Int J Adv Manuf Technol (2013) 67:675–686 DOI 10.1007/s00170-012-4514-4

ORIGINAL ARTICLE

An experimental work on the effect of injection molding parameters on the cavity pressure and product weight Hamdy Hassan

Received: 28 May 2012 / Accepted: 11 September 2012 / Published online: 10 October 2012 # Springer-Verlag London Limited 2012

Abstract The injection molding process is one of the most efficient processes where mass production through automation is feasible and products with complex geometry at low cost are easily attained. In this study, an experimental work is performed on the effect of injection molding parameters on the polymer pressure inside the mold cavity. Also, the effect of these parameters on the final products' weight is studied. Different process parameters of the injection molding are considered during the experimental work (packing pressure, packing time, injection pressure, injection time, and injection temperature). Two polymer materials are used during the experimental work (polystyrene (PS) and lowdensity polyethylene (LDPE)). The mold cavity has a cuboidal form with two different thicknesses. The cavity pressure is measured with time by using pressure Kistler sensor at different injection molding cycles. The results indicate that the cavity pressure and product weight increase with an increase in the packing pressure, packing time, and injection pressure for all the analyzed polymers. They also show that the increase of the filling time decreases the cavity pressure and decreases the product weight in case of PS and LDPE. The results show that the increase of packing pressure by 100 % increases the cavity pressure 50 % in the case of PS and 70 % in the case of LDPE. They also show that the increase of injection pressure by 60 % increases the cavity pressure 36 % in case of PS and 90 % in case of LDPE at an injection temperature of 220 °C. The results indicate that process parameters have an effect on the product weight for LDPE greater than PS. The results obtained specify well the H. Hassan (*) Mechanical Engineering Department, Assiut University, Assiut, Egypt e-mail: [email protected] H. Hassan TEMPO, UVHC, 59313, Valenciennes, France

developing of the cavity pressure inside the mold cavity during the injection molding cycles. Keywords Injection molding . Cavity pressure . Weight . Polystyrene . LDPE

1 Introduction Plastic injection molding is a widely used and complex process and is a highly efficient technique for producing a large quantity of plastic products, with high production requirement and tight tolerance. It is well-known as the manufacturing technique to produce products with various shapes and complex geometry at low cost [1]. The quality of parts produced by injection molding is a function of plastic material, part geometry, mold structure, and process conditions [2, 3]. The plastic injection molding process is a cyclic process. It begins with feeding the resin and the appropriate additives with polymer material from the hopper to the heating system of the injection molding machine. Then, the mold cavity is filled with the hot polymer melt at injection temperature (filling stage). After the cavity is filled, additional polymer melt is packed into the cavity at a higher pressure during the packing stage (post-filling stage) to compensate the expected shrinkage when the polymer solidifies. This is followed by “cooling stage” where the mold is cooled until the part is sufficiently rigid to be ejected. The last step is the ejection stage where the mold is opened and the part is ejected. Then, the mold is closed again to begin the next cycle [4, 5]. The quality of the final molded part, which may be characterized in terms of dimensional stability, appearance, and mechanical properties, is highly dependent on the processing variables [5]. Defects in the products, such as warpage, shrinkage, sink marks, and residual stress, are caused by many factors during the production process. These defects are detrimental to the quality

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and accuracy of the products. Therefore, it is of critical importance to effectively control the influencing factors during the molding procedure [4]. During the injection processing, there is a strict correlation between process parameters and the quality of product produced. For instance, improper settings of process parameters will induce defects on the products, such as warpage, shrinkage; sink mark, and residual stress [6, 7]. The cavity pressure profile and its repeatability remarkably influence the quality of the molded part, especially on its mass, dimensional stability, mechanical behavior, and the surface quality [8]. The cavity pressure of a complete injection molding cycle can be categorized in four stages [9]. The pressure is still nonexistent at the initial stage, but increases somewhat due to the filling of a small amount of feedstock into the mold. The pressure keeps building up to the maximum value at the third stage and the cavity is quickly filled with molten feedstock. In the final stage, the screw can offer a holding pressure but is unable to move forward because the runner and the gate of the mold are nearly frozen. Besides exerting a static pressure by the screw, the injection molding pressure can also be achieved by a backward movement of the mold wall [10]. Injection molding pressure is one of the key processing parameters mentioned in several research works. Zhang et al. [11] showed that the cavity pressure would gradually decay and vanish eventually if the temperature of the mold is low enough. The pressure drop (the difference between injection pressure and cavity pressure) is due to the solidification of feedstock and the friction at the mold surface. Kostic et al. [12] showed that the packing pressure is one of the main parameters which affect the cavity pressure and final product quality. In addition, several material properties change as the cavity pressure increases. They found that the higher the holding pressure is, the greater the cavity pressure will be. The residual pressure of the injection molding part is still detectable if the holding pressure is exerted until the mold opens. The cavity pressure is influenced by the temperature of the mold and increases as the temperature of the mold increases. A physically based adaptive controller has been developed experimentally for cavity pressure control during the packing phase by Mehdi Rafizadeh et al. [13]. Their results indicate that this control yields accurate results for the injection molding of polyethylene. Wen-Cheng et al. [9] studied the effects of injection molding pressure and holding time on the properties of injection-molded (IM) parts, and the pressure–volume–temperature relationship of the plastic ingredient and alumina feed stocks was investigated. The properties of IM parts include the dimension, surface fatness, green density, and sintered density. The results reveal that the pressure–time traces where the gate pressure is increased from 22 to 117 MPa, and the holding time up to 75 s illustrates the trend of the property change under these pressures. High holding pressures and longer holding time would be favored to increase molded dimension and to decrease the surface

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sinking of green pieces. The data also show that the pressures and holding time exert no significant influence on the bulk density of green parts. Huang [8] presented a novel method by which quick and accurate decisions concerning the ideal switchover time can be made. He adopted a simple gray model to predict instantaneously the volumetric-filling point when monitoring the cavity pressure profile in each molding. Cavity pressure and mold surface temperature were measured and recorded by pressure and temperature–pressure sensors by Kurt et al. [14]. The results of their experimental work indicate that cavity pressure and mold temperature are the dominant factors determining the quality of the final product in plastic injection molding. The measurement of the temperature at the center of the cavity during the molding of PP material using an embedded thermocouple probe was presented by Cuong Le et al. [15]. Their measurements, done at various pressures, showed an increase in the crystallization plateau temperature depending on the cavity pressure. The change in the measured crystallization temperature allows the identification of the pressure dependence on the Hoffman–Lauritzen crystallization kinetic parameters. The obtained results also showed a decrease in cavity pressure duration related to the increase in the gate-freezing speed as a result of the pressure increase. Angstadt and Coulter [16] investigated the relationship between cavity pressure and part quality in the injection molding process. According to their research, the use of cavity pressure as an indicator of part quality has significant potential. In this paper, the effect of the injection molding parameters (packing pressure, packing time, injection pressure, injection time, and injection temperature) on the cavity pressure is studied experimentally. Also, their effect on the weight of the injection molding products is performed. The experimental work is performed on an injection molding machine of type Demag Erotech 25/ 280-80 as shown in Fig. 1a. The cavity pressure was measured for various injection molding's cycles. The experimental work was performed for two polymer materials: polystyrene (PS) and low-density polyethylene (LDPE). The mold cavity has a cuboidal form with two different thicknesses. The experimental work is carried out for two injection temperatures: 220 and 240 °C for LDPE and 220 and 235 °C for PS.

2 Experimental work The experimental work was carried out on an injection molding machine of type Demag Erotech 25/280-80 concept having a maximum injection pressure 2,870 bar, with cylinder diameters for plastication18, 22, and 25 mm and maximum weight of the product 48 g as shown in Fig. 1a. During the experimental work, two types of polymer material are used:

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a

a

Injectionmolding machine

Amplifier with recorder Pressure signal input to output the amplifier

Acquisition system

Computer recording the signal output

b

Pressure sensor Mold cavity Fixed part

Fig. 2 a Polystyrene product (PS). b Fixation of pressure transducer inside the mold

b

Output pressure

signals from the amplifier are 0–10 DC voltages which are transmitted to the recorder and the data acquisition system. Universal indicator: This is used to display the different measurement signals on an output indicator. Acquisition data: Data output from the amplifier is collected using a Hewlett-Packard (HP) 3852A data acquisition/control system equipped with two HP 44711 24-channel. Computer: It is used to record the output reading of the acquisition system through an interface cart by the help of lab view program. Kistler6159A: The cavity pressure is measured in the mold cavity by the quartz sensor for mold cavity pressure type Kistler 6159A, which has a front of 1.0 mm diameter as shown in Fig. 2b. The pressure transducer has a sensibility of ±2.5 bar and it is able to register changes of the pressure as a function of time with the resolution up to 0.01 s. The pressure acting directly on the entire front of the sensor is transferred to the quartz measuring element, which produces an electrical charge proportional to the pressure.

Acquisition system Recorder

Injection molding machine

Mold temperature

Computer Amplifier

Fig. 1 a Experimental setup. b The experimental layout

& &

BP polystyrene PS-LD 45-210 (PS). B-ALCUDIA LDPE (PE-017) produced by highpressure autoclave technology (LDPE). Further details on the injection molding machine and polymer materials used in the study are found at [17]. The experimental setup as shown in Fig. 1a, b consists of: Mold: It is constructed from steel 40 CMD and it is constructed from two parts (fixed part and movable part). Each part has squared cross-sectional area of dimensions 200×200 mm2. The mold cavity is located at the movable part of the mold, and it has the form of cuboids with dimensions of 80×80 mm2. The mold cavity has two different thicknesses of 3 and 1.5 mm. Figure 2a shows the product produced by the injection molding and the cavity pressure is measured at the back of the center of the thick part. Amplifier (type Kistler 5039A): It is used to convert the electrical charge of the pressure sensor signals yielded by piezoelectric into proportional voltages. The output of the amplifier is transferred to the center of acquisition system and another output is transferred to a universal indicator. The output

The polymer material inside the hopper passes to the plastification cylinder where it is melted to the injection temperature and injected to the mold cavity through the runner and injection gate. The hot plastic material injected inside the cavity is cooled by cooling water circulating through four cooling channels inside the fixed and movable parts. At the end of the cooling stage, the product is ejected out of the machine to start a new cycle. During the experimental work and for each case studied, the following procedures are performed: & &

Adjust the cooling water inlet to have the same inlet temperature and flow rate for all cases. Adjust the process parameters (injection temperature, cooling time, injection time, etc.) of the injection molding machine by the machine regulator to suit each studied case and wait until reaching steady-state values

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& & & & &

& &

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especially for inlet polymer temperature before starting the injection molding cycle. Wait sufficient time until all the thermocouple readings by the recorder have the same values to ensure that the mold temperature is initially at the same temperature of water inlet. Run the acquisition data system for reading all of the input values (mold temperature, inlet temperature of cooling water flow rate, and cavity pressure). Run the lab view program through the computer to record the output data from the acquisition data system in a separate file inside the computer. Run the injection molding machine to start mold cycles until finishing the required cycles for each case studied. During all case studied, verifying that the values of the flow rate and inlet water temperature are constant during each experimental case by verifying their value through the indicator. Wait until the reading of all thermocouple gives the same reading of the initial value of the inlet water temperature. Repeating the same previous procedures to start another case.

3 Results and discussion The molded polymers are affected by many machine parameters and process condition. Incorrect process parameter settings will cause bad quality of surface roughness, decreases dimensional precision, product warpage, unacceptable wastes, and increases lead time and cost [18]. Optimization of all earlier mentioned parameters increases product quality and at the same time increases the machine life and decreases the process cost. Therefore, finding the optimized parameters is highly desirable. So, an experimental work is carried out on the effect of the packing pressure, packing time, injection pressure, and injection time on the cavity pressure and product weight. The experimental work is carried out for two polymer materials: PS and LDPE. The work is performed for two injection temperatures of the polymer material: 220 and 240 °C for LDPE and 220 and 235 °C for PS. The other parameters used in the experimental work are shown in Table 1 except those which will be illustrated in the text. 3.1 Effect on cavity pressure The quality and the cost of the products produced by the injection molding are affected by many parameters. One of these main parameters is the cavity pressure where its profile during injection molding cycles is a primary factor affecting the final part quality [19]. This profile and its repeatability remarkably influence the quality of the molded part, especially on its mass, dimensional stability, mechanical behavior, and the surface quality [8]. In addition, knowledge of the cavity

pressure is useful for estimating part shrinkage and clamping force requirements. Cavity pressure value affects on the product warpage and the residual stresses produced during the injection process, as well as significant changes in the physical properties of parts. So, it is important to study the cavity pressure and the main parameters' influence on its profile because: 3.1.1 Effect on packing pressure During the packing stage, additional melt is fed into the mold cavity to compensate for the shrinkage during subsequent cooling. Usually, the filling stage takes a relatively short period of time, while the post-filling (package) stage requires much longer time. It is widely recognized that the microstructure and its properties and quality, for example, birefringence, residual stresses, density distributions, and product shrinkage and warpage, are closely related to the packing and cooling conditions. Therefore, an understanding of the cavity pressure during post-filling stages is crucial for successful determination of the ultimate mechanical properties and real process control [20]. The effect of changing of packing pressure on the cavity pressure for different injection temperatures in case of PS and PE is shown in subpanels a and b of Fig. 3, respectively. Figure 3 indicates that the increase of packing pressure increases the cavity pressure for all analyzed polymer materials. It also shows that the increase of the injection temperature increases the cavity pressure and the value of cavity pressure in case of LDPE is greater than its value for PS. These results will be clear with the analysis of the cavity pressure evolution with time for different packing pressures for PS and LDPE shown in subpanels a and b of Fig. 4, respectively. Figure 4 is illustrated at an injection temperature of 220 °C. Figure 4 shows that at high packing pressure, the cavity pressure increases with time and a sensible value of the pressure inside the cavity is found during the injection, packing, and cooling stages. This means the cavity pressure pushes the pressure sensor during all these stages. Table 1 Process parameters used in the experimental work Process parameter

Value

Cooling water inlet temperature Packing time Packing pressure (PS) Packing pressure (LDPE) Injection pressure Injection time Ejection time Injection velocity Cooling time Rotation velocity

19 °C 3s 45 MPa 50 MPa 60 MPa 1.53 s 7.6 s 30 mm/s 14 s 100 rpm

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Fig. 3 a Effect of packing pressure on the cavity pressure for PS. b Effect of packing pressure on the cavity pressure for LDPE

At low packing pressure, the cavity pressure reaches to its maximum value and then decreases with time in case of PS and it vanishes at the end of the packing stage in case of LDPE. In case of PS, the development of the cavity pressure inside the mold cavity could be divided into three stages; during the filling stage, the pressure increases to a certain value, and at the beginning of the packing stage, the pressure increases to its maximum value. Then, the pressure maintains or decreases depending on the packing pressure value. From the results of Fig. 4, it could be predicted that the product shrinkage decreases with increasing the packing pressure. Also, the shrinkage in case of LDPE is greater than the shrinkage of PS, so we find that the cavity pressure decreases rapidly or vanishes in case of LDPE as shown in Fig. 4b. Figure 4 shows that the cavity pressure reaches its maximum value after the injection time in case of LDPE and at packing time in case of PS. From the analysis of Fig. 4, the history of the development of the cavity pressure inside the mold cavity is well specified for PS and LDPE. Figure 3 indicates that the increase of packing pressure by 100 %

Fig. 4 a Evolution of the cavity pressure with cycle time for different packing pressures for PS (Tinjection 220 °C). b Evolution of the cavity pressure with cycle time for different packing pressures for LDPE (Tinjection 220 °C)

increases the cavity pressure by 50 % in case of PS and 70 % in case of LDPE at injection temperature of 220 °C. 3.1.2 Effect of injection pressure The mold filling process is a very important step in the resin transfer molding, governing the performance of the final product and tool design. It is affected by several parameters comprising mold geometry, inlet gate(s) location, injection pressure or flow rate, resin rheology, permeability of fibrous media, and temperatures of mold wall and inlet resin. Higher injection pressure decreases the mold filling time; however, it may cause mold deformation and fiber washout [21]. The effect of the injection pressure on the cavity pressure for different injection temperatures in the case of PS and LDPE

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Fig. 5 a Effect of injection pressure on the cavity pressure for PS. b Effect of injection pressure on the cavity pressure for LDPE

is shown in subpanels a and b of Fig. 5, respectively. Figure 5 shows that the increase of the injection pressure increases the cavity pressure, and the cavity pressure in case of LDPE is greater than that of PS. It also shows that the cavity pressure increases with increasing the injection temperature because the increase of the injection temperature increases the specific volume and decreases the viscosity of the polymer. Hence, the fluidity of the polymer to enter the mold cavity increases. These results will be well illustrated by the analysis of the development of the pressure inside the mold cavity at different injection pressures for PS and LDPE as shown in subpanels a and b of Fig. 6, respectively. Subpanels a and b Fig. 6 demonstrated the injection temperatures of 235 and 240 °C, respectively. Figure 6a shows that for PS and at low injection pressure, the cavity pressure increases with time to the end of cycle time because the packing pressure is greater than injection pressure. For high injection pressure, the cavity pressure increases with injection time, and during the packing stage,

Fig. 6 a Evolution of the cavity pressure with time for different injection pressures for PS (Tinjection 235 °C). b Evolution of the cavity pressure with time for different injection pressures for LDPE (Tinjection 240 °C)

the cavity pressure decreases slightly and then returns to increase to its higher value. This is due to the packing pressure in this case which is smaller than the injection pressure. But in the case of LDPE, the cavity pressure increases until the end of packing stage and then decreases because the shrinkage of the LDPE is higher than the shrinkage of PS [22, 23]. Figure 5 shows that for injection temperature of 220 °C, the increase of injection pressure of 60 % increases the cavity pressure by 36 % in case of PS and 90 % in the case of LDPE. 3.1.3 Effect of packing time In the filling of the mold cavity, in order to yield a good quality product, molten polymer must fill the cavity without making any air entrapments, weld lines, or any other

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defects. After filling, additional polymer melt is pushed into the cavity to compensate for any possible shrinkage caused by a phase change of the material. This can be accomplished by increasing the packing pressure or increasing the packing time which increases the cycle time. Figure 7a, b shows the effect of packing time on the cavity pressure at different injection temperatures for PS and LDPE, respectively. Figure 7 shows that the increase of the packing time increases the cavity pressure and hence decreases the material shrinkage. Figure 8a, b shows the evolution of the cavity pressure at an injection temperature of 220 °C with cycle's time at different packing times for PS and LDPE, respectively. Figure 8a shows that at lower value of packing time, the cavity pressure increases to its higher value and then decreases with an increase in the cycle time. This is because the injected material inside the mold cavity is not sufficient to compensate the material shrinkage. This effect is similar to the effect of lower packing pressure on cavity pressure as shown in Fig. 4a. In the case of LDPE and at higher packing time, the material enters the cavity during the packing stage

Fig. 8 a Evolution of the cavity pressure with time for different packing times for PS (Tinjection 220 °C). b Evolution of the cavity pressure with time for different packing times for LDPE (Tinjection 220 °C)

increases the cavity pressure. This inlet material maintains cavity pressure constant during the injection molding cycle as shown in Fig. 8b because it compensates the material shrinkage during cooling. Figure 7 shows that for injection temperature of 220 °C, the increase of packing time nine times increases the cavity pressure by 21 % in case of PS and the increase of packing time four times increases the cavity pressure by 20 % in the case of LDPE. It also show that the cavity pressure increases with an increase of the injection temperature at all packing times. 3.1.4 Effect of injection time

Fig. 7 a Effect of packing time on the cavity pressure for PS. b Effect of packing time on the cavity pressure for LDPE

Even though an injection molding cycle can last up to several minutes, it is now recognized that the properties of the surface and subsurface zones are largely affected by the flow and

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thermal conditions prevailing during the filling stage of the process [24]. The cycle time is one of the major factors to determine the efficiency of the injection molding process. In view of the economic potential inherent in faster molding cycles, substantial effort has been centered on optimizing the movement of the various parts of the machine [25]. The effect of the injection time on the cavity pressure at different injection temperatures for PS and LDPE is shown in subpanels a and b of Fig. 9, respectively. During injection molding process, the increase of the injection time is followed by decreasing the injection pressure to have the same mass input to the mold cavity during injection stage. Hence, the cavity pressure decreases with an increase in the injection time due to the decrease of injection pressure as shown in Fig. 9. Figure 9 also shows that the injection temperature has an effect on the cavity pressure in case of LDPE greater than its effect in case of PS especially at low injection time. This is due to the shrinkage in case of LDPE is greater than that of PS. The increase of the injection time in case of LDPE by about 83 % decreases the cavity pressure by 23 % at injection temperature of 235 °C and 36 % at injection temperature of 240 °C as shown in Fig. 9b.

Fig. 9 a Effect of injection time on the cavity pressure for PS. b Effect of injection time on the cavity pressure for LDPE

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Figure 9a shows that the increase of the injection time 83 % decreases the cavity pressure about 23 % at injection temperature 220 °C for PS. The evolution of the cavity pressure with cycle time at injection temperature of 220 °C for PS and LDPE is shown in subpanels a and b of Fig. 10, respectively. Figure 10 shows that the increase of injection time decreases the cavity pressure, but it has no great effect on the cavity pressure profile for all polymer materials analyzed. It also shows that the increase of the injection time decreases the total cycle time. 3.2 Effect on product weight During the experimental work, the weight of the product produced by the injection molding process was measured after it is completely solidified. The weight of the product

Fig. 10 a Evolution of the cavity pressure with time for different injection times for PS (Tinjection 220 °C). b Evolution of the cavity pressure with time for different injection times in case of LDPE (Tinjection 220 °C)

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During the injection molding process, after the cavity is filled, the cooling continues and material in the cavity undergoes contraction, creating space for additional material. If the packing pressure is imposed after filling, additional material is supplied. Obviously, proper packing will reduce volumetric shrinkage and the linear shrinkage itself, yielding parts with dimensions closer to those of the cavity. However, lower shrinkage is sometimes obtained as a trade-off for other properties, i.e., internal stress level or warpage due to uneven shrinkage [22]. The effect of packing pressure on the product weight produced by injection polymer at different injection temperatures for PS and LDPE is shown in subpanels a and b of Fig. 11, respectively. Figure 11

shows that increasing the packing pressure increases the material weight and hence decreases material shrinkage grace of the material added due to increasing the packing pressure. It also shows that at the same parameters' input, the weight of the product increases with decrease of the injection temperature because the density of the product material increases with a decrease its injection temperature. The difficulty of polymer injection at low temperature required an additional force of the injection molding machine because of the higher density and viscosity of the polymer material. Figure 11 shows that the weight of the PS product is higher than the weight of the LDPE product. It also shows that the effect of injection temperature on the product weight increases with an increase in the packing pressure. At the injection temperature of 220 °C, the increase of the packing pressure by about 120 % increases the product weight by about 11 % for PS as shown in Fig. 11a, b shows that an increase of packing pressure 200 % increases the product weight about 8 % in case of LDPE.

Fig. 11 a Effect of packing pressure on the product weight for PS. b Effect of packing pressure on the product weight for LDPE

Fig. 12 a Effect of injection pressure on the product weight for PS. b Effect of injection pressure on the product weight for LDPE

was measured with a balance having a sensibility of ±0.01 g and the weight of each product was measured several times and the average weight was taken. 3.2.1 Effect of packing pressure

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3.2.2 Effect of injection pressure

3.2.3 Effect of packing time

Increasing the injection pressure decreases the filling time and hence decreases the cycle time, but this requires an additional force from the injection molding machine to compensate the material shrinkage due to its cooling inside the mold cavity. Figure 12a, b show the effect of injection pressure on the product weight produced by injection polymer at different injection temperatures for PS and LDPE, respectively. Figure 12 shows that the increase of the injection pressure increases the product weight because it increases the cavity pressure as illustrated in Fig. 5 and hence decreases the material shrinkage. Also, an increase in the injection pressure increases the density of the injected material and hence increases the product weight. Figure 12 shows that at the injection temperature of 220 °C, the increase of the injection pressure 65 % increases the product weight by 6 % in case of PS and of 15.7 % in case of LDPE.

Packing time is an important parameter for the overall quality of the injection-molded part. Continuous increase in the packing time and follow part weight evaluation is a method for finding out the quality of the product. The effect of packing time on the product weight at different injection temperatures for PS and LDPE is shown in subpanels a and b in Fig. 13, respectively. Figure 13 indicates that the increase of the packing time increases the product weight at all injection temperatures because of the further material added during this time. This added material compensates the shrinkage of the polymer material due to product cooling inside the cavity. Figure 13a shows that the maximum increase of the product weight in case of PS is 4 % due to the increase of the packing time eight times. The increase of packing time three times increases the product weight by 4.1 % in case of LDPE as shown in Fig. 13b.

Fig. 13 a Effect of packing time on the product weight for PS, b Effect of packing time on the product weight for LDPE

Fig. 14 a Effect of injection time on the product weight for PS. b Effect of injection time on the product weight for LDPE

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3.2.4 Effect of injection time Increasing the filling time decreases the flow rate of the polymer material inside the mold cavity and hence increases the ability of the trapped air inside the cavity to escape from the mold. At the same time, an increase in the solidified percent of the polymer material during the filling especially at high values of the injection time and the solidification during the filling stage impedes the compensation of shrinkage process [7]. Figure 14a, b shows the effect of injection time on the product weight at various injection temperatures for PS and LDPE, respectively. Figure 14a shows that the increase of the injection time decreases the product weight in the case of PS. It also shows that the maximum variation of the product weight is about 4 % with maximum variation of injection time three times. The increase of the injection time approximately leads to a decrease in the product weight in case of LDPE as shown in Fig. 14b. Figure 14b indicates that the maximum variation of the product weight is about 14 % due to maximum variation of the injection time less than twice. From the results, we find that the effect of the process parameters on the product weight in case of LDPE is more than their effect in case of PS because the shrinkage due to cooling of the LDPE is greater than the shrinkage of PS.

4 Conclusion An experimental work is carried out to study the effect of the injection molding parameters on the cavity pressure and product weight. The process parameters considered during the experimental work are cooling time, injection pressure, packing pressure injection temperature, and injection temperature. Two polymer materials are considered in this work: PS and LDPE. The results indicate that the cavity pressure and product weight increase with increasing the packing pressure, injection pressure, and packing time for PS and LDPE at all injection temperatures. They also show that increasing the injection time decreases the cavity pressure for all polymer material analyzed and decreases the product weight for PS. The experimental results show that the cavity pressure increases and the product weight decreases with increasing the injection temperature. They also show that for injection temperature of 220 °C, the increase of injection pressure by 60 % increases the cavity pressure by 36 % in case of PS and 90 % in the case of LDPE. The increase of packing time nine times increases the cavity pressure by 21 % in case of PS and the increase of packing time four times increases the cavity pressure by 20 % in the case of LDPE. From the results, we can predict that the shrinkage in

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the case of PS is lower than the shrinkage of the LDPE. The results obtained illustrate well the evolution of the cavity pressure inside the mold cavity for different process parameters of the injection molding.

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