Influence of Injection Moulding Parameters on Tensile Strength of Injection Moulded Part
Raos, P, Stojcis J
ISSN 1339-2972 (On-line)
Influence of Injection Moulding Parameters on Tensile Strength of
Injection Moulded Part
Pero Raos1 – Josip Stojsic2
1
2
Mechanical Engineering Faculty in Slavonski Brod, Email: praos@sfsb.hr
Mechanical Engineering Faculty in Slavonski Brod, Email: jstojsic@sfsb.hr
Keywords
Abstract
Injection moulding
Polyethylene
Tensile Strength
Polymeric materials processing is one of the fastest growing industries in the world. One of the most important method for
processing polymeric materials is injection molding. The properties of an injection molded part depend upon the working material
and upon the processing parameters as injection pressure, holding pressure, melting temperature, injection speed etc. As part of the
paper, the samples of polyamide were made by adjusting various parameters (injection pressure, injection speed). Afterward, the
tensile properties of obtained specimens were analysed. The mathematical models that show dependence of tensile properties on
the injection molding parameters were obtained as the result of analysis.
Article
History
Received 04 July 2014 | Revised 15 August 2014 | Accepted 05 September 2014
Category
Professional Paper
Citation
Raos P, Stojsic J (2014) Influence of Injection Moulding Parameters on Tensile Strength of Injection Moulded Part. Journal of Manufacturing and Industrial Engineering, 13(34):1-3, http://dx.doi.org/10.12776/mie.v13i3-4.412
INTRODUCTION
Injection moulding is one of the most common processes used
to produce plastic parts. It is a cyclic process of rapid mould
filling followed by cooling and ejection. A variety of materials
both plastic and non-plastic can be used as feedstock. However,
the machine must be configured for the type of material used.
The material, which is generally available as grains or powder, is
plasticised in an injection unit and injected into a clamped
mould under high pressure (500-1500 bar). The main advantage
of injection moulding is that it is a very economical method of
mass production. Ready parts with tight tolerances can be
produced in one step, often completely automatically. In
general after-processing is not necessary. It is also possible to
integrate different functions into one part to avoid the
formation of different components that would be more
expensive, e.g., the base of a typewriter with integrated
guidance and fixing elements, the springy components of a
printer element, a lens with integrated prisma to stop down a
beam of light [1].
Figure 1 a, b Injection moulded parts [2, 3]
The basic parts of an injection moulding are:
− injection unit,
− machine base with hydraulics,
− control unit,
− clamping unit with mould.
http://dx.doi.org/10.12776/mie.v13i3-4.412
An example of a commercially available injection moulding
machine with basic parts that make up a machine is shown in
Fig. 2.
Figure 2 Main parts of injection moulding machine [4] (a – injection unit,
b – clamping unit, c – power unit, d – control unit)
The properties of an injection moulded part depend upon
the working material and upon the processing parameters as
injection pressure, holding pressure, melting temperature,
injection speed etc.
The injection pressure and holding pressure selected
must be as high as necessary to fill the cavity sufficiently
fast, completely and efficiently, but, on the other hand, as low
as necessary to produce low-stressed injection moulded
components and avoid difficulties when the components are
ejected from the mould [1].
For thin-walled parts, the optimum filling speed is higher
than for thick-walled parts, so as to obtain
uniform filling of the moulding through the flow head. Too
low a filling speed causes a greater temperature variation
between those parts of the preform near the gate and those far
from it, due to increased cooling off of the compound while the
1
Raos, P, Stojcis J
Influence of Injection Moulding Parameters on Tensile Strength of Injection Moulded Part
cavity is being filled. The higher viscosity of the colder
compounds also requires higher injection pressures, which in
turn require stronger locking pressures [1].
The aim of this paper is determine influence of two
processing parameters (injection pressure and injection speed
on tensile strength of injection moulded part.
EXPERIMENTS
Injection moulding machine: Testing samples were prepared on
Dr. Boy injection moulding machine shown on Fig. 3, with
technical data shown in Table 1.
Table 1 Technical data BOY XS injection moulding machine [5]
Hydraulic power unit
Clamping force
Distance between tie bars
Min. stroke volume
Max. stroke volume
Max. injection pressure
Screw diameter
Max. injection speed
10 kN
160 mm (horizontal) x 205 mm
(diagonal)
3
0.1 cm
3
8.0 cm
3128 bar
12 mm
3 -1
24 cm .s
Table 3 Conventional sequence in the central-composite design of
experiment (real values of factor levels)
No.
of sample
1
2
3
4
6
7
8
9
10
11
12
Run No.
9
2
7
5
10
6
1
11
4
12
3
Injection pressure
[bar]
600
700
600
600
700
741
500
600
500
459
600
Injection speed
3
[cm /s]
8
10
15
15
20
15
20
15
10
15
22
Figure 4 a, b Testing of samples
RESULTS AND DISCUSION
Figure 3 “BOY XS” injection moulding machine
Studied material: Material used for the test samples is
polyethylene known under the trade name MG9621B produced
by „BOREALIS. The main material properties are given in Table 2.
m
Table 2 Properties of MG9612B [6]
Density
Melting flow rate (190 °C/2.16 kg)
Tensile strength (50 mm/min)
Melting temperature
Shrinkage
962 kg/m3
12 g/10 min
26 MPa
200 – 260 °C
1 – 2 % (depending on
sample thickness and
processing parameters)
Injection moulding of testing samples: Table 3 shows the
conventional sequence in the central-composite design of
experiment with the real values of factors and the sequence of
conducting the experiment. Injection pressure is in the range
from 459 to 741 bar and injection speed range is from 8 to 22
3
cm /s. The shape of test sample is 5B according to the Standard
HRN EN ISO 527-1: 2012.
Testing of sample: Universal tensile testing machine (Fig. 4 a,
b) was used for testing of tensile properties. Five test samples
for each state of experiment were used for testing (60 samples).
The testing speed was 5 mm/min.
2
In Table 4 are shown mean values of response (max. force).
Mean value is calculated for five repeated measurement. After
testing of sample and calculated mean force for each state of
experiment tensile strength was calculated. The tensile strength
is calculated by the following equation:
F
A
[MPa]
(1)
where is F – force in N and A – cross-section of test sample
2
2
in mm . Cross-section of test samples are 12 mm .
Table 4 Conventional sequence in the central-composite design of
experiment (real values of factor levels)
No. of sample Force [N]
1
169.8
2
163.4
3
169.8
4
171.6
6
165.2
7
159.8
8
180.8
9
167.6
10
179.8
11
187
12
167.2
Tensile strength [MPa]
14.15
13.616
14.15
14.3
13.76
13.316
15.06
13.96
14.983
15.583
13.93
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Factor Coding: Actual
Tensile strength (MPa)
Design points above predicted value
Design points below predicted value
15.583
Analysing the obtained data (Table 4) it was determined the
13.316
minimum value of tensile strength is 13.31 MPa and
15.58 MPa
X1 = A: Injection pressure
X2 = B: Injection speed
is maximum. Mean value of response is 14.239 MPa,
standard
deviation (amount of variation or dispersion from the mean
value) is 0.656.
Regression model and analysis of variance of the model:
Table 5 presents the report for the analysis of variance of the
selected linear regression model, which shows the dependence
of tensile strength polyethylene sample on the input variables.
Model
A – injection
pressure [bar]
B – injection
3
speed [cm /s]
Residual
Lack of fit
Pure error
Cor total
Sum of
Squares
4.31
4.31
0.0015
0.42
0.35
0.062
4.73
df
2
1
1
9
6
3
11
F
value
46.68
93.33
p
value
< 0,0001
< 0,0001
0.01015
0.022
0.8854
2.83
0.2113
Member of model A is significant in the model (p value –
probability for F variables that is 93.33 is less than 0.05).
Member of model B, is not significant (p value for F variable that
is 0.022 is greater than 0.05). Due to statistically insignificant
factors, a reduced model was made. A method of backward
elimination, which is based on the sequential testing of the
significance of input variables using the F-test, was chosen for
the reduced model.
Table 6 ANOVA for response surface of reduced linear model – tensile
strength
Source
Model
A – injection
pressure [bar]
Residual
Lack of fit
Pure error
Cor total
Sum of
Squares
4.31
4.31
0.42
0.35
0.062
4.73
df
1
1
10
7
3
11
Mean
Square
4.31
4.31
F
value
103.45
103.45
p
value
< 0,0001
< 0,0001
0.042
0.051
0.021
2.43
0.2500
14
13.5
13
10
12
741
684
628
571
515
458
Injection pressure, bar
Figure 5 3D surface view of reduced linear model – tensile strength
CONCLUSION AND FUTURE DIRECTION OF RESEARCH
Injection moulding is one of the most common cyclic
processes used to produce plastic parts. Today more than onethird of all polymeric materials are injection moulded.
Parameters of injection moulding as melting temperature,
mould temperature, clamping force, injection pressure, injection
speed etc., significantly affect the properties of moulded parts.
In this paper the impact of two parameters (injection pressure
and injection speed) on tensile strength of polyethylene
injection moulded part was determined.
The analysis of variance showed that the injection pressure
has the significant influence on the tensile strength, while the
injection speed has no effect on the observed property at the
significance level α = 0.05. The regression analysis revealed the
expressions that show the function dependence of injection
pressure on tensile strength of polyethylene injection moulded
part. Further research will go in the direction of getting
appropriate mathematical model that will determine the
dependence of mechanical properties on other processing
parameters of injection moulding.
REFERENCES
[1]
[2]
[3]
Member of model A is significant in the model (p value –
probability for F variables that is 103.45 is less than 0.05). The
lack of fit is here with the probability of F variable 0.2500. This
probability is greater than 0.05 and the null hypothesis that the
deviation from the model is not significant, i.e. that the model
estimates the reality well, is accepted. The coefficient of
2
determination R is the portion of explained variability (how
much the regression y deviates from the mean) in the total
variability (how much the real y deviates from the mean), and is
0.91. Expression 2 illustrates the reduced regression model with
the real values of factors.
18
20
[4]
tensile strength=18.64 – 0.074 x injection pressure
15
14.5
Injection speed, cm3/s 16
Mean
Square
2.16
4.31
0.046
0.059
0.021
20
16
15.5
14
Table 5 ANOVA for response surface – tensile strength
Source
Raos, P, Stojcis J
T e n s ile s t r e n g t h , M P a
Design-Expert®
Software
Influence of Injection Moulding Parameters on Tensile Strength of Injection
Moulded
Part
[5]
[6]
Goodship V (1994) Pratical Guide to injection Moulding,
Rapra Technology, Shawbury, United Kingdom.
Mold Rite, Inc., Woodinville: Home, available on:
http://www.moldriteinc.com/.
CNMoulding,
Shanghai:
Home,
available
on:
http://www.plastic-injectionmoulding.com/.
Šercer M (2004) Smjerovi razvoja ubrizgavalica za
injekcijsko prešanje plastomera. Polymeri, 24:74-78.
Dr. Boy GmbH & Co. Kg: Products, available on: http://drboy.de/de/product/boy-xs/.
Nomis
d.o.o.
Granulati,
available
on:
http://www.nomis.hr/admin/_upload/_files/MG9621.pdf.
LIST OF USED SYMBOLS
ANOVA – ANalysis Of VAriance
(2)
The graphic representation of reduced model is shown in Fig
5.
http://dx.doi.org/10.12776/mie.v13i3-4.412
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