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Abstract

Results of experiments focused on assessment, in injection molding, of the effects of independent variables (input parameters) on dependent variables: longitudinal shrinkage (Sw), perpendicular shrinkage (Sp) (Fig. 2 and 3) and weights (m) of the moldings made of semi-crystalline polyoxyme-thylene (POM) or amorphous polystyrene (PS) were presented. Experiment design consisted of 27 systems investigated and 5 input parameters: mold temperature (Tf), injection temperature (Tt), clamping pressure (pd), cooling time (tch) and injection speed (vw) (Table 2 and 3). Diversification of values of perpendicular shrinkage along the way of plastic flow in mold cavity (Fig. 4) has been found. Statistic analysis of the results based on estimation of parameters of regression equations (2) describing the variability of dependent values investigated as the functions of independent variables (Table 6). Analysis of correlation was done as well and the results were presented in the form of matrix of correlation (Table 4 and 5). It results from the data presented that with the aim to control the shrinkage value and molding weight in the industrial practice it is most advantageous to change the clamping pressure as essential easy-to-change parameter.
Change in injection moulded parts shrinkage and weight as a function of
processing conditions
P. Postawa, J. Koszkul
Department of Polymer Processing and Production Management, University of Technology Czestochowa
Armii Krajowej Ave. 19c, 42-200 Czestochowa, POLAND
Abstract
Results of experiments focused on assessment, in injection molding, of the effects of independent variables (input parameters) on
dependent variables: longitudinal shrinkage (SP), perpendicular shrinkage (SP) and weights (m) of the moldings made of semi-crystalline
polyoxymethylene (POM) or amorphous polystyrene (PS) were presented. The conditions of injection molding which are result of
working of many factors connected with injection molding machine, design of mould, received processing conditions and processed
polymer affect on physical state formed molding. All this decides about mechanical, thermal and useful properties. The results of
experiment, which had in view the estimation of influence of parameters of processing on shrinkage (longitudinal and perpendicular)
injection molding parts made from semi crystalline polymer POM and amorphous PS. The investigations was realized using prepared
design of experiment, which consisted with 27 arrangements and 5 entrance volumes: mold temperature, injection temperature, cooling
time, hold pressure, injection speed. Differentiation of perpendicular shrinkage across injection cavity was shown. The analysis of
received results hugged the search (estimation) of the equations of regression, which describe the changeability researches volumes in
function of processing conditions. Analysis of correlation was done as well and the results from the data presented in the form of matrix
of correlation. It results from the data presented that with the aim to control the shrinkage value and middling weight in the industrial
practice it is most advantageous to change the clamping pressure as essential easy-to change parameter.
Keywords: injection moulding, polyoxymethylene, polystyrene, moulding shrinkage, processing conditions, statistic analysis
1. INTRODUCTION
The conditions of injection which are a result of many
factors connected with the injection moulding machine, mould,
the processed plastic and the assumed injection process
conditions affect the physical state and the structure of the
injection moulding piece. These, on their part, decide on its
mechanical, thermal, usable and other properties [1, 2, 3, 4-8].
There are a lot of advanced investigations on the
improvement in manufacturing methods whose aim is to boost
the quality of the goods that are produced, with the high
repeatability at the same time and to build new, more accurate
control systems for processing machines. The issue of the control
and use of experimental techniques in injection process modelling
by means of screw extruders was broadly discussed in the
publications by the team led by S. Płaska [9, 10, 11, 12].
From the conducted literature survey [1, 5, 13, 14, 15, 16,
17, 18] it appears that the scale of processing shrinkage is
dependant on many factors, connected with both manufacturing
process itself and the shape of the injection moulding piece as
well as with the type of the plastic that it is made of. Within the
article, some results of the investigations on the manufacturing
shrinkage changes and the mass of the injection moulding piece
have been conducted for different processing conditions.
2. EMPIRICAL PART
The purpose of the investigations was to determine the
influence of chosen input parameters (injection moulding
conditions) on the output quantities (such as shrinkage and mass
of the injection moulding piece) and further, using experimental
techniques and statistical methods for the data analysis, to present
the relation between them in the form of function. The
investigations have been led for two chosen plastics:
partly crystalline copolymer POM of M8 type with the
commercial name Santial, manufactured by Rhodia
Engineering Plastics (France) [19]
amorphous polystyrene PS of 678E Cl type (transparent)
with commercial name Styron, manufactured by DOW
Chemicals (UK) [20].
Samples for investigations have been made using injection
method by means of Krauss Maffei KM 65-160 C1 machine
which was equipped with special mould for manufacturing
samples for thermoplastics shrinkage investigations [21-25]. The
shape and the dimensions for the mould cavity of the injection
moulded piece has been adjusted to the requirements of the ISO
standards [26,27]. The dimensions of the investigated samples
have been determined at specially prepared workstation equipped
with the measuring sensor which enabled reading the sample
dimension exact to a 0.001millimeter. The shape of the injection
moulded piece used for the investigations are presented in the
Figure 1.
The measurements of the plastic pressure in the mould
cavity has been conducted by means of Kistler piezoelectric
sensor. It enabled:
to register changes of the pressure as a function of time
with the resolution up to 0.01s
to determine the value of the maximal pressure inside the
mould cavity,
accurate determination of the injection time (defining the
point of switching the injection pressure to the clamp
pressure).
The investigations included the determination of the
following quantities:
mass of the injection moulded pieces m,
average value for the longitudinal shrinkage,
w
S
average value for the transverse shrinkage,
p
S
diversity of the processing shrinkage value in each zones
of the injection moulded piece.
SP_3
SP_2
SP_1
SW_3
SW_2
SW_1
Point of
pressure
mesurament
Fig. 1. Injection molding with marked lines of determinations of
longitudinal shrinkage (SW_1, SW_2, SW_3) and
perpendicular one (SP_1, SP_2, SP_3)
The measurements of the processing shrinkage for the whole
samples population used for the investigations were performed 3
months after the moment of their preparation. For each of the
investigation design arrangement, 10 samples have been
prepared. Within the investigations, the influence of the injection
conditions (such as injection temperature, mould temperature,
cooling time, injection rate and clamp pressure) have been
defined. On the basis of initial tests, 5 input parameters have been
specified; they were being changed within the limits presented in
the Table 1. The maximal clamp duration time [5] has been
defined during tests at a level of 12 sec. and that value has been
assumed as a value valid for the whole investigations, while the
Table 1. Range of variability of input parameters of PS or POM
processing
value of injection time has been defined individually for each
design arrangement.
2.1. Preparation of the design of experiment
The plan according to which the samples have been prepared
has been worked out on the basis of the literature on theory of
design of experiments [11, 12, 28, 29] and the module (DoE
Design of Experiment) of STATISTICA software for statistical
data analysis by StatSoft. Owing to the fact that two-value
designs enable to receive the description of the phenomena only
at the level of a linear function, which is less accurate for this
type of tests, one of the central composition design has been
chosen for investigations.
After the determination of the number of input parameters
and the range of their changeability, the design of experiment was
generated by STATISTICA software. Detailed designs, according
to which the samples have been prepared on the basis of two
plastics, POM and PS, have been presented in the Table 2.
Table 2. Input parameters of particular experiments of POM and PS injection molding experimental design
Parameter, symbol
Range of variability of parameters
POM
PS
Mould temperature, Tf, oC
Polymer temperature, Tw, oC
Cooling time, tch, s
Injection speed, vw, mm/s
Holding pressure, pd, MPa
30-70
175-210
10-56
20-120
300-600
30-70
200-240
15-56
20-120
200-500
POM
PS
Tf, oC
Tt, oC
tch, s
vw, mm/s
pd, MPa
Tf, oC
Tt, oC
tch, s
vw, mm/s
pd, MPa
40
187,5
21,5
45
52,5
30
210
22
45
35
40
187,5
21,5
95
37,5
30
210
22
95
25
40
187,5
44,5
45
37,5
30
210
44
45
25
40
187,5
44,5
95
52,5
30
210
44
95
35
40
202
21,5
45
37,5
30
230
22
45
25
40
202
21,5
95
52,5
30
230
22
95
35
40
202
44,5
45
52,5
30
230
44
45
35
40
202
44,5
95
37,5
30
230
44
95
25
60
187,5
21,5
45
37,5
50
210
22
45
25
60
187,5
21,5
95
52,5
50
210
22
95
35
60
187,5
44,5
45
52,5
50
210
44
45
35
60
187,5
44,5
95
37,5
50
210
44
95
25
60
202
21,5
45
52,5
50
230
22
45
35
60
202
21,5
95
37,5
50
230
22
95
25
60
202
44,5
45
37,5
50
230
44
45
25
60
202
44,5
95
52,5
50
230
44
95
35
30
195
33
70
45
20
220
33
70
30
70
195
33
70
45
60
220
33
70
30
50
180
33
70
45
40
200
33
70
30
50
210
33
70
45
40
240
33
70
30
50
195
10
70
45
40
220
11
70
30
50
195
56
70
45
40
220
55
70
30
50
195
33
20
45
40
220
33
20
30
50
195
33
120
45
40
220
33
120
30
50
195
33
70
30
40
220
33
70
20
50
195
33
70
60
40
220
33
70
40
50
195
33
70
45
40
220
33
70
30
3. STATISTICAL ANALYSES METHODOLOGY
The purpose of multidimensional analysis of regression is to
determine the quantitative relations between the investigated
values and the variables which directly influence them, to assess
the results of their activity and, to predict the behaviour of the
investigated variables [11].
Getting the knowledge on the interrelation between certain
factors of the investigated processes and phenomena is performed
by modelling the considered phenomenon and statistical
estimation of the model on the basis of the observation. Each
model equation reflects only the mechanism for only one
investigated variable, i.e. it expresses the relation where the
investigated factor does not change depending on the fact which
values are taken by the variables which describe them
(independent variables) [12].
3.1. Regress equation (model equation)
The variable (characteristic) which is described by the model
is called a dependent (investigated) variable. However, the
variable which appear in the model, which explain the regularity
of the tested variable are called the independent (explanatory)
variables.
First stage to build the model is to specify the scope of the
investigations and to specify dependent and independent
variables. An important task is proper selection of explanatory
(independent variables) and the choice of appropriate form of
model equations. The function may be in form of simple linear
dependence or more compound form by adding power component
and factors’ interactions.
Another stage is to gather the data and to select them
properly on the basis of which it becomes possible to estimate
model parameters. General form of such a model containing
linear and square term and factor’s interactions has been written
as an equation (1):
exxxz mm
.......
22110
In the analysed process of injection each input data are
described as following:
mould temperature: Tf =x1,
plastic temperature: Tt =x2,
cooling time: tch =x3,
injection rate: vw =x4,
clamp pressure: pd =x5,
Therefore, the equation (1) will take a final form:
epvptvtpTvT
tTpTvTtTTT
pvtTT
pvtTTz
dwdchwchdtwt
chtdfwfchftf
dwchtf
dvchtf
4535342524
2315141312
2
55
2
44
2
33
2
22
2
11
543210
Next stage of an analysis is estimation, i.e. looking for the
model parameters. On the basis of its results one can take
measures to limit the number of factors describing investigated
variable, which leads to simplification of the model. The value of
the adjustment coefficient R2 should be checked every time. If its
value, as a result of the simplification of the model, shows
significant drop, one should give up introducing further
simplifications.
3.2. Investigations results and analysis
In the beginning, the analysis of correlations of each
variables, both dependent and independent ones for both
polymers have been performed. The results have been presented
in tables 3 and 4.
Table 3. Matrix of correlation for POM Table 4. Matrix of correlation for PS
The analysis of correlations brings very useful results from
the point of view of the interpretation of reason-effect relations
which occur not only between the investigated values and the
input values (injection conditions) but also between the
investigated values.
While analysing data gathered in correlation tables, one can
come to a few conclusions. First of them is significant difference
between the values of coefficients received for POM and PS. In
case of POM, the change in mass of injection moulded piece is
strongly correlated with the value of the clamp pressure. Its
positive value shows that the increase in clamp pressure causes
the increase in moulded piece mass. On the other hand, the mould
temperature is of little influence on moulded piece mass. The
value of both longitudinal and transverse processing shrinkage
depends significantly on the clamp pressure value in this case
the values of the correlation coefficient reach even the value of
0,956. Hence, the crucial influence on the mass and processing
shrinkage change is exerted only by one value clamp pressure.
One can notice at the same time that all dependent variables
are very strongly correlated with each other. Their correlation
coefficients exceed the value of 0,95.
Other than these results have been received in case of
polystyrene the amorphous plastic. The value of clamp pressure
is of less influence on the moulded piece, while the injection
temperature is of much importance. The value of shrinkage, both
longitudinal and transverse, is almost is equally dependent on
three values: mould temperature, injection temperature and clamp
pressure.
On the basis of the gathered results of investigations on
changes in moulded piece mass and shrinkage as a function of
POM
m, g
w
S
, %
p
S
, %
PS
m, g
w
S
, %
p
S
, %
Tf, °C
-0,044
0,078
-0,026
Tf, °C
-0,044
0,078
-0,026
Tt, °C
0,239
-0,171
-0,156
Tt, °C
0,239
-0,171
-0,156
tch, s
0,004
-0,066
0,010
tch, s
0,004
-0,066
0,010
vw, mm/s
0,038
-0,128
-0,100
vw, mm/s
0,038
-0,128
-0,100
pd, MPa
0,935
-0,918
-0,956
pd, MPa
0,935
-0,918
-0,956
processing conditions, several statistical analyses and essential
calculations have been performed by means of STATISTICA
software, which enabled to determine the coefficient of the model
equation (2) for all investigated variables. The received
ij
coefficient values presented in Tables 5 and 6 enable to determine
the investigated variable value in any point of the space limited
by input parameters. One can therefore determine for example the
value of the injection moulded piece mass for any parameter:
injection temperature, mould temperature, cooling time, injection
rate and clamp pressure.
Part of regression
equation
POM
PS
m
p
S
w
S
m
p
S
w
S
0
-
11,72680
5,370757
9,451467
12,90085
4,785549
7,649767
1
Tf
-0,01087
-0,002901
-0,021823
-0,01113
-0,020004
-0,009644
2
Tt
0,02758
-0,012867
-0,043809
-0,00155
-0,033780
-0,062332
3
tch
-0,00528
0,001100
0,012043
0,02801
-0,009358
-0,010404
4
vw
0,01140
-0,007405
-0,009266
-0,01317
0,004562
0,007127
5
pd
0,04796
-0,067240
-0,087725
-0,03897
0,004353
0,012931
11
Tf
-0,00015
0,000106
0,000121
-0,00004
0,000076
0,000026
22
Tt
-0,00007
0,000016
0,000076
-0,00001
0,000071
0,000141
33
tch
-0,00011
0,000076
0,000107
-0,00001
0,000031
0,000036
44
vw
-0,00002
0,000012
0,000018
-0,00001
0,000002
0,000008
55
pd
-0,00018
0,000328
0,000279
0,00001
0,000097
0,000128
12
Tf Tt
0,00010
-0,000015
0,000058
0,00002
0,000056
0,000033
13
Tf tch
0,00018
-0,000140
-0,000141
-0,00005
0,000007
0,000028
14
Tf vw
0,00002
0,000008
0,000003
0,00004
-0,000008
-0,000026
15
Tf pd
-0,00003
0,000004
0,000054
0,00025
0,000010
-0,000028
23
Tt tch
0,00003
-0,000003
-0,000055
-0,00009
0,000029
0,000036
24
Tt vw
-0,00002
0,000009
0,000020
0,00006
-0,000014
-0,000026
25
Tt pd
0,00001
0,000098
0,000180
0,00018
-0,000049
-0,000091
34
tch vw
0,00001
0,000002
-0,000021
-0,00002
-0,000005
-0,000008
35
tch pd
-0,00009
0,000014
0,000005
-0,00013
0,000017
-0,000025
45
vw pd
-0,00013
0,000064
0,000063
0,00007
-0,000050
-0,000050
R2
0,97348
0,94928
0,97421
0,9731
0,8695
0,9258
Table 4. Regression coefficients of equation (2) for POM Table 5. Regression coefficients of equation (2) for PS
High values of the received adjustment coefficients R2
(higher than 0,9) prove that the model equations describe
properly (with small deviation) the dependence between the
change in the investigated values and processing parameters
received during experimental investigations.
3.3. Diversity of the processing shrinkage
From the literature survey it appears that during plastic flow,
the drop in pressure appears as a result of increasing flow
resistance [2, 3, 4]. Depending on the processing conditions that
have been assumed, some differences in the pressure registered in
the individual regions of the injection moulded piece appear; they
depend on the shape and cooling method for the moulded piece. It
causes the appearance of different plastic crystallisation
conditions which eventually leads to the appearance of different
longitudinal and transverse processing shrinkage. The value of
the clamp pressure has significant influence on the packing
degree and therefore the plastic density.
While analysing the value of the longitudinal shrinkage in
the places marked as SW_1, SW_2 and SW_3 (Fig. 1) no
significant differences between them have been noticed, both for
polystyrene and polyacetal. However, the significant differences
have been found between values of transverse shrinkage
measured close by the nozzle (SP_3), in the middle of the
moulded piece (SP_2) and at the end of plastic flow way (SP_1).
In the Figure 3 and 4 the charts showing the values of transverse
shrinkage in two places of the moulded piece i.e. SP_1 and SP_3
for the whole population of the samples have been presented. One
should mention the fact of appearance of stronger transverse
shrinkage in the part of the moulded piece which is at the farthest
distance from the nozzle (SP_1 point). The regularity appears for
two plastics for all the investigated samples.
One can easily notice from the presented charts the
differences between the run of the curves of changes in transverse
shrinkage values as a function of changing processing conditions.
These significant differences result from different sensitivity of
the investigated plastics to the change in processing conditions.
These differences have been also revealed by the correlation and
Pareto analysis.
The absolute differences of the shrinkage in each place on
moulded piece made of POM are 8 10%. Taking small size of
the moulded piece into consideration, the value is a considerable
one. It should be expected that for the bigger moulded pieces and
long distances of plastic flow, the differences may appear even
bigger.
In case of polystyrene (Fig. 3) the diversity of the moulded
piece shrinkage is a little lower than for the moulded piece made
of POM (Fig. 4). Very distinct difference, much bigger than in
case of POM is seen only for the change of the mould
temperature from 60 to 40°C. The absolute difference in the
shrinkage value between points SP_1 and SP_3 amounted then to
over 20%.
Fig. 3. Perpendicular shrinkage changes of all PS samples
investigated
Fig. 4. Perpendicular shrinkage changes of all POM samples
investigated
For the moulded pieces made of POM the 7%-drop in value
of transverse shrinkage has been noticed between points SP_1
and SP_3, while for PS, despite three-times less shrinkage, the
drop amounted to about 14%.
The distinct differences in the values of transverse shrinkage
in the individual regions of the moulded pieces prove diverse
conditions of their solidification which appears during filling in
the mould cavity, clamp phase and during cooling.
This fact is one of the imperfections which appear during
processing of plastics and it is caused by the nature of rheological
and thermal phenomena which occur during the flow and cooling
of the plastic in the mould.
The drop in the pressure of clamp and the pressure of
injected plastic which appears during flow causes different
density for different regions of the injection moulded piece. In the
places of higher pressure the plastic density is also higher and
vice versa, which results in the diversity of the transverse
shrinkage.
4. CONCLUSIONS
Taking advantage of the experiment design theory, the
investigations on the influence of processing conditions and
processed plastic on chosen properties that characterize the
injection moulded piece, such as a mass, longitudinal shrinkage,
transverse shrinkage and diversity of the processing shrinkage
within the confines of one moulded piece have been performed.
Applying the statistical analyses, the dependencies (model
equations) which describe the investigated properties of the
moulded pieces as a function of five the most significant input
parameters have been determined [30, 31].
The appearance of the significant differences between
dependencies of the investigated values on the changing injection
parameters for these plastics have been found. The change in
mass and processing shrinkage of the injection moulded pieces
made of polyoxymethylen, which belongs to the group of
crystalline plastics, depends much on the clamp pressure and less
on the injection temperature.
However, in case of the amorphous polystyrene, the mass
and the processing shrinkage of the moulded pieces depend
mainly on the temperature of the injected plastic and the mould
and slightly less on the clamp pressure.
It has been also found that the most important parameters on
which the processing shrinkage and mass value depend are, in
case of POM the clamp pressure and the injection temperature
while in case of PS the clamp pressure, injection temperature and
the temperature. Hence, in manufacturing practice, in order to
control the values of the shrinkage and mass of the moulded piece
the most profitable way is to change the value of the clamp
pressure as a parameter which can be changed quickly (the
possibility to change the value within a cycle).
Moreover, the investigations show the appearance of the
diversity for the shrinkage in the individual regions of the
moulded pieces. The transverse shrinkage is higher for the points
situated farther from the nozzle than for the closer points. The
reason may be the fact of the appearance of different values of
the plastic temperature and pressure along the way of its flow in
mould cavity. Therefore it could be useful to supplement the
guidelines on the measurements of shrinkage of thermoplastic
materials included in ISO standard with additional measurement
points (especially in case of transverse shrinkage). The
arrangement of the cooling channels should also be highlighted; it
should ensure even distribution of the temperature on the surface
of mould cavity.
The received results of investigations as well as statistical
analyses may find their application as a help in optimisation
activities for plastics processing. However, in order to make use
of it and to correlate the presented results with the production of
another injection product, it seems to be necessary to perform
comparative investigations. These enable to compare the
similarities which result from the processing the same plastic
with the injection moulded piece of different shape.
P. 1 P. 21 P. 41 P. 61 P. 81 P. 101 P. 121
Numer próbki
0,35
0,40
0,45
0,50
0,55
0,60
0,65
0,70
0,75
Skurcz poprzeczny, %
SP_1
SP_3
1 Number of sample 135
p
S
P. 1 P. 26 P. 51 P. 76 P. 101 P. 126 P. 151 P. 176 P. 201 P. 226 P. 251
Numer próbki
1,3
1,4
1,5
1,6
1,7
1,8
1,9
2,0
2,1
2,2
Skurcz poprzeczny, %
P. 1 P. 21 P. 41 P. 61 P. 81 P. 101 P. 121
Numer próbki
0,35
0,40
0,45
0,50
0,55
0,60
0,65
0,70
0,75
Skurcz poprzeczny, %
SP_1
SP_3
1 Number of sample 270
p
S
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... It has been established that crystalline materials are more prone to shrinkage than amorphous materials [1,8,11]. The shrinkage phenomenon has been best investigated for the injection molding process, hence the majority of dependences describing the effect of molding conditions on shrinkage value have been formulated with regard to injection-molded parts. ...
... Injection molding parameters, in particular pressure and holding time as well as mold temperature and cooling time, have a significant effect on shrinkage [1,4,11,13]. Generally, if the injection pressure, holding pressure and holding time are increased, injection shrinkage of thermoplastics decreases, whereas increasing the mold temperature, melt temperature and injection rate leads to increased shrinkage. ...
... If the holding time is too short, which is usually the outcome of premature freeze-off, or if the holding pressure is too low, depressions or shrinkage cavities can occur. If the holding pressure is too high, it results in excessive increase in polymer pressure and its packing in the mold cavity, leading to scratches and cracking of the injection-molded part [1,4,5,11]. The temperature of the mold cavity surface and the wall thickness of the molded part determine the cooling rate uniformity. ...
... The value of the processing shrinkage depends on many factors, including the type of polymer material, the geometry of the injection molded part, the mold cavity design, as well as the processing parameters [3][4][5]. Compared to semi-crystalline polymers, amorphous polymers are characterized by greater dimensional stability. During the cooling of semicrystalline polymers, an ordered crystalline structure is formed, which results in a step reduction in the volume of the material in the mold [6]. ...
Full-text available
Article
Cellular injection molding is a common method of modifying polymer materials aimed at reducing the sink marks on moldings’ surfaces while reducing their weight. However, the dimensions of polypropylene (PP) samples as well as their mechanical properties after the injection molding process change as a result of re-crystallization. Knowledge of dimensional accuracy and awareness of the change in mechanical properties of products during conditioning are very important aspects in the polymer processing industry. The aim of this study was to assess the changes in the value of processing shrinkage and the size of the sink marks of porous PP moldings depending on the degree of porosity and the time since they were removed from the injection mold cavity. Studies of the structure and mechanical properties of moldings were carried out after several conditioning time intervals. The maximum conditioning time of samples was 840 h at 23 °C. Based on the analysis of the test results, it was found that the cellular injection molding process with the holding phase reduces the nucleation of gas pores, which results in a smaller reduction of sink marks than in the case of samples produced without the holding phase. However, PP moldings with a porosity degree equal to 8.9% were characterized by a higher shrinkage value after 1 h of conditioning, compared to moldings with porosity equal to 3.6%. The extension of the conditioning time also resulted in an increase in the value of linear shrinkage and the properties determined during tensile tests of solid and porous samples. Furthermore, in the case of samples with the highest porosity, the impact strength was reduced by about 30% after 840 h of conditioning compared to results obtained after 1 h.
... Injection of polymer melt into a cavity starts heat transfer between a mold and a plastic part which in turn, determines products quality, structural aspects and important physico-chemical parameters. Therefore, during plastic flow and solidification time polymer rheology and heat transfer play a key role in formation of structure [1][2][3]. To maintain quality of plastic parts, injection molding should be monitored and analysed by modern detecting systems. ...
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This study concerns the application of infrared camera for injection molding analysis by measuring temperatures of both injection molded parts and injection mold cavities in a function of injection cycles. The mold with two cavities, differing in thickness (1 and 3 mm), and a cold direct runner was used. Isotactic polypropylene homopolymer was utilized to produce parts. Mold temperature was set at 22°C and controlled by a water chiller. Five measuring points were determined: SP1, SP2 (placed in the 3 mm cavity), SP3, SP4 (located in the 1 mm cavity) and SP5 around an injection molding gate. Our investigations showed that the highest temperature is localized around SP2 point and the lowest at SP4. Also, it was proved that even after 62 injection molding cycles, temperatures of cavities were not stable, revealing their further increase with each cycle.
... Shrinkage is an extremely important parameter that must be taken into account during injection moulding, because it affects the structural features determined by the shape and dimensions of the moulded part. It also determines the applicability and aesthetic features of the plastic product [5][6][7]. ...
Full-text available
Conference Paper
The process of polymer injection moulding is complex and complicated, therefore it is characterised by a number of technological and material parameters that influence the product quality. Setting the best injection conditions requires knowledge of the processing properties of the material, mould shape and technological capacity of the injection moulding machine. Injection shrinkage, as undesirable phenomenon, causes changes of the shape and dimensions of the injection moulding; therefore, it is one of the most important problems that have to be solved, both at the phase of mould designing and at selecting the technological parameters of injection. The paper presents classification of injection moulding shrinkage as well as factors influencing the value of the shrinkage. The conducted experimental tests were related to selected topics connected with the injection process and took into account the change of the processed material characteristics (type and amount of filler) and change of certain, relatively the simplest to regulate, injecting parameters (injection time and cooling of the piece in a mould). Geometrical dimensions of the injection mouldings produced from polypropylene filled with glass fibre were measured using two methods: tactile (electronic measuring instrument) and contactless (3D scanning). The analysis of injection shrinkage illustrates that an effective influence on its value is possible by both choosing appropriate conditions of the injection within the technical range of the machine, as well as, if needed, the processing properties of material can be modified in order to obtain a moulded piece of set geometrical and strength properties. After the analysis of the results, the connection between the injection moulding shrinkage and the content of the filler and the chosen parameters of injection was established.
Article
In this paper a numerical analysis of in-mold constrained shrinkage of injection molded parts is presented, considering the residual stresses produced during the packing and cooling stages. Residual stresses are the main reasons of shrinkage and warpage of the injected parts. In regards to the viscoelastic characteristics of polymeric materials, mold constraints have noticeable effects on the final dimensions of the molded parts. A numerical analysis was developed and experimentally examined for constrained shrinkage using a case study: a plate containing the holes (as constraints). The results indicated a good agreement between the numerical solution and the experimental data.
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This review concerns the rheological research related to compounds obtained using powder injection molding (PIM) technology. PIM is a process for making metallic and ceramic items using forming method for thermoplastics. This technique allows the large-number production of relatively small (corresponding to a weight of around 100 grams) parts of complex shapes with reduced cost comparing to traditional metallurgy and increased efficiency by avoiding the use of extra processes. On the other hand, the number of process variables is very high, and their interactions are only partially understood. The knowledge of the flow properties is the key factor for successful injection molding. Its novelty consists in the polymer physics aspects of the problem, considering the compounds obtained in PIM process as polymer melts highly filled with powder particles.
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Article
In the literature review the precise injection molding is presented (Fig. 6) as a process realized in a working system consisting of the following elements: man-operator, injection mold (Fig. 1-3), injection molding machine, injection method, injection parameters (Table 1) and the type of polymer. The effects of all the factors on the final quality of moldings obtained and its repeatability were discussed in detail. The material structure, mechanical properties and other functional features, weight, shrinkage (Fig. 4 and 5), stress state and accuracy of dimensions of molding were taken into account.
Article
The pressure-specific volume-temperature (p-v-7 relationship of polymers are important for the analysis of injection molding process. In the study, the p-v-T data for polypropylene Malen P type J-400 were measured during slow isobaric cooling. From the results of these measurements, the p-v-T data of the amorphous and semi-crystalline phases were obtained. The crystallization kinetics of the sample was determined using PSC measurements and the results have been fitted to the Nakamura-Hieber model to obtain equation parameters. Then the p-v-T behavior corresponding to the high cooling rate used in molding process was obtained.
Article
Simulation calculations of contraction in volume (S-v) and strain of moldings made of isotactic polypropylene "Malen P" type J-400 prepared by injection molding, were presented (Fig. 1, 2). Relations between pressure, specific volume and temperature (p-v-T) obtained in various conditions of polymer cooling were applied. The simulations of the flow in the mold as well as of the contraction in volume and strain of molding were done with use of the program "Moldflow Plastics Insight ver. 4.1". Rheological 7-parameter Cross-WLF model and thermodynamic Tait's state equation were used for p-v-T data approximation (Table 1-2). The values of contraction and strain of moldings (Fig. 3-6), relating to various cooling rates, differ significantly. The differences reach 35%. It confirms the significant influence of p-v-T data used in the process simulation.
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Article
The descriptions of liquid polymer flow in the injection mold channels, presented in the literature data, have been discussed. The flow is unstable and non-isothermal. Most often the symmetrical model is used to describe it although this model is true only at determined stable conditions. When thermal or kinetic conditions at both sides of the channel vary (e.g. because of the differences of temperature or surface roughness) thermokinetic flow asymmetry occurs. This asymmetry may be also caused by the change of flow direction in the channels e.g. in the area where the sprue joins the runner or in the cavities with inserts, bosses and ribs. In multicavity molds the polymer stream can change the direction even several times that lead to non-uniform filling of cavities. As well the weld lines areas are areas of polymer flow disturbances. Asymmetrical and non-uniform flow affects the injection molding efficiency evaluated on the basis of determinations of functional properties and surfaces qualities of molded parts.
Article
A mathematical model of the injection molding process is constructed, and an experimental analysis is made of the filling of the mold cavity as the starting point of the optimization. Optimum programs for controlling the injection cycle are proposed, as well as an adaptive control system for the injection process which involves optimization of the internal pressure in the mold cavity.
Article
Experimental presentation of the problem of injection molding of thermoplastics (low density PE) optimization, using injection molding machines of one pressure degree and constant injection rate. The two-level fractional factorial design and the method of steepest descent, being together a sequence procedure of experimental investigations, were used. The solution takes into consideration five investigated parameters (mold temperature, temperature of a polymer, injection time, time of moldings cooling, pressure -Table 1) and one result parameter - after-shrinkage of moldings (s). Particular steps of optimization investigations were described, viz. survey measurements (Figs. 1 and 2), looking for extremum neighborhood (Tables 2 - 5) and determination of a model approximating s value at extremum neighborhood (Table 6, Fig. 4). Coordinates of the point of factorial space for minimal s value were determined (Table 7). It has been found that low temperature of injection mold, long time of moldings cooling and high value of pressure show the biggest effects on shrinkage decrease. The method described let make the optimization independent on injection molding machine construction as well as on character and number of optimization criteria.
Article
Three methods based on Taguchi's loss function are presented to ensure good quality in moldings and the cost involved is estimated in relation to polymer type. Cost components are specified for each method. Practical manufacturing data are used to estimate the cost relation to process capacity (Cp).
Article
Random-sampled test specimens of bulk polymers shaped by various techniques to achieve the desired geometrical shape and size prior to the test, may yield property data differing from those observed for the finished product despite the fact that both have identical chemical compositions. Methods recommended by Polish Standards and ISO Standards to prepare test specimens are presented. To prepare test specimens from thermoplastics, a modern thermostatted injection mold (Fig. 1) is described, which meets the requirements of ISO Standards. Various reported data are intercompared to show the effects of processing technique (injection molding, extrusion, pressing) on the properties of test specimens (Figs. 10 - 13) and the effects of specimen size dimensions on mechanical property data (Figs. 14, 15). Regardless of the testing laboratory and the time of the test, consistent and repeatable data can be obtained provided that test specimens are prepared in a controlled manner and specimen size and dimensions conform with those recommended by Standards.
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