A STUDY ON THE ENTRAPPED AIR BUBBLE IN THE PLASTICIZING PROCESS Hogeun Park, Bongju Kim, Jinsu Gim, Eunsu Han, and Byungohk Rhee, Ajou University, South Korea Abstract In injection molding, gas in the melt causes various defects in molded parts. As well as many other sources of gas in the injection molded parts, the entrapped air during the plasticizing process would be an important source of gas in the parts. The entrapped air bubbles in the screw channel were examined by the screw quenching experiment. To reduce time for investigating the effect of plasticizing condition on the bubble size distribution, a bubble detecting device with a capillary and pressure sensor was designed in this work. The result from the bubble detector experiment with different plasticizing conditions showed a similar trend which is observed in the samples in the screw quenching experiment. It proves the feasibility of the bubble detecting device to examine the bubble size distribution in the screw channel. Introduction Gas in the melt causes various defects in molded parts such as silver streak. According to Yokoi s research, silver streak is caused by gas bubbles generated during fountain flow [1]. Gas in the melt can be left inside molded part as bubbles without being properly ventilated. The bubbles are one of the top cosmetic defects in transparent parts and source of crack initiation in functional parts. Bubbles can result in birefringence and localized mechanical strength reduction [2]. by shutting up the nozzle to keep the state of plasticizing. A lot of air bubbles developed in the screw channels as shown in Figure 1. Also, it was found that the bubble formation changed by plasticizing condition. However, it was time consuming work to get the samples like in Figure 1. To measure the bubbles in the melt, a measuring method was devised and examined its feasibility. Figure 1. Bubbles entrapped in the screw channel from the sample in the quenching experiment Experiments Injection molding machine A hydraulic injection molding machine of Arburg was used in this experiment. Clamping force is 25 tons and the diameter of the screw is 25 mm. There are four barrel heaters and an additional heater is installed at nozzle. The Maximum metering stroke is 100mm and the maximum shot volume is 9 cm 3. The barrel and nozzle heaters, and a schematic diagram of the screw in the barrel are shown in Figure 2. In general, gas in the injection molding is caused by moisture vapor, resin volatiles, or decomposed additives [3]. Moisture vapor can be prevented by proper drying. Also, resin volatiles and decomposed additives can be improved by material modification. However, another source of gas in the melt is entrapped air in the plasticizing process. It has not drawn attention so far. If gas causes a defect such as silver streak, it has been fixed usually by drying. If there are bubbles in transparent parts, they are considered shrink holes (void). However, as production of functional parts increases the bubbles in parts begin to draw attentions. The functional parts do not allow the bubbles because they are potential fracture sources. Therefore, it is necessary to look at the plasticizing process carefully. In this work, air bubbles in the plasticizing screw channel were examined by quenching the material in the screw at various plasticizing conditions. In the screw quenching experiment, pressure in the barrel was kept high Figure 2. Photo of heaters (upper) and a schematic diagram of the screw in the barrel (lower) SPE ANTEC Anaheim 2017 / 1635
Entrapping mechanism of air bubbles in the screw To examine how the entrapped air bubbles like the ones shown in Figure 1 formed, the screw quenching experiment as described as below was performed to investigate the screw channel in the plasticizing process. Step 1: Plasticizing resin until a preset stroke reached. The process conditions are in Table 1. Step 2: Cooling the barrel with the screw by keeping the nozzle closed to preserve the pressurized state during plasticizing. Step 3: Removing the screw from the barrel. Pressurized state in the screw channel during plasticizing could be frozen to be observed by the experiment. Table 1. Condition of plasticizing process. Process condition Heater temperatures Screw circumferential speed 195-200 -205-200 -195 10 m/min GPPS (LG Chem.) Entrapped air bubbles in plasticizing process is observed from the Figure 1. In the figure, many small bubbles can be seen in the feeding zone. As it goes forward, the number of bubbles becomes smaller but the size becomes bigger. It gives us an idea that many small bubbles in the feeding zone merge to larger ones during flowing in the liquid state. the surface and the pellets were not fully squeezed enough to remove the gap. Depth of screw channels in feeding zone is 3.8 mm, and the size of ABS pellets used in this experiment is 2 ~ 3mm. Thus, the gaps among pellets are enclosed by the melt film to form the air bubble in the screw channel. In addition, it is hard to consider that these bubbles are voids. Because, void means shrinkage hole generally which occurs when polymer melts shrink during the cooling process []. Table 2. Depth of screw channels. Channel number Channel depth Section 22 19 1.8 mm Metering 18 1.8 mm 17 21.mm 16 2.5mm Compression 15 2.85mm 1 3.25 mm 13 3.6 mm 12 1 3.8 mm Feeding Effect of plasticizing condition To examine the effect of plasticizing condition on air bubbles formation, samples were obtained in the injection chamber in front of the screw tip. The length of the samples was 100 mm. The plasticizing conditions are listed in Table 3. It was found that bubble formation was affected clearly by the plasticizing condition, as shown in Figure. To examine the effect of plasticizing condition, the screw quenching experiment had to be repeated many times. However, it takes 3 or hours to complete the experiment for a sample. It was nearly impossible to examine the effect by the method in a broad range. Therefore, a method to measure the bubble size and frequency were devised. Table 3. Plasticizing conditions for the samples in Figure Process condition Heater temperatures Screw circumferential speed 195-200 -205-200 -195 5 m/min, 25 m/min MABS (LG Chem.) Figure 3. Cross section of a sample in the feeding zone In the feeding zone, there should be a lot of gap among pellets. As pellets are compressed by forward movement, they could be compressed. But the gap could not be easily removed unless temperature of pellets is high enough to soften the pellets. As the pellets move further along the feeding zone, thin melt film would form at the barrel surface. The melt film may wrap the spaces to form air bubbles. Cross section of a sample from the feeding zone is shown in Figure 3. In the picture, there is the thin film at (a) (b) Figure. (a) Bubbles in MABS samples from the injection chamber at screw circumferential speed = 5m/min, and (b) screw circumferential speed = 25m/min SPE ANTEC Anaheim 2017 / 1636
Design of the air bubble detector An air bubble detector was designed in this work. When an air bubble travels through a thin capillary, pressure drops substantially by the low viscosity of air. The compressed air in the capillary will move faster than the plastic melt. Then, air bubbles could be counted and the size could be approximated by the decreased amount of pressure drop. The schematic of the detector is shown in Figure 5. A capillary is fixed at the nozzle position and pressure sensor is installed just before the capillary. The cylinder with the capillary and pressure sensor is attached in the front of the barrel. Figure 5. Cross section of the detector attached in front of the barrel The capillary nozzle was designed to measure the pressure drop when the air bubble was passing through the capillary. If the melt is an incompressible Newtonian fluid, pressure drop equation can be derived from Hagen- Poiseuille equation. Q is flow rate of the fluid, µ is the fluid viscosity, R is a radius of the capillary and L is length of the capillary. π( P0 PL ) R Q = 8µL (Eq. 1) 8µQ P0 PL = ΔP = L R π The viscosity of air is very smaller than viscosity of polymer melt. Therefore, pressure in the chamber drops as the length of air bubble. (a) Figure 6. (a) Melt without air bubble through the capillary (b) Melt with air bubble through the capillary Three sizes of capillary were tested. The lengths were all the same as 10mm and the diameters were 0.5, 1.0, 2.0 mm, respectively. The smaller the diameter of capillary, the greater the amount of variation in pressure drop caused by air bubbles and the longer the testing time at the same speed. If the pressure exceeds the limiting value of the injection molding machine, a regular extrusion speed could not be achieved. It happened at the 0.5 mm capillary. Figure 7 shows the extrusion time vs. purging speed at the same shot volume. The result of 0.5 mm capillary showed a big deviation from the Theoretical curve. It means that the purging speed control was not reliable. In the experiment, 1.0 mm capillary was used. Theoretical extrusion time, t e can calculate with the following equation. 3 weight volume[ cm ] [ s ] = (Eq.) purging speed[ cm /s] te 3 (b) If the melt without air bubble flows through the capillary like in Figure 6 (a), pressure drop is as below. 8µQ Δ Pmelt = L (Eq. 2) melt R π However, if an air bubble in the melt travels through the capillary like in Figure 6 (b), pressure drop through the capillary can be calculated by the following equation. 8µ meltq 8µ airq Δ Ptotal = Lmelt + L (Eq. 3) air R π R π Figure 7. Extrusion time vs. purging speed for different capillary sizes. Air bubble detection experiment There is a previous result that the size and the number of bubbles in the MABS sample from the injection chamber varied by plasticizing condition, as shown in Figure. SPE ANTEC Anaheim 2017 / 1637
Bubble detection experiment was done at the same plasticizing condition for the same material, MABS from LG Chem. The purging speed that is purging speed was 5.0 cm 3 /s not to exceed the limiting pressure of the machine. The experiment conditions are listed in Table. Pressure was measured at 1 khz sampling speed. Table. Experimental conditions for bubble detection experiment Parameters MABS (LG Chem.) Heater temperatures 195-200 -205-200 - 195 Screw circumferential 5 m/min, 25 m/min speed purging speed 5.0 cm3 /s Capillary diameter 1.0 mm The enlarged pressure in Figure 8 (b) shows a typical pressure behavior in the pressure graph. As air bubble travels through the capillary, pressure drops very quickly because air moves much faster than the plastic melt as it expands through the capillary. The decreased pressure will recover slowly to the highest level due to high viscosity of the plastic melt. The pressure behavior was found in the graph as shown in Figure 8 (b). The region A in the figure represents pressure in a regular melt extrusion state and the small peaks are noises. The region B shows the pressure drop caused by the air bubble in the capillary. The region C shows pressure in the slow recovery state, in which plastic melt enters and fills the capillary. In the pressure data, the number of region B and C were counted to estimate the number of bubble. The size of a bubble can be represented by the amount of the pressure drop at the region B. It can be explained by (Eq. 3) and Figure 6 (b). If a huge bubble traveled through the capillary, L would become smaller because of the long length of melt the melt ( L air ). The pressure drop of the (Eq. 3) will decrease. Consequently, the pressure drop at the region B should be affected by a bubble size. The pressure drops were divided into 10 levels and indexed. The largest pressure drop level in the experiment was assigned to pressure drop index 10. In the experiment, 33 bar was the largest pressure drop. The pressure drop index is shown in Table 5. The amount of bubbles and its sizes of the screw quenching experiment at different plasticizing conditions which is shown in Figure can be compared as a histogram shown in Figure 9. A B Figure 8. (a) A pressure graph measured in the detector and (b) an enlarged pressure graph showing a typical pressure variation when air bubble travels through the capillary Low frequency component in the pressure signal was eliminated from raw pressure data by a high pass filter. Figure 8 (a) is the filtered pressure from the experiment. C Table 5. Range of pressure drop as the pressure drop index. Pressure drop Range of pressure drop [bar] index 0 < P 3 1 3 < P 6 2 9 < P 12 3 12 < P 15 15 < P 18 5 18 < P 21 6 21 < P 2 7 2 < P 27 8 27 < P 30 9 30 < P 33 10 The experiment of the air bubble detector showed a similar result with bubble formation in the screw quenching experiment with MABS. As shown in Figure 9, there were SPE ANTEC Anaheim 2017 / 1638
larger sized bubbles and the number of bubbles small in the condition of 5 m/min screw circumferential speed while there were much smaller sized bubbles and the number of bubbles are big in the faster plasticizing condition. The result showed the bubble detector designed in this work measured the bubble size distribution successfully. Frequency 350 300 250 200 50 Screw circumferential speed : 5(m/min) Scerw circumferential speed : 25 (m/min) References 1. H. Yokoi, N. Masuda, and H. Mitsuhata, Journal of s Processing Technology, 130-131, 328-333 (2002).. 2. R.A. Torres et al, Polymer Bulletin, 59, 251-260(2007) 3. J. Bozzelli, Plastic Technology, April 2015, 32 (2015).. D. Annicchiarico, s and Manufacturing Processes, 29, 662-682 (201) 5. L. Zhang, G. Zhao, G. Wang, G. Dong, H. Wu, InC. International Journal of Heat and Mass Transfer, 10, 126-1258 (2017). 6. J.H. Jeon, J.S. Gim, B.O. Rhee, SPE-ANTEC Tech. Papers, (2015). 0 1 2 3 5 6 7 8 9 10 Pressure drop index Figure 9. Histogram of bubble size represented by the pressure drop index. Conclusions As well as many other sources of gas in the injection molded parts, the entrapped air during the plasticizing process would be an important source of gas in the parts. The entrapped air bubbles in the screw channel were examined by the screw quenching experiment. The size and the number of bubbles varied by plasticizing condition. To investigate the effect of plasticizing condition on the bubble size distribution, a large number of experiments is required. However, the screw quenching experiment takes too long for the whole test. To reduce the test time, a bubble detecting device with a capillary and pressure sensor was designed in this work. By the behavior of bubble through the capillary a typical pressure variation appeared in the pressure graph. When a bubble travels through the capillary, it makes the pressure drop quickly as it expands toward the exit. And plastic melt of high viscosity enters the capillary with slow speed to recover the pressure at slower speed. The result from the bubble detector experiment with different plasticizing conditions is similar to bubble distribution which was we observed in the samples in the screw quenching experiment. It proves the feasibility of the bubble detecting device to examine the bubble size distribution in the screw channel. In the future, investigation about the bubble formation at various plasticizing conditions will perform to find an optimum condition with respect to the bubble entrapment. SPE ANTEC Anaheim 2017 / 1639