Analysis of PVT characteristics of polymer materials (Polymer PVT tester)

Time:2022-10-29 09:30:42 / Popularity: / Source:

Excellent physical properties of plastic materials and economic advantages of extremely high cost performance have fully covered various products in the field of human life. Due to development advantage of high cost performance, all material manufacturers have actively developed plastics in different application fields, ranging from life plastics, engineering plastics, special application high-performance plastics and biocompatible plastics, etc. In manufacturing method of plastic products, injection molding is widely used in various processes, which can realize economic benefits of mass and rapid production. In many plastic injection molding processes, it is necessary to rely on adjustment experience of processing process, to understand and analyze material characteristics in advance, so as to reduce adjustment time, reduce product quality defects, and improve production efficiency.
In injection molding process, relevant plasticity quality analysis includes rheological analysis, PVT analysis, heat transfer analysis, etc. This article will introduce PVT analysis part in detail as follows. PVT instrument can measure volume change data of plastic under temperature and pressure changes, so as to obtain PVT physical properties of material, then optimize injection product through effective mold flow analysis.

PVT Test Unit Architecture

Main data obtained by PVT measurement are pressure, temperature and specific volume (that is, reciprocal of density). PVT curve describes external change of volume (or specific volume) of material with temperature and pressure during transition between molten state and solidified state of plastic, showing correlation between the three values. During measurement process, instrument continuously collects required data, records raw data for user to analyze and calculate pattern details to meet predictive analysis needs of processing, production or other applications.
Company's PVT testing equipment follows ISO17744 standard. Material to be tested is placed in a set of temperature-controllable test cylinders with heating and cooling functions, a gasket is used to cover material to be tested to prevent leakage. Material to be tested can be granular or powdered. Upper and lower pistons are used to hold gasket. During test, pressure is applied to piston, material to be tested is pressurized and pressure-stabilized, and a high-precision travel sensor is used to obtain change of specific volume value (see Figure 1 for details). PVT test process and injection molding process in this operation mode relatively consistent.
plastic injection molding processes 
Figure 1: Schematic diagram of PVT test structure

PVT test process

PVT test instrument provides four test procedures, namely isobaric cooling, isobaric heating, isothermal variable pressure cooling and isothermal variable pressure heating. Generally, plastic test adopts isobaric cooling process. During test, under a fixed pressure, temperature is controlled to drop from melting temperature of plastic to normal temperature with a fixed slope. If there are different pressure sections, program will heat up to melting temperature again, and next pressure will be lowered again. During testing process, system will successively capture various data, finally compile PVT data and curves. Isothermal variable pressure cooling mode is to change set pressure segment in sequence under constant temperature state, record relevant value, then cool down to next temperature point, repeat above steps until test is over.
Isobaric heating is different from isobaric cooling process. It is to control temperature to increase from normal temperature to a set temperature with a fixed slope under a fixed pressure, and record various data. It is used for materials that will cure after heating, such as silica gel and rubber, etc. . As for isothermal variable pressure heating process, it also starts from room temperature, and changes set pressure segment in sequence. After recording various data, temperature gradually rises, and above steps are repeated until test is over.
plastic injection molding processes 
Figure 2: PVT curves of crystalline materials
plastic injection molding processes 
Figure 3: PVT curves of amorphous materials
PVT curve can be roughly divided into crystalline materials (as shown in Figure 2) and amorphous materials (as shown in Figure 3) according to material properties. Glass transition state of material can be observed from PVT curve, changing trend of specific volume value between molten region and solid region can be determined from melting point temperature. Changing trend of specific volume value between. Computer of instrument can export original data, then provide suitable processing parameters for analysis, so as to reduce defects of finished products such as shrinkage and warpage deformation that may be caused during injection process.

Summarize

With development of injection technology, requirements for dimensional accuracy of injection products are increasing. PVT parameters are an important indicator for improvement of injection technology. Combination of injection molding technology and computer software, hardware, through mold flow analysis, can help industry to quickly complete pre-analysis operation, effectively shorten test cycle, improve production efficiency of production line and ensure quality of finished product. In addition, with advancement of materials technology, various new polymer materials have come out one after another, their molecular combinations and structures are more diverse. It is necessary to master physical properties of various materials to facilitate commercialization of research and development results. ■

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