【Abstract】: This paper proposes a new online inspection method for plastic injection molding process. Ultrasonic attenuation behavior of amorphous plastics (GPPS) and crystalline plastics (PP) during injection molding was studied by using reflection and transmission behaviors of ultrasonic waves at heterogeneous interface, response characteristics to temperature and pressure, influence of process parameters (mold temperature and holding pressure) on ultrasonic attenuation behavior of amorphous plastics was discussed. Experimental results show that ultrasonic attenuation signal can accurately reflect information of injection, cooling, shrinkage and crystallization process of plastic melt, which can be used for on-line detection of injection molding process.
Plastic injection molding process is a complex dynamic change process. After plastic is melted, it is injected into a mold cavity of a certain shape, cooled and solidified, and then formed. Plastic melt is accompanied by drastic changes in temperature and pressure throughout process. It is of great significance to track and detect evolution law of plastic melt on-line in actual processing. At present, online detection technology of forming process mainly relies on traditional means such as thermocouples, pressure sensors, etc. Traditional detection methods not only have a single function, slow response time, poor sensitivity and stability, but also cannot reflect real situation of melt. Other online detection technologies, such as visualization technology, fluorescence technology, infrared technology and dielectric technology, can only be used in special systems and equipment, and are not suitable for industrial research. As a kind of mechanical wave, ultrasonic wave has advantages of convenient probe installation, no damage to detection object, fast signal feedback speed, and rich content. It has a very broad application prospect in polymer processing and inspection. In this paper, ultrasound is used as a research method to study ultrasonic response law of plastic melt in the whole process of injection molding, which provides a theoretical basis for optimization of product quality.

Ultrasonic energy propagates at a certain speed and direction in an elastic medium. When it encounters a heterogeneous interface with different acoustic impedance, it will reflect and transmit. This phenomenon can be used for ultrasonic testing. The most commonly used detection method is pulse reflection method, which uses same ultrasonic probe as transmitter and receiver. Schematic diagram of detecting plastic melt in mold cavity by pulse reflection method is shown in Figure 1.

 

A part of sound wave U0 emitted by ultrasonic probe is reflected at the first steel/plastic interface, denoted as U1. A portion of signal transmitted through plastic medium, reflected at second steel-plastic interface and transmitted through the first steel/plastic interface is denoted as U2. Let attenuation of sound wave in steel be 0, sound pressure reflection coefficient of sound wave at steel/plastic interface is r, transmission coefficient is 1+r, attenuation of sound wave by plastic medium is α. Sound pressures of U1 and U2 are shown in equations (1) and (2), respectively, and negative sign indicates inversion.

 

It can be seen from above formula that when interface reflection coefficient r is constant, size of signal peak U2 is only related to α. Attenuation information of plastic melt can be learned through change of peak height of signal peak U2, so as to grasp change of state of plastic melt in real time. When there is no plastic melt in mold cavity or plastic solidifies and shrinks away from mold to create a void, steel/plastic interface becomes a steel/air interface, because acoustic impedance of steel is 46*105g/cm2 than acoustic impedance of air 0.004*105g/ cm2 is several orders of magnitude larger, sound wave is almost completely reflected at interface, and U2 signal disappears.

Material: Polypropylene (PP F401), Lanzhou Petrochemical Company and Polystyrene (GPPS PG-33), Zhenjiang Qimei Chemical Co., Ltd.; Injection Machine: HTL90-F5B, Ningbo Haitai Plastic Machinery Co., Ltd.; Mould: Rectangular Box ( 160 mm * 100 mm * 30 mm) with an average thickness of 2.6 mm; mold temperature controller: STM-910-W series water and oil dual-use, Shini Electric Heating Machinery Co., Ltd.; Ultrasonic Card: UT-2001, Shanghai Siqueke Information Technology Co., Ltd. Pressure sensor is located at the top of mold cavity, and ultrasonic probe is installed on the outside of fixed mold. In order to ensure close contact with mold, probe is fixed with a spring, and oil is used as a couplant. Pulse reflection method is used for detection, and flaw detection gain is set to 30dB.

In this paper, amorphous plastic GPPS is taken as research object to study ultrasonic behavior of amorphous plastic in injection molding process. Nozzle temperature of injection machine was set to 210 ℃, mold temperature was 30 ℃, injection pressure and holding pressure were 3 MPa and 4 MPa, injection stroke was 63 mm, injection time, holding time and cooling time were 5s, 15s and 7s, respectively. Relationship curve of U2 signal peak height and cavity pressure and molding time during injection molding is shown in Figure 2.

 

As shown in Figure 2, peak height of signal has four different characteristic intervals with forming time. The first area (0~8.1s) and fourth area (13.5~15.0s) correspond to injection and plastic shrinkage and release stages, respectively. In second area (8.1~11.6s), peak-to-peak value of signal decreased, while in third area (11.6~13.5s) peak value increased.
The greater attenuation effect, the smaller peak-to-peak value of signal. Above phenomenon shows that attenuation effect in second and third regions first increases and then decreases. In a viscoelastic medium, friction between particles due to viscosity of medium will lead to loss of sound energy. Generally, the greater viscosity of medium, the stronger attenuation effect. After 8.1s, plastic injection process ends. With diffusion of heat, melt viscosity gradually increases, so peak height continues to decrease between 8.1 and 11.6s. As cooling and solidification continued, molecular chains were gradually frozen, viscoelastic absorption attenuation of ultrasonic signal weakened, resulting in an increase in peak-to-peak value of signal between 11.6 and 13.5 s.

This paper takes crystalline plastic PP as research object to study ultrasonic behavior of crystalline plastic in injection molding process. Injection stroke of injection machine is 68mm, pressure holding time and cooling time are 10s and 12s respectively, and other working parameters of injection machine are same as those in Section 2.2.1.

 

Relationship between U2 signal peak height and cavity pressure and molding time during injection molding is shown in Figure 3. Between 5.0 and 6.6 s, also due to diffusion of heat, melt viscosity gradually increased, peak height of signal decreased rapidly. However, compared with Fig. 2, ultrasonic behavior is different after 6.6 s. Peak value in Fig. 2 increases between 11.6 and 13.5 s, while signal peak in Fig. 3 shows a slow decreasing trend. This is mainly because amorphous plastic GPPS does not crystallize during cooling process, while PP is a crystalline plastic. With progress of cooling and solidification, precipitated crystal grains lead to uneven acoustic impedance of propagation medium, resulting in attenuation and scattering of sound waves, scattering attenuation offsets effect of reduced viscoelastic absorption attenuation, resulting in a slow decrease in peak-to-peak value of signal.

Under condition that other process parameters in Section 2.2.1 remain unchanged, ultrasonic behavior of amorphous plastic GPPS under holding pressure of 1MPa, 2MPa and 3MPa was investigated in this paper.

 

Figure 4 shows relationship between signal peak height and forming time of amorphous plastic GPPS under different holding pressures. It can be seen from figure that in injection stage (0~8.1s), change trends of ultrasonic signals under three holding pressures are basically same, because they have same injection parameters. After injection stage, with increase of holding pressure, density field of plastic melt in mold cavity becomes larger, shrinkage rate during cooling and solidification becomes smaller, resulting in a longer time for plastic melt to shrink and leave mold.

In this paper, mold temperature was changed by cooling water, ultrasonic behavior of amorphous plastic GPPS was investigated when mold temperature was 30℃ and 60℃, respectively. Relationship between signal peak height and forming time at different mold temperatures is shown in Figure 5.

 

It can be seen from Figure 5 that under the two mold temperature conditions, peak-to-peak height of signal has experienced a process of first increasing, then decreasing, then increasing and then decreasing, but there are also some differences. Between 8.1 and 11.6s, change trend of peak height is basically same, but attenuation of high mold temperature (60℃) is always smaller than that of low mold temperature (30℃) at the same time, which is also due to fact that in viscoelastic medium, the higher viscosity, the more severe decay. Under same injection temperature, viscosity of melt in mold cavity with high mold temperature is lower at same time. At the same time, it takes longer for melt to transform from viscoelasticity to elasticity at high mold temperature. Therefore, when mold temperature is 30℃, signal peak-to-peak height starts to rise from 11.6 s, while when mold temperature is 60℃, signal peak-to-peak height starts to rise after 11.9s. In addition, due to fast curing rate, articles with a low mold temperature will complete curing process and shrink away from mold before articles with a high mold temperature.

(1) Ultrasonic detection has advantages of strong penetrating ability, high detection sensitivity, harmless to human body, and flexible use. In this paper, the whole process of injection molding is tested online by using reflection and transmission behavior of ultrasonic waves at heterogeneous interface and the response characteristics to temperature and pressure.
(2) Peak-to-peak value of ultrasonic signal of the amorphous plastic shows a trend of decreasing first and then increasing. When it is in a molten state, with increase of viscosity, viscoelastic absorption attenuation gradually increases. When cooling and solidification reaches a certain level, molecular chain gradually freezes, and viscoelastic absorption attenuation weakens.
(3) Peak-to-peak value of ultrasonic signal of crystalline plastic is gradually decreasing. Crystal grains precipitated in cooling process of crystalline plastic lead to scattering attenuation of sound wave, effect of scattering attenuation offsets weakening of viscoelastic absorption attenuation.
(4) Since injection parameters of plastic melt are same, changing trends of ultrasonic signals at injection stage under different holding pressures are basically same. However, the higher holding pressure, the denser plastic melt in mold cavity, the later plastic melt shrinks and leaves mold.
(5) At the same time, high mold temperature slows down curing rate, so sound attenuation of high mold temperature is always lower than that of low mold temperature, and it takes longer for melt to transform from viscoelasticity to elasticity at high mold temperature. In addition, article with a lower mold temperature shrinks away from mold before article with a higher mold temperature completes curing process.