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UNDERSTANDING SELECTION OF INJECTION MOULDING MACHINE |
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Home Things to Know in Injection Moulding of Plastics Pitfalls in Injection Moulding Myth in Injection Moulding A Guide to Injection Moulding of Plastics Foreword Archives of Articles by Prabodh C. Bolur Selection of IM Machine Engineering Basics for Injection Moulding Machine PlastIndia Photos Useful Links Author Contact Author Millineum's MoldFlow User Meet Answers to YOUR Questions  |
Continued from previous page. POWER CONSUMEDSince power is product of pressure (kg / cm2 ) and rate (cc / sec.) actual power consumption is product of
and
through out the moulding cycle. Peak of filling pressure and peak of injection rate do not reach simultaneously when wall thickness is large. Peak of filling pressure and peak of injection rate may reach simultaneously when wall thickness is low, thereby demanding more power. Lower wall thickness demands higher injection rate and the filling pressure can reach higher values when higher injection rate is in operation. Therefore, injection unit should have enough power rating to handle such moulding. This not being realised by many. They ignore injection rate specification of the new machine from Japan / Taiwan / Hongkong and find that they are not able to mould what was normally possible in SP series machine of same clamp force. The POWER consumption is
To mould any part whatever power is required will have to be made available by the machine. However the energy efficiency deals with the reduction of wastage of power. Hydraulic system provides dynamic power and heater control system provides thermal power. Efficiency of hydraulic system can be enhanced by using followings:
POWER SUPPLY FREQUENCYIn our country power supply frequency is 50Hz. whereas, in many other countries like Japan, Taiwan, Hongkong, USA , etc. power supply frequency is 60Hz. Prime mover for hydraulic pump is 3 phase Induction Motor whose RPM is directly proportional to frequency of supply and inversely proportional to number of poles of motor. RPMs for different poles and Hz. given below:.
It can now be observed that:
DRIVE POWERThe size of the power unit is decided by plasticising rate and injection rate both having influence on quality and production rate. Fast cycling machine requires higher drive power. Accumulator is used to increase the injection speed (rate) to enable parallel operations of ejector and core pull . The option of three different drive power is possible
CONNECTED POWER & POWER CONSUMPTION.The connected power is decided by the power requirement for plasticising and injection. Moulding cycle has consist of 70% cooling time. 15 to 20% injection time and remaining 10 to 15% is taken up by mould close / open and ejection. During cooling time after dosing is over the machine is idling. The power consumed during idling is normally 4 to 8% for Induction motor. This can be considered as power wasted. This wastage occurs during the balance period of cooling time after the plasticising is over. This wastage can be negligible if the cooling time is just enough for plasticising to be over. The power wastage during idling is reduced by usage of multiple pumps which can be unloaded or loaded depending on the demand for power. The sequencing of loading and unloading is programmed in the software of microprocessor controls. The best energy efficiency is obtained by using variable delivery pump. Thus energy efficiency or less wastage of power depends on hydraulic system of the machine. This calls for evaluating hydraulic system.
The power consumed depends on product of actual pressure (read on pressure gauge and not set pressure) and injection speed. These parameters depend on geometry of part and viscosity of melt. If the connected power is lower than it may not be enough to fill larger parts of thin wall thickness (long flow ratio) even though the machine may have enough shot capacity. The lower connected power does not indicate lower consumption of power. It indicates lower capability for moulding bigger size of parts with large flow ratio. It defines the limitation of machine. Low powered machine can enhance the injection power by incorporating hydraulic accumulator. This increases the cost of machine and also hassles of maintenance of accumulator. MOULD CLAMP SYSTEMSThe injections moulding machines with hydraulic clamp as well as toggle clamps are being manufactured in Europe, U.S., and Japan for last few decades. MOULD SETTINGMould setting is quit simple in hydraulic clamp machine whereas in toggle clamp machine toggle mechanism is required to be shifted on the tie bars. Proper clamp adjustment demands more skill and judgment on the part of workmen. This is a very favourable advantage which user of both types of machines can confirm. MAXIMUM DAYLIGHT AND MOULD OPEN STROKEMaximum daylight as well as mould open stroke depends on toggle geometry whereas hydraulic clamp machine can be built easily for larger daylight by simply providing longer cylinder and longer tie bars. The longer mould open stroke can be provided by using longer clamp cylinder and ram. Therefor it is easier to satisfy special requirements of customers. In hydraulic clamp machine Mould open stroke = Maximum day-light minus Closed mould height. Hydraulic clamp machines have enabled the moulders to set the mould easily by shifting the limit-switches. This feature has been liked by the users of machines. FRICTIONAL RESISTANCE TO MOVE THE MOULDIn hydraulic clamp machine frictional resistance are encountered at
Therefore these seals have long operational life. It does not require any special lubricants. In-fact the hydraulic clamp does not require any attention and has always been proved more reliable. There are eight toggle pins / bush joints in double toggle machines. Obviously wear-tear proportional to operational speed would take place at the joints as cent percent fail-safe lubricants are not available. Since there are many machined parts in toggle any inaccuracies in these parts would result in mismatch of cylinder piston with cross head bore. This would result in unequal wear-tear of toggle pin / bush joints. It can also result in loss of parallelity between movable and stationary platens. SPEEDMould close operation is carried out through small diameter bore cylinder and mould open operation is also carried out through the ram end of clamp cylinder. Equivalent cylinder is used in toggle machines to move the mould. Therefor to perform mould close / open operations oil requirement would be similar.The speed control is provided by hydraulic valves i.e. proportional valve or pressure compensated flow control valve in both types of machines. In spite of natural kinetics of toggle mechanism speed control in hydraulic system can not be eliminated. FR series machine has such an excellent mould safety feature that mould close terminates and opens (about-turn) even if a paper is trapped between the mould. Therefore lack of natural kinetics is never felt in SP as well as FR machines. CLAMP FORCEIn both the machines application of clamp force is against the deformation resistance of tie bars, mould and platen. In hydraulic machine the force is applied by a column of oil under pressure behind the ram of clamp cylinder. The oil is drawn in by gravity flow or sucked in during mould close operation. The oil column is pressurized by intensifier and the clamp force is retained by the non return / check valve with out consuming any additional power. The clamp force can be varied and set precisely at desired value by adjusting pressure switch or pressure control valve to suit the requirement of mould. The pressure intensifier has never posed any maintenance problem even after 7-10 years in SP series machines. When toggle links are folded by pulling in of hydraulic cylinder, the mould is opened. Similarly when toggle links are straightened by pushing out of hydraulic cylinder, the mould is closed. The mould is clamped only when toggle unit is positioned in such a way so that in the fully straightened position of toggle links the two halves of mould meet. This alone does not confirm the clamping of the mould. Therefore special skill is required to adjust the position of toggle on the tie-bars so that mould is compressed slightly when the toggle links are fully stretched. Mould setting. therefore, is skill dependent job. The toggle mechanism provides excess clamping force and "naturally" it can not be varied. To make clamp force variable or settable complicated and delicate strain measuring instrument with critical mechanism at high cost is required to be added. Even this is indirect measurement of clamp force which can not be accurate. RESPONSE TO PEAK PRESSURE IN CAVITYAs the melt is injected (with speed) in to the cavity the cavity pressure steadily increases. At appropriate instant speed phase is terminated and pressure phase takes over. The injected melt expands in the cavity to reach peak pressure. This peak pressure multiplied by the projected area of mould cavity gives the force opposing the set clamp force. The cavity pressure drops as the melt cools in the cavity. As long as the clamp force is equal to the peak cavity force both types of clamp system behave in the same manner. However, as soon as the opposing force generated in the cavity exceeds the set value the behaviour of Toggle clamp and Hydraulic clamp differs. In Hydraulic clamp unite the clamp force depends on the compressibility of hydraulic oil. Therefore rigidity of system need not be as high as in toggle clamp unit. Moreover if the cavity pressure increases it is indicated on the Tonnage indicator alerting the operator to search for the reasons in the process settings. In Toggle clamp unit increase in clamp force, during peak pressure in cavity, depends on the rigidity of the mould and tie-bars of machine. A small expansion of the mould due to increase in it's temperature can increase stretching of the tie-bars and thereby increase the clamp force. A further expansion of mould can lead to breakage of tie bars. For these reason toggle clamps are designed for 10% higher clamp force.  Previous page / Next page Technical Papers by Prabodh C. Bolur Understanding Energy Consumption in Injection Moulding Machine(1994)Understanding Heat Exchange in Injection Moulds (1994) Ideal Moulding Shop (2001) Understanding Selection of Injection Moulding MachineThis paper was part of authors lectures at CIPET since 1980. It has been regularly updated. Technological Solution to Injection Moulding of Plastics (1999) Technological Tools for Part Design, Mould Design and Mould Fabrication (1999) Extrusion of Thermoplastics(1998) Archives. Home, A Guide to Injection Moulding of Plastics, Archives of Articles,Usefullinks.  if you are concerned with plastics molding, Order your copy NOW. |