Monitoring In-situ Density Change for In-process Measurement and Control of Hot-pressing

The hot-press is undoubtedly a key piece of equipment in composite panel production. Process control of the pressing procedure can he divided into preprocess, in-process, and postprocess measurement and control stages. This paper presents an in-process measurement and control technique during the pressing cycle and discusses application possibilities based on the study. There are different curing conditions between the core layer and the sutiace layers of apanel, resulting in the core layer always having higher in-press expansion than the surface layers. This in-press expansion can be monitored by measuring in-situ density changes as the press platens open. It is proposed that when a predetermined value of controlled expansion is reached, the press process controller can immediately stop normal press opening and prolong the pressing cycle to ensure sufficient resin cure and prevent substandard production. Results showed that both the prolonging and secondary closing procedures could improve panel performance and prevent substandard production. In-process measurement and control of hot-pressing is made possible by monitoring internal density changes during press opening. T h e hot-press is undoubtedly a key piece of equipment in composite panel production. I t determines the performance of products and the processing efThere is a philosophy that says in simple terms: when the press closes, 90 percent of the cost of production has been incurred and 96 percent of the oriented is usual in the hoard manufacturing process, all of these factors interact. Wood-based composites such as OSB, plywood, laminated veneer lum(MDF), and particleboard are typically cured under pressure in multi-platen ficiency of the whole production line. ber (LVL), medium density fiberhoard strandboard (OSB) panels must he good panels hecase quality can not he added in the warehouse (4). In pressing, anumher of factors are involved, including the resin type and level, wax type and level, pressing temperature, wood species and furnish geometry, mat density and distrihution, mat moisture level and distrihution, press closure rate, and pressure. As FORESTPRODUCTSJOURNAL VOL. presses using thermosetting adhesives. Most wood-based composites processing done toddy is controlled by a fixed “time and temperature recipe” based on empirical observation of the process variables and resin chemistly (5). These recipes must he overly conservative to allow for the variability inherent in the process. The result is a costly compromke in both process time and product quality. In addition to the common lot-to-lot variations associated with adhesive systems, the processing of woodbased composites is further complicated by variations in wood species, moisture content (MC), pH, and seasonal plant environmental effects. Process control of the pressing procedure can he divided into preprocess, inprocess, and postprocess measurement and control (8). Preprocess measurement and control may he required to monitor or control the materials or parts entering the process or to control the product or the process based on input measurements (2). The measurement may include physical or chemical chracteris-