journal regarding rubber moulding defects

1. PI (proportional integral) controller or on/off settings, type and
thickness of insulation plates, platen heater and the molding
process, as previously described. Virtual molding will auto-
matically calculate heat flow through all the mold components,
and the heat lost from the cavities due to radiation, while the
mold is open. It will also show why the mold will need to be
shut down after 50 cycles because it is losing too much heat
from insufficient heaters and insulation.
This does not mean part design evaluations are not possible,
or that they are not valuable. It simply means that in order to
replicate what happens in production, more information is re-
quired. In order to know how much pressure is required to fill
the cavity in a controlled way, temperature and shear rate de-
pendent viscosity information, actual local mold temperature
information, and a realistic 3D representation of the mold, part
and runner are all required. The difference here is that virtual
molding software is developed specifically for this approach,
and traditional flow simulation software is not. For example, if
a mold has two different mold inserts, one made of P20 steel
and another made of MoldMax HH (ref. 1), they will both ab-
sorb and dissipate heat at different rates, which is calculated
during the virtual molding simulation, as are the effects on the
curing reaction. Not only one molding cycle, but multiple
molding cycles, are calculated to match the real world molding.
This distinction is very important.
A virtual molding analysis either uses existing CAD for the
mold/parts/runner, or they can be created inside the software if
they are not available. Typically, .step assembly files are used,
but other file types such as .STL or .SAT are also possible, as
are some native formats. Once CAD is imported, the entire as-
sembly is automatically converted into millions of calculation
points for the multi-physics software to compute the continu-
ously changing environment. This process is often referred to
as “meshing” (ref. 2) (figure 1). These millions of calculation
points are populated throughout all the components of the mold
Evaluating the root causes of rubber molding
defects through virtual molding
by Matt Proske and Harshal Bhogesra, Sigma Plastic Services
Injection (and transfer) molding of elastomers is a complex
operation. It may not seem very complicated, but when you
look into the details, it really is. The combination of material,
process, mold design and molding machine capabilities is
among the primary ingredients for this “stew” of sorts. Every
new mold, however, is like making a new stew; it might not
turn out the way you planned. Then there is a big pot of bad
stew that nobody wants, customers that are hungry, and you are
back to the drawing board to figure out what did not work out
as planned and how to fix it. Was it the carrots or the store I
bought them from? Was it the temperature or the time? Let us
face it, we have been making stew for 4,000 years and rubber
parts for a little more than 100. We do not have 4,000 years to
develop recipes for every possible combination of molded
product. In this article, we explore a different way to evaluate
molding problems to help unlock some of the mystery behind
these secret recipes for quality molded elastomers.
Virtual molding is a unique approach to molding simulation
technology which combines the most relevant molding aspects
so they can interact in a simulation together as they do in live
production. Temperature, time and shear rate dependent elasto-
mer material properties, thermo-physical mold, insert and insu-
lation properties, electrical heaters, wattage and thermocouple
location(s) comprise the complete injection molding process.
Fill speed, pressure limit, melt temperature, thermocouple set
point temperatures, mold opening and closing times, and post
curing processes are all critical pieces of information used by
the simulation for the virtual molding production. Combining
all of the inputs in one model allows for a comprehensive
evaluation of potential production issues. The following ques-
tions are answered in this article:
• What is virtual molding?
• How does it work?
• What inputs are required?
• What do the outputs (results) look like?
• Which problems can be evaluated?
The first important question is: Did the simulation results
come from a part design evaluation (emulation), or did they
come from a virtual molding simulation? Readers beware: A
part design evaluation typically only consists of some basic
material properties, a part geometry and a gate location. The
validity of the results based on this approach in a production
environment is skeptical at best. There is just not enough infor-
mation for the results to reflect what happens in a molding
machine. Virtual molding, on the other hand, is a fully coupled
3D heat and fluid flow program capable of simulating a real
production trial, including how the mold is preheated and what
process is used for the first 5, 10 or 100 production cycles. It
requires runner geometry, mold components, BOM (bill of
materials), electrical heater wattages, thermocouple locations,
Figure 1 - geometry of the mold before
meshing (l), and after meshing (r)
318
-106
-212
-318
Z (mm)212
318
Y(mm)
Y
X
Z
Part
Gate
Movable mold
Ejector pin
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2. the cavity, which means certain areas fill much earlier than oth-
ers. When this happens, the local pressure of the early filled
areas goes too high and the mold flashes anyway. This typi-
cally results in a process which requires the machine to slow the
injection speed as the cavity fills to avoid generating this high
local pressure. This can make it difficult to fill the other areas
of the cavity. Accurate mold temperature plays a significant
role in the ability to properly understand what will happen. Will
the rest of the part fill and without scorch? This is why virtual
molding considers the complete mold and multiple consecutive
molding cycles. The mold temperature is always changing, and
mold temperature during the first cycle is quite different from
the mold temperature after the 100th cycle. Also, if the mold
cavity temperatures are evaluated at the 100th cycle, it will not
be the same everywhere. Certain areas will be cold at the same
time other areas are hot. So in every cycle, heat is exchanged
between the mold, parts, heaters and other components. All of
these heat flow calculations are occurring simultaneously, and
these hot and cold spots inside the mold will lead to uneven or
uncontrolled curing of the rubber part. Some areas will cure
faster, while other areas will cure slower, depending on the
local temperature of the mold surface in contact with each area
of the part. The late areas of curing will increase the cycle time,
reduce the quality of the parts or even require a post molding
curing process to cure completely. As one can see, injection
molding elastomers is actually a very complex process.
While the virtual molding simulation is in process, various
graphs like mold temperature versus time, or heater power
versus time, are available (figures 2a and 2b). This information
in seconds. Each calculation point is contained by an element
which contains initial information, such as specific volume,
material type and starting temperature (depending on what
geometric domain it is part of).
Prior to starting the simulation, the molding process must be
described. It is broken into two distinct sections, including pre-
heating and production. The preheating phase starts the mold at
room temperature and activates the electrical heaters based on
the thermocouple locations. Each heating zone is controlled in-
dependently. Once the integrated PI controller finds a solution for
how much wattage each heater will require to maintain the set
point temperature, the simulation can proceed to the production
phase. This phase consists of previously described process infor-
mation, as well as any post molding curing process information
(if applicable). The same molding process can be repeated over
and over again, or the production cycles can be interrupted, the
process changed and further production cycles calculated.
These two steps assure that the thermal gradient of the mold
is accurately established. If there is a problem getting the mold
up to the desired temperature or keeping it there, the heaters
will run at full power and the mold temperature will decline,
just as it can in reality. The result will be a mold running cold
and uncured parts when the mold opens. It is also possible to
prescribe the best locations for thermocouples and to identify
where higher or lower wattage heaters are needed. Different
thickness insulation plates or blankets can also be evaluated. It
is clear that mold temperature is not only critical for curing, but
also for filling. Therefore, the questions arise: Is the mold tem-
perature under control, or does it just exist and we find a way
to make production despite this lack of control?
Once the mold is up to temperature, the cold rubber flows
into it. The rubber temperature rises due to its interaction with
the higher temperature mold and shear heating. Shear heating is
a phenomenon where internal friction within the rubber, while it
is flowing, creates heat and locally reduces the melt viscosity.
This can affect how much pressure is required to fill the runner
and cavity, and can create unexpected filling patterns. If the rub-
ber temperature rises too much, either due to shear heating or
mold temperature, it can begin to cure during cavity filling. This
creates problems with mechanical strength because crosslinking
progressed too far for the material to bond sufficiently with
other merging melt fronts. This is typically referred to as scorch.
If the rubber cures too much during filling, the viscosity will
rise and some areas may not fill properly. This can also be related
to trapped air in the cavity and poor venting. Trapped air can
produce bubbles, burn marks, non-fill or poor mechanical proper-
ties. Venting must be properly located and properly sized. When
air is trapped, the air volume decreases due to encroaching higher
pressure rubber. As the air pocket volume decreases, its tempera-
ture rises. If too much air was trapped, the air pressure will be-
come so high that the air temperature will rise above the burning
temperature of the rubber, and voila, a burn mark is created.
Filling pattern and melt pressure are critical to a robust pro-
cess. We must have enough machine pressure to fill the cavity,
but enough clamp force to keep the mold closed at the same
time. If not, the fill pressure must be reduced so the mold does
not flash (as much). In some cases, the filling is not balanced in
Figure 2b - heater power versus time
Power(watts)
2,000
1,600
1,200
800
400
0
10,000
0
2,000
4,000
6,000
8,000
14,000
12,000
Time (seconds)
Relative power
Figure 2a - mold temperature versus time
Temperature(C)
249
248
247
246
245
244
243
242
10,000
0
2,000
4,000
6,000
8,000
14,000
12,000
Time (seconds)
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3. provides important insights to process engineers about poten-
tial problems related to temperature control of the mold. Know-
ing if there are enough heaters, proper thermocouple locations
or appropriate insulation for the mold is a powerful position to
be in before the mold is even built.
Once the mold is brought up to temperature, the production
cycles begin. If the mold continues to lose heat during the pro-
duction cycles, even when all the heaters are operating at
maximum power, it is a clear sign the mold needs more (or
higher wattage) heaters or insulation. From these graphs, we
can clearly visualize this would be a production issue. Solving
this issue in a real mold is an expensive ordeal. A new heating
system must be designed in an existing mold. Retrofitting
molds like this is never optimum because there are already so
many constraints regarding the existing design. This may be
combined with more effective insulation around the mold, but
this can also be accomplished virtually.
There can also be a wide variety of filling related issues
commonly found in injection molded elastomers. Each image
contains a color scale (user scale) at the right side of the mold-
ed product. Values always decrease from top (red) to bottom
(blue). Colors in the molded product correspond to values in
the user scale.
Figure 3 shows the filling of a thick-walled cylindrical part
with two small gates on the top surface (left), and a thin-walled
part gated with an optical grade LSR (right). Both scales represent
temperature to show heat exchange differences between thick and
thin walled parts. No real heat exchange occurs in the thick part
because the rubber does not yet have contact to the walls com-
pared to the thin part where the thermal gradient is stronger.
Melt “jets” into the cavity because the melt does not engage
with the surface of either the mold or the insert, so it does not
slow down. Rather, it enters the cavity and travels through the
part to the opposite side. This behavior can also be attributed to
high velocity or low viscosity (or a combination) of the flowing
polymer. Jetting can create trapped air, but the most common
problem is that it leads to uncontrolled filling susceptible to
small process or material changes.
Trapped air also creates certain challenges. Initially, it ends
up in the final part, which is, of course, the problem. The chal-
lenge is finding out where it came from and how to get rid of
it. Figure 4 shows where contamination from air is located in
the part. The user scale displays the percent concentration of
air. Red areas have higher concentrations.
Was the air trapped in the nearby rib because the melt
flowed too quickly below the rib and encapsulated the air? Or
maybe the venting was not sufficient, and the air pressure in the
cavity was just too high? Actually, it was not either of these.
Figure 3 - jetting inside part cavities
Temperature
(°C)
161.1
Empty
160.0
153.6
147.1
140.7
134.3
127.9
121.4
115.0
108.6
102.1
95.7
89.3
82.9
76.4
70.0
98.66
Temperature
(°C)
139.2
Empty
128.0
120.6
113.3
105.9
98.6
91.2
83.9
76.5
69.2
61.8
54.5
47.1
39.8
32.4
25.1
25.05Y
Z
X
YZ
X
ShinEtsu optical lense
Filling, temperature
1.028s, 13.01%
Figure 4 - contamination from air inside
the part
Air
entrapment
(%)
88.79
Empty
10.00
9.29
8.57
7.86
7.14
6.43
5.71
5.00
4.29
3.57
2.86
2.14
1.43
0.71
0.00
0.01
Y
Z
X
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4. The second series of images (figure 5) shows where it came
from. Providing clarity of the root cause of molding issues
makes virtual molding not only a predictive tool, but a teaching
tool.
The highest concentration of air (shown in red) was initially
trapped inside the ribs at the gated side of the part. Now, vir-
tual molding becomes a communication tool to show others
exactly what they need to see. This elastomer part has adequate
venting around the parting line. But the image shows that
sometimes, even when we have adequate venting, it may not be
sufficient to allow all of the air to escape through the vents,
depending on the filling pattern.
In this example, air contaminates the rubber and becomes
trapped inside the part due to its filling pattern. It does not even
reach the parting line where the vents are located. In this case,
the part design or the gate location should be changed, if pos-
sible, in order to get all the air out of the cavity. Another option
might be to include vented ejector pins at the base of the ribs.
Virtual molding allows such variants to be modeled and com-
pared.
Filling imbalances in a multi-cavity runner system can be
tricky to nail down (figure 6). The user scale represents veloc-
ity to identify which areas are moving faster than others. Near
the end of the fill, the red areas indicate highest velocity to-
wards the unfilled cavities. Once the prematurely filled cavities
are full, all of the incoming melt is directed towards the unfilled
cavities, resulting in higher velocity. This image was taken
when the cavity is 96% filled, and we observe that the center
four cavities are completely filled, while the remaining four
cavities are still unfilled, even when the flow length for all of
them is the same.
The mold is hotter at the center, so the steel swells more and
the runners are bigger (unlikely), or the mold temperature is
higher and it affects the viscosity (getting warmer); actually, the
viscosity is affected by the local shear rate the rubber experi-
ences. When rubber flows faster at one specific location than
the location immediately next to it (figure 7), there is a shear
rate difference. The higher the shear rate, the greater the fric-
tional heat and the larger the effect on viscosity.
The issue is that the viscosity is not affected uniformly ev-
erywhere; it is only affecting the material at the higher shear
rate. This creates a problem that lower viscosity material flows
Figure 5 - progression of melt with trapped air inside the part
Air
entrapment (%)
99.27
Empty
10.00
9.29
8.57
7.86
7.14
6.43
5.71
5.00
4.29
3.57
2.86
2.14
1.43
0.71
0.00
0.01
Air
entrapment (%)
Empty
10.00
9.29
8.57
7.86
7.14
6.43
5.71
5.00
4.29
3.57
2.86
2.14
1.43
0.71
0.00
98.68
0.01
Air
entrapment (%)
94.81
Empty
10.00
9.29
8.57
7.86
7.14
6.43
5.71
5.00
4.29
3.57
2.86
2.14
1.43
0.71
0.00
0.01
Air
entrapment (%)
Empty
10.00
9.29
8.57
7.86
7.14
6.43
5.71
5.00
4.29
3.57
2.86
2.14
1.43
0.71
0.00
89.20
0.01
Y
Z
X
Y
Z
X
Y
Z
X
Y
Z
X
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5. more easily under pressure, so the low viscosity areas flow
faster and filling imbalances are created. If the imbalance is big
enough, some cavities will fill too early, resulting in high pres-
sure and potential flashing at those cavities. Again, the process
would require slowing the injection speed towards the end of
filling, possibly resulting in unfilled cavities. These issues can
only be fixed if they are quantifiably understood. I would not
attempt to trial and error my way through this kind of issue. I
did once, many years ago, and that left a terrible taste in my
mouth; worst stew ever.
Scorch results show how much material is cured during fill-
ing. In the rubber molding industry, sometimes the gate dimen-
sions are reduced in order to initiate high shear during filling.
Due to increased shearing, the polymer temperature will rise,
which will ultimately jumpstart the curing of the material dur-
ing the filling process. It is easier to identify the areas with
higher and lower scorch using virtual molding, as shown in
figure 8.
The user scale for scorch value is set to 2%, meaning the
scale result of 1 shows the curing degree of 2% or more is fully
achieved. Yellow areas indicate a higher degree of scorch (or
premature crosslinking). Scorch values can be lower if the mold
temperatures are reduced, or if the material is sheared less.
Weld lines are created where the two melt flow fronts meet
inside the part. Computer-generated tracer particles are auto-
matically deposited as each weld line is formed (figure 9). The
user scale displays temperature to convey the melt front tem-
peratures when they engage. These tracer particles aid in visu-
alizing what is happening behind the flow front or beneath the
surface skin. They are used during the filling, packing/holding
and curing phase to evaluate the molding conditions, such as
mixing polymer, stagnation or re-direction of the polymer flow
Figure 6 - filling Imbalances inside an
eight-cavity tool
Unbalanced velocity Absolute velocity
cm/s
65.65
Empty
21.00
19.50
18.00
16.50
15.00
13.50
12.00
10.50
9.00
7.50
6.00
4.50
3.00
1.50
0.00
7.547e-007
Y
Z
X
Figure 7 - differences in temperature and viscosity inside the runner system
Temperature Low
High
Temperature
(°C)
164.8
Empty
135.0
133.2
131.4
129.6
127.9
126.1
124.3
122.5
120.7
118.9
117.1
115.4
113.6
111.8
110.0
100
Viscosity
Low
High Dyn. visc.
(Pa·s)
1.767c-004
Empty
12.500
12,143
11,786
11,429
11,071
10,714
10,357
10,000
9,643
9,285
8,929
8,571
8,214
7,857
7,500
2,538
Y
Z X
Y
Z X
Figure 8 - areas with higher and lower
scorch inside the part
Scorch
(-)
Empty
0.2800
0.2600
0.2400
0.2200
0.2000
0.1800
0.1600
0.1400
0.1200
0.1000
0.0800
0.0600
0.0400
0.0200
0.0000
0.3013
1.535e-007Y
ZX
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6. through the weld line regions.
Tracer particles not only show the location of the weld lines,
but also provide specific information to determine if they will
be comparatively strong or weak. Important parameters such as
the temperature, curing degree, pressure, contact angle and
velocity at which the flow fronts meet are considered by the
weld line strength results. This information is used to determine
how effectively the flow fronts will fuse together during the
molding process.
Figure 10 displays the curing degree inside the part when
the mold opens at the end of the molding cycle. The user scale
represents curing degree in percent. Orange and red areas have
achieved a higher degree of cure (90-95%) compared to the
blue areas (25%).
The parts are sliced to visualize the inside core of the wall
thickness. From the outside surface, the parts are fully cured,
but inside the core of the wall thickness, they are still uncured.
In order to cure these parts completely, they might need to be
placed inside an oven after molding. Secondary curing is also
fully coupled to the production phase and simulated in virtual
molding. Decisions can be made very early in the design about
whether or not we prefer to have a longer cure cycle, higher
mold temperature or a post ejection curing process. The virtual
molding “oven” is also calculating the curing progression with
respect to time and temperature. The curing state is monitored
to calculate production rate and energy cost prior to building
the mold.
Part of a comprehensive virtual molding environment in-
cludes the ability to consider over-molded inserts placed into
the cavity during each cycle. These inserts are ejected with the
part and often used to provide higher mechanical properties.
The metal inserts can be preheated or placed into the mold at
ambient temperature. Once placed, the insert exchanges heat
with the mold at a rate defined by their different temperatures,
thermo physical properties and amount of surface contact.
However, the heat exchange is not uniform (figure 11). The
user scale displays temperature and is used to convey the mes-
sage that the insert is not always the same temperature every-
where due to thermal exchange that occurs between the insert
and the mold/part.
On the surfaces in contact, there is heat transfer from con-
duction, but the exposed surfaces exchange heat with the envi-
ronment through radiation. This non-uniform temperature also
produces non-uniform expansion, and it can lead to potential
shut-off or other dimensional issues. Over-molded inserts can
distort, depending on pressure during filling or due to a non-
uniform shrinkage of the rubber that occurs during the curing
and cooling process (figure 12). The user scale shows displace-
ment from the original insert shape in mm. Original shape ap-
pears transparent for reference.
Figure 9 - tracer particles deposited at
the weld line
Temperature (°C)
160.0
155.0
150.0
145.0
140.0
135.0
130.0
125.0
120.0
115.0
110.0
105.0
100.0
95.0
90.0
180
85.55
Y
ZX
Figure 10 - curing degree inside the part
at ejection
Percent cured (%)
Empty
95.00
90.00
85.00
80.00
75.00
70.00
65.00
60.00
55.00
50.00
45.00
40.00
35.00
30.00
25.00
95.34
0.0004138Y
Z
X
Figure 11 - non-uniform temperature
gradient in the metal insert
Temperature
(°C)
Empty
124.0
118.0
112.0
106.0
100.0
94.0
88.0
82.0
76.0
70.0
64.0
58.0
52.0
46.0
40.0
138.5
60.97
Y
Z
X
Insert temperature rise
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7. It also results from the insert design not having enough
strength to withstand this pressure from the rubber part. Time
in contact also plays an important role in the thermal exchange
between insert(s) and mold, especially if multiple inserts need
to be placed, one at a time. This produces significant variation
and should be avoided.
Deflection of core pins during filling due to high pressure
imbalances are also difficult to quantifiably prove and fix.
When the melt pressure on one side of a core pin is higher than
the other side, there is a potential for the pin to bend. If it will,
it is because the pin does not have the mechanical strength to
resist the net pressure.
Having a material database of temperature dependent ther-
mo-physical and mechanical properties of a wide variety of
mold materials makes these calculations possible. If the pin will
bend, the filling pattern will need to change to reduce the pres-
sure imbalance (figure 13). The user scales represent melt pres-
sure (1) during filling and resulting core pin deflection (r) in
mm. The deflection is magnified for clarity.
Material data for the mold, inserts, insulation, heaters, and a
variety of polymers and elastomers, are already present in the
virtual molding database. However, many, if not most, elasto-
mers are custom materials whose properties can be measured
through a material testing laboratory.
Temperature, pressure, shear rate and time dependent prop-
erties are required for the virtual molding simulation, and these
properties include thermal conductivity, specific heat capacity,
rheology, pressure volume temperature (PVT) curves and cur-
ing kinetics.
All types of elastomers are possible, including natural rub-
ber (NR), nitrile rubber (NBR), hydrogenated nitrile butadiene
rubber (HNBR), fluoroelastomer (FKM), ethylene propylene
diene monomer rubber (EPDM), styrene-butadiene rubber
(SBR) or liquid silicone rubber (LSR). It is imperative to have
accurate material data in order to achieve accurate results.
While measuring these material properties, it is also impor-
tant to capture the data to cover the entire processing range for
the particular elastomer. If the material will be processed at
120°F, then rheology should be measured at 100, 120 and
140°F to cover material behavior at, above and below the initial
temperature. If the mold will operate at 350°F, then curing de-
gree curves should be measured at 330, 350 and 370°F.
Various models are provided to fit the material data inside
the virtual molding database. Rheology models include Cross-
Arrhenius, Carreau WLF, Carreau Yasuda WLF and interpo-
lated viscosity. For reaction kinetics, common models are
Kamal and Deng-Isayev. Similarly, various models are includ-
ed for fitting the PVT, for curing shrinkage calculations (ref. 3),
and reactive viscosity data.
Overall, virtual molding is a completely different approach
to understanding injection molded elastomers because the en-
tire mold, material properties and process are fully coupled and
calculated over multiple consecutive molding cycles to match
the real world production environment. Automatic meshing
and process specific user interfaces support such comprehen-
sive calculations.
Various tools like x-ray, clipping, scale, zoom, rotate,
etc., can be used to visualize, evaluate and communicate the
molding issues and the root causes for quantifiable solu-
tions. The examples shown clearly identify multiple poten-
tial elastomer molding issues and some potential solutions.
Virtual molding is a unique approach which makes produc-
tion visible.
References
1. Virtual Molding: http://www.virtualmolding.us/.
2. https://materion.com/Products/Alloys/MoldMAX-Alloys/In-
jection-Molding.aspx.
3. https://www.researchgate.net/publication/224453190_Com-
prehensive_material_characterization_of_organic_packag-
ing_materials.
Figure 12 - distortion of the metal insert
Total displacement (mm)
Empty
0.3605
0.3356
0.3106
0.2857
0.2607
0.2358
0.2108
0.1858
0.1609
0.1359
0.1110
0.0860
0.0611
0.0361
0.0111
0.3605
0.01115
Y
Z
X
Figure 13 - part at 50% filled (l); core pin
deflection during filling (r)
Pressure (bar)
Empty
103.2
95.9
88.6
81.3
74.0
66.7
59.4
52.1
44.8
37.5
30.2
22.9
15.6
8.3
1.0
103.2
1
Y
Z
X
Displacement X
(mm)
Empty
+0.0002
-0.0227
-0.0457
-0.0686
-0.0916
-0.1145
-0.1374
-0.1604
-0.1833
-0.2063
-0.2292
-0.2522
-0.2751
-0.2980
-0.3210
0.0000218
-0.321
Y
Z
X
26 RUBBERWORLD.COM
12RW - 20,21,22,23,24,25,26.indd 26 3:36:21 PM