Injection Molding Process Summary

Injection molding is a mass conserving (MC) process in which thermoplastic material is fed into a heated barrel, mixed, and molten plastic is forced into a mold cavity where it cools and hardens to the configuration of the mold cavity, which is the inverse of the desired shape. Injection molding is very widely used for manufacturing a variety of parts, from the smallest component to entire body panels of cars. Hence this unit process life cycle inventory (uplci) is to establish representative estimates of the energy consumption and overall environmental impact from the injection molding unit process. The injection molding unit process life cycle inventory (uplci) profile is for a high production manufacturing operation, defined as the use of processes that generally have high automation and are at the medium to high throughput production compared to all other machines that perform a similar operation. This is consistent with the life cycle goal of estimating energy use and environmental impact representative of efficient product manufacturing. Injection Molding is a cyclic process with variation of load through-out the cycle. The variation of the load depends on several factors of the mould design (final part shape), polymer used and process parameters set. Therefore presentation of overview of energy consumption becomes quite complex.
Injection molding is one of the most widely used and versatile methods of polymer processing. The process begins as solid polymer beads are feed from a hopper into the heated barrel of an injection molding machine. The polymer is plasticized in the barrel by the heat provided by heater bands and through shear forces exerted on the polymer by the turning of a screw in the barrel. Figure 1 shows the heating barrel and injection unit of a typical injection molding machine. Once enough polymers have been melted in the barrel, the screw also acts as a ram to inject the polymer into a cooled and pressurized mould.  The injection molding process in most new machines is electrically controlled by a built-in computer and the motors are electrically driven. The choice of injection molding machine type (hydraulic, hybrid or electric) has a substantial impact on the specific energy consumption (SEC). The conventional measure of process energy use is the SEC, which is expressed as the number of kWh used to process 1 kg or lb of material (kWh/kg or kWh/lb). SEC is the ratio of energy consumed per molding cycle to the weight of molded part in kg. This ratio could be maintained as low as possible provided efficiency aspects of machine selection pump and controls are understood while setting the machine. The SEC values for hydraulic, hybrid and electric machines are 5.3, 3.7 and 3.5 kWh/kg respectively (Thirez Thesis). In electric machines the hydraulic elements are replaced, so there is no problem in hydraulic oil leakage and pollution, so is the operation noise. There are savings in energy and electric power consumption is lower. It is of higher accuracy than ordinary hydraulic injection molding machine with highly clean.

Figure MC2.1 Injection Molding Machine (Photograph from Insurtech International Corp. Europe.)

 

Figure MC2.2 Injection Molding Process Schematic (Todd et al., 1994)

            Figure MC2.3 shows an overview of the developed environmental-based factors for injection molding forming process. During the operation, energy is input to an electrically driven injection molding machine in the form of electricity supplied to the heater bands, the electric motors and the electric controls. Energy is lost from the system in the form of motor losses, radiation and convection from the heater bands, cooling losses, and as an increase in the enthalpy of the formed plastic. The polymer demands specific kilo calories of heat for changing its state from solid to melt. The required heat comes from shear action in the compression zone of the screw, the heat-in-put through pre-heating and the heat-in-put through heaters on the barrel. One servo motor rotates the screw, another moves the screw along the axis and a third moves the toggle clamp to open and close the mold. Injection molding machines consists mainly two parts: the injection unit and the clamping unit which can be seen Figure MC2.2. Injection pressures can range from 3 to 2000 MPa and clamp tonnage from 5 to 10,000 tons (Muccio, 1994).

           

Injection Molding Process Energy Characteristics

           
The injection molding machine injects molten plastic into a mold piece. The plastic then cools, as it is packed into the mold. The entire process of an injection molding machine cycle can be broken down into four main energy consuming phases: plasticizing, injecting, cooling and clamping/unclamping. All four parts of the cycle perform different operations and have different pressure requirements, meaning that they have different power consumption. Because high production injection molding is a semi-continuous process, the lci is based on a representative operational sequence, in which

1. When the machine is turned on, the computer and fans start up, creating an initial power draw. The computer and fans will be running for the entire time that the machine is on. The polymer pellets will be filled into the hopper. This is at the level of Basic energy.
2. During the plasticizing (melting polymer) the heaters or turned on, which heat the plastic pellets and cause them to melt. This process has an initial power draw while the heaters warm up. After the heaters have finished heating, they have a small power draw, while the heaters keep the plastic at a constant temperature. This latter power draw is present until the heaters are turned off, after the part has been removed. The time required to heat the plastic is around 10 to 30 min prior to the machine being ready to produce parts. This is at the level of Basic energy.
3. After the heaters have warmed up, the servo motors are started (unloaded motors), which controls the feed screw (in charge of injecting the molten plastic into the mold). The machine is ready to start the actual process of making the part.
4. The actual process can be broken down into clamping, injecting, cooling and packing and unclamping stage.  Each of these stages requires a power draw, with the clamping stage requiring the largest power draw to the entire process.
5. Part are taken away to be typically sent forward to another manufacturing unit process.

 

Waste energy is the energy consumed by the machine that does not go into the formation of the part. Furthermore, during the actual production of parts, energy is lost through different ways. For example, during the heating process, the heater not only heats the plastic, but also heats the environment as well. This results in energy that does not go into the production of the part, but that is lost to the environment. Other losses include heated plastic that is not injected into the mold and the excess clamping pressure beyond that absolutely needed to keep the mold pressed together. The minimum energy needed to create the product can be calculated by considering the mass of polymer needed for one part and determining the energy needed to first heat the plastic to liquid form, then transition it into the shape of the part, and finally the energy to cool the part. Thus, the energy wasted in the actual production of the part is the difference between the energy used in production and the energy calculated for the production of the part.

 

 

 

Thermoplastic                 Yield Strength    Elastic Modulous      Heat deflection                                             

(MN/m2)                (MN/m2)                 Temp (0C)

High density
Polyethylene                                      23                    925                              42

High impact
Polystyrene                                        20                    1,900                           77

Acrylonitrile-
butadiene styerene (ABS)                  41                    2,100                           99

Acetal                                                 66                    2,800                           115

Polyamide                                          70                    2,800                           93

Polycarbonate                                    64                   2,300                           130

Polycarbonate
(30% glass)                                        90                    5,500                           143

Modified
Polyphenylene
Oxide (PPO)                                 58                    2,200                           123

 Modified PPO
(30% glass)                                  58                    3,800                           134

Polypropylene
(40% talc)                                    32                    3,300                           88

Polyester
Teraphtalate
(30% glass)                                               158                  11,000                         227

 

The energy consumption of brake forming is calculated as follows.

Etotal = Pbasic * (tbasic ) + Pidle * (tidle) + Pbrakeforming * (tbrakeforming)                                         (1)

where power and time are illustrated in Figure MC1.5.

A. Parameters effecting the Energy required for brake forming

An approximate importance of the many variables in determining the brake forming energy requirements was used to rank parameters from most important to lower importance as follows:

1. Thickness of the material
2. Properties of the material
3. Length of bend line
4. Speed of the punch
5. Bend shape (U, V, channel, modified channel)
6. Springback or energy release (relation of rated hp to actual energy use)
7. bottom force
8. geometry
9. production rate
10. friction at the punch, die, and workpiece interfaces.
11. machine energy during standby and idling

From this parameter list, only the top 5 were selected for use in this unit process life cycle with the others having lower influence on energy. Energy required for the overall brake forming process is also highly dependent on the time taken for idle and basic operations.

 

Table MC1.2. Typical tensile strengths of some materials (Tschatsch, 2006)

Material	Tensile strength, Psi (Mpa)
	Soft	Hard
Lead	3625-5800 (25-40)		-
Tin	5800-7250(40-50)		-
Aluminum (99.0%)	13,350(92)		246,564(1700)
High-tension aluminum alloy Type 4	32,640(225)		69,618(480)
Duralumin	36,990(255)		69,618(480)
Zinc	21,755(150)		36,260(250)
Copper	31,183-39,885(215-275)		43,511-58,015(300-400)
Brass (70:30)	47,140(325)		76,870(530)
Brass (60:40)	53,950(372)		71,068(490)
Phosphor bronze	58,015-72,518(400-500)		72,518-108,778(500-750)
Bronze	58,015-72,518 (400-500)		72,518-108,778 (500-750)
Nickel silver	50,763-65,266(350-450)		72,518-101,526 (550-700)
Cold rolled iron sheet	46,412-55,115(320-380)		-
Steel, 0.1%C	46,412(320)		58,015(400)
Steel, 0.2%C	58,015(400)		72,518(500)
Steel, 0.3%C	65,266(450)		87,023(600)
Steel, 0.4%C	81,221(560)		104,427(720)
Steel, 0.6%C	104,427(720)		130,534(900)
Steel, 0.8%C	130,534(900)		159,542(1100)
Steel, 1.0%C	145,038(1000)		188,550(1300)
Silicon steel sheet	79,770(550)		92,275(650)
Stainless steel sheet	92,275-101,526(650-700)		-
Nickel	63,816-72,518(440-500)		82,672-91,374(570-630)

Table MC1.3. Recommended minimum bend radius for commercial quality steel sheet, strip and plate (Groover, 2003)

Material	Soft	Hard
Aluminium alloys	0	6T
Brass, low leaded	0	2T
Magnesium	5T	13T
Steels: low C, low alloy and HSLA	0.5T	4T
Austenitic stainless steels	0.5h	6T
Titanium	0.7T	3T
Titanium alloys	2.6T	4T

 

Idle Energy

Energy-consuming peripheral equipments included in idle power (Pidle) are shown in Table MC1.1. The idle power characterizes the load case when there is relative movement of the tool and the work-piece without changing the shape of the body (e.g. axis movement) - Handling. For brake forming, the handling times are the air time of approach and retraction after forming. The idle time (tidle) is the sum of the handling time (thandling) and the brakeforming time (calculated above as tforming, equation 2), see Figure MC1.5. For brake forming machines, the handling times are the air time of approach and retraction after forming. We can calculate the handling times and energy as follows.

Idle energy = [timehandling + timeforming]* Pidle                                                               (7)

During the brake forming process the punch is considered to be at an offset of at least 6 times the thickness of workpiece. Every time while brake forming the punch comes down from a height of 6 times the thickness of the workpiece and again retraces back to an offset position after completing the process. Approach time is 6T divided the approach speed, which depends on machine specification. While the retraction time may be longer than the brake forming time, this is estimated as the sum of the approach and brake forming times and divided by the return speed, which depends on the machine specification
Time for handling is
Approach + retraction times = timehandling                                                                                           (8)

timehandling= (6T)/Approach speed + (6T+D)/return speed

The brake forming time was previously calculated, equation 2  
timeidle = thandling + tforming                                                                                               (9)
From these calculations the idle energy for a single hole is

E (Joule/bend) = (tapproach + tretraction) + tforming]* Pidle                                                    (10)

The average idle power Pidle of automated CNC press brake machines is between 1,200 and 15,000 watt*. (* This information is from the CNC manufacturing companies, see Appendix 1). Approximately Handling time will vary from 0.1 to 10 min. 

Basic Energy

The basic energy of a press brake is the demand under running conditions in “stand-by mode”. Energy-consuming peripheral equipments included in basic energy are M1, M2 and PC from Table MC1.1. There is no relative movement between the tool and the work-piece, but all components that accomplish the readiness for operation (e.g. machine control unit (MCU), unloaded motors, servo motors, pumps) are still running at no load power consumption. Most of the automated CNC press brake machines are not switched off when not forming and have a constant basic power.
The average basic power Pbasic of automated CNC press brake machines is between 800 and 8,000 watt* (* From CNC press brake manufacturing companies the basic power ranges from 1/8th to 1/4th of the maximum machine power, (see Manufacturers Reference Data in Appendix). The largest consumer is the hydraulic power unit.
Ostwald, 1986 has shown that the time to load a blank or part into a press and then remove the part is proportional to the perimeter of the rectangle which surrounds the part. This time can be given by tload/unload = 3.8 + 0.11 (L + W) (seconds)
Where L, W = rectangular envelope length and width, cm.

From Figure MR 1.5, the basic time is given by
Tbasic = tload/unload + thandling + tforming                                                                                                    (11)
where   thandling + tforming = tidle as determined in equation 9.

With only the following information the unit process life cycle energy for brake forming can be estimated.

1. material of part being bent and Table MC1.2
2. thickness of the material
3. length of the sheet along the bend line
4. angle of the bend (see illustrations page 9)
5. punch speed, using representative manufacturers’ values, see Appendix

B. Method of quantification for mass loss:

For ordinary press-brake operations such as bending and simple forming, coolant oil is less commonly used. Hydraulic press brakes use fluid power to do work. In this machine, high pressure liquid called hydraulic fluid is transmitted throughout the machine to various hydraulic motors and hydraulic cylinders. In addition to transferring energy, hydraulic fluid needs to lubricate components, suspend contaminants and metal filings for transport to the filter, and to function well to several hundred degrees Fahrenheit.  Hydraulic fluid replacement occurs so infrequently that on per bend or per 1,000 bend basis, this mass loss is neglected.
Lubricant oil is commonly (but not always) used on the metal surface in contact with the die.  Lubricant is applied along the bend line and then is some subsequent processing step, it is removed before a final product is used.  In order to link this mass loss directly to brake forming, it is included here.  Note the energy or ancillary waste for lubricant removal (solvent degreasing, rag wipe, etc) would be captured in the uplci of those processes and only the lubricant mass is assigned to brake forming. Lubricant applied and removed is estimated (Madavan, Wichita State University, 2009, personal communication) as 5cm width x L (cm) x (2.54/1000) cm thickness x 0.9 g/cm3 = 0.11 g/cm length of bend.

Case Study on Brake forming process

In this report we analyze the detailed energy consumption calculations in brake forming process. The forming process is performed on BAILEIGH CNC press brake machine (BP – 5060). The machine specifications are listed below:

Table MC 1.4. Machine specification

Product Details

            For this example we are assuming a stainless steel (soft) sheet as the work piece. The work piece is of sheet-metal part of 3mm thick and 20mm bend length is bent to an included angle of α = 1200 and a bend radius of 7.5 mm in a V-die. The objective of the study is to analyze the energy consumption in press brake machine. The die opening is 24 mm. The metal has tensile strength of 675 Mpa.

Process Parameters

            The forming conditions and the process parameters are listed in Table MC1.5.

Table MC1.5. Process Parameters for Example Case

Process Conditions
Sheet thickness (T)	0.12 in
Ultimate Tensile strength (UTS)	96,900 psi
Die opening (W)	0.94 in
Bend angle (A)	120 deg
Bend radius (R)	0.30 in
Bend Length (L)	0.79 in
Die closed depth, D	0.27 in

 

Brake forming process

            During forming operation the tool is considered to be at an offset of 18mm (6 times the workpiece thickness) above the workpiece. During brake forming the tool comes down from a height of 18mm. It retracts (D+18) mm back to the offset position after completing the forming process.

Time, Power and Energy calculations for V-bend

 

            The total processing time can be divided into the 3 sub groups of basic, idle, and brake forming time.

Brake forming time:
The time for bending is determined by
tforming = (D)/V (sec)
Where V is the bending speed in mm/sec, and D is the die depth in mm.
D = depth of close die = 0.28 in (0.12*2.3, see page 9)
V = 0.28 in/sec

Time to bend will be,
tforming = (0.28 in)/0.28 in/sec)
= 1 sec/bend
Energy required for each bend,
E = D * F
The force required to bend a 20-mm long sheet of soft stainless steel 3mm thick can be estimated using the following calculation:
F = S L T2 K/ W                                                                                                           
F =    96,900 psi * (0.79) * (0.12)2 * 1.33    = 1,560 lbf = 7,000 N
0.94
Brake forming energy for each bend,
E = 7,000 N * 0.0069 m = 0.048 kJ
Power required P = F * V
= 7,000 * 0.007 = 0.0.049 kW/bend

Handling Time:
The air time for bending is approach and retract time
Approach time = 18/80 = 0.225 sec
Retracts time = (18 + D) /60 = 24.9/60 = 0.415 sec
Total air time = 0.64 sec
Total idle time = tbf + tair
= 1 + 0.64 = 1.64 sec
Idle power from Appendix 1 can be assumed as = 2.5 kW
Idle energy = 2.5 * 1.64 = 4.125 kJ/bend

Basic time:
Loading and unloading time t = 3.8 + 0.11 (L + W)
= 3.8 + 0.11 (2 + 5)
= 4.57 sec = tl/u
Pbasic = 1.25 kW
Ebasic = Pbasic * tbasic
Tbasic = tl/u + tidle = 4.57 + 1.64 = 6.2 sec
Ebasic­ = 1.25 * 6.2 = 7.8 kJ/bend

Total Energy
Etotal = 0.207 + 4.125 + 7.8 = 12 kJ/bend

Mass Loss
Lubricant loss is 0.011 g/cm length *2 cm length = 0.022 g lubricant loss/bend

Summary:

This report presented the models, approaches, and measures used to represent the unit process life cycle inventory (uplci) of brake forming operations. The only major environmental characteristics are is the energy consumption of the press brake and lubricant loss. Calculations for product manufacturing are presented, based on knowing only the bend length and the material bent. The life cycle of brake forming is based on a typical high production scenario (on a CNC press brake machine) to reflect industrial manufacturing practices.
The energy can be calculated from a basic list of variables, likely to be known for each part to be brake formed

1. material of part being bent and Table MC1.2
2. thickness of the material
3. length of the sheet along the bend line
4. angle of the bend, illustrated on page 9
5. punch speed, using representative manufacturers’ values

References Cited

 

1. Abele, E.; Anderl, R.; and Birkhofer, H. (2005) Environmentally-friendly product development, Springer-Verlag London Limited.
2. George, F.S; and Ahmad, K. E. (2000) Manufacturing Processes & Materials, 4th Edition, Society of Manufacturing Engineers.
3. Kalpakjian, S.; and Schmid, S. (2008) Manufacturing Processes for Engineering Materials, 5th Edition, Prentice Hall.
4. Ostwald, P. (1991) Engineering cost estimating, 3rd Edition, Prentice Hall.
5. Piacitelli, W.; Sieber, et. al. (2000) Metalworking fluid exposures in small machine shops: an overview, AIHAJ, 62:356-370.
6. Schuler GmbH. (1998) Metal forming Handbook, 1st Edition, Springer.
7. Thiriez, A. An environmental analysis of injection molding, Master’s thesis at Massachusetts Institute of Technology, Cambridge, MA, USA:2006
8. Todd, R.; Allen, D.; and Alting, L. (1994) Manufacturing processes reference guide, Industrial Press, New York.
9. Tschatsch, Heinz. (2006) Metal forming practice: Processes-machines-tools, Springer.
10. Wlaschitz, P. and W. Hoflinger. (2007) A new measuring method to detect the emissions of metal working fluid mist, Journal for Hazardous Materials, 144:736-741.

 

Appendices

Manufacturers Reference Data

The methodology that has been followed for collecting technical information on CNC machines has been largely based in the following:

The documentation of the CNC press brake and the technical assistance collected from the manufacturing companies through the internet. Several interviews with the service personnel of the different CNC manufacturing companies have been carried out. After collecting the information from the different companies it has been put together in the relevant document that describes the different approaches the different companies have regarding the technical information on the CNC press brake machine. Telephone conversations allowed us to learn more about basic power and idle power. Companies that involved in our telephone conversations are Baileigh, Ronmack, Trumpf and Cincinnati. These companies manufacture different sizes of CNC machines, but this report shows the lower, mid and highest level of sizes. For our case study we picked machine at the mid level.

Specifications	BAILEIGH
Model Number	BP-3360	BP-5060	BP-9078
Bending force, kN	330	500	900
Approach Speed, mm/sec	80	80	80
Bending Speed, mm/sec	7	7	7
Return Speed, mm/sec	60	60	60
Main motor, kW	2.2	3.7	7.4
Motor 2, kW	0.4	0.4	0.4
3 Axes motor output(X,Y,Z), kW	0.75	0.75	0.75

 

Specifications	RONMACK
Model Number	RM-2050	RM-3100	RM-8000
Bending force, kN	600	1500	5000
Approach Speed, mm/sec	120	120	120
Bending Speed, mm/sec	9	9	9
Return Speed, mm/sec	100	80	80
Main motor, kW	5.5	11	37
Motor 2, kW	1	1	1
3 Axes motor output(X,Y,Z), kW	1	1	1

 

Specifications	TRUMPF
Model Number	TB V-50	TB V-130	TB V-320
Bending force, kN	560	1440	3570
Approach Speed, mm/sec	150	150	150
Bending Speed, mm/sec	12	12	12
Return Speed, mm/sec	120	120	120
Main motor, kW	6	18	35
Motor 2, kW	1	1	1
3 Axes motor output(X,Y,Z), kW	1	1	1

 

Specifications	CINCINNATI
Model Number	90MX6	175MX10	350MX12
Bending force, kN	500	1200	4000
Approach Speed, mm/sec	300	275	250
Bending Speed, mm/sec	30	25	20
Return Speed, mm/sec	200	180	150
Main motor, kW	15	20	25
Motor 2, kW	2	2	2
3 Axes motor output(X,Y,Z), kW	1	1	1