Review A Comprehensive Study of Energy Conservation in Electric-Hydraulic Injection-Molding Equipment

Transcription

1 Review Comprehensive Stdy of Energy Conservation in Electric-Hydralic Injection-Molding Eqipment Hongjan Zhang 1, *, L Ren 1, Yan Gao 1 and Baoqan Jin 2 1 College of Electrical and Power Engineering, aiyan University of echnology, aiyan , China; renl528@163.com (L.R.); gaoyantylg@163.com (Y.G.) 2 Key Laboratory of dvanced ransdcers and Intelligent Control Systems, Ministry of Edcation, aiyan University of echnology, aiyan , China; jinbaoqan@tyt.ed.cn * Correspondence: zhanghongjan@tyt.ed.cn; el.: cademic Editor: Leonardo P. Chamorro Received: 29 September 2017; ccepted: 27 October 2017; Pblished: 2 November 2017 bstract: n injection-molding machine (IMM) is eqipment that prodces all kinds of plastic prodcts. t present, the global prodction of IMMs amonts to more than 30 million nits each year, and its total prodction acconts for 50% of all plastic molding eqipment. Now, the main energy consmption eqipment of plastic processing plants consists in IMMs. herefore, energy conservation research on IMMs has become rgent. his paper initially introdces the crrent development of IMMs. he working principle and the distribtion of energy consmption of IMMs are then analyzed in detail. In addition, the methods and characteristics of the energy conservation technology in hydralic control circits and electrical control circits are developed. Meanwhile, the recovery and the rese of braking energy of IMMs are proposed. Moreover, some control strategies are discssed in order to improve energy efficiency. Finally, challenges and prospects for hydralic IMMs are carried ot. his paper ths provides a comprehensive review on energy-saving technology of electric-hydralic injection-molding eqipment for researchers. Keywords: plastic injection-molding machine; energy conservation technology; variable freqency drive control; energy optimization; energy regeneration 1. Introdction Injection-molding machines (IMM) constitte the most crcial plastic processing machinery. hey can prodce a variety of plastic prodcts and varios eqipment parts, so it is widely sed in national defense, electronics, atomotive, transportation, packaging, agricltre, edcation, health, and all areas in daily life [1]. he application sitation in the end prodct market is shown in Figre 1. With the rapid growth of economy and the rapid expansion of plastic consmption, the demand for IMMs is growing. Crrently, the global injection-molding indstry contines to grow at an annal growth rate of 7.43% [1]. otal global prodction of IMMs reached p to 293,000 nits in 2014 [2]. Plastic IMMs have now constitte the main energy consming eqipment of plastic processing plants. China s IMM indstry has become the world s largest prodcer and consmer. However, over 90% of the energy costs in injection molding are acconted for by electricity [3,4], so energy consmption remains high. It is estimated that these injection-molding plants in the United States consme arond 30 billion kwh of electricity annally [5,6]. In China, it is estimated that they consme more than 210 billion kwh of electricity [7]. By applying a systemic approach to redce energy consmption on every possible part, it is possible to achieve energy-saving control, from the selection of materials and molds to drying and cooling [8]. herefore, even a small improvement in each part will have a significant impact on the total energy efficiency of the machine. Energy conservation has become a research focs of IMM systems. Energies 2017, 10, 1768; doi: /en

2 Energies 2017, 10, of 20 Figre 1. he application sitation in the end prodct market of injection-molding machines (IMMs). IMMs are classified primarily by the type of driving system: all-electric, hydralic, or hybrid [9]. Hydralic machines sally se hydralic pmps and control valves to adjst their flows and pressres, so they can offer higher injection rates, larger drive torqe, and longer hold time. Hydralic IMMs are mainly sed in high-power and ltra-high-power loads, becase hydralics have an energy density roghly five times higher than that of electric motors [10]. ll-electric machines se only high-speed servo motors, so they can achieve these merits of lower energy consmption, short cycle time, and higher precision. However, their application is restricted de to servo motor (SM) power and the cost of large-power machines. Crrently, the largest clamping force of all-electric IMM is 10,000 kn, while the largest clamping force of hydralic IMMs is p to 80,000 kn [11], and all-electric IMMs are only sed in small- and medim-power machines. Hybrid machines se a combination of hydralic pmps and servo motors or variable freqency motors, so they are characterized by both advantages and are widely sed. Now, all-electric IMMs still accont for only 15 20% of sales in Erope, while hydralics and hybrids accont for 80 85%. In the US, jst nder 50% are all-electric, and the rest are hybrid or hydralic [12]. In China and Germany, hydralics and hybrids accont for more than 80% [10]. herefore, energy-saving research on hydralic and hybrid IMMs is especially important. Some stdies have focsed on certain drive controls of the electrical and hydralic circits [13 17], and some papers have presented control strategies for saving energy [18 21]. However, the latest development of energy-saving technology in IMMs has not been flly reviewed in detail. With the development of control technology, power electronics technology, and variable freqent drive technology, they will provide new ideas and frther promote the development of energy saving in IMMs. In this paper, the energy conservation technology of an electric-hydralic IMM will be reviewed. Firstly, the applications and featres of IMMs are introdced in Section 1. he evolvement of IMMs is presented and the distribtion of energy consmption analyzed in Section 2. Certain methods on hydralic control circits and electric control circits for energy-saving are then discssed in Section 3. Control strategies and energy monitoring are stdied in Section 4, and the energy-recovery technology is proposed in Section 5. Challenges and prospects are then sggested. he aim of this review is to provide a comprehensive perspective on the energy conservation technology of IMMs for researchers. 2. Evolvement and Energy Consmption istribtion of Injection-Molding Machines s a kind of plastic forming eqipment, an electric-hydralic IMM consists of an injection nit, a clamping nit, a hydralic control nit, a heating nit, a cooling nit, an electrical control nit, a feeding device, and the body [22]. It has several main fnctions that contain plastification, barrel heating, platen movement, injection, clamping, packing & holding, ejection, and barrel retraction [23]. he operation program may not be exactly the same for different plastic IMMs, bt this action

3 Energies 2017, 10, of 20 can be sbstantially attribted to a basic program. Figre 2 shows the composition, appearance, and process of a hydralic IMM. clamping mechanism Injection mechanism B B B B B 1Clamping Cylinder 2Ejector Cylinder 3Carriage Cylinder 4Injection Cylinder 5Hydralic motor P P P P P Process Flow Mold closing Frame forward Injection Packing Cooling Plastication Frame back Mold opening Ejection time Figre 2. he composition, appearance, and process of a hydralic injection-molding machine (IMM). ring the process of injecting and forming, plastic materials are first added to the barrel, and they are then heated by the heating element and begin to melt. With the rotation of the screw, the plastic materials are conveyed forward and pshed onto the head of the screw nder pressre. he pre-plastic is completed along with the formation of screw retreating and back pressre forming. hen, the clamping mold mechanism is driven and closed nder the control of the clamping cylinder. t the same time, the screw is driven to rotate nder the pshing of the injection cylinder, so the molten materials in the chamber are injected into the mold cavity. hen the molten materials are solidified and shaped after packing and cooling. t last, the mold is opened by a clamping mechanism, and the shaped prodcts are ejected nder the driving of the ejector. prodction cycle is ths completed. he energy consmption is distribted in all aspects [24,25]. Energy consmption of each part is different for each IMM and depends on part weight, cycle time, machine size, machine type, and machine efficiency. ccording to the system shown in Figre 2, the power sorce ses the asynchronos motor (SM) to drive the fixed displacement pmp, and or research grop carried ot this experimental stdy. Model WK400 injection-molding machine was selected to prodce a kind of mine cable hook prodcts. Power consmption of the IMM was tested dring one dty cycle [26]. he working cycle of an IMM consists of six stages. hey are, respectively, clamp closing, injection table moving forward, injection, pack and hold, plastification and cooling, and clamp opening. he distribtion of power consmption of a hydralic IMM in a work cycle is shown in Figre 3 [26]. Figre 3a shows the power crve, and Figre 3b shows the distribtion of power consmption. In Figre 3, 1 represents clamp closing, 2 represents frame forward, 3 represents injection, 4 represents packing and holding, 5 represents plastification and cooling, 6 represents clamp opening. With the data analysis from Figre 3, dring the working process of an IMM, the energy consmption ratio of clamp opening, clamp closing, and cooling is very small. However, the energy consmption mainly focses on the processes of injection, plastification, and cooling for the hydralic IMM in a work cycle. Especially, in plastification and cooling links, the reqired load power is relatively small, while the system provides a relatively large amont of power. herefore, plastification and cooling provide the greatest opportnities for saving energy.

4 Energies 2017, 10, of Power Consmption P/kW time t/s (a) 40% 20% 14% 10% 0 Power Consmption 38% 25% 14% 9% 7.5% 6.5% (b) Figre 3. he distribtion of power consmption of a hydralic injection-molding machine (IMM) in a work cycle: (a) he power crve, (b) he distribtion of power consmption 3. rive echnology of Hydralic IMM eveloping a highly efficient hydralic IMM is an rgent task, so all aspects of energy conservation research, sch as sing green polymers, redcing energy sage, and lowering clamp forces and injection pressre, are carried ot. In the axiliary eqipment, replacing conventional heater bands with radiant IR heaters, infrared elements from Rex Materials Inc. (Fowlerville, MI, US) can be sed to convert all heating zones. Haitian Plastics Machinery Grop Co. Ltd. (Ningbo, China) has offered Smart Heat barrels as an option on all its IMMs, both all-electric and electric-hydralic hybrids [27]. In addition, Franklin L. Qilmba, Lyndon K. Lee, Wei-Jen Lee, and lan Harding sed alternative synthetic hydralic flid to take the place of conventional flid in the work process of IMM, which had a significant effect on redcing energy consmption by 16.7% in one test period [16] Energy Conservation Research on Hydralic Circits In hydralic driving sorce, conventional hydralic IMM consists of a fixed pmp and a pressre and flow (P/Q) proportional valve, where the pressre and flow are adjsted by P/Q proportional valve. If an asynchronos motor driven by a constant pmp is sed, the flow is only adjsted by a proportional throttle valve according to system reqirements. Extra oil flows back into the tank throgh the bypass of the proportional overflow valve [28,29]. he strctre diagram of a fixed displacement pmp drive system is shown in Figre 4. his control method meets the control of clamping speed, injection speed, packing pressre, screw speed, and top otpt movement reqirements of IMMs [28]. t the same time, it improves the control precision and stability of an IMM. However, it also leads to energy losses associated with flow throgh one fixed pmp and one proportional valve. Especially in the packing process of IMM, this kind of loss is the largest. In the 1980s, an electro-hydralic load-sensing control technology was developed by combining a variable pmp with an electro-hydralic proportional valve. he schematic diagram of a load-sensing control system is shown in Figre 5. Velocity adjstment of the actator is achieved by adjsting the proportional throttle valve and variable pmp flow simltaneosly. he control method can make pmp otpt flow match the load flow by adopting differential pressre signals of import and export oil throgh the proportional throttle valve, thereby redcing energy losses [30]. closed loop control system was designed that combined a load sensitive pmp with a servo valve. practical test proved that energy saving and performance had been achieved [31].

5 Energies 2017, 10, of 20 Injection molding machine q p q p Control system Q s P s M P m Ω Figre 4. he strctre diagram of a fixed displacement pmp drive system. Control system p set + - p f q set + - q f p q Injection molding machine p q Back pressre valve B Q s B P W 1 P P s V 1 W 2 Figre 5. he schematic diagram of a load-sensing control system. By adopting the load-sensing control, all the flow-related energy loss was eliminated. However, there was still a large throttle loss, especially in the high-speed stage, becase of a fixed working pressre in the proportional throttle valve. o eliminate the energy loss of this part, the high response variable pmp was driven by a direct closed-loop control of pressre and flow. herefore, the electro-hydralic system controlled by a P/Q compond proportional pmp was proposed [32]. he control diagram is shown in Figre 6. he latest development trend of IMMs is the integration of electrical and hydralic technology, which ses variable freqency drives to control motor speed, so the motor trns at a precisely specified speed to pmp the correct oil pressre in this process. he IMM system driven by a variable freqency motor will be 50% more energy-efficient than conventional hydralics and 20 30% more energy-efficient than modern servo-hydralics [33]. herefore, the novel electro-hydralic technology provides a new way for energy-saving control in IMMs [34]. he German emag company developed and prodced a new kind of electro-hydralic machine, which is driven by three servo motors and a proportional pmp. his scheme is called the hybrid control, whose diagram is shown in Figre 7. In this scheme, a plastification cylinder is directly driven by servo motor, and injection cylinder and clamping cylinder are driven by constant displacement pmp controlled servo motor. However, other cylinders are driven by a proportional pmp. his plan can greatly redce the energy consmption of this IMM system [26].

6 Energies 2017, 10, of 20 Injection molding machine p p q q P s Q s Control system Figre 6. he electro-hydralic system controlled P/Q compond proportional pmp. B P Clamping Cylinder Ejector Cylinder Injection Cylinder Carriage Cylinder B B M 3~ P q p P M 3~ p p Control Unit Figre 7. he hybrid control scheme of an electric-hydralic IMM Energy Conservation Research on Electrical Circits With the development of power electronic technology, control technology, and microelectronics technology, variable freqency technology of alternating crrent (C) motor has been rapidly developed and applied. If variable freqency technology is applied to a hydralic system, the velocity of a hydralic pmp can be adjsted by changing the motor speed, and the flow of hydralic pmp can be reglated. In view of the hydralic circit shown in Figre 4, variable freqency drive control is adopted for the indction motor. he variable freqency drive control system consists of a variable freqency controller, an asynchronos motor, and a fixed displacement pmp. ssming that the inpt power of an asynchronos motor is P1, the otpt power is Pm, the iron loss is PFe, the copper loss power is PC, and the stray loss is Pa, the efficiency ηam is expressed as = = (1) synchronos motor speed n is expressed as

7 Energies 2017, 10, of 20 = 60 (1 ) (2) where f is the power freqency, np is the nmber of pole-pairs of motor, and s is the slip ratio of the motor. hen, the otpt power of asynchronos motor Pm is expressed as = 9.55 = (1 ) where M is the torqe of the asynchronos motor. ssming that the flow of hydralic pmp is qp, the pressre is pp, the volme loss power is PV, and the torqe loss power is PM, the otpt power Pp is expressed as = = (4) (3) he flow of hydralic pmp qp is dedced as = = 60 (1 ) (5) where V is the displacement of the hydralic pmp. From Eqation (5), we learn that the flow of the pmp cold be changed by adjsting the C power freqency [35]. herefore, the otpt power of hydralic pmp Pp is also converted as 60 = ( )= (6) (1 ) From the analysis of the hydralic circit in Figre 4 in Section 3.1, there are two parts of losses: overflow loss and throttle loss. he overflow loss power PO is expressed as he throttle loss power P is expressed as = ( ) (7) =( ) (8) where qv is the the flow of the valve, and pv is the pressre of the valve. ssming that the loss of the hydralic cylinder itself is ignored, the otpt power of hydralic system Ps is eqal to the load power PL, and Ps can be expressed as = = (9) ccording to Eqations (1), (4), and (9), the otpt power of the hydralic system Ps can be dedced as = (10) ccording to Eqations (6) and (9), the otpt power of the hydralic system Ps can also be dedced as 60 = (1 ) (11) he efficiency of the whole system ηs is calclated as = = (12) where ηp is the efficiency of the hydralic pmp, and ηc is the efficiency of the hydralic system. In the practical application system, load is constantly changing. Similarly, the reqired power PL of the load is varied for the IMM system. Especially in the stages of holding pressre and cooling, the reqired load power PL becomes smaller. When the load power PL is redced, if the otpt power of the motor has been exported the rated power, the otpt power of the hydralic pmp will remain nchanged. In order to maintain

8 Energies 2017, 10, of 20 the otpt power Ps and the load power PL of the hydralic system eqal, according to Eqation (9), we can see that the overflow loss power PO and throttle loss power P will increase. herefore, it will reslt in a greater energy waste and a lower efficiency. When the load power PL is redced, if variable freqency technology is adopted in the asynchronos motor, the freqency is lowered by adjsting inverter. he speeds of the motor and the hydralic pmp drop, and the otpt power of hydralic pmp Pp, also decreases. herefore, the otpt power of the hydralic system decreases as the load power decreases. he inpt power atomatically adapts to load power, so the power consmption of this system will be greatly redced. hereby, when inpt power atomatically tracks load power, the otpt flow and pressre can achieve a good match to system reqirements, so this variable freqency technology can greatly redce power consmption and improve system efficiency. When freqency control is adopted, motor soft start can be achieved so as to redce motor wear and increase motor operational life span. Besides, the power factor correction is designed, so as to increase the active power of the electric network and save power consmption. ccording to the prodcts of injection, the inverter energy saving can be p to 20 70% [36]. If the variable speed control technology is applied in an electro-hydralic IMM, it can effectively redce its energy consmption, improve process control, and redce wear and tear on the motor [37 40]. In recent years, some research instittions and scholars have developed variable speed technology for energy-saving and have made a series of achievements. In the 1990s, Heldser pt forward a variable speed control for a direct pmp control cylinder circit, redcing the power loss, especially with respect to partial loads or no load. speed adjstable C servo motor and a fixed displacement pmp were combined to make the electric hydrostatic drive system easy to control, and to improve the dynamic performance of the system. hen, some comparative stdies of energy efficiency were condcted in variable displacement control and variable speed control. Variable speed control cold obviosly achieve less energy consmption and efficiency as high as 98.4%. By developing and testing the application, the reslts showed that, in a working cycle, the total efficiency of an IMM driven by freqency conversion can reach p to 52%, while the total efficiency of an IMM driven by ordinary hydralic pressre is less than 25% [41 43]. In 1999, Heldser pioneered a variable speed control in an electro-hydralic IMM, which is sed to optimize the efficiency of packing and cooling processes, saving energy [44]. From 1999 to 2011, Professor Long proposed variable speed driven differential cylinder control circits, which are sed for energy-saving control of electro-hydralic IMMs [45,46]. he principle to control differential cylinders with two speed variable pmps is shown in Figre 8. m F x x f - x set + Pmp 2 SM 3~ - + i f i set n set + - n f SM 3~ Pmp 1 i set -+ i f n set + - n f Figre 8. he principle to control differential cylinders with two speed variable pmps.

9 Energies 2017, 10, of 20 In 2008, akayki Imamra focsed on the efficiency and power consmption of IMM servo driven by a variable speed pmp control system, which was compared with the traditional asynchronos motor-driven variable displacement pmp system [47]. However, becase of the slow dynamic response of asynchronos motors, sch motors are sbstitted for servo motors in order to improve the dynamic response, control accracy, and low-speed performance of the system. herefore, the variable freqency drive is vigorosly promoted and applied in IMMs [45]. he new electro-hydralic control technology has opened p new avenes for efficient and energy-saving IMM control. In 2009, Mao-Hsing Chiang adopted permanent magnet synchronos motor speed control to replace the traditional throttling and volme control, which redced the power consmption of IMMs [48]. In 2011, Gan Cheng s grop and Ying Ji s grop separately designed an IMM control system where the servo motor was adopted to drive a fixed displacement pmp [49,50]. In 2011, Professor Peng adopted a servo motor driving a fixed displacement pmp system to achieve energy-saving control [51]. In 2013, the energy conversion efficiency of the variable speed driving IMM system was analyzed and stdied by merican scholar F. L. Qilmba [16]. In 2011, or research grop condcted some research for five control schemes, which are respectively listed here [52]. Scheme 1 is an asynchronos motor driving a fixed displacement pmp. Scheme 2 is a variable speed asynchronos motor driving a fixed displacement pmp. Scheme 3 is an asynchronos motor driving a variable displacement pmp. Scheme 4 is a variable speed asynchronos motor driving a variable displacement pmp. Scheme 1 is an C servo motor driving a servo pmp. he control principle diagram and energy consmption analysis are shown in Figre 9. Figre 9(a1) shows the control system of the fixed displacement pmp driven by a variable speed asynchronos motor (SM). Figre 9(a2) shows the energy consmption distribtion where there is overflow loss and throttling loss. When the motor is not adjsted by the variable freqency control, the system overflow loss is relatively large (hat is the sm of the shaded rea 1 and 2). When the motor is adjsted by variable freqency control, the overflow loss redces to rea 2. Figre 9(b1) shows the control system of a variable displacement pmp driven by a variable speed asynchronos motor. Figre 9(b2) shows that there is throttling loss. Figre 9(c1) shows the control system of a servo pmp driven by a servo motor (SM). Figre 9(c2) shows the energy consmption distribtion, and there is no overflow and throttling losses. iming to the above five schemes, the same IMM was adopted, and same prodct prodced. Experimental tests were respectively carried ot. able 1 shows the power consmptions of IMM for the five control schemes in a work cycle time. Control Schemes able 1. Power consmptions of an IMM with five different control schemes (kw). Clamp Closing (7s) Frame Forward (8s) Injection (7s) Pack, Hold (4s) Plastification and Cooling (30s) Clamp Opening (5s) otal Power Consmption (61s) Scheme / Scheme / Scheme / Scheme / Scheme / By comparing Scheme 1 with Scheme 2, we can clearly see that power consmption of Scheme 2 redces 26.8 kw in a work cycle time. By comparing Scheme 3 with Scheme 4, the power consmption of Scheme 3 redces 7.5 kw in a work cycle time. ccording to the above analysis and data comparison, it can be very clearly obtained that variable freqency control can redce energy consmption. However, in Scheme 5, after an C servo motor displaces an asynchronos motor, power consmption becomes the smallest. Becase the efficiency of an C servo motor is higher than the efficiency of an asynchronos motor. hen, according to Eqation (12), the conclsion is drawn that the energy efficiency of the servo pmp system driven by the C servo motor is the highest.

10 Energies 2017, 10, of 20 Qantitative Pmp q v q p p v p p ω SM P m p p synchronos Motor inverter (a1) P n q s q s ata Collection Freqency Converter q Overflow Loss q' hrottling Loss p q p 1 2 q v ctive Power 0 (a2) p v p p p q v p v Variable isplacement Pmp p p ω p p SM n synchronos Motor ata Collection P inverter (b1) n set PLC Q s Q s Freqency Converter q q p q v hrottling Loss ctive Power 0 p (b2) p v p p Servo Pmp q p p p ω SM p p P m Servo Motor (c1) q s Servo controller q s p P p P P n ata Collection q q p ctive Power p p 0 p (c2) Figre 9. he control principle diagram and energy consmption analysis: (a1) he control system of the fixed displacement pmp driven by asynchronos motor (SM), (a2) Energy consmption distribtion for Figre 9(a1) shown system, (b1) he control system of a variable displacement pmp driven by asynchronos motor (SM), (b2) Energy consmption distribtion for Figre 9(b1) shown system, (c1) he control system of a servo pmp driven by a servo motor (SM), (c2) Energy consmption distribtion for Figre 9(c1) shown system 4. Control echnology of IMMs 4.1. Energy Conservation Control Strategy By improving system components, i.e., hydralic and electrical circits, energy-saving was achieved. However, in order to frther redce loss and improve system performance, certain control strategies have been stdied. In 2004, a stdy was carried ot to simltaneosly achieve good velocity control capability and high energy efficiency. he acceptable system consists of velocity control in a hydralic valve-controlled cylinder system and energy-saving control with both a variable displacement pmp system and a variable rotational speed pmp system. he feasibility of the integrated control system was ths shown [18]. In 2005, a decopling fzzy sliding-mode control strategy was proposed for the integrated control of clamping force and energy-saving for simltaneosly

11 Energies 2017, 10, of 20 improving the accracy of force control response and energy efficiency in a hydralic IMM [53]. In 2007, a low-cost and flly digital velocity/pressre controller integrated digital signal processor (SP) and analog-to-digital converter (C) was designed, which demonstrated fast response and precise tracking. n in-parallel interlaced sample-and-hold strctre was sed in this system to enhance the ability to resist noise and enlarge the sampling freqency of the C [54]. In 2009, Mao-Hsing Chiang investigated a variable rotational speed electro-hydralic pmp-controlled system to improve the response and energy efficiency of hydralic IMMs. In order to achieve better response and efficiency, he sed a constant displacement axial piston pmp. In addition, a control strategy of a signed-distance fzzy sliding mode control was sed. Better response and higher energy efficiency of the system were demonstrated in the experiments [48]. In the same year, Sho Wang developed a new fzzy controller for the servo motor driving fixed displacement pmp system. he research showed that the system was more robst than that of the traditional proportional-integral-derivative (PI) controller and that the speed tracking error decreased to the reqired injection accracy. his fzzy controller is now applied to the Haitian servo IMM [19]. t the same time, a non-dominated sorting genetic algorithm II was adopted by Zhe Wei, and this algorithm had better convergence near the actal Pareto-optimal front. he control strategy solved the design problem of mlti-objective performance optimization, and energy-saving was ths achieved in a large IMM [55]. In 2010, Lin and Lian developed a controller, called a hybrid self-organizing fzzy and radial basis-fnction neral-network controller, that can adjst the appropriate parameter vales of a self-organizing fzzy controller in a timely manner by selecting and changing both the learning rate and the weighting distribtion of the controller. his controller was sccessfl at redcing speed errors, maintaining the stability of the system, increasing the injection rate, and improving the qality of plastic prodcts [56]. In the same year, an energy control strategy based on logic gate was designed by ao Li for a parallel hydralic hybrid vehicle and was sed to realize the switching between different working modes, whose control goal was to achieve parameter optimization based on the principle of optimal energy-saving and improved the performance of the whole machine [57]. In 2011, Wang designed a gray fzzy proportional-integral (PI) controller, which integrated a predictive gray system, a robst fzzy ratiocination, and a traditional PI control algorithm. his controller achieved a more desirable performance and energy-saving control in an IMM driven by a servo motor [58]. In 2012, the adaptive fzzy PI compond control was pt forward by Gang Feng for variable speed motor-driven pmp control system, which achieved energy-saving and strong robstness of the system [59]. In the same year, Ning-Yn L adopted the genetic algorithm-based lexicographic method for realizing energy saving and ensring prodct qality at the same time. hs, energy tilization increased by abot 10% [60]. In 2014, Fenfen Qi adopted a nonlinear fzzy control strategy for a servo motor driving IMM system. he experiment verified the improvement in response speed, injection accracy, and energy efficiency [61]. In the same year, Bing X designed a PI controller with a model-based feed forward compensator and sed a parameter selection method simltaneosly for achieving desired force control accracy and low energy consmption. s a reslt, the energy consmption of this system, compared with that of the traditional system, was redced by 71%, and the robstness of the system was greatly enhanced [62]. Yong-Gang Peng pt forward a nonlinear model predictive control based on the nero dynamic optimization method instead of a traditional PI method, which was applied in an injection molding process for a servo motor driving constant pmp hydralic system. he reslts showed that online optimization, qick response, favorable controlling precision, and smaller overshoot were achieved [63]. ccording to the topology optimization based on the method of finite element analysis, Bin Ren proposed other optimization design scheme for stationary platen and chose a preferred alternative that can realize low energy consmption and high injection precision [20]. he thermal control of an injection molding system is a key isse in the development of high-efficiency injection molds. In 2015, Byeong-Ho Jeong, Nam-Hoon Kim, and Kang-Yeon Lee

12 Energies 2017, 10, of 20 proposed a SP-based PI control system for thermal control, which achieved high precision, better performance, and good stability at a low cost [21] Energy Monitoring System In addition, Bosch Rexroth developed a software and a control package to monitor and optimize energy tilization throghot the molding process (Bosch Rexroth G in Lohr am Main, Germany) [64]. he BB Grop introdced energy monitoring capabilities in driving motors [64]. Krass Maffei also developed motion control software, which atomatically optimized screw speed and other settings for energy efficiency. In addition, Engel discssed new ecobalance software to redce peak energy consmption atomatically [10]. In addition, energy monitoring can monitor varios parameters, operating the state and performance of IMM. It can also provide references for energy-saving methods. herefore, some researchers and companies began to stdy it. Star Master International Limited developed an energy monitoring system that cold monitor the operating capacity of the oil pmp motor by detecting the pressre and volme of IMM [65]. In 2010,. Weissman and. nanthanarayanan et al. made a first attempt, according to the compter-aided design (C) model of the parts, the material name, and the prodction volme, as well as the se of simlation software, to obtain an accrate estimate of the total energy consmption [66]. In 2011, a finite-state machine was sed to model energy consmption patterns of engineering processes, and a two-stage framework combining a Savizky Golay filter with neral network was proposed to classify energy patterns. his method achieved an accracy of 95.85% in identification of machine operation states and frther realized the prediction of machine operation states and energy consmption patterns [67]. In the same year, based on non-intrsive load monitoring, a discrete wavelet transform and a modified niversal threshold filter were sed to divide time-series data into segments by detecting stepwise changes. two-stage fzzy c-means was then sed to clster extracted segments into grops according to existing operation states [68]. his year Hanieh Mianehrow and li bbasian analyzed the factors that affected energy consmption throgh energy monitoring, and its aim was to provide methods for improving energy efficiency [69]. 5. Energy Regeneration 5.1. Recovery and Rese of Hydralic Braking Energy IMMs have mltiple operating nits and intermittent operation. ring operation, these operating nits freqently start and stop and freqently accelerate and decelerate, thereby reslting in a certain amont of braking energy. If there are no recovery measres, braking energy is consmed in the form of heat. Moreover, additional cooling links need to be added. herefore, in recent years, domestic and foreign scholars have been concerned with the recovery and rese of hydralic energy and electrical energy. More energy saving is possible with linear stop/go movements sch as clamp-open and clamp-close by regenerating electricity from braking instead of dissipating it as heat [70]. In 2004, Peter Jarosch analyzed the accmlator storage control according to a comparative stdy on the energy consmption of IMMs [71]. In 2006, Professor Qan adopted a variable speed pmp, accmlator, and proportional valve to control differential cylinders for the clamping mechanism of IMMs. Hydralic accmlator will recover energy when cylinder retrns, while it will release energy when cylinder protrdes. herefore, energy consmption is redced, and the tilization rate of energy improves [72]. In the same year, Robin Kent sed energy storage technology to redce the installed power and energy consmption of an IMM system [73]. In 2008 and 2010, a control idea of electro-hydralic variable speed based on energy reglation was proposed. he energy reglator consisted of an accmlator, a proportional throttle valve, and a relief valve, which was to redce the pressre shock dring the retrning and protrding of the cylinder. he experiment verified an energy-saving effect [74,75].

13 Energies 2017, 10, of 20 In 2010, Yang reformed traditional P/Q valve-controlled hydralic IMM circits and developed an energy saving modle with a hydralic accmlator for redcing energy consmption [76]. In 2013, Ningbo Haitian Plastic Machinery Co. Ltd., which was the leading enterprise in the prodction of plastic IMMs in China, sed an accmlator to store oil dring the cooling stage and to provide oil dring the injection process, so the energy tilization rate was improved [77] Recovery and Rese of Electric Braking Energy Research on recovery and rese of electric braking energy is widely applied in many different fields, so it is also a research hotspot on the energy-saving control of IMMs. Electrical braking energy is fed to the power grid, which is an effective soltion for energy recovery and tilization. However, this mode will case power grid voltage flctations and increase harmonic content sch that the qality of the power grid will decrease [14,78]. herefore, exploring a green recovery device of braking energy has been the goal of the majority of researchers. Sper capacitor energy storage is a common mode of electrical energy storage. he sper capacitor has high power, a strong otpt capacity, excellent charge discharge performance, fast response, and a long service life, so it is widely sed as an energy storage device. In 2002, ran-gomez employed a flyback bi-directional direct crrent to direct crrent (C/C) converter circit with a high voltage/power transmission ratio to perform the closed-loop control of direct crrent (C) bs voltage. herefore, it can achieve energy storage and the release of the sper capacitor [13]. In 2006, the nsaldobreda Research epartment has carried ot a stdy and experimentation on an innovative energy storage system of an electrical vehicle. It consists of sper capacitor modles and achieves energy optimization and high energy recovery dring the braking phase [79]. In 2008, Cheng stdied the energy recovery and tilization of a sper capacitor for special saloon carriage [80]. In the same year, Professor Cao stdied the inflence of the sper capacitor and battery hybrid power spply and analyzed the recovery and tilization of braking energy for electric vehicle [81]. In 2009, Brabetz applied the sper capacitor for electric vehicles to improve the energy efficiency of the system [82]. In the same year, Rao introdced a sper capacitor to the high voltage and large capacity variable freqency controller for achieving motor brake energy storage and rese, while improving system performance [83]. In 2010, Mishima omokaz sed an energy storage device that combined a psh pll bi-directional C/C converter with a sper capacitor for achieving bi-directional control of energy and redcing transmission loss [84]. akahashi provided a novel control scheme of an energy recovery system with an electric doble-layer capacitor and a bi-directional C/C converter. n experiment verified a 49% energy-saving effect for the control system [85]. he system strctre diagram of energy recovery and rese is shown in Figre 10. In the same year, X sed a nmber of sper capacitor modles in a series to adapt to different power spply system for traction power grid applications [86]. Li applied a sper capacitor in an excavator for achieving potential energy regeneration and tilization [87]. In 2013, ipankar e focsed on the design of the C/C power converter for achieving energy recovery and tilization control of the sper capacitor [88]. In the same year, Z. eng applied a sper capacitor to an elevator for the recovery of brake energy and analyzed the lower inflence of power flctation on the power grid qality [89]. In 2014, the control system of a peak power shaving that consisted of a mltiphase bidirectional C/C converter and sper capacitors was designed. When the injection motor was at rest, energy passed throgh the mltiphase bidirectional C/C converter and was stored in the sper capacitors. When the injection motor accelerated, energy was released. Conseqently, the peak power was shaved by 70%, and significant energy conservation was ths shown [15]. In 2015, a hybrid energy storage system based on a mltiport C/C converter, aimed at a C micro grid system, was proposed. he hybrid energy storage system, composed of a sper capacitor and a storage battery, combined the advantages of the two and made p for the shortcomings of the weak energy storage capacity of the sper capacitor. herefore, the service life of the storage device was prolonged. Simltaneosly, in order to keep the sper capacitor voltage close to the rated vale

14 Energies 2017, 10, of 20 and ensre the battery energy distribtion balance, an adaptive energy control strategy based on the moving average filtering algorithm was pt forward [90]. In 2016, Wei and his grop stdied the sper capacitor storage energy system and proposed a control strategy based on a modlar mltilevel converter that achieved voltage balance control of energy storage components and doble loop control of the C bs side crrent [91]. P n i U d P pressre sensor + Inverter B C I U c U i C C Bi-directional C-C converter r C 6. Challenges and Prospects Figre 10. he system strctre diagram of energy recovery and rese. his paper mainly reviews hydralic drive circits, electric drive circits, control strategies, the regeneration of hydralic braking, and electric braking energy for hydralic IMMs. lthogh the energy-saving technology of IMMs has made some effective achievements and developments, the energy recovery technology of hydralic IMMs still faces several challenges. From the above stdy, energy regeneration is independently carried ot, respectively, on a hydralic link and an electrical link, so it can only achieve energy saving for a single link. herefore, if the braking energy from the hydralic and the electrical link simltaneosly is recycled and sed, it will be one of the effective ways for the improvement of energy efficiency. However, the two links carry ot energy recovery at the same time nder independent control, which will lead to system vibration or movement disorders, and even case the eqipment to not work properly. herefore, the coordination control of the hydralic and electrical links is the next problem to solve. For this problem, or research grop pts forward a coordinated control strategy of an electro-hydralic mixed energy recovery for hydralic IMMs. he principle of the coordination control strategy is shown in Figre 11. ring the operation of the hydralic IMMs, the system pressre, the accmlator port pressre, the motor speed, the motor crrent, and the C bs voltage are collected in real time and transmitted to the coordination controller. ccording to the actal signal and the given signal, and combining with load conditions, the coordination controller will otpt a control signal to drive the varios hydralic cylinders. ssming that the clamping cylinder is driven and extended, the flid in the rod cavity of cylinder flows into the accmlator throgh the check valve. hen, the accmlator will absorb the braking energy and shock cased by the sdden stop of the clamping cylinder and the contact between the front and rear molds. t the same time, by the real-time acqisition of the C motor crrent direction and the C bs voltage, the coordination controller comprehensively jdges the operating stats of C motor. In addition, the coordination controller is to control the bidirectional C/C converter, thereby

15 Energies 2017, 10, of 20 realizing the atomatic energy storage and release of the spercapacitor. When the mold clamping is completed, the coordination controller controls the carriage cylinder to move forward. he coordination controller then controls whether Switch Valve V1 or Switch Valve V2 is connected according to the system pressre and the accmlator port pressre. If the accmlator port pressre is larger, and the difference between the system pressre and the accmlator port pressre is too small, the coordination controller drives Switch Valve V1. hen, the accmlator spplies oil to the hydralic pmp, and the coordination controller controls the switch valve to connect the low pressre port B of the directional valve to the oil tank. t the same time, the controller adjsts the motor speed and the otpt power according to the system pressre, the motor speed, and the load changes. herefore, the otpt power will match the reqired power of the load in real time, thereby to achieve minimm otpt power. Clamping Cylinder Ejector Cylinder Carriage Cylinder Injection Cylinder Plastification motor Bi-directional C/C PWM B B B B Rectifier filter C /U Inverter PWM i/u C M n/u p s Coordination Controller / C V1 p a / V2 set to directional valve Figre 11. he principle of the coordination control strategy. In accordance with the above control law, the electro-hydralic IMM in trn completes injection pack and hold plastification and cooling clamp opening clamp closing frame forward periodic work. herefore, the coordination control of an electro-hydralic mixed energy recovery for hydralic IMMs is achieved. If the coordination control is applied in electric-hydralic injection-molding eqipment, the system efficiency can be frther improved. However, the application of servo motor drive sytem is narrowed, becase the servo motor power is limited. In practice, the application of some high-power electric-hydralic injection-molding eqipment is more extensive, and there are more conditions of partial loads or no load. herefore, there is a large energy-saving space. he above energy conservation control technology will be able to greatly redce their energy consmption if it is sed for the high-power electric-hydralic injection-molding eqipment system. nd then the speed control of the high-voltage high-power motor is reqired. However, the emergence and development of high-voltage freqency conversion technology make the settlement of this problem possible. he next step, if the hydralic pmp control system is combined with high-voltage freqency conversion technology, can not only increase the system drive power bt also redce the energy consmption of the high-power electric-hydralic injection-molding eqipment system.

16 Energies 2017, 10, of 20 his will promote the development of high-power electric-hydralic injection-molding eqipment. In smmary, electric-hydralic injection-molding eqipment is moving toward low-loss, high-efficiency, and green energy-saving development and will have a wide range of application prospects. cknowledgments: he athors wold like to thank the anonymos reviewers for their valable comments. his research work is spported by the National Natre Science Fondation of China (Grant No ). thor Contribtions: Hongjan Zhang and Baoqan Jin conceived and designed the stdy; L Ren and Yan Gao developed the design of the illstration. ll athors contribted to the detailed srvey of the literatre and the state of the art, which were essential for the completion of this review paper. Conflicts of Interest: he athors declare no conflict of interest. bbreviation IMM injection-molding machine C analog-to-digital converter P/Q pressre and flow PI proportional integral derivative C alternating crrent PI proportional integral SM asynchronos motor C Compter-aided design PLC programmable logic controller C/C direct crrent to direct crrent C direct crrent PWM plse width modlation SM servo motor / analog to digital SP digital signal processor / digital to analog References 1. Erne, B. Injection Molding Machines Benefit from echnology-netral esign pproach. vailable online: (accessed on 26 Jne 2013). 2. Global and China Injection Molding Machine Indstry Report, vailable online: (accessed on 23 ecember 2016). 3. Energy Management in Plastics Processing. vailable online: echnical%20notes%20-%20energy%20(1%20-%208).pdf (accessed on 22 March 2017). 4. Understanding Energy Consmption in Injection Molding Machine. vailable online: (accessed on 25 March 2016). 5. China Plastics Injection Machine Market Panorama Srvey and evelopment rend Research Report dring Years. vailable online: (accessed on 25 gst 2015). 6. Energy Utilization in Injection Molding. vailable online: _tilizatn_in_inj.pdf (accessed on 25 ecember 2016). 7. In the Next Five Years, the Consmption of Injection Molding Machines Is Likely to Exceed 2 imes the Power Generation Capacity of the hree Gorges. vailable online: (accessed on 17 October 2016). 8. Godec,.; Rjnic-Sokele, M.; Šercer, M. Processing parameters inflencing energy efficient injection molding of plastics and rbbers/utjecaj parametara prerade na energijski inkovito injekcijsko pre anje plastike i gme. Polimeri 2013, 33, Liebig, G. Comparison of the Performance of a Hybrid and an ll-electric Machine. Knststoffe/Plast. Er. 2002, 7, Scht, J.H. oes Low Constant Pressre Injection Molding Work? vailable online: (accessed on 31 gst 2015). 11. kasaka, N. esign Method by se of Control Similarity Principle applied for a Load Simlator of Electric-motor riven Injection Molding Machine. In Proceedings of the International Conference on Control, tomation and system, Seol, Korea, October 2008; IEEE: Piscataway, NJ, US, 2008; doi: /iccs Scht, J.H. Eco-Molding: More Power for Less Energy: How Mch More Efficient Can Injection Molding Machines Get? Plastics Engineering: Sheboygan, WI, US, vailable online: plasticsengineering/march2015/coverstory_ecomolding.html (accessed on 5 pril 2016).

17 Energies 2017, 10, of ran-gomez, J.L.; Enjeti, P.N.; Joanne,.V. n approach to achieve ride-throgh of an adjstable speed drive with flyback converter modles powered by sper capacitors. IEEE rans. Ind. ppl. 2002, 3, doi: / Rodrigez, J.R.; ixon, J.W.; Espinoza, J.R.; Pontt, J.; Lezana, P. PWM regenerative rectifiers: State of the art. IEEE rans. Ind. Electron. 2005, 52, doi: /ie kiyoshi, H.; Hiraki, E.; anaka,.; Okamoto, M.; Matso,.; Ochi, K. Peak Power Shaving of an Electric Injection Molding Machine with Spercapacitors. IEEE rans. Ind. ppl. 2014, 50, doi: /i Qilmba, F.L.; Lee, L.K.; Lee, W.J.; Harding,. Improving hydralic system energy efficiency with high performance hydralic flids. IEEE rans. Ind. ppl. 2014, 50, doi: /i homas,.r.; Speight, R.G.; nnareddy, M. Using injection molding simlation to develop environmentally friendly prodcts. Plast. Rbber Compos. 2013, 37, doi: / x Chiang, M.H.; Lee, L.W.; sai, J.J. he concrrent implementation of high velocity control performance and high energy efficiency for hydralic injection molding machines. Int. J. dv. Manf. echnol. 2004, 23, doi: /s Sho, W.; Ji, Y.; Weian, Z.; Min, K. Fzzy control of the ram velocity in energy-saving servo injection molding machines. In Proceedings of the International echnology and Innovation Conference, Xi an, China, October 2009; IE: London, UK, 2009; doi: /cp Ren, B.; Zhang, S.Y.; an, J.R. Strctral Scheme Optimization esign for the Stationary Platen of a Precision Plastic Injection Molding Machine. Chin. J. Mech. Eng. 2014, 27, doi: /cjme Jeong, B.H.; Kim, N.H.; Lee, K.Y. Optimized igital Proportional Integral erivative Controller for Heating and Cooling Injection Molding System. J. Electr. Eng. echnol. 2015, 10, doi: /jee Socks, M. he Promise of ll-electric Injection Molding Machines: Promise Kept? CEEE Smmer Stdy on Energy Efficiency in Indstry 2005; Energy Efficiency in Indstry: Washington, C, US, pp vailable online: (accessed on 5 pril 2016). 23. Li, P. Modern Injection Molding Machine Control, 1st ed.; China Light Indstry Press: Beijing, China, 2004; pp , ISBN Madan, J.; Mani, M.; Kevin, W.L. Characterizing Energy Consmption of the Injection Molding Process. In Proceedings of the SME 2013 International Manfactring Science and Engineering Conference, Madison, WI, US, Jne Competition Bbbles Up gain in Physical Foam Molding. vailable online: (accessed on 8 pril 2016). 26. Zhang, H.J. Research on Variable Speed Pmp Controlled ifferential Cylinder and Low Energy Consmption Injection Molding Machine. Ph.. hesis, aiyan University of echnology, aiyan, China, Energy Efficiency in Plastics Processing Practical Worksheets for Indstry. vailable online: (accessed on 20 pril 2016). 28. Li, H.B. Zh, X.H.; Zh, Z..; Sn, H.P. he Energy-saving pplication for Injection Molding Machine. Chin. Hydral. Pnem. 2007, 8, e doi: /j.issn Moog-Plastics-Injection-Molding-Overview-en. vailable online: (accessed on 18 pril 2017). 30. Wen, S.P.; Jiang, J.Q. he latest development and application of energy saving control techniqe for injection molding machine. Eng. Plast. ppl. 2006, 34, doi: /j.issn (In Chinese) 31. Electrohydralic Control in Plastic Parts Prodction. vailable online: ydralicvalves (accessed on 25 pril 2017).