## 1 Introduction  
Plastic is one of the most widely used synthetic materials in the world [1]. This wide-spread use can be attributed to its unique properties, which allows it to be easily processed and integrated into end-user products. Being light weight and relatively low cost make plastics more suitable than its metal counterpart for many industrial applications. Today, plastic plays a dominant role in industries such as automotive, packaging, home appliances, and medical. A large percentage of these plastic parts are produced via injection molding, thereby making this polymer processing method one of the most important processes in the plastic industry [2]. The popularity of the plastic injection molding process stems from its ability to viably produce high volumes of plastic parts of varying complexity in a short time frame [3]. Plastic injection molding is a process in which molten plastic material is injected into a shaping mold, cooled, solidified, and ejected [4]. An injection molding machine consists of three major parts, namely; the injection unit, the mold assembly unit, and the clamping unit [5]. There are two types of clamping units commonly used in the plastic injection molding industry. These units are the ram clamping unit and the toggle clamping unit [6, 7]. The ram clamping unit is used to produce precision parts while the toggle clamping unit is used for high-speed injection molding. Toggle clamping units are typically more energy efficient than ram clamping units, and hence are predominantly used in industry. The scope of this research focuses on the toggle clamping unit of the injection molding machine. The plastic injection molding process begins when the shaping mold is closed via the toggle clamping system. Once the mold is closed, molten plastic material is injected and packed into the mold cavity. The plastic material then undergoes a cooling process, resulting in the formation of a solid part. In the final step, the toggle clamping system opens the shaping mold, allowing for the easy ejection of the molded part. In an industrial setting, the plastic injection molding machine is required to operate for extended periods. Given the nature of the injection molding process, the toggle clamping system must undergo repetitive clamping-unclamping cycles. To maintain optimum productivity, the toggle must therefore always be properly lubricated. If the lubrication condition deteriorates beyond the hydrodynamic regime, the friction generated due to metal to metal contact will accelerate the wear of the toggle, eventually causing failure and ultimately, machine downtime [8]. Gleaner reported that poor lubricant maintenance practices of the injection molding machine can reduce a factory's productivity by 5-20% [9]. To date, substantial research effort has been dedicated to understanding and improving the toggle mechanism design and lubricant circulation. [PERSON] and [PERSON] investigated and proposed improvements for the friction effect at pin joints for the five-point double-toggle clamping mechanisms of injection molding machines [10; 11]. [PERSON] et al. explored the key design parameters of a five-point, double-toggle clamping mechanism [12]. Unfortunately, to date there has been little or no published literature on systems or methods capable of performing real-time monitoring of the lubricating condition of the toggle clamping system. To avoid catastrophic lubricant failure of the toggle, machine operators have traditionally relied on their experience paired with pre-calculated empirical lubricant life estimation, as a guide for determining the re-lubrication timing of the toggle. By adhering to this approach, machine operators often adapt improper lubrication practices. Machine operators often find themselves either wasting lubricant by over lubricating the toggle, or damaging the toggle by failing to provide sufficient lubricant on a timely basis. This approach lacks robustness as it does not provide any information on the real-time lubrication condition of the toggle clamping system.