- Email: [email protected]
Influence of injection parameters on the formation of blush in injection moulding of PVC
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 4 ( 2 0 0 8 ) 1–7
journal homepage: www.elsevier.com/locate/jmatprotec
Review
Influence of injection parameters on the formation of blush in injection moulding of PVC ´ J. Llado´ ∗ , B. Sanchez Department of Mechanical Engineering, CPS, University of Zaragoza, 50018 Zaragoza, Spain
a r t i c l e
i n f o
a b s t r a c t
Article history:
This paper presents results of the study of influence of injection parameters of PVC fittings
Received 21 September 2007
moulding on the formation of blush. The aim was to determine the cause of the blush and
Received in revised form
predict its evolution in relation to the injection rate and melt temperature, based on Finite
3 December 2007
Element (FE) moulding software. The computation model was fitted by means of experimen-
Accepted 16 December 2007
tal tests carried out with a prototype fitting mould. Once the cavity pressure was verified, the analysis of results provided by the software such as the shear stress distribution around the gate and the flow front temperature enabled the identification of the injection rate as
Keywords:
the principal reason for the flaw, the melt temperature being a secondary factor. © 2007 Elsevier B.V. All rights reserved.
PVC Blush Injection pressure Flow rate
Contents 1. 2.
3. 4.
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Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Experimental method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Material and part geometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Pressure sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3. Injection machine and injection parameters. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Injection modelling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. Experimental results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2. Numerical results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3. Comparison of experimental and theoretical results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3.1. Pressure analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3.2. Flow front temperature. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3.3. Shear stress . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Corresponding author. ´ E-mail address: [email protected] (J. Llado). 0924-0136/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2007.12.063
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j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 4 ( 2 0 0 8 ) 1–7
5.
1.
Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Introduction
Polyvinyl chloride (PVC) is most commonly used in the construction sector because of its excellent weather, chemical and flame resistance properties. In recent years, its use has been growing in specialty injection moulding markets such as domestic appliances, business machines, medical devices and consumer electronics. Despite its advantageous properties, the injection of PVC is regarded as troublesome by some moulders (Gruber and Gockowski, 1992), because this material often decomposes and burns during processing, releasing hydrochloric acid, which rusts the equipment; in addition, surface defects can appear. To overcome the perception of the difficult injection of this material, the processability of PVC compounds has been improved by development of highflow (low-viscosity) PVC grades through the use of acrylic processing aids. These aids promote PVC fusion, modify the melt rheology and/or provide lubrication. Some processing aid products are designed to serve one of these functions while others provide a combination of functions (Stevenson and Einhorn, 1993; Disson and Girois, 2003). Furthermore, significant advances have also been made in processing equipment and conditions. Previous experiments have also indicated that the study of processing parameters is very important because just setting them correctly covers about 90% of the problems encountered in PVC injection, such as degradation (Garcia et al., 2004a) or surface defects (Weir, 1994). Companies always strive to produce high-quality parts while lowering their costs; however, significant time delays and increasing costs may occur if part design is not carefully evaluated or the injection process is not completely understood. The factors involved in the injection moulding process that have a great influence on the final quality of plastic products can be classified into the following four categories: materials, moulding machine, model design and processing conditions (Min, 2003). Each factor, accurately chosen and controlled, requires a multi-disciplinary knowledge to improve and optimize the final product, but the process of filling a cavity mould with a plastic melt is complex due to significant interactions between variables, and this requires particular attention when setting up the machine. Injection moulding software packages are widely used to analyse product performance and processability and represent a powerful tool to evaluate the moulding conditions with a high level of detail because they provide a lot of predicted data that are not normally available during experimental tests (Spina, 2004). A thorough analysis of the numerical results enables potential problems in product injection to be identified and resolved by proposal of a new set of process variables that improve the part quality, although huge amounts of computer-generated data and complex nonlinear interactions between all process variables frequently make the selection of the optimal design and process parameters very difficult (Turng and Peic, 2002).
7 7 7
This work refers to an industrial problem, that is, the blush that appears around the gate in the injection of PVC fittings. The target was to determine the cause of the blush and predict the evolution of this flaw in relation to the injection rate and melt temperature, using Moldflow numerical simulation software. To adjust the computation model, a prototype fitting mould was designed and experimental tests were performed, where the pressure inside the sprue bushing is controlled. The pressure data obtained during the injection trials were used to obtain the viscosity curves to be applied in the numerical simulation.
2.
Experimental method
2.1.
Material and part geometry
The material used in this work was provided by Solvay. The PVC compound injected was Benvic IR705, a standard material used for the injection of PVC fittings. This study is directed towards understanding the industrial problem of the blush around the gate during injection moulding of PVC fittings. These parts are cylindrical and usually have a centre sprue gate from which a radial flow goes through the cavity. It was decided to design a centre-gated semicylinder, as shown in the mould in Fig. 1; this is easier to eject and mimics the radial flow produced on the fittings. The dimensions of the part were chosen by the company Pipelife Hispania S.A., which was interested in studying this problem. The thickness chosen was 8 mm, the width 200 mm, and the length 200 mm. The sprue was 70 mm long, with an inlet diameter of 6 mm and the angle of the conical section was 3◦ .
2.2.
Pressure sensors
For a better understanding and control of the injection process, it is important to understand the evolution of some variables that can be checked with the results provided by injection moulding packages. The plastic pressure in the mould and the melt temperature are the two variables. At present, there are no techniques available to measure the actual melt temperature profile in the cavity without affecting the flow (Garcia et al., 2004b). A previous attempt was made by the authors to measure the melt temperature with an infrared sensor, mounted at the same level as the mould wall, but this sensor was not able to measure the maximum melt temperature in the central layer of the flow, and it only provided the evolution of the temperature near the mould wall, where there was a temperature gradient. So, experimentally, the thermal degradation of the PVC is difficult to determine. In this work, to ensure the quality of PVC injected parts, the quantitative assessment of cavity pressure history is obtained using two Kistler 6157BSP0 quartz sensors placed at locations
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 4 ( 2 0 0 8 ) 1–7
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Fig. 1 – Cavity mould.
P1 and P2 in the sprue bushing (Fig. 2). The lower part of the sprue bushing is rectangular to avoid the relative rotation of this element that could preload the sensors when the nozzle leans against the mould. The voltage provided by both sensors is amplified and transmitted through an Adaptor DAS16 multi-channel acquisition system to a PC (Fig. 3), where the data are registered by means of DFPLUS software, where some values must be supplied such as the number of injection cycles, the number of values recorded per second and the amplifier features.
2.3.
Injection machine and injection parameters
The injection moulding trials were performed at Pipelife Hispania’s facilities, with a Cincinnati Milacron 250 injection moulding machine, specific for PVC injection, with maximum
values of clamp force, injection pressure and velocity equal to 153 kN, 1400 kg/cm2 and 368 cm3 /s, respectively. The processing conditions evaluated were the injection speed and the melt temperature. Two melt temperatures were selected in the range recommended for this material, 205 ◦ C and 195 ◦ C, measured with a portable infrared thermometer when the material left the nozzle. Fig. 4 shows the cylinder and nozzle temperatures corresponding to each case. Initial tests were carried out to determine the injection speeds at each temperature. The selection of the injection speeds was based on the fact that they were within the usual range of flow rates used in the injection of PVC fittings, and that the material was not burned at the highest injection rate. At 205 ◦ C melt temperature, injection trials were performed at injection rates of 30, 25, 22, 20, 18 and 15%, which means a flow rate ranging from 113 to 20 cm3 /s. In the case of 195 ◦ C, the injection rates
Fig. 2 – Location of pressure sensors.
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Fig. 3 – Measurement chain. Fig. 5 – Part mesh.
As in the experimental injection trials, eleven simulations were performed using the same melt temperatures, injection rates and packing conditions.
Fig. 4 – Cylinder and nozzle temperatures.
were 25, 22, 20, 18 and 16%, which correspond to a flow rate range between 70 and 19 cm3 /s. After the filling stage, to avoid sink marks, all parts were packed during 5 s, with a packing pressure equal to 80 bar. Five parts were injected at each injection velocity, and every pressure profile registered by both pressure sensors was recorded in each test.
3.
Injection modelling
4.
Results and discussion
4.1.
Experimental results
For both temperatures and all injection rates, the pressure profiles recorded by both sensors are qualitatively similar. As an example, Fig. 7 shows a typical pressure profile measured by both sensors during the injection of the part at 195 ◦ C and 25% injection rate. As soon as the flow front reaches the sensor, measurement starts and two stages can be observed. Initially, during the filling of the sprue, the curve rises sharply due to the large drop in pressure. Then the second step corresponds to the filling of the part, where pressure increases slightly since the part is thick and it does not offer too much resistance to be filled. Finally, the beginning of the packing phase can be seen where the pressure increases sharply again. If all records are analysed, it is observed that the maximum pressure at the
Moldflow FEM software has been used to simulate the injection trials. First, the part was meshed, as shown in Fig. 5. The sprue was modelled using 7 cold runner elements that fed a semicylindrical cavity of 276 shell triangular elements. The properties of the PVC Benvic IR705 are listed in Table 1, and the critical shear rate and degradation temperature are about 0.2 MPa and 215 ◦ C, respectively. Fig. 6 shows the viscosity plotted against the shear rate, at different temperatures, provided by the material supplier.
Table 1 – Properties of the PVC Solvay Benvic Property
Value 3
Density (kg/dm ) Conductivity (W m−1 ◦ C−1 ) Glass transition temperature (◦ C) Specific heat (J kg−1 ◦ C−1 ) Shrinkage (%)
1.32 0.13 79–80 1767 0.6
Fig. 6 – Viscosity data.
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 4 ( 2 0 0 8 ) 1–7
Fig. 7 – Experimental and numerical pressure evolution at 195 ◦ C and 25% injection rate.
end of the filling stage is higher at 195 ◦ C than at 205 ◦ C, and that pressure increases as injection rate increases. In addition to the pressure records, the parts injected in each test were examined to observe the evolution of the cosmetics of the part. Inspection of the samples showed the appearance of a circular white mark around the gate. For both melt temperatures, the largest defect appears at the highest injection rate and the blush diminishes as the injection rate decreases and the melt temperature is higher. The flaw mainly depends on the injection rate, the melt temperature being a secondary factor. So the shear produced at high injection rates seems to be the main reason for the blush. The influence of thermal degradation will be determined by means of injection simulation.
4.2.
Numerical results
Initial simulations performed with the rheological characteristics provided by the material supplier, showed a difference of about 30% between the experimental and theoretical pressure data at the end of the filling stage. The reliability of the numerical simulation depends mainly on the rheological data of the PVC. Therefore, to characterise the behaviour of the polymer during injections at 195 ◦ C and 205 ◦ C, the maximum pressure values recorded by both sensors at the end of the filling of the sprue have been used to calculate the viscosity function according to Poiseuille’s law, as Eq. (1) states: =
4r4 P 8QL
(1)
where (Pa s) is the viscosity, r (m) and L (m) are the average radius of the sprue and the distance between both sensors, respectively, Q (m3 /s) is the flow rate and P (Pa) is the drop in pressure. The adjustment of the viscosity curves at 195 ◦ C and 205 ◦ C is performed by fitting a potential function to the experimental values obtained with Eq. (1). If these two experimental viscosity curves are compared with the rheological data of the PVC obtained by means of a rheometer (Fig. 8), it can be observed that the viscosity data at 195 ◦ C coincide
5
Fig. 8 – Viscosity data comparison.
with the 200 ◦ C viscosity curve. This discrepancy can result from the fact that the method performed with the rheometer is much more controlled than the injection tests where the control of the melt temperature is not so precise, because machine variables such as barrel temperature, back pressure, screw RPM, screw geometry, clearance between the screw and the barrel, all affect the melt temperature. As a consequence, to correlate the information recorded during the injection of the PVC prototype parts with the numerical results from the FEM injection software, it has been considered convenient to perform the numerical simulations using the viscosity curves determined from the injection tests. Pressure evolution at 195 ◦ C and 25% injection rate, obtained with the experimental rheological curves, is shown in Fig. 7. The behaviour of this variable is the same as in the experiment, and the filling of the sprue and the cavity part can also be distinguished. The best agreement between the experimental and numerical pressure data is achieved at the end of the filling stage. With regard to the shear stress, if this variable is higher than its critical value of 0.2 MPa, the material degrades and this is reflected in the poor surface cosmetics (Serrano et al., 1995). At 195 ◦ C and 25% injection rate, Fig. 9 shows the distribution of the shear stress, where a circular, degraded zone appears around the gate due to the shear after the critical value has been reached.
4.3. Comparison of experimental and theoretical results The correlation between the theoretical and experimental results enables the reliability of the PVC fitting numerical model to be checked. This integration is made through comparison of the pressure at both sensor locations, and once the cavity pressure has been verified, the analysis of the shear stress zone around the gate and the flow front temperature in the central layer will enable the causes of the blush to be identified and determine the influence of the injection parameters on that defect.
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Fig. 11 – Maximum pressure evolution at 195 ◦ C.
Fig. 9 – Shear stress distribution around the gate.
model will be used to analyse the shear stress and flow front temperature results provided by the software.
4.3.2.
Fig. 10 – Maximum pressure evolution at 205 ◦ C.
4.3.1.
Pressure analysis
Figs. 10 and 11 show the evolution of the maximum pressure values at the end of the filling stage for both melt temperatures. The same qualitative behaviour of the experimental and numerical results can be observed. The quantitative difference is in the range 7–16% for both sensors at 205 ◦ C, and in the ranges 8–11% and 9–17% for pressures 1 and 2, respectively, at 195 ◦ C. Thus, cavity pressure measurements in the sprue bushing have provided effective control of the injection process and a good agreement between numerical and experimental results has been achieved. The computational
Flow front temperature
To determine if the blush can be produced by thermal degradation, the numerical temperature results obtained by Moldflow FEM software in the zone around the gate were analysed. Table 2 shows the maximum values in the central layer for all injection simulations. The theoretical results are higher than their respective injection melt temperatures and this indicates that when the material flows along the mould, its temperature increases due to shear, this effect being more important at 195 ◦ C as the viscosity function at this temperature is higher. It is interesting to note that the injection speed is a significant parameter since it turns out that the maximum temperature depends on it. During injection, the maximum temperature in the melt varies slightly for both inlet temperatures, and in any case, the maximum value exceeds the degradation temperature of 215 ◦ C therefore, the blush is not produced by thermal degradation.
4.3.3.
Shear stress
The blush at the gate can be predicted by the FE simulation through the shear stress. If the value of this variable is higher than the critical shear stress then the material is degraded and a white mark can be seen around the gate. Due to the circular shape of the defect, Figs. 12 and 13 show the diameter of the blush of the injected parts compared with the diameter of the shear stress zone where the value is higher than the critical value. For both melt temperatures, one can clearly
Table 2 – Maximum flow front temperature calculated using Moldflow 205 ◦ C
195 ◦ C
Injection rate (%)
Max. temperature (◦ C)
Injection rate (%)
Max. temperature (◦ C)
30 25 22 20 18 15
212.2 211.6 211.0 210.5 209.5 208.6
25 22 20 18 16
203.2 202.7 202.0 201.1 199.4
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mine which process conditions are most suitable to remove this flaw. 1. Results provided by computer simulation show that shearinduced degradation is responsible for the flaw, and that there is no thermal degradation. 2. The blush shows a strong dependence on the injection rate and decreases as the injection rate decreases. 3. Melt temperature can be considered as a secondary factor although higher melt temperatures are more suitable because the shear-induced degradation is lower than that at lower temperatures. 4. The simulation model can be used to analyse the effect of other dimensions of the sprue on the blush and to reduce the number of injection trials. Fig. 12 – Blush diameter comparison at 205 ◦ C.
Acknowledgement The authors would like to thank Pipelife Hispania S.A. for experimental support in performing the injection tests.
references
Fig. 13 – Blush diameter comparison at 195 ◦ C.
see that the blush has a strong dependence on the injection rate, and that the size of defect diminishes as the injection rate decreases. At 195 ◦ C, the size of the blush is larger, and this fact is more noticeable in the FE prediction than in the experimental injection however, the dependence on melt temperature is not significant. In addition, the blush at the gate and the shear stress above the critical value disappear at the same injection rate. It has virtually disappeared at 18% injection rate and 205 ◦ C, and the blush totally disappears at 16% injection rate and 195 ◦ C. Thus, the shear produced at high injection rates is the main reason for the blush, and injection at higher temperature diminishes the surface defect.
5.
Conclusions
Integration of computer simulation and injection trial results allowed the cause of the blush to be defined and to deter-
Disson, J.-P., Girois, S., 2003. Acrylic process aids for PVC: From theoretical concepts to practical use. J. Vinyl Technol. 9 (4), 177–187. Garcia, J.L., Koelling, K.W., Xu, G., Summers, J.W., 2004a. PVC degradation during injection molding: experimental evaluation. J. Vinyl Technol. 10, 17–40. Garcia, J.L., Koelling, K.W., Summers, J.W., 2004b. Computational prediction of PVC degradation during injection molding in a rectangular channel. Polym. Eng. Sci. 44, 1295–1312. Gruber, C.M., Gockowski, N.L., 1992. Molders’ perceptions of vinyl injection molding compounds. J. Vinyl Technol. 14 (2), 78–82. Min, B.H., 2003. A study on quality monitoring of injection-molded parts. J. Mater. Process. Technol. 136, 1–6. Serrano, M., Little, J., Chilcoat, T., 1995. Critical shear rate for the injection molding of polycarbonate, polystyrene and styrene acrylonitrile. In: Proceedings of Antec’ 95, Boston, I, pp. 357–365. Spina, R., 2004. Injection moulding of automotive components: comparison between hot runner systems for a case study. J. Mater. Process. Technol. 155–156, 1497–1504. Stevenson, J.C., Einhorn, R., 1993. Understanding the functions of acrylic aids in rigid PVC injection moulding. J. Vinyl Technol. 15 (4), 244–251. Turng, L.-S., Peic, M., 2002. Computer aided process and design optimization for injection moulding. J. Eng. Manuf. 216, 1523–1532. Weir, S., 1994. Predicting surface defects in injection molded PVC components. J. Vinyl Technol. 16 (4), 231–234.
journal homepage: www.elsevier.com/locate/jmatprotec
Review
Influence of injection parameters on the formation of blush in injection moulding of PVC ´ J. Llado´ ∗ , B. Sanchez Department of Mechanical Engineering, CPS, University of Zaragoza, 50018 Zaragoza, Spain
a r t i c l e
i n f o
a b s t r a c t
Article history:
This paper presents results of the study of influence of injection parameters of PVC fittings
Received 21 September 2007
moulding on the formation of blush. The aim was to determine the cause of the blush and
Received in revised form
predict its evolution in relation to the injection rate and melt temperature, based on Finite
3 December 2007
Element (FE) moulding software. The computation model was fitted by means of experimen-
Accepted 16 December 2007
tal tests carried out with a prototype fitting mould. Once the cavity pressure was verified, the analysis of results provided by the software such as the shear stress distribution around the gate and the flow front temperature enabled the identification of the injection rate as
Keywords:
the principal reason for the flaw, the melt temperature being a secondary factor. © 2007 Elsevier B.V. All rights reserved.
PVC Blush Injection pressure Flow rate
Contents 1. 2.
3. 4.
∗
Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Experimental method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Material and part geometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Pressure sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3. Injection machine and injection parameters. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Injection modelling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. Experimental results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2. Numerical results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3. Comparison of experimental and theoretical results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3.1. Pressure analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3.2. Flow front temperature. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3.3. Shear stress . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Corresponding author. ´ E-mail address: [email protected] (J. Llado). 0924-0136/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2007.12.063
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5.
1.
Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Introduction
Polyvinyl chloride (PVC) is most commonly used in the construction sector because of its excellent weather, chemical and flame resistance properties. In recent years, its use has been growing in specialty injection moulding markets such as domestic appliances, business machines, medical devices and consumer electronics. Despite its advantageous properties, the injection of PVC is regarded as troublesome by some moulders (Gruber and Gockowski, 1992), because this material often decomposes and burns during processing, releasing hydrochloric acid, which rusts the equipment; in addition, surface defects can appear. To overcome the perception of the difficult injection of this material, the processability of PVC compounds has been improved by development of highflow (low-viscosity) PVC grades through the use of acrylic processing aids. These aids promote PVC fusion, modify the melt rheology and/or provide lubrication. Some processing aid products are designed to serve one of these functions while others provide a combination of functions (Stevenson and Einhorn, 1993; Disson and Girois, 2003). Furthermore, significant advances have also been made in processing equipment and conditions. Previous experiments have also indicated that the study of processing parameters is very important because just setting them correctly covers about 90% of the problems encountered in PVC injection, such as degradation (Garcia et al., 2004a) or surface defects (Weir, 1994). Companies always strive to produce high-quality parts while lowering their costs; however, significant time delays and increasing costs may occur if part design is not carefully evaluated or the injection process is not completely understood. The factors involved in the injection moulding process that have a great influence on the final quality of plastic products can be classified into the following four categories: materials, moulding machine, model design and processing conditions (Min, 2003). Each factor, accurately chosen and controlled, requires a multi-disciplinary knowledge to improve and optimize the final product, but the process of filling a cavity mould with a plastic melt is complex due to significant interactions between variables, and this requires particular attention when setting up the machine. Injection moulding software packages are widely used to analyse product performance and processability and represent a powerful tool to evaluate the moulding conditions with a high level of detail because they provide a lot of predicted data that are not normally available during experimental tests (Spina, 2004). A thorough analysis of the numerical results enables potential problems in product injection to be identified and resolved by proposal of a new set of process variables that improve the part quality, although huge amounts of computer-generated data and complex nonlinear interactions between all process variables frequently make the selection of the optimal design and process parameters very difficult (Turng and Peic, 2002).
7 7 7
This work refers to an industrial problem, that is, the blush that appears around the gate in the injection of PVC fittings. The target was to determine the cause of the blush and predict the evolution of this flaw in relation to the injection rate and melt temperature, using Moldflow numerical simulation software. To adjust the computation model, a prototype fitting mould was designed and experimental tests were performed, where the pressure inside the sprue bushing is controlled. The pressure data obtained during the injection trials were used to obtain the viscosity curves to be applied in the numerical simulation.
2.
Experimental method
2.1.
Material and part geometry
The material used in this work was provided by Solvay. The PVC compound injected was Benvic IR705, a standard material used for the injection of PVC fittings. This study is directed towards understanding the industrial problem of the blush around the gate during injection moulding of PVC fittings. These parts are cylindrical and usually have a centre sprue gate from which a radial flow goes through the cavity. It was decided to design a centre-gated semicylinder, as shown in the mould in Fig. 1; this is easier to eject and mimics the radial flow produced on the fittings. The dimensions of the part were chosen by the company Pipelife Hispania S.A., which was interested in studying this problem. The thickness chosen was 8 mm, the width 200 mm, and the length 200 mm. The sprue was 70 mm long, with an inlet diameter of 6 mm and the angle of the conical section was 3◦ .
2.2.
Pressure sensors
For a better understanding and control of the injection process, it is important to understand the evolution of some variables that can be checked with the results provided by injection moulding packages. The plastic pressure in the mould and the melt temperature are the two variables. At present, there are no techniques available to measure the actual melt temperature profile in the cavity without affecting the flow (Garcia et al., 2004b). A previous attempt was made by the authors to measure the melt temperature with an infrared sensor, mounted at the same level as the mould wall, but this sensor was not able to measure the maximum melt temperature in the central layer of the flow, and it only provided the evolution of the temperature near the mould wall, where there was a temperature gradient. So, experimentally, the thermal degradation of the PVC is difficult to determine. In this work, to ensure the quality of PVC injected parts, the quantitative assessment of cavity pressure history is obtained using two Kistler 6157BSP0 quartz sensors placed at locations
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Fig. 1 – Cavity mould.
P1 and P2 in the sprue bushing (Fig. 2). The lower part of the sprue bushing is rectangular to avoid the relative rotation of this element that could preload the sensors when the nozzle leans against the mould. The voltage provided by both sensors is amplified and transmitted through an Adaptor DAS16 multi-channel acquisition system to a PC (Fig. 3), where the data are registered by means of DFPLUS software, where some values must be supplied such as the number of injection cycles, the number of values recorded per second and the amplifier features.
2.3.
Injection machine and injection parameters
The injection moulding trials were performed at Pipelife Hispania’s facilities, with a Cincinnati Milacron 250 injection moulding machine, specific for PVC injection, with maximum
values of clamp force, injection pressure and velocity equal to 153 kN, 1400 kg/cm2 and 368 cm3 /s, respectively. The processing conditions evaluated were the injection speed and the melt temperature. Two melt temperatures were selected in the range recommended for this material, 205 ◦ C and 195 ◦ C, measured with a portable infrared thermometer when the material left the nozzle. Fig. 4 shows the cylinder and nozzle temperatures corresponding to each case. Initial tests were carried out to determine the injection speeds at each temperature. The selection of the injection speeds was based on the fact that they were within the usual range of flow rates used in the injection of PVC fittings, and that the material was not burned at the highest injection rate. At 205 ◦ C melt temperature, injection trials were performed at injection rates of 30, 25, 22, 20, 18 and 15%, which means a flow rate ranging from 113 to 20 cm3 /s. In the case of 195 ◦ C, the injection rates
Fig. 2 – Location of pressure sensors.
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Fig. 3 – Measurement chain. Fig. 5 – Part mesh.
As in the experimental injection trials, eleven simulations were performed using the same melt temperatures, injection rates and packing conditions.
Fig. 4 – Cylinder and nozzle temperatures.
were 25, 22, 20, 18 and 16%, which correspond to a flow rate range between 70 and 19 cm3 /s. After the filling stage, to avoid sink marks, all parts were packed during 5 s, with a packing pressure equal to 80 bar. Five parts were injected at each injection velocity, and every pressure profile registered by both pressure sensors was recorded in each test.
3.
Injection modelling
4.
Results and discussion
4.1.
Experimental results
For both temperatures and all injection rates, the pressure profiles recorded by both sensors are qualitatively similar. As an example, Fig. 7 shows a typical pressure profile measured by both sensors during the injection of the part at 195 ◦ C and 25% injection rate. As soon as the flow front reaches the sensor, measurement starts and two stages can be observed. Initially, during the filling of the sprue, the curve rises sharply due to the large drop in pressure. Then the second step corresponds to the filling of the part, where pressure increases slightly since the part is thick and it does not offer too much resistance to be filled. Finally, the beginning of the packing phase can be seen where the pressure increases sharply again. If all records are analysed, it is observed that the maximum pressure at the
Moldflow FEM software has been used to simulate the injection trials. First, the part was meshed, as shown in Fig. 5. The sprue was modelled using 7 cold runner elements that fed a semicylindrical cavity of 276 shell triangular elements. The properties of the PVC Benvic IR705 are listed in Table 1, and the critical shear rate and degradation temperature are about 0.2 MPa and 215 ◦ C, respectively. Fig. 6 shows the viscosity plotted against the shear rate, at different temperatures, provided by the material supplier.
Table 1 – Properties of the PVC Solvay Benvic Property
Value 3
Density (kg/dm ) Conductivity (W m−1 ◦ C−1 ) Glass transition temperature (◦ C) Specific heat (J kg−1 ◦ C−1 ) Shrinkage (%)
1.32 0.13 79–80 1767 0.6
Fig. 6 – Viscosity data.
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Fig. 7 – Experimental and numerical pressure evolution at 195 ◦ C and 25% injection rate.
end of the filling stage is higher at 195 ◦ C than at 205 ◦ C, and that pressure increases as injection rate increases. In addition to the pressure records, the parts injected in each test were examined to observe the evolution of the cosmetics of the part. Inspection of the samples showed the appearance of a circular white mark around the gate. For both melt temperatures, the largest defect appears at the highest injection rate and the blush diminishes as the injection rate decreases and the melt temperature is higher. The flaw mainly depends on the injection rate, the melt temperature being a secondary factor. So the shear produced at high injection rates seems to be the main reason for the blush. The influence of thermal degradation will be determined by means of injection simulation.
4.2.
Numerical results
Initial simulations performed with the rheological characteristics provided by the material supplier, showed a difference of about 30% between the experimental and theoretical pressure data at the end of the filling stage. The reliability of the numerical simulation depends mainly on the rheological data of the PVC. Therefore, to characterise the behaviour of the polymer during injections at 195 ◦ C and 205 ◦ C, the maximum pressure values recorded by both sensors at the end of the filling of the sprue have been used to calculate the viscosity function according to Poiseuille’s law, as Eq. (1) states: =
4r4 P 8QL
(1)
where (Pa s) is the viscosity, r (m) and L (m) are the average radius of the sprue and the distance between both sensors, respectively, Q (m3 /s) is the flow rate and P (Pa) is the drop in pressure. The adjustment of the viscosity curves at 195 ◦ C and 205 ◦ C is performed by fitting a potential function to the experimental values obtained with Eq. (1). If these two experimental viscosity curves are compared with the rheological data of the PVC obtained by means of a rheometer (Fig. 8), it can be observed that the viscosity data at 195 ◦ C coincide
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Fig. 8 – Viscosity data comparison.
with the 200 ◦ C viscosity curve. This discrepancy can result from the fact that the method performed with the rheometer is much more controlled than the injection tests where the control of the melt temperature is not so precise, because machine variables such as barrel temperature, back pressure, screw RPM, screw geometry, clearance between the screw and the barrel, all affect the melt temperature. As a consequence, to correlate the information recorded during the injection of the PVC prototype parts with the numerical results from the FEM injection software, it has been considered convenient to perform the numerical simulations using the viscosity curves determined from the injection tests. Pressure evolution at 195 ◦ C and 25% injection rate, obtained with the experimental rheological curves, is shown in Fig. 7. The behaviour of this variable is the same as in the experiment, and the filling of the sprue and the cavity part can also be distinguished. The best agreement between the experimental and numerical pressure data is achieved at the end of the filling stage. With regard to the shear stress, if this variable is higher than its critical value of 0.2 MPa, the material degrades and this is reflected in the poor surface cosmetics (Serrano et al., 1995). At 195 ◦ C and 25% injection rate, Fig. 9 shows the distribution of the shear stress, where a circular, degraded zone appears around the gate due to the shear after the critical value has been reached.
4.3. Comparison of experimental and theoretical results The correlation between the theoretical and experimental results enables the reliability of the PVC fitting numerical model to be checked. This integration is made through comparison of the pressure at both sensor locations, and once the cavity pressure has been verified, the analysis of the shear stress zone around the gate and the flow front temperature in the central layer will enable the causes of the blush to be identified and determine the influence of the injection parameters on that defect.
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Fig. 11 – Maximum pressure evolution at 195 ◦ C.
Fig. 9 – Shear stress distribution around the gate.
model will be used to analyse the shear stress and flow front temperature results provided by the software.
4.3.2.
Fig. 10 – Maximum pressure evolution at 205 ◦ C.
4.3.1.
Pressure analysis
Figs. 10 and 11 show the evolution of the maximum pressure values at the end of the filling stage for both melt temperatures. The same qualitative behaviour of the experimental and numerical results can be observed. The quantitative difference is in the range 7–16% for both sensors at 205 ◦ C, and in the ranges 8–11% and 9–17% for pressures 1 and 2, respectively, at 195 ◦ C. Thus, cavity pressure measurements in the sprue bushing have provided effective control of the injection process and a good agreement between numerical and experimental results has been achieved. The computational
Flow front temperature
To determine if the blush can be produced by thermal degradation, the numerical temperature results obtained by Moldflow FEM software in the zone around the gate were analysed. Table 2 shows the maximum values in the central layer for all injection simulations. The theoretical results are higher than their respective injection melt temperatures and this indicates that when the material flows along the mould, its temperature increases due to shear, this effect being more important at 195 ◦ C as the viscosity function at this temperature is higher. It is interesting to note that the injection speed is a significant parameter since it turns out that the maximum temperature depends on it. During injection, the maximum temperature in the melt varies slightly for both inlet temperatures, and in any case, the maximum value exceeds the degradation temperature of 215 ◦ C therefore, the blush is not produced by thermal degradation.
4.3.3.
Shear stress
The blush at the gate can be predicted by the FE simulation through the shear stress. If the value of this variable is higher than the critical shear stress then the material is degraded and a white mark can be seen around the gate. Due to the circular shape of the defect, Figs. 12 and 13 show the diameter of the blush of the injected parts compared with the diameter of the shear stress zone where the value is higher than the critical value. For both melt temperatures, one can clearly
Table 2 – Maximum flow front temperature calculated using Moldflow 205 ◦ C
195 ◦ C
Injection rate (%)
Max. temperature (◦ C)
Injection rate (%)
Max. temperature (◦ C)
30 25 22 20 18 15
212.2 211.6 211.0 210.5 209.5 208.6
25 22 20 18 16
203.2 202.7 202.0 201.1 199.4
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mine which process conditions are most suitable to remove this flaw. 1. Results provided by computer simulation show that shearinduced degradation is responsible for the flaw, and that there is no thermal degradation. 2. The blush shows a strong dependence on the injection rate and decreases as the injection rate decreases. 3. Melt temperature can be considered as a secondary factor although higher melt temperatures are more suitable because the shear-induced degradation is lower than that at lower temperatures. 4. The simulation model can be used to analyse the effect of other dimensions of the sprue on the blush and to reduce the number of injection trials. Fig. 12 – Blush diameter comparison at 205 ◦ C.
Acknowledgement The authors would like to thank Pipelife Hispania S.A. for experimental support in performing the injection tests.
references
Fig. 13 – Blush diameter comparison at 195 ◦ C.
see that the blush has a strong dependence on the injection rate, and that the size of defect diminishes as the injection rate decreases. At 195 ◦ C, the size of the blush is larger, and this fact is more noticeable in the FE prediction than in the experimental injection however, the dependence on melt temperature is not significant. In addition, the blush at the gate and the shear stress above the critical value disappear at the same injection rate. It has virtually disappeared at 18% injection rate and 205 ◦ C, and the blush totally disappears at 16% injection rate and 195 ◦ C. Thus, the shear produced at high injection rates is the main reason for the blush, and injection at higher temperature diminishes the surface defect.
5.
Conclusions
Integration of computer simulation and injection trial results allowed the cause of the blush to be defined and to deter-
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