stretching. This reduces the complexity of the material flow path and avoids localized overstretching or accumulation.
The stretching rate must be matched to the material's fluidity. Excessively rapid stretching can lead to material breakage or thinning of the wall thickness, while excessively slow stretching can cause the material to cool and harden, preventing it from fully filling the mold. Experimentation can determine the optimal stretching rate range, which can be dynamically adjusted during the molding process to accommodate the material flow requirements of different areas.
Mold surface treatment and maintenance are essential for ensuring long-term wall thickness uniformity. Excessive mold surface roughness increases material flow resistance, resulting in uneven wall thickness; surface scratches or wear can cause material accumulation or insufficient filling. Therefore, molds must be regularly polished to ensure surface roughness remains below a specified value. High-hardness, corrosion-resistant mold materials should be used to extend mold life. Furthermore, mold cleaning and maintenance must be standardized to prevent residual material or impurities that may affect subsequent molding quality.
Through systematic measures such as mold design optimization, precise process parameter control, equipment coordination and dynamic monitoring, material property matching, molding sequence optimization, and mold maintenance, positive and negative pressure molding machines can achieve highly precise control of wall thickness uniformity in the production of medical plastic packaging.
This process requires not only advanced equipment and technology but also the integration of multidisciplinary knowledge in materials science, fluid mechanics, and mold engineering to establish a comprehensive quality control system from design to production, ultimately meeting the stringent safety, sealing, and dimensional accuracy requirements of medical packaging.
