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PHYSICS OF INJECTION MOLDING MACHINE
DIPTY LAL JUDGE MAL PRIVATE LTD
OCTOBER – NOVEMBER 2015
RAGHAV GUPTA
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BACKGROUND
Plastics are relatively low in cost, and due to ease of manufacture, versatility, plastics are used in an enormous and expanding range of products, from making daily house hold items like buckets to high tech application in Automobiles, Aviation & Space Industry. Plastics are playing important role in replacing conventional materials such as wood, stone, leather, paper, metal, glass, and ceramics. Plastic Parts are reusable and can be recycled to create new plastic items. Plastics are inseparable part of our daily life and per capita consumption of plastics is an important economic indicator for any country’s growth.
One of the processes of making plastic parts is through process of Injection molding. In this process plastic granules are heated to a specified temperature of around 200 centigrade to bring it into molten state. Then molten plastic material is injected with very high pressure into a mold /die for making desired plastic part.
The Process is described graphically in below mentioned picture.
I have taken up a project to study physics involved in working of Plastic injection molding machine.
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Basic Pr inciples of Physics involved in Injection Molding machine are l is ted as under
1. ENERGY
2. ELECTRIC MOTOR
3. MACHINE
4. PUMP
5. PRESSURE
6. SPEED
7. POWER
8. CLAMPING FORCE
9. EFFICIENCY
10. HEAT EXCHANGE
11. COOLING TIME
12. PROCESS OF EJECTION
13. HEAT EXCHANGE IN MOLDING
14. THERMAL STABILITY
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1. ENERGY
Energy is capacity to do work. Power is rate of doing work. Power = force x (distance/time). This also called energy. Energy can neither be created nor destroyed but only transformed from one form to others. Injection molding machine is actuated by hydraulic system which has electrical three phase A.C induction motor as prime mover. Therefore in the machine electrical energy is transformed in to mechanical energy through hydraulic energy. The energy reaches the actuators in the form of pressure and volume flow. While transmitting power through hydraulic the loss of energy could be due to flow losses and friction. The compression of hydraulic oil develops frictional heat which has to be controlled by radiation or cooling. BREAK-UP OF ENERGY If we analyse power consumption in a typical molding cycle the break up of power consumption would be as follows. General Calculation of Energy consumption per Sec in injection molding: Mold open.
5.7 kW-sec . -
Mold close. 7.4 kW-sec.
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Mold clamp pressure build-up
3.68 kW-sec
Total mold movement 16.78. kW sec 12.51%
Injection unit forward 00.67 kW -sec 00.50%
Injection 10.50 kW sec 07.83%
Follow-up pressure 12.00.kW-sec 08.95%
Plasticising 69.00 kW-sec. 51.50%
Cooling / Inj-unit-retract/idling.
18.00.kW-sec 13.42%
Ejector forward 05.88 kW-sec 04.40%
Ejector retract 1.20KW-sec 00.90%
The Injection molding process involves-
• Melting of solid and shear sensitive polymer granules • Injecting the polymer melt in to the closed and clamped mold.
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• Cooling of melt inside the mold through circulation of cold water. • shear action in the compression zone of the screw and • the Heat in-put through pre-heating and • The heat in-put through heaters on the barrel. • Pressure. • Flow rate.
The heat is drained off through the water circulating in the mold during cooling period. The power is also required for the various movements in the machine.
• Mold open/close and clamp. • Injection unit movement. • Screw rotation.
2. ELECTRIC MOTOR (HYDROMOTOR IN INJECTION MOLDING)
The motor or an electrical motor is a device that has brought about one of the biggest advancements in the fields of engineering and technology ever since the invention of electricity.
“The electric motor is a device which converts electrical energy to mechanical energy”.
The very basic principal of functioning of an electr ical motor lies on the fact that force is experienced in the direction perpendicular to magnetic f ield and the current, when field and current are made to interact with each other.
In simple words we can say a device that produces rotational force is a motor.
Working principle of electric motor:
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HYDROMOTOR IN INJECTION MOLDING
The Hydro motor described below is a radial-piston type motor. The motor has a rotating cylinder shaft and stationary housing. The cylinder block is mounted in fixed roller bearings in the housing. Even numbers of pistons are located in bores inside the cylinder block. The valve plate directs the incoming and outgoing oil to and from the working piston. Each piston is working against a cam roller. Because of the hydraulic pressure the pistons pushes the cam rollers when going down on the cam ring and transmits no power when going up. This principle produces torque and therefore the shaft starts rotating..
Hydro motor for 350 Ton machine
3. INJECTION MOLDING MACHINE / MOLD
Machine can not create energy by it self. Therefore, machine requires a prime mover. In most of the machines prime mover is an Electrical motor - 3 phase Induction motor in case of Injection Molding Machine. In this machine electrical energy is converted to mechanical energy through hydraulics
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3-Phase induction Motor
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PRIME-MOVER
Electric motor is coupled to hydraulic pump. Pump has flow rating -say 100 litres per min. or 25 gpm. Pump delivers oil flow in the system. Oil flow meets resistance causing resultant pressure in the system. The hydraulic system includes Relief Valves for pressure limit, Flow control Valves for speed and Directional valves for directing oil to Actuators - Cylinder for linear movement and Hydro motor for rotary movement. For each to and fro movement there must be one cylinder as actuator. Therefore there are cylinders for mold open/ close, injection unit forward / retract, Injection / retract. And Hydro-motor for screw rotation,
4. PUMP
A pump is a device that moves f lu ids (liquids or gases), or sometimes slurries, by mechanical action. Pumps can be classified into three major groups according to the method they use to move the fluid- direct lift, displacement, and gravity pumps.
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Pumps operate by some mechanism (typically reciprocating or rotary), and consume energy to perform mechanical work by moving the fluid.
In injection molding Pump delivers oil flow in the system at rated flow rate till the resistance reaches say 150 bar. Thereafter further increase in load pressure decreases the flow rate. Many in the molding shop floor wrongly believe that pump creates pressure. Pump creates flow rate not pressure. Therefore, hydraulic system pressure is not allowed to go beyond 150 bar by Main Relief Valve in the system.
5. PRESSURE
Pressure is a one of the critical factor in injection molding. When melt is pushed by the screw tip through nozzle in to the mold then the melt moves inside the mold and hence resistance to flow builds up and melt pressure at screw tip increases. This is known as Screw Back Pressure also known as Plast ic Pressure.
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It should be noted that melt pressure is
• Directly proportional to viscosity. (it varies with temperature pvT diagram), • Directly proportional to length of flow, • Inversely proportional to wall thickness of flow.
6. SPEED
Injection rate = As V cc/sec. Injection rate is available in the spec table. Here velocity V of screw is same as velocity of hydraulic piston of injection cylinder.
On hydraulic side AhV = oil flow rate at hydraulic cylinder.
Ah is normally does not appear in the machine specification table. However, Max pressure rating (say 1400 bar) at screw is available under Injection Unit spec. Most of the hydraulic system pressure is recommended to limit at 150 bar. Therefore 150 bar can be considered as max hydraulic pressure corresponding to max melt pressure. Now it is possible to compute diameter or area of injection piston if required.
7. POWER
POWER= F V =(F/A) (AV) =P Q
F is force and Q is flow rate
Max Work that can be done by the machine with out accumulator =
=(max. melt pressure) x (max injection rate)
It means for a given screw diameter, higher injection rate means higher power and hence higher hp of electric motor.
In injection molding power consumption is not constant but it is cyclic. Pressure peak occurs for a short duration. Moreover during peak load after switchover point, flow rates are lower. Therefore, machine manufacturer may use slightly lower hp electric motor, which can sustain short peak load.
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Thin walled injection molding demand higher injection rate at higher pressure. Therefore power consumption has to be higher than that of medium or thicker walled molding.
8. CLAMPIING FORCE
When a injection mold is fixed in a molding machine and molten plastic is injected into the interior of the cavity from the injection nozzle, a high filling pressure acts on the inside of the cavity. Since the parting surfaces of the mold try to expand outward due to this pressure, it is necessary to clamp the mold so that it does not open instantaneously.
It is easy to imagine that flash will be generated if the parting surfaces open even very slightly. The force of keeping the mold closed tightly is called the "required mold clamping force". The unit for the required mold clamping force is N (Newton’s), or kgf,
At the time of designing a new mold, it is necessary to obtain by theoretical calculations what is the optimum required mold clamping force that the injection molding machine has to have for the mold to be installed in it. For example, if a required mold clamping force of 100 tf was obtained by calculations, if this mold is installed in an injection molding machine with a 75 tf capacity, the molded product will be full of flash thereby making it impossible to carry out the molding operation. Further, if the mold is installed in a molding machine with a 300 tf capacity, even if the molding operation is possible, since usually the hourly cost of a 300 tf machine is higher than that of a 100T machine, the molding operation becomes high in cost.
The required mold clamping force of a mold can be calculated using the following equation.
F = p×A/1000, where, F: Required mold clamping force (tf),
p: pressure inside the cavity (kgf/cm2), and A: total projection area (cm2)
Here, p will have a value in the range of 300 to 500 kgf/cm2. The value of p varies depending on the type of plastic, molded item wall thickness, cavity surface temperature, molding conditions, etc. To be more accurate, it is recommended to incorporate a pressure sensor inside the cavity, and to collect guideline data from actual measured values. Also, A is the total projection area of the cavity and the runner with respect to the parting surface. Therefore, the value of A varies depending on the number of items molded and on the placement of the runner.
Example of a Calculation
Consider calculating the required mold clamping force when four molded items are obtained using PBT plastic with 30% glass fibbers added.
Let us assume that the assumptions for calculation are that the pressure inside the cavity is P = 300 kgf/cm2, the projection area of one cavity is A1 = 15.3 cm2, and the projection area of the runner is A2 = 5.5 cm2.
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F = p×A/ 1000
= 300×(15.3×4+5.5)/1000
= 20.01(tf)
Therefore, an injection Molding machine that has a required mold clamping force of about 20 tf is required. Giving some margin, it is considered optimum to select an injection molding machine with a 25 to 30 tf rating
9. EFFICIENCY
Efficiency of the machine is defined as the ratio of work output to wok input. It can not be one. It has to be lower than one to account for power losses due to friction and other causes. Efficiency of various types of hydraulic circuit is discussed in the book " A Guide to Injection Molding of Plastics".
10. HEAT EXCHANGE (COOLING TIME)
Injection molding process is cyclic in characteristic. Cooling time is about 50 to 75% of the total cycle time. Therefore, optimising cooling time for best performance is very important from quality and productivity point of view.
Cooling time is proportional to square of wall thickness. Therefore part design should ensure more or less uniform wall thickness through out the part.
Heat Exchange in mold
During every injection molding cycle following heat transfers take place:
• from the hot melt to mold steel (heat input to the mold) and • from mold steel to coolant flowing through cooling channel of the mold. (heat
removal from the mold)
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If heat input is more than heat removal, then the mold temperature would keep on increasing from cycle to cycle. Therefore molding quality would not be constant from cycle to cycle. The molding quality would be erratic- i.e. varying from cycle to cycle. Therefore, there is a need to balance between the heat input and heat removal in the mold after the desired mold surface temperature is reached. In other words, removal of heat by circulating coolant through the mold cooling channel would arrest the rise of mold temperature above the desired value. In practice, it may not be possible to maintain constant mold temperature with respect to time. However, the mold temperature would fluctuate between two values around the desired value.
During injection molding cycle heat flow takes place from polymer melt to mold steel by
• effective thermal diffusivity of polymer melt and • Conduction.
This heat is to be removed by circulating cooling fluid through the cooling channels in core as well as cavity during cooling period in order to maintain the desired temperature. Uneven temperature of the mold surface results (uneven shrinkage) in parts with molded-in stresses, warped sections, sink marks, poor surface appearance and varying part dimensions from cycle to cycle and even cavity to cavity.
11. COOLING TIME/CIRCUIT:
Injection molding process is cyclic in characteristic. Cooling time is about 50 to 75% of the total cycle time. Therefore, optimizing cooling time for best performance is very important from quality and productivity point of view.
Cooling time is proportional to square of wall thickness. Therefore part design should ensure more or less uniform wall thickness throughout the part.
Part design should also ensure that the melt flow is uniform in all direction from the gate and melt should reach the boundary of the part more or less at the same time
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12. PROCESS OF EJECTION
This is done with an ejection system. When the mold opens the part is pushed out, Force must be used because the part shrinks and sticks to the mold. The mold can be shut again after ejection and another shot can be injected for the process to begin again.
Ejection is the process is nothing but eject parts from the mold during production, generally pins are used for ejection, Designers should be aware of the need to accommodate such pins.
In some process uses ejector pins of various sizes to push the plastic part out of the mold after it has solidified. The sizes and arrangement of these pins are selected to minimize the impact on your part design.
Figure is an example of the illustration we provide early in the process of designing the mold so that the location and size of both the gate/s and ejector pins can be approved.
13. THERMAL STABILITY
Heating, Shearing, and Melt ing the Material:
• Conversion of Mechanical Energy by Shearing the Plastic (90%) • Conduction of Heat From the Heater Bands Through the Barrel and Into The Resin
(10%)
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CONCLUSION
An injection-molding machine is an incredible amalgam of various
principles of science and physics. I was truly amazed to learn about its
mechanisms and its parts. During the process, I realized the
importance and applications of these principles in various spheres of
manufacturing. I also wondered whether the people who had
discovered the various phenomena ever imagined that these would
today be combined for manufacturing goods of daily importance.
Through this experience with core manufacturing I came to an
understanding that basic principles of physics can have unlimited
possibilities and thus I was deeply inspired to dig deeper into this vast
and amazing world of science through Industrial Engineering.
ACKNOWLEDGMENTS
I wish to express my sincere thanks to Mr. Gaurav Jain, Director at
DLJM, for his continuous support and encouragement. I sincerely
appreciate his guidance and allowing me to intern at DLJM for my
project. I am also thankful to Mr. PK Raman, Plant Head at DLJM, who
has helped in documenting different steps of project. I take this
opportunity to express my deep sense of gratitude for his invaluable
guidance, constant encouragement, constructive comments,
sympathetic attitude and immense motivation, which has sustained
my efforts at all stages of this project work.
