In the production of high-density polyethylene medicinal ointment boxes, wall thickness uniformity is a key indicator affecting product quality and functional stability. If wall thickness deviations exceed the standard range, it may lead to problems such as insufficient sealing, decreased compressive strength, or material waste. Therefore, process optimization requires multi-stage collaborative control.
Raw material selection and pretreatment are fundamental to wall thickness uniformity. High-density polyethylene raw materials must maintain a uniform molecular weight distribution to avoid melt flow fluctuations due to batch-to-batch variations. If recycled material is added, it must be ensured that it is thoroughly mixed with virgin material to prevent localized flow variations caused by differences in physical properties. For example, one company significantly improved raw material uniformity by introducing a high-speed mixer to extend the mixing time between recycled and virgin materials, providing a stable foundation for subsequent processes.
The precision of the extrusion equipment directly affects wall thickness control. The screw of the high-density polyethylene medicinal ointment box extruder needs regular maintenance to prevent uneven extrusion speeds due to wear; the heating system must employ zoned temperature control technology to ensure that temperature deviations in different sections of the barrel are controlled within a reasonable range, preventing melt flow fluctuations caused by temperature differences. The coaxiality of the die and mandrel needs to be checked regularly using laser calibration equipment to prevent preform bending due to equipment deviation, which could lead to uneven wall thickness. For example, one company significantly reduced the circumferential wall thickness fluctuation range of the preform by upgrading its die design and adopting high-precision machining processes.
Dynamic adjustment of process parameters is crucial. During the extrusion stage, appropriate extrusion temperature and screw speed must be set according to the melt index of the raw material. If the temperature is too high, the melt fluidity will be too strong, easily causing the preform to sag; if the temperature is too low, the melt plasticity will be insufficient, potentially causing blockage or surface roughness. The traction speed must be matched with the extrusion speed to avoid uneven preform stretching due to speed differences. For example, one company significantly reduced the longitudinal wall thickness deviation rate by introducing a servo drive system to achieve real-time linkage between traction speed and extrusion speed.
The application of a wall thickness adjustment device can further improve uniformity. Circumferential wall thickness adjustment is achieved by setting adjusting screws around the die circumference and adjusting the gap according to the preform bending direction. For example, when the preform bends to the left, the left screw needs to be tightened to reduce the gap, while the right screw is loosened to balance the pressure. Longitudinal wall thickness adjustment relies on a programmable control system. A servo valve drives a hydraulic cylinder to move the mandrel up and down, adjusting the die clearance according to a preset program. After adopting this technology, one company saw a significant reduction in the longitudinal wall thickness variation, resulting in a substantial improvement in product yield.
Mold temperature management significantly impacts wall thickness uniformity. Excessive mold temperature leads to insufficient preform cooling, causing uneven shrinkage; excessively low temperature may cause rapid surface hardening of the preform due to rapid cooling, hindering internal material flow. Typically, mold temperature needs to be controlled within a reasonable range, achieved precisely through a mold temperature controller. For example, one company, by embedding a temperature sensor within the mold and combining it with a PID control algorithm, reduced the temperature fluctuation range, significantly improving wall thickness uniformity.
Optimizing the cooling system can reduce residual stress. Cooling water needs to be evenly distributed to avoid localized overcooling or overheating. One company modified a traditional straight-through cooling water path to a spiral water path, resulting in more uniform cooling water flow and extending the cooling time to ensure complete preform solidification. Furthermore, blow molding air pressure needs to be adjusted according to the raw material characteristics to avoid preform shrinkage due to insufficient pressure or cracking due to excessive pressure. Real-time monitoring and feedback adjustments during the production process are the final guarantee. Continuous monitoring of the preform wall thickness using an online thickness gauge, followed by data feedback to the control system, allows for automatic adjustments to extrusion speed, traction speed, or wall thickness adjustment parameters. For example, one company, after introducing a laser thickness gauge, achieved data acquisition and, combined with a closed-loop control system, significantly improved wall thickness uniformity. Regular sampling and data analysis are equally important. By statistically analyzing wall thickness distribution patterns, process parameters can be further optimized, creating a virtuous cycle of continuous improvement.
