Troubleshooting: Injection Molding How to Solve Uneven Clamping

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Troubleshooting:
Injection
Molding
ng:
Troubleshooting:
Injection
Injection
MoldingMolding
How to
Solve
Uneven
Clamping
Uneven
How to Solve
Uneven
Clamping
Clamping
By John
W. Bozzelli,
Injection
Molding Solutions
tion
By John W.
Molding
Bozzelli,
Injection
Molding
Solutions
Solutions
Print This Article
Print This Article
Of of
thevariables
hundreds
of
variables
involved
in injection
molding,
clampingand
mechanisms
and Learn
platens More
often Learn
More
es
Of the involved
hundreds
involved
in
injection
in injection
molding,
clamping
molding,
mechanisms
platens
clamping
often
mechanisms
and
get than
less attention
than they
Good clampis
performance
is critical
to minimize
ey
get less
deserve.
attention
they deserve.
Good
Gooddeserve.
clamp
clamp
performance
performance
critical to minimize
cycle
times, cycle
is times,
critical to minimize c
Visit the
optimizeand
part
quality, process
and maintain
processAlignment
consistency.
Alignment
of
theplatens,
mold halves,Visit
platens,
the of
maintain
optimize part quality,
process
maintain
consistency.
consistency.
of the
mold halves,
Alignment
the mold halves
Injection
and
clamping
system
is
therefore
critical.
Injection
Molding
ZoneMolding Zone
erefore
and clamping system
critical.
is therefore critical.
Company Info
Company Info
When problem
an alignment
from
theline,
moldthe
parting
line,
or the clamping
em
When stems
an alignment
from
stemsproblem
from
the
thestems
moldmold
parting
parting
platens,
orthe
theplatens,
clamping
line,
the platens, or the clam
Sensor
Inc., damage,
mechanism
and
tiebars,
the
can bemachine
unnecessary
wear,
mold
damage, Sensor
broken
Products
Inc., Products
he
mechanism
results
and
tiebars, the
can
results
can
be
be results
unnecessary
unnecessary
wear,machine
mold damage,
machine
broken
wear,
mold
tiebars,
mold
flash,
and scrap parts.
Madison, NJ Madison, NJ
crap
tiebars, mold
parts.
flash,
and
scrap
parts.
PARTING
LINE ALIGNED?
ED?
PARTING LINE
ALIGNED?
One way
to ensure
proper
mating
of halves
the
tool parting
halves at
parting
line
is simply
by having
level
molding
machine
and parallel
One
mating
way to ensure
proper
of
mating
the
of the
tool
tool
halves
at the
linethe
at
is simply
the
by
having
parting
a level
moldinga machine
line
and parallel
is simply
by ha
platens.
Leveling
a machine
three
machinist
Start
by getting
all
the
adjustment
screws
on
theofleveling
feet of
the the ad
ne
platens.
requires
Leveling
a machine
requires
three
three requires
machinist
machinist
levels.
Start bylevels.
getting
levels.
all the
adjustment
Start
screws
on the
by
leveling
getting
feet
the
all
press
to the Place
same
torque.
one
level
across
the tiebars
or platen
support
near
the
fixed platen,
across
or
Place
press to the same
one
torque.
level
one
levelPlace
across
across
the
tiebars
or
the
platen
support
tiebars
near the
fixed or
platen,
platen
another
acrossanother
thesupport
tiebars
or the tiebars near
the
platen
support
near
the
back
end of and
the a
machine,
and
a third level
on alevel
tiebar
or platen
support.
Onetoperson goes toon a tie
ack
end
the
machine,
and
a
third
lengthwise
platen support
near of
the
back
end of
the
machine,
third level
lengthwise
on lengthwise
a tiebar or platen
support.
One
person
goes
the
appropriate
leveling
foot
and
raises
lowersit
it as
directed
by another
eyeing
the levels.
oot
and
raises
or
lowers
as
directed
by
another person eye
the appropriate
leveling
foot and
raises
or lowers
it as or
directed
by
another
person
eyeing person
the levels.
Flash
is a problem
common
part
a host
of possible
answers.
If
you
have
shorts
flash,
you
couldorhave
a mold or clamping
oblem
Flash is a common
with
part
awith
host
aproblem
host of with
possible
of
answers.
possible
If you
have shorts
answers.
and
flash,
you and
could
have
If
a you
mold
clamping
have
shorts an
alignment
This
may
not
be the
case
if you
are are
running a
high-speed,
thin-wallproblem
product—the
problem
may
alignment
not
problem.
be
This problem.
may
the
not be
the
case
case
if you
are
if
running
you
a high-speed,
thin-wall
running
product—the
a high-speed,
may be
simplymay
not be simply notthin-wa
clamp
To help
establish
whether
flash
stems
a mold
or clamp
alignment
issue,
check
parting-line
mating.
In
elp
enoughestablish
clamp enough
force. To
help force.
establish
whether
whether
flash stems
flash
from
a mold
stems
orfrom
clamp
alignment
from
issue,
checka
parting-line
mold
mating.
or
In
clamp
align
onefor
typical
check
for uniform
at
theblueing
parting
line, applied
blueing to
agent
applied
to one
mold half
rm
one typical
clamp
check
uniform
pressure
clamp
pressureclamp
at thepressure
at
partingthe
line,
parting
agent
one
line,
mold half
will
blueing
transfer
towill
thetransfer
other
agent
if to the other if
applied
there
is contact
at
the
parting
but
it will not
distinguish
low force.
or high
touch
force. information
More detailed
information
be
ting
there is contact
line,
at the
parting
but
line,
it
but
itwill
will line,
not distinguish
not
between
distinguish
low orbetween
high touch
between
More
detailed
low
can
be or can
high
touc
obtained
with pressure-indicating
paper, which
changes
color
to
amount
ofThe
touch
force.
The to
shade
color change
can
dicating
obtained with pressure-indicating
paper,
paper,
which
which changes
color
changes
relative to
the relative
amount
color
of the
touch
force.
relative
shade
of color
changeof
the
can
amount
o
be correlated
e.
be correlated with
pressure.with pressure.
Pressure-sensitive
(suchfrom
as Pressurex
from Sensor
Sensor
Products)
comes
in a broad
indicating
range
from 2 psi.
psi to
such
as
Pressurex
from
Products)
comes
in psi.
a I broad
Pressure-sensitive
paper
(such aspaper
Pressurex
Sensor Products)
comes
in a broad
indicating
range
from 2 psi
to 43,000
I 43,000
recommend
a range
of 7000
tofor
18,000
psi
for
parting-line troubleshooting.
00
recommend
to a 18,000
range of 7000
to 18,000
psi
psi for
parting-line
parting-line
troubleshooting.
troubleshooting.
To check
for good
parting-line
touch
force pressure-sensitive
with pressure-sensitive
paper,
cover
parting
linepaper,
with
thethe
paper,
the mold up the pa
-line
touch
force
with
cover
To check for
good
parting-line
touch
force with
pressure-sensitive
paper, cover the
parting
linethe
with
the paper,
close
moldclose
up
for
5 sec,
then
open
anduniformity
check
for color
uniformity
onon
the do
paper.
If you
do
not touch
have uniform
touch
force on
the
partingnot
line, the have u
heck
for 5 sec, then
for
open
and
color
check
for color
uniformity
on the
paper. If
you
not
the
have
uniform
paper.
force on
If
the parting
you
line,
do
the
problem
is either
the
mold
orthe
the
clamp. Ifmachine
machine
level and
the
platens
are the
parallel,
then
the
mold
parting
line is the
d problem
or the
clamp.
If
the
is
level
and
the
platens
are pa
is either
the
mold
or the
clamp.
If
machine
isthe
level
and theisplatens
are
parallel,
then
mold
parting
line
is the
source of misalignment.
Die-height
adjustment
will or
only
increase
orincrease
decrease
itor
cannot
adjust clamp-pressure
ie-height
source of misalignment.
Die-height
adjustment
adjustment
will
only increase
will
decrease
only
clamp
force, itclamp
cannotforce,
adjust
clamp-pressure
decrease
clamp fo
uniformity,
unless
platens are bowing.
ens
uniformity,are
unless
thebowing.
platens
arethe
bowing.
CLAMPISSUES
& TIEBAR ISSUES
UES
CLAMP & TIEBAR
Molders
find
that
clamp
not lock up
completely
after The
moldnormal
closing.
The normal
to
check the parting
hat
Molders
the
sometimes
clamp
findsometimes
that the clamp
will
will the
notnot
lock upwill
completely
lock
after
up
mold closing.
completely
response
is after
to response
check theisparting
mold
closing.
lineflash
and
look
for flash
or build-up
that maybe
be
mold Iffrom
Ifthe
the
will
notmaking
lock up
after making closing
uild-up
there
that
may
preventing
mold
from
line and look for
or
build-up
there
that maythere
be preventing
thepreventing
mold
from the
closing.
the closing.
clamp will
not clamp
lock up
after
parts for
a relatively
and
there
is nothing
parting
line
preventing
is line
a hint more
of something
parts
time
for a relatively
and
short
there
time andshort
theretime
is
is
nothing
nothing
on the
parting on
linethe
on
preventing
the
lockup,
parting
this is lockup,
a hint ofthis
something
preventing
serious. more serious.
loc
One
place
to start
is to note theoftemperature
of of
the
mold
and of the and of the
te
One place
the
to start
temperature
is
to note
the temperature
the of
mold and
the
the
mold
platen.
If thethan
mold
is running
hotter
than
180
F,
it could
warm the platen.
ing
platen. Ifhotter
the mold
is running
hotter
than 180
180
F, it could
F,
warm
it
the
could
platen.
warm the platen.
Check
for platen
expansion
at the
top (while
tiebars by
(while
warm)
by measuring by measuring
n
Check
at
for platen
the
expansion
top
at the
tiebars
top tiebars
(while
warm)
measuring
warm)
the distance
between
the
top
bars, then
compare
it to the
distance
top
the distance
bars,
between
the
then
top
bars, then
compare
compare
it to the
distance
it
to
the distance
between
the bottom
tiebars.
Take your measurement
near the
stationary the stationary
rs.
between
Take
the bottom
tiebars.
your
Take your
measurement
measurement
near the stationary
near
platen.
Since
the
bottom
of
the
fixed
platen
is
secured
to
the
frame
the
of
the
fixed
platen
is secured
toof the
frame of the
platen.
Since the
bottom of the
fixed platen is secured
to the frame of the
machine,
it
restricted
tofrom
some
extent
from thermal expansion.
machine,
some
it is restricted
extent
toissome
extent
from
thermal
thermal
expansion.
expansion.
However,
top
of the
fixed
platen
has no such
restriction
and will
xed
However,platen
the top
of the the
fixed
has
platen
has
no
no such
such
restriction
and
restriction
will
and will
as the platen
warms.
cana
create
a situation
where the where the
ms.
expand as
This
the expand
platen can
warms.
This
create
can
createThis
a situation
situation
where
the
Massive
as they“wrap”
are, platens
“wrap”
aroundMassive
the mold
Massive
as they are, platens
around
the mold
as
distance
between
the
topthe
tiebars
near
the
platen that
is larger
than that
tiebars
distance between
the
near
top
the
near
fixed
fixed
platen
is fixed
larger
platen
is
larger
than
that
between
thetiebars
bottom
tiebars.
As the
moving
platenthan
is pushed toward
the
due to uneven application of clamp pressure. Toggles
fixed platen to close the mold, the now pushed-out upper tiebars can
apply pressure in the corners, hydraulic clamps do so
restrict the movement of the moving platen. If this is occurring, a telltale in the center.
sign is galling or scratches on the sides of the tiebars. The solution is to
keep the platens relatively cool.
Troubleshooting - Injection Molding: How to Solve Uneven Clamping
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Many processors feel that insulating the mold from the platen solves this problem, but it just extends the time before thermal
differences between upper and lower platen appear. The insulator actually still conducts heat to the platen, though at a slower
rate. Molders should put the insulator board between the clamping plates and the mold, not between the clamping plates and the
Troubleshooting - Injection Molding: How to Solve Uneven Clamping
between the bottom tiebars. As the moving platen is pushed toward the
fixed platen to close the mold, the now pushed-out upper tiebars can
restrict the movement of the moving platen. If this is occurring, a telltale
sign is galling or scratches on the sides of the tiebars. The solution is to
keep the platens relatively cool.
Page 2 of 3
due to uneven application of clamp pressure. Toggles
apply pressure in the corners, hydraulic clamps do so
in the center.
Many processors feel that insulating the mold from the platen solves this problem, but it just extends the time before thermal
differences between upper and lower platen appear. The insulator actually still conducts heat to the platen, though at a slower
rate. Molders should put the insulator board between the clamping plates and the mold, not between the clamping plates and the
platens.
Another way to prevent this problem is to actively cool the platens. Molders can gun-drill the clamping plates for cooling lines and
control them to a moderate temperature. It is rare to see a machine with cooling lines in the platens, but if cooled, the platen will
not thermally expand as much, and process consistency will be improved as the platens will be thermally stabilized during startups
or shutdowns. Gun-drill the clamping plate and use a fluid temperature-control unit (TCU) to keep the platens at or below about
130 F.
Tiebars can also be thermally affected by hot air generated during processing. Since hot air rises, the top tiebars may get warmer
than the bottom bars. Such a temperature difference may mean that the clamp won’t compress the parting line evenly. For
example, if the tiebars are 10 ft long, and the upper tiebars are 15° F warmer than the bottom bars, the top bars would “grow”
0.012 in. longer than the bottom bars. This creates a difference in clamp force top to bottom. Mold cavities may flash due to the
top tiebars being hotter and therefore longer than the bottom bars.
HOW MUCH FORCE?
It is important to know the clamp force required for a job and to use the
smallest appropriate machine. Molders should find the minimum clamp
force that also minimizes flash. Too much clamping force on a small mold
causes the mold to bow in the middle. Less clamping force will provide
more uniform pressure. An industry rule of thumb is 2 to 6 tons per square
inch of projected area. Easy-flowing plastics should be in the lower range,
while stiff-flowing resins like polycarbonate or acrylic are at the higher end
of the range. If the nominal wall is thin or there is a long flow path,
required tonnages go up dramatically.
It may be hard to believe, but platens—as massive as they are—can “wrap”
Pressure-sensitive paper shows much more than
around a mold, especially if too small a mold is mounted on a large platen.
blueing agent would about uneven clamp pressure at
Molders should make sure that the mold occupies 70% or more of the
the parting line that results in flash. (Pressurex paper
distance between tiebars to avoid this.
from Sensor Products was used.)
Platen “wrapping” can occur regardless of whether the clamp force comes from a toggle or from hydraulic cylinders. It occurs also
with tiebarless presses and with hydromechanical, two-platen types in which hydraulic cylinders pull, rather than push, the platens
together. For toggle clamps, the moving platen “feels” the clamping force in the corners, while for a fully hydraulic clamp, the force
is applied in the middle of the moving platen.
You can see evidence of platen wrap when the mold is removed from the press and there is loose rust on the platen where the
mold hung. This rust lies on the surface of the platen, not pounded into it as would be expected. If a mold has been in a press for
more than a few hours, when it is removed you see only the outline of the mold.
Platen bowing can lead to cracked cavities due to metal fatigue. The problem is so prevalent that special “negative-pressure” TCUs
can be used that suck coolant through the mold rather than push coolant through it. This keeps water from getting into the
cracked cavity and ruining the part.
Another solution taken by some mold builders is to cut off the corners of a mold in an attempt to get the clamp force more evenly
distributed across the mold face. This provides a bit better clamp pressure uniformity, as the corners are not the pressure points.
Also, mold builders will “pre-stress” the mold with center support pillars that are longer or taller than side pillars by 0.002 to 0.004
in. The mold will bow slightly prior to clamp-up and injection.
About The Author
John W. Bozzelli has taught seminars on plastics design and processing for more than 30 years. He has extensive experience in
polymer development and processing from more than 20 years with Dow Plastics. He is the founder of Injection Molding
Solutions/Scientific Molding in Midland, Mich., a provider of in-plant training, troubleshooting, and consulting services. Tel: (998)
832-2424 or e-mail: [email protected]
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Plastics Technology - April 2009
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4/7/2009