Measurement techniques in injection molding

Introduction

Injection molding is a widely favored method for accurately fabricating objects with intricate geometries. However, the machine, melt, and product fluctuate from shot to shot due to multiple devices and processes, and complex dimensions in mold, presenting numerous challenges in producing high-consistent products. Although the parameters sensing information such as plasticization temperature, injection pressure, holding pressure, injection velocity, and so on, can be directly obtained in the injection molding machine, they do not involve the detailed machine, melt, and component information but fluctuate with the manufacturing environment, influencing the process robustness in injection molding. Many state-of-the-art measurement technologies have been developed to overcome those limitations. In this article, the recent progress in measurement techniques in injection molding will be discussed.

The need for real-time measurement

During the plasticization and injection process, the polymer is molten; the melt viscosity, a significant material property, reflects the flow resistance and can help to adjust material processing in a mold; thus, it is necessary to be monitored. After that, the mold closure happens; the clamping units move and the tie bars are under tension; the kinematics/dynamics of the clamping mechanism and the clamping force are the main variables that influence the mold closure (i.e. machine status) consistency. In the packing and cooling processes, the melt cools and the product starts to form; the pressure/temperature in the cavity after the velocity/pressure (V/P) switchover position affects the polymer shrinkage, causing product quality variations. 

In the end, the injection-molded component is ejected; the force of the ejectors should be carefully detected or the surface quality of the product may be affected. Detecting and maintaining online replication in the machine, melt, and product helps achieve the desired dimensions and properties of products throughout a continuous material processing line. Simultaneously, it is essential to monitor, refine, and control process parameters to ensure optimal product quality. In recent times, sensing technologies such as in-cavity sensors, nozzle sensors, tie-bar sensors, ultrasonics, magnetic levitation, and so on, have greatly propelled the advancement of measurement techniques in injection molding. Significant progress has been reported in gaining the machine status, molten resin flow state, and product quality data for each shot.

The stability of the machine status impacts the material processing. Fluctuating equipment conditions lead to variations in mold separation, cavity pressure, and clamping force, affecting the melt flow and molded product quality on a processing line. Especially for the long-term continuous operation of injection molding machines in industries, the machine status becomes a more critical issue because the tie bar and mold may wear out, posing a significant challenge to maintaining a stable condition. Considerable progress has been made in the development of machine status-related parameters in recent years. Parameters such as the clamping force, demolding friction force, and clamping mechanism concerning kinematics and dynamics, have undergone substantial advancements. The clamping force plays an important role in mold separation and impacts the tie-bars and mold’s lifespan. An inappropriate magnitude of clamping force can lead to problems such as broken tie bars or product flashes. Maintaining a consistent and appropriate clamping force facilitates a stable combination of moving and stationary molds, creating an optimal environment for effective melt filling. In the ejection process, the ejectors exert direct force on the surface of the molded components. Improper application of friction force can lead to product shrinkage and surface damage during the demolding phase.

To mitigate these issues, the friction force should be carefully applied and monitored. The clamping mechanism, particularly for a five-point double-toggle unit, influences the movement of injection molding machines and the connection of moving and stationary molds. Its kinematics and dynamics directly impact the production efficiency and force amplification during mold-closing/melt-filling processes. Achieving excellent clamping actions is essential to maintain the consistency of the clamping process. The machine status measurement becomes necessary to ensure optimal performance for manufacturing. Numerous studies have been reported on those topics in recent years.

Characterizing the molten resin flow state is crucial to investigating pseudoplastic behavior during the manufacturing process in injection molding. During the filling and packing phases, the melt is pushed into the mold from the plasticization unit. However, the flow state of the melt may deviate from the desired value pre-set in the barrel due to factors such as the distance between the barrel and mold and the presence of the nozzle. The melt cools down as it travels from the barrel to the mold; the molecular friction in the polymer also induces viscous dissipation. Furthermore, the hot-runner system usage can complicate the evaluation of the mold condition, bringing great challenges to maintaining melt uniformity in injection molding. Previous solutions have primarily focused on melt temperature- or pressure measurements via in-cavity sensors in injection molding.