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Study on the clamping force measurement and partial load regulation technology of injection molding machine
CIRP Journal of Manufacturing Science and Technology 19 (2017) 19–24
Contents lists available at ScienceDirect
CIRP Journal of Manufacturing Science and Technology journal homepage: www.elsevier.com/locate/cirpj
Study on the clamping force measurement and partial load regulation technology of injection molding machine Rao Binqia,* , Zhou Hongweib , Ouyang Huabinc , Wan Yanjiana , Zhang Yuanhuia , Wu Jingyangb a b c
School of Mechanical and Electrical Engineering, China JiLiang University, Hangzhou 310018, China Tederic Machinery Co., Ltd., Hangzhou, China School of Mechanical and Electrical Engineering, Shanghai DianJi University, Shanghai 201306, China
A R T I C L E I N F O
Article history: Available online 6 April 2017
Keywords: Clamping force Partial load rate Parallel degree of the template (mold) Thread type Partial load rate adjustment
A B S T R A C T
Tie bars are the most important parts of injection molding machine, and the partial load of tie bars will directly affect the product quality. According to the measurement of strain and stress of the tie bars, the partial load rate of tie bars was calculated, and the influence of different types of thread shape and template (mold) parallelism on the partial load rate of tie bars were analyzed. Experiments reveal that the partial load rate is changed within 0.8%–3.8%, and the partial load rate gradually decreases with the increase of the clamping forces. Besides, different types of the thread of tie bars have little influence on the partial load rate, while the parallel degree of the template (mold) has great influence on the partial load rate of the tie bars. Further experiments show that the partial load rate is located in 0.79%–1.81% when the parallelism of template (mold) is good, and the partial load rate of tie bars between 8.59%–11.46% when the parallelism of template (mold) is poor. Finally, the partial load adjustment system of tie bars were also designed to make the force of tie bars more uniform and the partial load rate can be reduced by detecting the partial load rate of the tie bar and using the closed-loop control. © 2017 CIRP.
Introduction The molding clamping unit is one of the most important parts of injection molding machine, which directly affects the dimensional accuracy and the quality of products. The optimized clamping force and uniform load on four tie bars are important indexes to evaluate the performance of the injection molding machine [1]. The uniform load of tie bars ensures quality of products, plays a protective role and extends the life of the mold and injection molding machine. Current molding clamping units are inherent with several limitations [2–4], for example, the nonparallel template (mold) caused the nonuniform load of the tie bars, and the unstable clamping force may cause tie bars fracture. Therefore, several efforts are needed to relieve the above limitations [5–9]. Firstly, the accuracy should be optimized and enhanced for the measurement of the clamping forces; secondly, the partial load needs to be optimized and adjusted for uniform clamping force of the tie bar.
* Corresponding author. E-mail address: [email protected] (B. Rao). http://dx.doi.org/10.1016/j.cirpj.2017.03.001 1755-5817/© 2017 CIRP.
The measuring of clamping force is to convert the stress of tie bar into strain according to the principle of strain electrical measuring, and electrical measuring method is to change nonelectricity (force, stress, temperature, etc.) into electricity by the sensing element according to the amplification and processing of the measurement circuit, which can be reflected in the instrument reading device of a measuring method. Finally, the stress of the parts, mechanism and working condition can be analyzed [10]. Traditional measurement method is by using embellishment strain gauge to measure the strain and calculate clamping force of tie bars [11,12]. The traditional measurement method has many disadvantages such as the strain gauge paste difficultly and the strain gauge can be used only once which not only takes long time to prepare the paste work, but also requires critical measurement environment. The values of the experiments were presented as follows: according to measuring the strain and stress of the four tie bars, the stress and partial load rate of the tie bars are shown in real time, when the partial load rate exceeds a certain value, the device will alarm, at the same time, the partial load adjustment system start work to adjust separately the stress of every tie bar to ensure the stress of the four tie bars within the reasonable limits, so as to
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B. Rao et al. / NULL 19 (2017) 19–24
improve the injection molding machine performance and ensure the quality of plastic product. The analysis of the influence of thread types and parallel degree of template (mold) on the stress and partial load rate of the tie bars, determine the influence of thread types and parallel degree of template (mold) on the stress and partial load rate, the values of this were that to avoid the uneven load caused by the uneven template, provide the basis for the design of the template parallelism. Stress measurement of tie bar and method of partial load calculation Experimental device and apparatus The clamping force measurement technique used in this paper is magnetic force measurement, the measuring device is called magnetic enclosed type clamping force measuring device (QE-1008, SENSORMATE, Switzerland). The structure and installation of the magnetic measuring device are shown in Fig. 1, as shown in Fig. 1(a), the measuring device consists of steel shell, strain gauge, magnet, rubber pad, adjusting bolt, adjusting nut, connecting line, etc., the strain gauge is placed between the two magnets in stainless steel foil, it stands absolutely in axial line on the surface, strong magnetic force through the bolt, nut and a rubber cushion make the strain gauge firmly affixed to the surface of the tie bars, so as to produce a strong interaction with friction between the surface of strain gauge and steel foil and tie bar, the
pressing force of the strain gauge can be adjusted by turning on the nuts of the magnets. Each measuring case is connected respectively with the input of the digital display device, the deformation variable is converted into the change of electric quantity, after the measurement circuit amplification and processing, the measured strain value is then displayed directly in “microstrain” or “kN” or “t”. The measuring device is a whole one, when the experiment is ended, the measuring device can be removed by force which exceed the magnetic force, so the measuring device and its application is reproducible. Template (mold) parallelism will directly affect the deflection of the tie bars. According to mechanical principle, nonparallel template (mold) will directly affect the horizontal direction pulling force of the tie bars. Terminal of the tie bars are thread structure were fixed on template by the nut, and the thread types are trapezoidal thread and triangle thread. In order to investigate the influence of parallel degree of template (mold) and thread types on the stress and partial load rate of the tie bars, the two model machine T300 with different thread and T250 with different parallel degree of mold were used in the experiment. Experiment procedure As shown in Fig. 1(b), four tie bars are fixed on the front template, movable template and back template through the nut under the action of the cylinder. The hydraulic cylinder imposes force on the clamping unit, and the clamping unit imposes pull to
Fig. 1. Structure and installation of the magnetic measuring device.
B. Rao et al. / NULL 19 (2017) 19–24
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the movable template and the mold, when the two mold which installed on the movable template and front template stick tightly, the four tie bars are imposed huge force, according to the principle of mechanics, the four tie bars stretched with a certain deformation. Meanwhile, the strain gauge which are tightly attached on the surface of the tie bars have the same deformation with the tie bars [13]. The strain data of four tie bars are collected by eight measuring cases which are installed symmetrically on the surface of the corresponding tie bars. According to the measured strain, length, diameter, the elastic modulus of the material of tie bar and Formula (1), clamping force of every tie bar can be calculated by Formula (1), and the partial load rate can be calculated by the Formula (2). F i ¼ pd Ee=4 2
ð1Þ Fig. 2. Relationship between clamping force and partial load rate.
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi n
1 n1
P¼
S ðF i FÞ2
i¼1
F
100%
ð2Þ
where d is the diameter of tie bar (mm), E is the elastic modulus of the material of tie bar (kN/mm2), e is the strain of tie bar, P is the partial load rate (%), Fi represents the clamping force corresponding to the tie bars (i = 1–4, kN), F is the average value of the tie bar force (kN), n is the tie bars number (n = 4). With the T300 model machine, the strain and stress of tie bar with trapezoidal and triangular thread were measured, and the strain and stress of tie bars under the condition of good and bad parallel degree of template were measured with the T250 model machine. One notes the condition of good parallel degree of template (mold) is that the parallel degree of template (mold) conforms to the national or industrial technical standards, while the condition of bad parallel degree of template (mold) does not meet the national or industry technical standards. Results and discussion Relationship between clamping force and partial load rate of the tie bars The relationship between the clamping force and partial load of the tie bars is shown in Table 1 and Fig. 2. From Table 1 and Fig. 2, it can be seen that the partial load rate of tie bars is between 2.53% and 3.82%, and the partial load rate of tie bars gradually reduced with the increasing of the clamping force. When the sum of clamping force up to 1670 kN, the partial load rate was 2.53%, while the sum of clamping force decrease to 1010 kN, the partial load rate up to 3.82%. Because the partial load rate are related with F and the difference between F and Fi, with the increasing of clamping force, F increase accordingly with the same proportion; while the difference between F and Fi increase accordingly with the smaller proportion, the increase of numerator
is less than the increase of denominator in Formula (2); second, with the increasing of clamping force, the template (mold) is pressed flatter and closer, the load is evenly distributed on the four tie bars, so the partial load rate of the tie bar is gradually reduced. The influence of thread types on the force and partial load rate of tie bars With the T300 model machine, the strain and stress of tie bars with the trapezoidal and triangular thread types were measured, and the clamping force and partial load rate were calculated with the results shown in Table 2 and Fig. 3. The experiment results show that the partial load rate decreased with the increased clamping force. When the clamping force reduces to the minimum value of 1900 kN, the partial load rate reached a maximum value as 2.72%. When the clamping force increases up to 2960 kN, the partial load was 1.10%, and the clamping force of four tie bars are more uniform with the partial load rate of tie bar below lower than the national standard (8%). The types of thread have little effect on the partial load rate. Considering the partial load rate, trapezoidal thread and triangle thread are both applicable. The influence of template parallelism on partial load rate The influence of different parallel degree of template (mold) on the clamping force and partial load rate of tie bars with T250 models machine are shown in Table 3 and Fig. 4. The above results reveal that parallel degree of template (mold) has great influence on the clamping force and partial load rates of the tie bars. With the good parallel degree of template (mold), the partial load rate of tie bars within 0.79%–1.81% is far below the national standard. With the poor parallel degree of template (mold), the partial load rates of tie bar are between 8.59% and 11.46%, all partial load rates are more than 8% of the provisions of
Table 1 The clamping force and partial load rate of tie bars. Clamping force of each tie bar (kN) The first tie bar
The second tie bar
The third tie bar
The forth tie bar
262.8 309.3 329.4 349.6 390.1 430.5
254.2 300.0 321.5 340.8 381.1 421.5
253.5 298.2 314.4 332.9 368.5 407.5
239.5 282.5 304.7 326.7 370.3 410.5
Sum of clamping force (kN)
Partial load rate (%)
1010 1190 1270 1350 1510 1670
3.82% 3.74% 3.31% 2.94% 2.66% 2.53%
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Table 2 Clamping force and partial load rate of tie bar with different thread types (T300). Clamping force of trapezoidal thread (kN)
Clamping force of triangular thread
The first tie The second bar tie bar
The third tie bar
The forth tie bar
Total clamping force
Partial load rate (%)
The first tie bar
The second tie bar
The third tie bar
The forth tie bar
Total clamping force
Partial load rate(%)
470 550 640 690 720
480 560 650 700 730
490 560 660 700 730
1900 2210 2590 2770 2890
2.72 1.73 1.48 1.38 1.33
510 600 670 710 740
500 600 670 710 740
490 590 660 700 730
510 610 680 720 750
2010 2400 2680 2840 2960
1.91 1.36 1.22 1.15 1.10
460 540 640 680 710
Fig. 3. Comparison of clamping force and partial load rate of different thread types. Fig. 4. The influence of parallel degree of the template on the partial load rate.
the state. The reason is that nonparallel template (mold) makes clamping force to the four tie bars distribution not uniform and leads high rate of partial load.
11
Design of the partial load adjustment system of clamping forces The optimized clamping forces and uniform load of the four tie bars are critical to assess the performance of the injection molding machine. Only when the load is distributed uniformly, the product quality can be ensured, and the injection molding machine can be protected with extended lifetime. If the force of four tie bars are uneven with excessive deviation, controller makes response and automatically drives the system work when necessary to avoid the clamping force beyond its limit value. The schematics of partial load adjustment system is shown as Fig. 5, and the part enlarged view of the partial load adjustments of the tie bars is shown in Fig. 6. The partial load adjustment system mainly consists of back template, movable template, front template, toggle component, hydraulic motor, center ring, controller, four tie bars and tie bar adjustment system. The tie bar adjustment system comprises servo motors, front nuts, driving
8 9
10 12
2
4
1
5
6 3
7
Fig. 5. Schematic diagram of the system.
gear and eight strain gauges fixed to the four tie bars. The strain gauges and tie bars share the same positions and direction. The
Table 3 Force and partial loading rate with different parallel degree of template (mold). Template (mold) in a good degree of parallelism
Template (mold) in a bad degree of parallelism
The first tie bar (kN)
The second tie bar (kN)
The third The forth Total tie bar (kN) tie bar (kN) clamping force (kN)
Partial load rate (%)
The first tie The second bar (kN) tie bar (kN)
The third The forth Total tie bar (kN) tie bar (kN) clamping force (kN)
Partial load rate (%)
450 520 590 630 670
440 520 590 630 660
460 530 600 640 670
1.81 1.10 0.84 0.79 0.87
450 530 600 630 690
390 470 540 570 620
11.46 9.49 9.08 8.59 9.22
450 530 590 630 660
1800 2100 2370 2530 2660
390 480 550 570 630
340 420 480 510 550
1570 1900 2170 2280 2490
B. Rao et al. / NULL 19 (2017) 19–24
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setting clamping force is consistent, the sum clamping force is calculated through the combination of the signal from the eight sensitive strain gauges. If the actual clamping force is smaller than the setting value, the hydraulic motor will adjust the position of the back template immediately, so that the clamping force can be increased. If the actual clamping force is larger than the setting clamping force, an alarm signal will be issued, and then the hydraulic motor will adjust the position of the back template to reduce the clamping force. Conclusions and outlook of the research
Fig. 6. Part enlarged view of the tie bar adjustments system. (1) Back template, (2) movable template, (3) front template, (4) toggle mechanism, (5) tie bar, (6) strain gauge, (7) servo motor, (8) front nut, (9) driving gear, (10) hydraulic motor, (11) center ring gear, (12) back nut.
outer peripheral face of the front nut is a gear, servo motor is further connected with drive gear which is coupled with the front nut. The controller consists of storage unit, arithmetic unit, comparator and control instruction units which are connected with servo motors and strain gauges. The flow chart of partial load adjustment system is shown in Fig. 7. The principle of the system works as follows: eight sensitive strain gauges are installed on the four tie bars. The central controller collect the signal from the eight sensitive strain gauges, compares the signal of micro voltage value with the set value, calculates the clamping force of each tie bar and the sum clamping force of four tie bars, and finally obtains the partial load rate according to the clamping force. If clamping force of one of the tie bars is too large, the adjustment instruction is initiated and drive the servo motor, riving gear and front nut rotating, thereby release the deformation and reduce the force of one of the tie bar. In order to determine whether the current actual clamping force and the
By measuring the strain of the tie bars, the force condition was obtained, and the partial load rate was calculated. The partial load rate decreases with the increasing of the clamping force under normal circumstances. The partial load rate of tie bar is between 0.8%–3.8%, which is lower than the national standard value of 8%, demonstrating the partial load rate is in line with the requirement. The trapezoidal thread and the triangle thread on the tie bar have almost no influence on the partial load. Parallel degree of templates (mold) have large impact on the partial load, the partial load rate of tie bars is located in 0.79%–1.81% when the parallel degree of template (mold) is good, the partial load rate of tie bars between 8.59%–11.46% when the parallel degree of template (mold) is poor. The novel clamping force adjustment structure on each tie bar was designed. when the partial load rate exceeds the setting value, the partial loading rate of the tie bar would decrease to the regulation range through the adjustment structure to ensure the quality of the plastic product. The future trends and developments would be optimization design and automatic control of key parts of the injection molding machine according to the measurement of clamping force. For instance, we can carry out optimization design of the tie bars. The current dimension of tie bars is a bit larger because of the imprecise control of partial load, and if the partial load can examined and controlled accurately, the dimension of tie bars can be reduced accordingly. Second, we can protect and control the mold automatically, when the mold occurs emergent situation in
Fig. 7. Flow chart of partial load adjustment system.
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the mold-closing process, the partial load will increase suddenly, and through the partial load detection system, we can control the mold’s movement and protect the mold. Acknowledgment The authors gratefully acknowledge the support of the Zhejiang Key Discipline of Instrument Science and Technology (JL130103). References [1] Li, D.Q., 2005, Principle, Method and Application of the Modern Plastic Injection Molding. Shanghai Jiaotong University Press. [2] Huang, B.M., 2001, Main Problems in Clamping Systems of Injection Machines and Answers to Them. China Plastics, 9:76. [3] Huang, B.M., Xu, Z.B., 2002, Comparison and Analysis of All-electric Injection Molding Machine Precision and Clamping Device. Engineering Plastics Application, 30:47–49. [4] Yin, H., Lu, G.D., Wang, J., 2009, Comparison and Analysis of All-electric Injection Molding Machine Precision and Clamping Device. China Plastics, 11:1–6.
[5] Yang, Y., Wang, B.C., Fu, J.G., 2011, Comparison and Analysis of All-electric Injection Molding Machine Precision and Clamping Device. Light Industry Machinery, 29:113–116. [6] Huang, M., Tsung, Y., 2012, Key Design Parameters and Optimal Design of a Five-point Double Toggle Clamping Mechanism. Nature Structural & Molecular Biology, 18:941. [7] Luo, Y.Q., Shi, Y.Y., Wang, J., 2008, New Precision Injection Molding Machine Clamping Structure. Light Industry Machinery, 26:10–14. [8] Chen, X.F., 2008, Material Injection Molding Machine Double Toggle Clamping Mechanism. South China University of Technology. [9] Wu, J.Y., 2015, Injection Molding Machine Clamping Mechanism and Clamping Force Balance Adjustment Method. Chinese Patent, 2012103272117, 2015-0729. [10] Liu, X.H., Song, S.L., 2006, Testing and Error Analysis of Plastic Injection Molding Machine Clamp Force. Journal of Guangxi University of Technology, 17:53–55. [11] Peng, N.Q., 2002, Measurement Techniques of Injection Molding Machine Clamping Force. Mechanical & Electrical Engineering Technology, 31:115–116. [12] Kang, C.F., 2009, Research on Mold Closing and Locking Force Control of Electric Molding Machine. Engineering Plastics Application, 37:71–74. [13] Chen, J.W., Schlaepher, B., 2010, New Measurement Techniques of Injection Molding Machine Clamping Force. Engineering Plastics Application, 38:75–77.
Contents lists available at ScienceDirect
CIRP Journal of Manufacturing Science and Technology journal homepage: www.elsevier.com/locate/cirpj
Study on the clamping force measurement and partial load regulation technology of injection molding machine Rao Binqia,* , Zhou Hongweib , Ouyang Huabinc , Wan Yanjiana , Zhang Yuanhuia , Wu Jingyangb a b c
School of Mechanical and Electrical Engineering, China JiLiang University, Hangzhou 310018, China Tederic Machinery Co., Ltd., Hangzhou, China School of Mechanical and Electrical Engineering, Shanghai DianJi University, Shanghai 201306, China
A R T I C L E I N F O
Article history: Available online 6 April 2017
Keywords: Clamping force Partial load rate Parallel degree of the template (mold) Thread type Partial load rate adjustment
A B S T R A C T
Tie bars are the most important parts of injection molding machine, and the partial load of tie bars will directly affect the product quality. According to the measurement of strain and stress of the tie bars, the partial load rate of tie bars was calculated, and the influence of different types of thread shape and template (mold) parallelism on the partial load rate of tie bars were analyzed. Experiments reveal that the partial load rate is changed within 0.8%–3.8%, and the partial load rate gradually decreases with the increase of the clamping forces. Besides, different types of the thread of tie bars have little influence on the partial load rate, while the parallel degree of the template (mold) has great influence on the partial load rate of the tie bars. Further experiments show that the partial load rate is located in 0.79%–1.81% when the parallelism of template (mold) is good, and the partial load rate of tie bars between 8.59%–11.46% when the parallelism of template (mold) is poor. Finally, the partial load adjustment system of tie bars were also designed to make the force of tie bars more uniform and the partial load rate can be reduced by detecting the partial load rate of the tie bar and using the closed-loop control. © 2017 CIRP.
Introduction The molding clamping unit is one of the most important parts of injection molding machine, which directly affects the dimensional accuracy and the quality of products. The optimized clamping force and uniform load on four tie bars are important indexes to evaluate the performance of the injection molding machine [1]. The uniform load of tie bars ensures quality of products, plays a protective role and extends the life of the mold and injection molding machine. Current molding clamping units are inherent with several limitations [2–4], for example, the nonparallel template (mold) caused the nonuniform load of the tie bars, and the unstable clamping force may cause tie bars fracture. Therefore, several efforts are needed to relieve the above limitations [5–9]. Firstly, the accuracy should be optimized and enhanced for the measurement of the clamping forces; secondly, the partial load needs to be optimized and adjusted for uniform clamping force of the tie bar.
* Corresponding author. E-mail address: [email protected] (B. Rao). http://dx.doi.org/10.1016/j.cirpj.2017.03.001 1755-5817/© 2017 CIRP.
The measuring of clamping force is to convert the stress of tie bar into strain according to the principle of strain electrical measuring, and electrical measuring method is to change nonelectricity (force, stress, temperature, etc.) into electricity by the sensing element according to the amplification and processing of the measurement circuit, which can be reflected in the instrument reading device of a measuring method. Finally, the stress of the parts, mechanism and working condition can be analyzed [10]. Traditional measurement method is by using embellishment strain gauge to measure the strain and calculate clamping force of tie bars [11,12]. The traditional measurement method has many disadvantages such as the strain gauge paste difficultly and the strain gauge can be used only once which not only takes long time to prepare the paste work, but also requires critical measurement environment. The values of the experiments were presented as follows: according to measuring the strain and stress of the four tie bars, the stress and partial load rate of the tie bars are shown in real time, when the partial load rate exceeds a certain value, the device will alarm, at the same time, the partial load adjustment system start work to adjust separately the stress of every tie bar to ensure the stress of the four tie bars within the reasonable limits, so as to
20
B. Rao et al. / NULL 19 (2017) 19–24
improve the injection molding machine performance and ensure the quality of plastic product. The analysis of the influence of thread types and parallel degree of template (mold) on the stress and partial load rate of the tie bars, determine the influence of thread types and parallel degree of template (mold) on the stress and partial load rate, the values of this were that to avoid the uneven load caused by the uneven template, provide the basis for the design of the template parallelism. Stress measurement of tie bar and method of partial load calculation Experimental device and apparatus The clamping force measurement technique used in this paper is magnetic force measurement, the measuring device is called magnetic enclosed type clamping force measuring device (QE-1008, SENSORMATE, Switzerland). The structure and installation of the magnetic measuring device are shown in Fig. 1, as shown in Fig. 1(a), the measuring device consists of steel shell, strain gauge, magnet, rubber pad, adjusting bolt, adjusting nut, connecting line, etc., the strain gauge is placed between the two magnets in stainless steel foil, it stands absolutely in axial line on the surface, strong magnetic force through the bolt, nut and a rubber cushion make the strain gauge firmly affixed to the surface of the tie bars, so as to produce a strong interaction with friction between the surface of strain gauge and steel foil and tie bar, the
pressing force of the strain gauge can be adjusted by turning on the nuts of the magnets. Each measuring case is connected respectively with the input of the digital display device, the deformation variable is converted into the change of electric quantity, after the measurement circuit amplification and processing, the measured strain value is then displayed directly in “microstrain” or “kN” or “t”. The measuring device is a whole one, when the experiment is ended, the measuring device can be removed by force which exceed the magnetic force, so the measuring device and its application is reproducible. Template (mold) parallelism will directly affect the deflection of the tie bars. According to mechanical principle, nonparallel template (mold) will directly affect the horizontal direction pulling force of the tie bars. Terminal of the tie bars are thread structure were fixed on template by the nut, and the thread types are trapezoidal thread and triangle thread. In order to investigate the influence of parallel degree of template (mold) and thread types on the stress and partial load rate of the tie bars, the two model machine T300 with different thread and T250 with different parallel degree of mold were used in the experiment. Experiment procedure As shown in Fig. 1(b), four tie bars are fixed on the front template, movable template and back template through the nut under the action of the cylinder. The hydraulic cylinder imposes force on the clamping unit, and the clamping unit imposes pull to
Fig. 1. Structure and installation of the magnetic measuring device.
B. Rao et al. / NULL 19 (2017) 19–24
21
the movable template and the mold, when the two mold which installed on the movable template and front template stick tightly, the four tie bars are imposed huge force, according to the principle of mechanics, the four tie bars stretched with a certain deformation. Meanwhile, the strain gauge which are tightly attached on the surface of the tie bars have the same deformation with the tie bars [13]. The strain data of four tie bars are collected by eight measuring cases which are installed symmetrically on the surface of the corresponding tie bars. According to the measured strain, length, diameter, the elastic modulus of the material of tie bar and Formula (1), clamping force of every tie bar can be calculated by Formula (1), and the partial load rate can be calculated by the Formula (2). F i ¼ pd Ee=4 2
ð1Þ Fig. 2. Relationship between clamping force and partial load rate.
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi n
1 n1
P¼
S ðF i FÞ2
i¼1
F
100%
ð2Þ
where d is the diameter of tie bar (mm), E is the elastic modulus of the material of tie bar (kN/mm2), e is the strain of tie bar, P is the partial load rate (%), Fi represents the clamping force corresponding to the tie bars (i = 1–4, kN), F is the average value of the tie bar force (kN), n is the tie bars number (n = 4). With the T300 model machine, the strain and stress of tie bar with trapezoidal and triangular thread were measured, and the strain and stress of tie bars under the condition of good and bad parallel degree of template were measured with the T250 model machine. One notes the condition of good parallel degree of template (mold) is that the parallel degree of template (mold) conforms to the national or industrial technical standards, while the condition of bad parallel degree of template (mold) does not meet the national or industry technical standards. Results and discussion Relationship between clamping force and partial load rate of the tie bars The relationship between the clamping force and partial load of the tie bars is shown in Table 1 and Fig. 2. From Table 1 and Fig. 2, it can be seen that the partial load rate of tie bars is between 2.53% and 3.82%, and the partial load rate of tie bars gradually reduced with the increasing of the clamping force. When the sum of clamping force up to 1670 kN, the partial load rate was 2.53%, while the sum of clamping force decrease to 1010 kN, the partial load rate up to 3.82%. Because the partial load rate are related with F and the difference between F and Fi, with the increasing of clamping force, F increase accordingly with the same proportion; while the difference between F and Fi increase accordingly with the smaller proportion, the increase of numerator
is less than the increase of denominator in Formula (2); second, with the increasing of clamping force, the template (mold) is pressed flatter and closer, the load is evenly distributed on the four tie bars, so the partial load rate of the tie bar is gradually reduced. The influence of thread types on the force and partial load rate of tie bars With the T300 model machine, the strain and stress of tie bars with the trapezoidal and triangular thread types were measured, and the clamping force and partial load rate were calculated with the results shown in Table 2 and Fig. 3. The experiment results show that the partial load rate decreased with the increased clamping force. When the clamping force reduces to the minimum value of 1900 kN, the partial load rate reached a maximum value as 2.72%. When the clamping force increases up to 2960 kN, the partial load was 1.10%, and the clamping force of four tie bars are more uniform with the partial load rate of tie bar below lower than the national standard (8%). The types of thread have little effect on the partial load rate. Considering the partial load rate, trapezoidal thread and triangle thread are both applicable. The influence of template parallelism on partial load rate The influence of different parallel degree of template (mold) on the clamping force and partial load rate of tie bars with T250 models machine are shown in Table 3 and Fig. 4. The above results reveal that parallel degree of template (mold) has great influence on the clamping force and partial load rates of the tie bars. With the good parallel degree of template (mold), the partial load rate of tie bars within 0.79%–1.81% is far below the national standard. With the poor parallel degree of template (mold), the partial load rates of tie bar are between 8.59% and 11.46%, all partial load rates are more than 8% of the provisions of
Table 1 The clamping force and partial load rate of tie bars. Clamping force of each tie bar (kN) The first tie bar
The second tie bar
The third tie bar
The forth tie bar
262.8 309.3 329.4 349.6 390.1 430.5
254.2 300.0 321.5 340.8 381.1 421.5
253.5 298.2 314.4 332.9 368.5 407.5
239.5 282.5 304.7 326.7 370.3 410.5
Sum of clamping force (kN)
Partial load rate (%)
1010 1190 1270 1350 1510 1670
3.82% 3.74% 3.31% 2.94% 2.66% 2.53%
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Table 2 Clamping force and partial load rate of tie bar with different thread types (T300). Clamping force of trapezoidal thread (kN)
Clamping force of triangular thread
The first tie The second bar tie bar
The third tie bar
The forth tie bar
Total clamping force
Partial load rate (%)
The first tie bar
The second tie bar
The third tie bar
The forth tie bar
Total clamping force
Partial load rate(%)
470 550 640 690 720
480 560 650 700 730
490 560 660 700 730
1900 2210 2590 2770 2890
2.72 1.73 1.48 1.38 1.33
510 600 670 710 740
500 600 670 710 740
490 590 660 700 730
510 610 680 720 750
2010 2400 2680 2840 2960
1.91 1.36 1.22 1.15 1.10
460 540 640 680 710
Fig. 3. Comparison of clamping force and partial load rate of different thread types. Fig. 4. The influence of parallel degree of the template on the partial load rate.
the state. The reason is that nonparallel template (mold) makes clamping force to the four tie bars distribution not uniform and leads high rate of partial load.
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Design of the partial load adjustment system of clamping forces The optimized clamping forces and uniform load of the four tie bars are critical to assess the performance of the injection molding machine. Only when the load is distributed uniformly, the product quality can be ensured, and the injection molding machine can be protected with extended lifetime. If the force of four tie bars are uneven with excessive deviation, controller makes response and automatically drives the system work when necessary to avoid the clamping force beyond its limit value. The schematics of partial load adjustment system is shown as Fig. 5, and the part enlarged view of the partial load adjustments of the tie bars is shown in Fig. 6. The partial load adjustment system mainly consists of back template, movable template, front template, toggle component, hydraulic motor, center ring, controller, four tie bars and tie bar adjustment system. The tie bar adjustment system comprises servo motors, front nuts, driving
8 9
10 12
2
4
1
5
6 3
7
Fig. 5. Schematic diagram of the system.
gear and eight strain gauges fixed to the four tie bars. The strain gauges and tie bars share the same positions and direction. The
Table 3 Force and partial loading rate with different parallel degree of template (mold). Template (mold) in a good degree of parallelism
Template (mold) in a bad degree of parallelism
The first tie bar (kN)
The second tie bar (kN)
The third The forth Total tie bar (kN) tie bar (kN) clamping force (kN)
Partial load rate (%)
The first tie The second bar (kN) tie bar (kN)
The third The forth Total tie bar (kN) tie bar (kN) clamping force (kN)
Partial load rate (%)
450 520 590 630 670
440 520 590 630 660
460 530 600 640 670
1.81 1.10 0.84 0.79 0.87
450 530 600 630 690
390 470 540 570 620
11.46 9.49 9.08 8.59 9.22
450 530 590 630 660
1800 2100 2370 2530 2660
390 480 550 570 630
340 420 480 510 550
1570 1900 2170 2280 2490
B. Rao et al. / NULL 19 (2017) 19–24
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setting clamping force is consistent, the sum clamping force is calculated through the combination of the signal from the eight sensitive strain gauges. If the actual clamping force is smaller than the setting value, the hydraulic motor will adjust the position of the back template immediately, so that the clamping force can be increased. If the actual clamping force is larger than the setting clamping force, an alarm signal will be issued, and then the hydraulic motor will adjust the position of the back template to reduce the clamping force. Conclusions and outlook of the research
Fig. 6. Part enlarged view of the tie bar adjustments system. (1) Back template, (2) movable template, (3) front template, (4) toggle mechanism, (5) tie bar, (6) strain gauge, (7) servo motor, (8) front nut, (9) driving gear, (10) hydraulic motor, (11) center ring gear, (12) back nut.
outer peripheral face of the front nut is a gear, servo motor is further connected with drive gear which is coupled with the front nut. The controller consists of storage unit, arithmetic unit, comparator and control instruction units which are connected with servo motors and strain gauges. The flow chart of partial load adjustment system is shown in Fig. 7. The principle of the system works as follows: eight sensitive strain gauges are installed on the four tie bars. The central controller collect the signal from the eight sensitive strain gauges, compares the signal of micro voltage value with the set value, calculates the clamping force of each tie bar and the sum clamping force of four tie bars, and finally obtains the partial load rate according to the clamping force. If clamping force of one of the tie bars is too large, the adjustment instruction is initiated and drive the servo motor, riving gear and front nut rotating, thereby release the deformation and reduce the force of one of the tie bar. In order to determine whether the current actual clamping force and the
By measuring the strain of the tie bars, the force condition was obtained, and the partial load rate was calculated. The partial load rate decreases with the increasing of the clamping force under normal circumstances. The partial load rate of tie bar is between 0.8%–3.8%, which is lower than the national standard value of 8%, demonstrating the partial load rate is in line with the requirement. The trapezoidal thread and the triangle thread on the tie bar have almost no influence on the partial load. Parallel degree of templates (mold) have large impact on the partial load, the partial load rate of tie bars is located in 0.79%–1.81% when the parallel degree of template (mold) is good, the partial load rate of tie bars between 8.59%–11.46% when the parallel degree of template (mold) is poor. The novel clamping force adjustment structure on each tie bar was designed. when the partial load rate exceeds the setting value, the partial loading rate of the tie bar would decrease to the regulation range through the adjustment structure to ensure the quality of the plastic product. The future trends and developments would be optimization design and automatic control of key parts of the injection molding machine according to the measurement of clamping force. For instance, we can carry out optimization design of the tie bars. The current dimension of tie bars is a bit larger because of the imprecise control of partial load, and if the partial load can examined and controlled accurately, the dimension of tie bars can be reduced accordingly. Second, we can protect and control the mold automatically, when the mold occurs emergent situation in
Fig. 7. Flow chart of partial load adjustment system.
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B. Rao et al. / NULL 19 (2017) 19–24
the mold-closing process, the partial load will increase suddenly, and through the partial load detection system, we can control the mold’s movement and protect the mold. Acknowledgment The authors gratefully acknowledge the support of the Zhejiang Key Discipline of Instrument Science and Technology (JL130103). References [1] Li, D.Q., 2005, Principle, Method and Application of the Modern Plastic Injection Molding. Shanghai Jiaotong University Press. [2] Huang, B.M., 2001, Main Problems in Clamping Systems of Injection Machines and Answers to Them. China Plastics, 9:76. [3] Huang, B.M., Xu, Z.B., 2002, Comparison and Analysis of All-electric Injection Molding Machine Precision and Clamping Device. Engineering Plastics Application, 30:47–49. [4] Yin, H., Lu, G.D., Wang, J., 2009, Comparison and Analysis of All-electric Injection Molding Machine Precision and Clamping Device. China Plastics, 11:1–6.
[5] Yang, Y., Wang, B.C., Fu, J.G., 2011, Comparison and Analysis of All-electric Injection Molding Machine Precision and Clamping Device. Light Industry Machinery, 29:113–116. [6] Huang, M., Tsung, Y., 2012, Key Design Parameters and Optimal Design of a Five-point Double Toggle Clamping Mechanism. Nature Structural & Molecular Biology, 18:941. [7] Luo, Y.Q., Shi, Y.Y., Wang, J., 2008, New Precision Injection Molding Machine Clamping Structure. Light Industry Machinery, 26:10–14. [8] Chen, X.F., 2008, Material Injection Molding Machine Double Toggle Clamping Mechanism. South China University of Technology. [9] Wu, J.Y., 2015, Injection Molding Machine Clamping Mechanism and Clamping Force Balance Adjustment Method. Chinese Patent, 2012103272117, 2015-0729. [10] Liu, X.H., Song, S.L., 2006, Testing and Error Analysis of Plastic Injection Molding Machine Clamp Force. Journal of Guangxi University of Technology, 17:53–55. [11] Peng, N.Q., 2002, Measurement Techniques of Injection Molding Machine Clamping Force. Mechanical & Electrical Engineering Technology, 31:115–116. [12] Kang, C.F., 2009, Research on Mold Closing and Locking Force Control of Electric Molding Machine. Engineering Plastics Application, 37:71–74. [13] Chen, J.W., Schlaepher, B., 2010, New Measurement Techniques of Injection Molding Machine Clamping Force. Engineering Plastics Application, 38:75–77.