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Proceedings of the ASME 2013 International Manufacturing Science and Engineering Conference
MSEC2013
June 10-14, 2013, Madison, Wisconsin
MSEC2013-1222
CHARACTERIZING ENERGY CONSUMPTION OF THE INJECTION MOLDING
PROCESS
Jatinder Madan
Systems Integration Division
National Institute of Standards and Technology
Maryland, USA
&
Sant Longowal Institute of Engineering &
Technology, Longowal, Sangrur, India
Email: jatinder.madan@nist.gov
Mahesh Mani
Systems Integration Division,
National Institute of Standards and Technology
Maryland, USA
&
University of Maryland
College Park, Maryland, USA
Kevin W. Lyons
Systems Integration Division
National Institute of Standards and Technology
Maryland, USA
ABSTRACT
Presently available systems for sustainability assessment
do not fully account for aspects related to a product’s
manufacturing. In an effort to make more sustainable decisions,
today’s industry seeks reliable methods to assess and compare
sustainability for manufacturing. As part of the Sustainable
Manufacturing program at the National Institute of Standards
and Technology (NIST), one of our objectives is to help
develop the needed measurement science, standards and
methodologies to evaluate and improve sustainability of
manufacturing processes. As a first step towards developing
standard reference sustainability characterization
methodologies for unit manufacturing processes, in this paper
we focus on injection molding with energy as the sustainability
indicator. We present a science-based guideline to characterize
energy consumption for a part manufactured using the injection
molding process. Based on the study, we discuss the selection
of process parameters and manufacturing resources,
determination of cycle time, theoretical minimum energy
computations, and estimated energy computations for
characterizing the injection molding process.
Keywords: sustainable manufacturing; injection molding; cycle
time; theoretical minimum energy; energy consumption;
information models.
INTRODUCTION
Sustainability assessment systems do not fully account for
aspects related to a product’s manufacturing, such as manufac-
turing resources, process parameters and cycle time. In an effort
to make more sustainable decisions, today’s industry seeks reli-
able methods to assess and compare sustainability for manufac-
turing.
The sustainability assessment is made by way of key sus-
tainability performance indicators, such as energy and air emis-
sion. The Organization for Economic Co-operation and Devel-
opment (OECD) [1] defines 18 key performance indicators
(KPIs) for sustainable manufacturing, which include water in-
tensity, energy intensity, renewable energy intensity, and others.
Energy is an important KPI for sustainable manufacturing and
is the focus in this paper.
In this paper, we selected the injection molding process for
characterizing energy. Injection molded parts are widely used in
consumer products and industrial equipment. For example, the
injection molded components constitute 42 % and 33 % in toys
and medical equipment components, respectively. The extent of
energy use in manufacturing processes in general, and the injec-
tion molding process in particular, are well recognized [2, 3].
According to Gutowski et al. [4], injection molding processes
use energy in the order of 10 MJ/kg, not counting auxiliary
operations such as compounding and drying, which is compa-
rable to other processes like machining [5]. The overall energy
consumption in the US injection molding industry on a yearly
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basis amounts to 2.06 x 108 GJ. This is comparable to the ener-
gy consumption for sand casting and to the entire electricity
production of some developed countries [6].
The injection molding process consists of melting raw pol-
ymer granules, also referred to as plasticating1 [7], and injecting
the molten polymer into a mold (or die). A typical injection
mold has a cavity, which is a negative of the part being pro-
duced. When the cavity is filled with plastic, it is cooled and the
plastic becomes solid material resulting in a positive compo-
nent. After the injected molten polymer is solidified, the mold,
which consists of two halves, is opened, and the solidified part
is ejected out by force. Figure 1 shows a schematic diagram of
the injection molding machine.
Figure 1: Injection molding process schematics [8]
To help US industries, one of the objectives of the Sustain-
ability Manufacturing program of NIST is to develop measure-
ment science standards and methodologies to evaluate and im-
prove sustainability of manufacturing processes [3]. As a pre-
cursor, this paper presents a science-based guideline to charac-
terize energy consumption for the injection molding process.
The organization of this paper is as follows. The next
section presents a summary of the work related to energy
consumption in the injection molding process alongside
research gaps and objectives of this paper. The following
section discusses the stages of the injection molding process,
and presents a brief overview of the proposed guideline with
the help of its schematic. Subsequent sections deal with
different steps of the proposed guideline, namely: (i) select
initial process parameters, (ii) define cavity details and
determine other process parameters, (iii) select injection
molding machine, (iv) determine cycle time and theoretical
minimum energy, and (v) estimate energy consumption. Next, a
summary of the different parameters influencing energy
consumption for the injection molding process is presented.
1 Plasticating refers to conversion of plastic granules to flow-able melt. It
happens inside the screw barrel assembly of the injection unit in the injection
molding machine.
o The plastic granules move inside the screw channel when screw is rotated.
o The screw has three zones: feed, compression and metering.
o In the compression zone the material is gradually compressed.
o Plastic material under shear changes its viscosity (Shear Thinning)
o Melt is then homogenized in metering zone.
Lastly, we conclude with a brief discussion of future research
directions.
RELATED WORK
In this section, the literature related to the determination of
energy consumption in the injection molding processes is
discussed at different levels of industry, factory, machine and
process. A brief summary of the research gaps and objectives of
the present work is also presented at the end of this section.
Industry sector level: Thiriez and Gutowski [6] studied the
impact of the type of injection molding machine on the specific
energy consumption (SEC2). The SEC typically varies from
13.2 MJ/kg for electrically powered machines to 19.0 MJ/kg for
those using hydraulic systems. The Rigid Plastics Packaging
Group [9] quantified total energy requirements, energy sources,
atmospheric pollutants, waterborne pollutants, and solid waste
for two plastic fabrication processes, namely injection molding
and thermoforming. The scope of the study was to generate a
life cycle inventory (LCI) database for products made by injec-
tion molding and thermoforming in the North America.
Factory level: Lu et al. [10] developed a process modeling
parameter optimization algorithm using a genetic algorithm
(GA) based on the lexicographic method. The implementation
of their framework reduced the energy consumption for a la-
boratory scale test. Pun et al. [11] proposed a multiple-criteria
methodology for evaluating environmental impacts in the plas-
tic injection molding. They identified various indicators of en-
vironmental impact assessment and established a multiple-
criteria rating matrix. The study provided injection molding
manufacturers with a means to assess the environmental per-
formance and perform benchmarking analysis. Muroyama et al.
[12] suggested that discrete event simulation (DES) and life
cycle assessment (LCA) functionality can be combined to ana-
lyze the utilization and processing of manufacturing resources
in a factory setting. They demonstrated DES and LCA’s ability
to facilitate decision-making and, optimize the injection mold-
ing process in terms of productivity and energy use.
Machine level: Kanungo and Swan [13] investigated the
energy consumption of all electric and hydraulic injection
molding machines. They compared various aspects like energy
consumption, cost, throughput, and process parameters affect-
ing energy consumption.
Process level: Qureshi et al. [14] presented an empirical
approach to characterize the relationship between energy con-
sumption and process variables for the injection molding pro-
cess. Riberio et al. [15] presented a thermodynamic model that
estimates the energy consumption for any injection molded part
based on its geometry and the material. They initially consider
the efficiency of the system to be 100 % and then take different
values of machine efficiency to compare estimated and actual
energy consumption. Weissman et al. [16] developed a method-
ology to compute estimates for the total energy consumption
for manufacturing of injection molding parts. It utilizes part
2 SEC is the energy consumption per unit of throughput. For example
energy consumption per unit mass.
Feed zone
Compression
zone
Metering
zone
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information to determine shot size and cycle time. Based on the
actual power consumption of the different drives, the energy
usage is determined for a specific injection molded part. Kalla
et al. [17] provided a methodology to collect unit process life
cycle inventory (uplci) for the injection molding process using
the CO2PE! framework [18].
Research gaps
Despite very useful work of the researchers, based on our
review, we identified some research gaps mentioned below.
i. Most of the reported research provides industry, factory,
or machine level analysis, which cannot accurately esti-
mate the energy performance at the process level.
ii. Studies at the process level are useful to collect data but
have limited application for estimation and benchmark-
ing a wide range of materials and manufacturing re-
sources.
iii. Pre and post operations, such as drying, which are close-
ly associated with the process, have not been considered.
This paper attempts to fill above mentioned research gaps.
To overcome the research gaps, there is a need to develop a
science-based guideline, which includes the required pre and
post operations to estimate energy consumption for the injec-
tion molding process. Various factors related to part design,
manufacturing resources, material and process planning need to
be considered for energy estimation. The guideline should sup-
port multiple methods for comparing injection molding pro-
cesses based on the theoretical minimum energy requirements.
Objectives of the present work
The objectives the paper are:
Determine theoretical minimum energy required for the in-
jection molding process.
Establish guidelines for estimating the energy consumption
of the injection molding process, which also includes pre
and post operations.
The subsequent sections provide details of our proposed
guideline.
INJECTION MOLDING PROCESS STAGES AND BRIEF
METHODOLOGY OF THE PROPOSED GUIDELINE
In this section, we first provide a brief overview of the
injection molding process followed by a brief description of our
proposed methodology.
Stages of the injection molding process
The injection molding process is comprised of three stages:
drying, injection molding, and regrinding. In the drying stage,
the plastic beads and the reusable scrap are fed into the dryer,
where it is either set free of moisture or the moisture is brought
down to an acceptable level. In the second stage of injection
molding, the solid plastic mixture (plastic beads and reusable
scrap) is fed to the injection molding machine, where it is
melted and the actual injection molding process is carried out.
The injection molding process cycle consists of mold closing,
injecting, cooling, mold opening, and ejecting. Other operations
of feeding and melting, which take place within the injection
molding machine, operate parallel to the injection molding.
Lastly, in the third stage of regrinding, runner, gates, and any
other solid plastic, which is attached to the part, is removed for
regrinding.
Sustainability characterization methodology
Sustainability characterization methodology primarily
comprises performance metrics, process analytics and
supporting information models for sustainability [3]. In lieu of
pursuing a comprehensive sustainability characterization
methodology, we focus on the injection molding process with
energy as a performance metric and discuss corresponding
analytics to compute the theoretical minimums for
benchmarking and comparison purposes.
Figure 2 presents a schematic of our proposed guideline.
The schematic shows the interaction of the information
required for characterizing energy consumption for the
injection molding process. The steps of the guideline are
subsequently explained in later sections of the paper.
Figure 2: Schematic of the guideline for estimating injection
molding energy consumption
Injection molding
machine selection
Initial process parameters
determination
Injection
molding
material
information
Cavity details and
determining other
process parameters
Part design
information
Injection
molding
machines
information
Clamping unit
Injection unit
Theoret
ical minimum
energy computations
Estimated energy
consumption
Machine
/Auxiliary
equipment
performance
rating
Step 1
Step 2
Step 3 Step 4 Step 5
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The five steps of the proposed guideline are mentioned be-
low.
i. Determine initial process parameters
ii. Define cavity details and determine other process pa-
rameters
iii. Select injection molding machine
iv. Determine cycle time and theoretical minimum energy
requirements
v. Estimate energy consumption
An injection mold part design provides the necessary
details like part material, volume, weight, and wall thickness. In
Step 1, these details are used to find initial process parameters,
such as injection pressure and temperature. Step 2 is used to
determine other process parameters, such as the injection force,
and the required number of cavities. In Step 3, based on the
cavity details and process parameters, the selection of the
suitable injection molding machine is made. Step 4 helps
determine the process cycle time and theoretical minimum
energy requirements for the process. Lastly, Step 5 of the
guideline helps estimate the energy consumption taking into
account the process-specific manufacturing resources. The
following sections provide details of these steps.
SELECTING INITIAL PROCESS PARAMETERS
Injection molding process parameters like injection pres-
sure, injection temperature, and ejection temperature are related
to the material and, to some extent, to the geometric details of
the part. These parameters affect the energy required for the
injection molding process. The determination of these initial
process parameters is discussed.
Injection pressure: This is the pressure exerted on the melt in
front of the screw tip during the injection stage, when the screw
is acting like a plunger. It is a common practice to refer to the
corresponding pressure in the hydraulic cylinder as the injec-
tion molding pressure. Since a considerable amount of this
pressure is lost by the time the melt reaches the cavity, the max-
imum pressure of the standard barrel is calculated using Eq. 1
[19].
p~1.25× p (1)
where pmax (MPa) and pinj (MPa) are the maximum pressure
required from the injection molding machine and the required
injection pressure respectively. The required injection pressure
depends upon the type of the material and part characteristics. A
representative list of pressure requirements for some plastic
materials, which varies with part characteristics, is given in
Table 1.
Injection and mold temperatures: Injection temperature and the
temperature inside the mold are the key process parameters.
Recommended processing temperatures for some thermoplastic
materials are provided in Table 2.
Table 1: Injection pressure (pinj) for producing plastic parts [19]
Plastic
material3 Required effective injection pressure (MPa)
Easy flow
material,
heavy
sections
Medium flow
materials,
standard sec-
tions
High vis-
cosity
material,
thin sec-
tions
Thin-wall
parts, thin
wall injec-
tion
ABS 80 to 110 100 to 130 130 to150 150 to 200
CAB 80 to 110 100 to 130 130 to 160 N/A
POM
90
to 1
10
110
to
130
130
to
150
N/A
Table 2: Recommended injection and mold temperatures [19]
Plastic
material Injection temperature
(°C) Mold temperature
(°C)
ABS 200 to 260 40 to 60
CAB
180
to
220
40
to
80
POM 180 to 230 80 to120
CAVITY DETAILS AND SELECTION OF OTHER PRO-
CESS PARAMETERS
After the initial parameters are selected, the next step is to
decide the number of cavities in the mold (or die). As defined
earlier, cavity is a negative of the part being produced. In the
injection molding process, the mold may have a single cavity or
multiple cavities, which helps attain a higher production rate.
The number of cavities effects many parameters of the injection
molding process including injection volume (or shot volume),
projected area, and injection force. The method to compute
these parameters is discussed in the following paragraphs.
Shot volume: Shot volume for a cavity is the amount of molten
polymer required to fill the mold cavity, in each cycle. Since
most of materials shrink in volume when cooled, a shrinkage
allowance is given to the cavity. Therefore, shot volume pri-
marily should take care of shrinkage compensation due to cool-
ing and additional material required for gates and runners. To
compensate for shrinkage, the volume of a cavity (m3) can be
found using Eq. 2.
V= V(1 + ε100
) (2)
where ε is the percentage shrinkage rate of the polymer from
injection temperature to room temperature and V (m3) is the
volume of the injection molded part.
Furthermore, using Eq. 3 [20] the volume of the gating sys-
tem, which is a function of the part volume, can be calculated.
V= V1+ε100
+Δ100
(3)
whereΔ is the percentage of the part volume used for the gat-
ing system, and n is the number of cavities.
Cavity projected area: The cavity projected area is simply the
area of cross-section perpendicular to the withdrawal direction
(or opening direction of the mold). According to Boothroyd et
al. [20], the increase in projected area of the cavity due to pres-
3 ABS Polystyrene and styrene copolymers
CAB Cellulose acetate butyrate
POM Polyoxymethylene
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ence of gates and runners is proportional to the increase in vol-
ume. The cavity projected area is therefore found using Eq. 4.
A= A×
(4)
where (m2) and (m2) are the total projected area of
the cavity (along-with gating system) and the part, respectively.
Separating force: When the molten polymer is injected into the
cavity at high pressure, core and cavity halves of the die tend to
separate by a force, which is known as the separating force. The
separating force4 (MN) is calculated using Eq. 5.
F=p×A (5)
SELECTING THE INJECTION MOLDING MACHINE
After injection molding process parameters are determined,
the next step is to identify the production planning requirements
for selecting the appropriate machine. For estimating energy
consumption, the information about the specific injection mold-
ing machine is required. We assume that the number of cavities
is decided before the injection molding machine is selected.
Basically, an injection molding machine may be divided into
two units, namely a clamping unit and an injection unit. The
suitability of a die-cavity needs to be checked for both the
units. As an example, specifications of an injection molding
machine are given in Table 3.
Table 3: Specification of an injection molding machine [21]
Item
Unit
Machine ID
Clamp unit
Clamp
force
tf
5
100
Clamp
stroke
m
0.
35
Ejector
force
tf
3
Injection unit
Injection
unit
Injection Unit 1
Injection Unit 2
Injection
capacity
cm
3
38
78
130
78
162
254
Shot
volume
(PS)
g
35
72
120
72
149
234
Max. Inj
.
pressure
MPa
284
287
174
287
247
158
Plasticizing
capacity
g/s
6.1
11.1
23
11
23.1
33.3
The injection molding process parameters used to select a
suitable injection molding machine are discussed in the follow-
ing paragraphs.
4 The total separating force has to be increased if depth of the part is more
than 25 mm. For every additional 25 mm of depth, a 10 % increase in cavity
pressure is provided, and this aspect is considered while deciding the separating
force. [27]
5 tf is tonne force
Injection capacity: The injection capacity, (m3)
of the injection machine refers to the maximum possible design
displacement of the reciprocating screw and can be found using
Eq. 6 [19].
=D2
×π× S (6)
where D (m) is the diameter of the injection screw, and S (m) is
the injection stroke6.
Injection capacity is also used to find the realistically at-
tainable volume, (m3) of the injection molded cavity
using Eq. 7 [19].
V= k×V (7)
where k is a correction factor with a value between 0.7 and 0.8,
which indicates that the maximum possible injection volume
does not correspond to the maximum injection capacity. The
attainable injection volume is useful to check machine injection
capacity against the cavity requirements represented by shot
volume.
Clamp force: To keep the die closed during the injection
molding process, the machine clamp force should be higher
than the separating force.
Plasticizing capacity: Plasticizing capacity is defined as the
amount of plastic that can be melted, homogenized and heated
to processing temperature in the barrel, per unit of time [22].
The machine plasticizing capacity should be such that the re-
quired amount of melt is available, when the machine is ready
for the next cycle.
Clamp stroke: Clamp stroke of the machine should be higher
by 5 cm than the part height (or maximum dimension of the
part in the die-opening direction).
CYCLE TIME AND THEORETICAL MINIMUM ENERGY
COMPUTATIONS
In this section, components of the injection molding cycle
time are discussed, followed by the theoretical minimum
energy requirements.
Injection molding cycle time
Components of the injection molding cycle time are:
injection time, cooling time and mold resetting time. The
procedure to determine each of these components [20] is
mentioned in the following paragraphs.
Injection time: To find injection time, shot volume and average
flow rate need to be determined. Determination of shot volume
has already been discussed in the previous section. The
procedure to find the average flow rate is discussed below.
6 The optimum value of the feeding stroke S is generally taken between 1D to
2D to ensure good quality parts. The maximum utilizable shot weight
corresponds to feeding stroke of 3D.
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Maximum Flow rate (m3/s) Q =
× 10 (8)
where is machine injection power (kW), and (MPa) is
the recommended injection pressure.
However, in practice the average flow rate, Qavg (m3/s)
decreases as the mold is filled, which is found using Eq. 9.
Q= 0.5×Q (9)
Therefore, the injection time (s)
t=
(10)
Cooling time: This is the time taken by the molten plastic to
solidify inside the mold. Equation 11 is used to find the cooling
time, tc (s):
tc=hπ×α
× log4T−TT−T
(11)
where Tx is the recommended part ejection temp (°C), Tm is
recommended mold temperature (°C), Tinj is the polymer
injection temperature (°C), hmax is the maximum wall thickness
(mm), α is the thermal diffusivity of the material (cm2/s).
However, to make sure that the runners are solidified and
the solidified part comes clean from the mold, the minimum
cooling time is taken as 3s.
Mold resetting time: Mold resetting time is the time already
stated for the mold to open and close. The mold resetting time,
tr, is estimated using Eq. 12.
t= 1 + 1.75t
(12)
where td is the dry cycle time of the machine (s), S is the
maximum clamp stroke of the machine (cm) and, d is the depth
of the part (cm).
The total cycle time, ttotal (s) is sum of above three
components of the injection molding cycle.
Determining theoretical minimum energy
After different components of the cycle time are
determined, the next step is to find the theoretical minimum
energy required for each stage of the injection molding process.
We divide the energy consumption of the injection molding
process into the following:
i. Injection molding process energy
ii. Auxiliary operations energy
i. Injection molding process energy (Eprocess): In the injection
molding process a number of operations take place. These
operations are:
Melting the polymer
Injecting the molten polymer
Cooling
Clamping, opening, ejecting and closing the mold
The theoretical minimum energy required for these
operations is discussed in the following paragraphs. The energy
computation is for a single injection molding cycle (or shot).
Energy for melting the polymer (Emelt): Before polymer is
injected into the die, it is heated and melted. The energy
required for heating and melting is derived from two actions of
the injection molding machines: extrusion screw and barrel
heating. Equation 13 is used to find power used for heating and
melting [23].
P=ρQCT−T+ρQH (13)
where Po is the power required for melting (kW), Cp is the
heat capacity of the polymer (J/kg°C), ρ is specific density
(kg/m3), Tpol is the initial temperature of the polymer (°C), Qavg
is the volume rate of flow of the polymer (m3/s), and Hf is the
polymer heat of fusion (zero for amorphous polymer)
The energy required for melting the volume of plastic
required for one shot, E (kJ) is found using Eq. 14.
E= P×
(14)
Energy for injecting the polymer (Einj): According to
Johannaber [19], the injection or the plasticating unit of the
reciprocating screw injection molding machine has a major
influence on the quality of the final molded part. Its basic
function is to accept and convey free flowing solid plastic and
additives, perform melting, convey the melt along the screw,
mix the plastic and additives, possibly devolatilize the melt,
inject the melt into a shape providing cavity, and keep it there
under pressure. The theoretical energy required for injecting the
plasticated polymer into the cavity is the work done for filling
the cavity against pressure. The Eq. 15 is used to find the
injection energy, (kJ) [15].
E= p×V× 10 (15)
where pinj is the injection pressure required to fill the cavity
(MPa) with the polymer and Vshot (m3) is the shot volume.
Energy for cooling (Ecool): The molten polymer, which is
injected into the mold is cooled to a temperature, also known as
the ejection temperature. This means that a certain amount of
heat has to be taken out from the molded part. The amount of
heat to be taken out from the molded part, Hcool (J) is
represented by Eq. 16.
H=ρVCT−T+ρVH (16)
In the previous section, we discussed a method to find the
cooling time. The cooling power required to take out the heat
from the mold, (kW) is calculated using Eq. 17.
P =H t
(17)
The energy required for cooling, (kJ) depends upon the
COP (coefficient of performance) of the cooling equipment
used. In Eq. 18, we assume that the COP of the cooling
equipment is the theoretical maximum (based on Carnot’s
cycle) [24], which leads to theoretical minimum energy
requirements.
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E=
× 10 (18)
Energy for clamping, ejection and opening/closing (Ereset): It
has been suggested in the literature that the energy required for
opening/closing, clamping and ejection is 25 % of the process
energy [25]. Equation 19 is used to find Ereset (kJ).
E= 0.25(E+E+E) (19)
ii. Auxiliary operations energy: As discussed previously, some
auxiliary operations are required alongside the injection
molding, which need to be included for energy computations.
The minimum theoretical energy required for these auxiliary
operations is discussed in this section.
Energy for drying of polymer (Edrying): According to Strumitto
et al. [26], the measure of energy consumption in a drying
process is the unit energy consumption for the evaporation of 1
kg of moisture. Theoretically, the amount of heat required to
evaporate 1 kg of moisture under standard conditions is 2200 kJ
to 2700 kJ, which is also referred to as the specific moisture
evaporation rate (SMER), as shown in Eq. 20.
SMER =
(20)
The upper limit of this value refers to the removal of bound
moisture. However, the only drying regime in which such a
result could be obtained is the adiabatic equilibrium in which
there is no heating of a solid body and accompanying moisture.
However, there are a number of factors which affect the
energy efficiency of the drying process. In the majority of the
cases, the most efficient approach for drying intensification is
likely to be the combination of several methods. By using
techniques such as mechanical centrifuging and heat recovery,
it is possible to achieve the SMER, which is higher than the
theoretical maximum. However, here we limit the scope to the
theoretical possibilities only.
Moisture content of different types of plastics and their
other properties related can be found from material databases or
resources such as Johannaber, 2007 [19]. A representative list of
material properties relevant for drying is given in Table 4.
Table 4: Moisture related properties of plastics [19]
Material
Test
method
(DIN)
Max. water
absorption in
standard
climate (%)
Max. water
absorption in
water bath
(%)
Max.
moisture
during
injection (%)
ABS 53495 0.3 to 0.5 0.7 0.2
CA
53472
3.5 to
5
.0
3.8
to
5
.0
0.2
CAB
53472
2.0 to
2.5
2
.0 to
2.5
0.2
From these properties, the estimated moisture in the plastic
can be computed; alternatively, the moisture content can be
found by applying the suggested testing method. The
theoretical energy required for drying, E (kJ) can be
determined using Eq. 21.
E=(M × m)/SMER (21)
Where M is the mass of the plastic loaded in the dryer (kg), and
m is the percentage of moisture in the plastic.
Energy required for loading, coloring and blending: The raw
plastic beads and regrind are first loaded in the machine hopper.
Blending may be defined as combining two or more types of
materials to give a uniform mixture. Blending units are actually
metering devices that allow a specified amount of ingredients to
be combined with a specific batch of plastic materials, before
they are fed into the molding machine hopper. For injection
molders, the blending may be required to achieve proper color
combinations, combine regrind with virgin pellets, adding
ingredients to improve flow, reduce sticking or enhance the
base material in a variety of ways [27, 28].
The blending can be done manually, which of-course does
not consume any energy. We therefore assume theoretical
minimum energy for this process to be zero. However, due to
inconsistencies of the manual method, automated, integral
blending systems are often used.
Energy for trimming: In the trimming operation, extra material
attached to the solid part, like runner, gate, and fins, are
removed from the part by shearing operation. The energy used
in the trimming operation, E (kJ) depends on the shear
strength of the material and the surface area of the trimming
portion.
E=τ×A× d× 10 (22)
Where, is the shear strength of the polymer (MPa), is
the surface area of the trimming portion (m2), and is the
depth of trim portion (m).
COMPUTING ESTIMATED ENERGY CONSUMPTION
The estimated energy consumption in the injection molding
process is higher than the theoretical minimum energy
requirements. Some of the energy is lost during transmission
from the energy-supplying unit to the energy-consuming unit
(or operation). Furthermore, there is some energy loss, when
the energy-supplying unit converts one form of energy to
another. For example, an electric motor converts electrical
energy to the rotational energy. We classify these energy losses
into two categories:
i. Energy loss during transmission
ii. Energy loss at drive unit
Energy loss in transmission
In the previous section, we discussed different operations,
which take place in the injection molding process. These
operations need energy and some of the energy is needed
during transmission. For example, it is estimated that
approximately 25 % of the injection pressure [19] is lost when
switching injection pressure to the holding pressure. The
maximum pressure therefore should be more than the required
pressure by 25 %, which contributes to the energy loss for
injection. Similarly, there is a loss of heat energy when heating
and melting of the polymer and cooling of the mold. These
energy losses are added to the theoretical minimum energy
requirements of the various operations of the injection molding
process.
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Energy loss at the drive unit
Injection molding machines impose a very high demand on
the drive and control systems, because of the fact that the
plastic injection molding process is complex and the machine
cycle times are mostly short [19]. Though the modern
electromechanical drives have clear advantages, single electro-
hydraulic drives, consisting of an electric motor and a hydraulic
pump, are still in use in many places [29]. Energy loss for both
of these drives is discussed in this section.
Electro-mechanical drives: Electro-mechanical or the all-
electric injection molding machines, as they are commonly
called, do have the advantage of precision programmed
operations. In the new injection molding machines, all axes are
driven by the electric servo-motors. These axes control different
operations of the injection molding machines.
The advantages of having the all-electric injection molding
machines are that power is supplied in line with the
consumption, has minimal idling losses and good efficiency.
Approximately 20 % of the energy is used for control. The
reason for this is the power electronics of the servo-motors,
which consume much of the energy [19]. The electric drives
have the efficiencies from 0.87 to 0.95 [19]. It is also found that
75 % of the energy required for plastication comes from the
rotation of the screw, and the remaining 25 % by the heating
elements [23]. Furthermore, in addition to the process related
energy consumption, all injection molding machines consume
energy for some basic equipment like the display, fan, and other
equipment, which remain running throughout. The energy
consumption for an injection molding part manufacturing can
therefore be computed using equations 23-26.
If we take into account the injection molding process only,
the energy used for a single shot is sum of the energy required
for all the drives, as given in Eq. 23.
E=.
+
+
+
.
× 1.2 +P× t (23)
where Eshot is the energy required for injection molding a single
shot, is the power required for the basic energy consuming
units (kW), when machine is in stand-by mode, and ηinj, ηreset,
ηcooling, ηheater are the efficiencies of the different units for
injection, resetting, cooling and heating, respectively.
The auxiliary equipment like dryer, loader/blender, and
trimmer, are operational for a fraction of the time compared to
the injection molding machine. The energy required for
auxiliary operations can be found using Eq. 24.
E=
+
× t× f+
× t× f× W+
(24)
where Ploader and Pblender are the power rating of the loader and
the blender (kW), Wloader and Wblender are the weight of charge in
loader and blender (kg), ηdryer is the efficiency of the drying
unit, Edrying is the energy required for the drying operation (kJ),
and floader and fblender are the fraction of time the loader and
blender are on. Similarly, ηtrimmer is the efficiency of the
trimmer.
Lastly, the energy consumption per part (kJ/part) is
determined by aggregating all the above components of energy
consumption and dividing by number of parts in a single shot as
shown in Eq. 25.
=+
(25)
where n is the number of cavities in the die.
Electro-hydraulic drives: Unlike the electromechanical drives,
which use a different drive unit for each axis, the electro-
hydraulic drives use a single drive unit and power is then
distributed to the different energy-consuming units or
operations. The basic operations of the injection molding
process remain the same. The flow of energy from the electric
motor, up to the energy-consuming operation goes through
various systems of the machine, like the hydraulic pump,
control valve, pipes and manifolds, and hydraulic motors. The
hydraulic motors are responsible for actually delivering the
power to the desired operation of the injection molding process.
Inefficiency at each stage of the power flow contributes to the
power loss. The efficiency of a hydro-mechanical system is
found by using Eq. 26 [19].
η=η×η×η (26)
where ηtot, ηmech, ηhydr, ηvol are the total, mechanical, hydraulic
and, volumetric efficiency, respectively, of the hydraulic
system.
To achieve higher efficiency, it is therefore necessary to
use the individual components with smaller losses, especially
for the hydraulic pumps and motors. The efficiency of a
hydraulic system is further reduced by modifying valves,
length, cross-section and flexibility of the pipeline, sharp
elbows, compressibility of the hydraulic fluid, and external
leaks.
The major difference between the electric injection
molding machine and the hydraulic one is that in hydraulic
machines the electric motor, which powers the hydraulic unit, is
always on. Because of this reason there are some idling losses.
The energy consumption of the hydraulic injection molding
machine therefore can be summarized as shown in Eq. 27.
E=(P+P)× t+E (27)
where is the power consumption (kW), when the injection
molding machine is idle, is the basic power consumption,
and is the energy required for completing the injection
molding cycle (kJ).
The equations derived in the previous section are also
applicable in the case of electro-hydraulic injection molding
machines, except for the values of the total efficiency available
for each of the operations.
FACTORS INFLUENCING ENERGY CONSUMPTION
In this section, we identify the relationship of various
influencing parameters with different components of the energy
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consumption. The components of the energy use are divided
into two: injection molding and auxiliary operations. In Table 5,
these parameters are organized according to their dependence
on different sources, i.e., part geometry, part material,
material/process, others and manufacturing resources. For
example, the influence of part material information on energy
use can be easily identified from the table, which shows the
relationship of different material properties like heat capacity,
thermal diffusivity, and shear strength with different
components of energy. The motivation behind the schematic of
the guideline presented in Figure 2 and the classification of
various influencing parameters presented in Table 5 is to
facilitate development of supporting information models for
computing energy consumption as a scope of future work.
Table 5: Energy consuming operations and affecting parameters
CONCLUDING REMARKS
This study is a step towards developing a generic
sustainability characterization methodology for unit
manufacturing processes. In this paper, the guideline to
characterize energy consumption of the injection molding
process was discussed. We discussed five steps of the proposed
guideline for computing estimated energy consumption. We
also presented a schematic which shows the various steps of the
guideline and the parametric information requirements for
characterizing energy for the injection molding process.
Classification of the parameters which influence energy
computations was also presented. The proposed guideline
overcomes the shortcomings of the available methods by: (i)
extending the system boundary of the injection molding process
to cover other stages like drying, blending, dosing, and
trimming, (ii) considering factors related to part design,
material, and process planning to compute theoretical minimum
energy and, (iii) making estimates of energy consumption using
manufacturing resource information. The proposed guideline
will be useful for estimating and benchmarking diverse
manufacturing processes and products made using the injection
molding process.
Another motivation behind the schematic presented, and
the classification of various influencing parameters, is to
facilitate the development of structured information models for
computing energy consumption. Development of structured
information models will help seamless information flow
between design and manufacturing domains, which would
greatly help develop design-for-energy tools.
The following are the potential future research directions
that need to be explored.
Showcase effectiveness of the proposed guideline with ac-
tual case studies.
Define system boundaries of the injection molding process
and auxiliary operations.
Include other inputs and outputs (material, water usage and
waste) of the process, along with energy consumption.
Develop guidelines to determine the efficiency of the
manufacturing resources, which vary according to the
technology use and process conditions.
Define structured information models for seamless flow of
information across design and manufacturing domains.
Address the issue of uncertainty in the analytical models.
Develop a standard reference methodology for sustainabil-
ity characterization of unit manufacturing processes.
DISCLAIMER
Mention of commercial products or services in this paper
does not imply approval or endorsement by NIST, nor does it
imply that such products or services are necessarily the best
available for the purpose.
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