Measurement And Control Of Polymer PVT Relationships Based On Injection Molding Equipment

PVT (pressure-specific volume-temperature) relationships of polymers are the main basis for flow simulation of injection molding parts, injection mold design, injection molding process control and analysis. It plays a more and more important role in polymer processing, especially in precision injection molding. In this study, a novel on-line PVT measurement based on injection molding equipment was developed, and the PVT relationships of several polymer materials were measured. Then, the measured polymer PVT relationships was analyzed using polymer PVT EOS (equation of state) by non-linear regression and curve fitting, parameters of the EOS and fitted data were obtained, and a PVT database was established. Finally, a novel on-line PVT control method based on injection molding equipment was presented, several injection molding process control techniques were developed, the experimental results proved the validity of these new control methods to improve the quality and repeatability of the injection molded parts. The main works are as follows: 1. A novel PVT on-line testing method and equipment were developed. This new measurement is based on an injection molding machine, and it can be used to obtain PVT data of polymers directly with a special testing mold under normal processing conditions. The testing temperature range was from normal temperature to 160℃, the testing pressure range was from atmospheric pressure to 200MPa, the heating rate could reach 15℃/min, and the repeatability of the measurement was within 0.3%. These conditions agreed with real industrial processing conditions, therefore the obtained data could reflect real polymer PVT relationships under normal processing conditions.The experiments with previous PVT on-line testing equipment confirmed the feasibility of the new measurement. The PVT data of several polymers including amorphous polymers (ABS, PS) and semi-crystalline polymers (LDPE, PP, PA6) were obtained. Then modified testing equipment was developed based on the previous one, the experimental results confirmed the advantages of the modified testing equipment in temperature range and sealing, and the PVT data of several brands of PP, PP with 21% talc and PA66 with 33% glass fiber were obtained. The advantages and disadvantages of the novel testing method were also summarized. The influence of the testing sample geometry and additive to polymer PVT relationships was investigated. The influence of polymers with same family name but different grade, different measurements, process and conditions to polymer PVT relationships was analyzed.2. Based on the on-line PVT testing results, the modified two-domain Tait EOS was used to solve the experimental PVT data of ABS, PS, LDPE, PP, PA 6 and PA66. The parameters of the modified two-domain Tait EOS and fitting data were obtained. Then the PVT database was established, which enriched data resource in polymer processing and provided the data support for development of injection molding CAE simulation software in China.A modified two-domain Spencer EOS was developed to describe the PVT relationships of amorphous polymers. Compared with the original Spencer EOS, it could describe the PVT relationships of amorphous polymers more accurately. Compared with the other empirical EOS, it is much easier to use.The on-line PVT data with the modified two-domain Tait EOS were used in injection molding numerical simulation. Compared with the PVT data measured by piston-die technique, the shrinkage and warpage simulation results proved that using the PVT data measured by the on-line measurement could obtain much more accurate simulation results, which embodied the advantage of the on-line PVT measurement.3. Based on the injection molding process control techniques, polymer PVT relationships control technique based on injection molding equipment was presented, and several new injection molding process control techniques (including V/P switchover by cavity pressure, V/P switchover by cavity temperature, holding end-point control by cavity pressure, holding end-point control by cavity temperature, on-line PVT relationships control technique/holding process control technique with cavity temperature, multi parameters control technique) were developed. The influence of mold temperature, transducers position, different process parameter, and the characteristics and properties of these new techniques were investigated. Experimental results indicated that each of these new techniques could improve the quality and repeatability of the injection molded parts in different levels. Especially, the on-line polymer PVT relationships control technique could improve the part mass repeatability from 0.19683% of conventional technique to 0.05494% without mold temperture control.