Rubber Injection Molding Press
Basic Theory of Operation
Rubber feed stock, usually in continuous strip form, is drawn into the feed port
and ground into a paste, or plasticized, by the action of the feed screw.
Because the friction of the feed screw shearing the rubber against the wall of the
screw barrel generates heat, the feed port, screw and screw barrel are equipped
with liquid temperature control circulation units (TCU).
Depending on the individual compound of the rubber feed stock, it may be
necessary to chill the feed port to prevent feed strip melt and breakage.
Slow screw speeds and high durometer feed stock may require increased TCU
temperature settings in the screw and barrel to soften the rubber for feeding.
High screw speeds produce excessive heat in the feedstock, necessitating lower
TCU temperature settings to prevent premature cure and / or feed strip breakage
problems.
The inject unit is fitted with a manually adjusted backpressure valve which
controls the resistance exerted on the rubber as the screw forces it into the
injection barrel. Increasing this backpressure can eliminate entrained air from
the compound, but excessive backpressure will cause rubber to backup in the
feed port rather than feeding properly.
The turning action of the screw forces the rubber through a ball check valve and
into the injection barrel. An injection ram can then force the rubber from the
injection barrel through a nozzle and into the mold.
The extreme pressures generated by the injection ram virtually liquefy the rubber
and generate a great deal of heat. The friction of its rapid passage through the
nozzle further heats the rubber. In order to control this heat production, the
injection barrel and nozzle are also assigned to a TCU controlled zone.
Adjustment of these temperatures is similar to those of the screw unit. Faster
inject speeds and faster curing compounds require lower temperatures to prevent
premature curing. Higher durometer feed stock and lower inject speeds may
require higher TCU temperatures to sufficiently soften the rubber for injection.
This pre-heating and liquification of the rubber allows quicker filling of more
complex mold cavities with faster curing compounds and thus greatly reduces
cycle times, giving the productivity multiplication which is one major advantage of
injection molding.
In both the screw and inject units, certain combinations of compound parameters
and machine settings reach a physical limitation beyond which satisfactory
results cannot be obtained.
Typically, high screw speeds and high inject speeds (generating excess heat)
coupled with high durometer (high friction), fast curing (high temperature
sensitivity) compounds and perhaps narrow passages in the mold cavity will
unavoidably produce premature curing and “burned” parts.
In this situation, one or more of the limiting parameters must be changed to
produce satisfactory results.
For instance, reducing the screw speed will often alleviate the tendency to
produce lumps of prematurely cured compound. Also, a mold with large cavities
and small feed runners may be impossible to fill with a fast curing compound.
This is because a slow inject will not have time to fill the cavities before the mold
heat begins to cure the compound, but a fast inject through the small runners will
produce so much friction heating that the compounds will begin to cure in the
runner system before it reaches the cavities. The only solutions to such a
problem may be to switch to a slower curing compound, lower the mold
temperature, and / or machine out the runner system to a larger cross-sectional
area.
As the screw feeds rubber into the injection barrel, the volume is monitored by
the PLC and the screw is shut off when a preset fill volume is reached. (The
injection reloading action begins with the screw slowly ramping up to the preset
speed to minimize rubber strip breakage. Then the screw continues to run at the
preset speed until it is within approximately 95% of the preset fill value, at which
time it changes to a slow down speed to more accurately fill the barrel.) If the
rubber feed strip melts, breaks or reaches its end, the screw will stop and the
fault light will flash along with a message on the OIT screen alerting the operator
to a “Feed Port Alarm”.
Depending on the specific mold configuration and rubber feed stock chemistry, it
may be desirable to use the manual inject pushbutton to purge any partially
cured compound from the injection nozzle and mold runners before closing the
mold. After this is done, the screw will automatically refill the inject barrel to the
preset volume. At this point the press is ready to begin an auto-cycle.
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To begin an auto-cycle, the operator verifies that all personnel are clear of the
press and actuates the two Cycle Start pushbuttons simultaneously (for the
purposes of clarity it is assumed that the press is in “Two-Hand Operation Mode”,
if the press is equipped with a light curtain system it is only necessary to activate
the Top Cycle Start pushbutton), holding them until the mold is fully closed and
the pressure column has moved under the center of the clamp bolster.
WARNING
The operator must verify that all personnel are
clear of the machine before activating any
control. The operator must not allow any
personnel to approach or touch the machine
until the Auto-Cycle has completed and the
mold has fully opened. In the event of any
malfunction or emergency, the operator must
press the “Emergency Stop” pushbutton before
the operator or any other personnel approach
or touch the machine. Failure to comply with
these precautions can result in
SERIOUS INJURY OR DEATH!
When the cycle start buttons are actuated, the PLC makes several readiness
checks (some of which are optional and may not be applicable to every press).
These checks include: Inject barrel full to preset volume, purge cycle finished, top
platen locked, mold fully open (to start cycle), ejectors retracted, shuttle
retracted, core bars clamped, gates and guards closed, light curtain clear,
hydraulic oil level and temperature normal, mold temperature within limits, etc.
If any of these checks fail, the lower platen will not close and a fault will be
displayed on the OIT screen accompanied by a flashing Fault Light. Operation
cannot resume until the fault is corrected.
Next, the clamp cylinders lift clamp bolster to close the mold. The speed and
pressure of these cylinders is adjustable by on-screen parameters. The speed
can switch from high to low at four on-screen adjustable points during the clamp
travel. This allows the mold to close quickly where there are no obstructions, but
slow down when approaching mold intermediate plates or where alignment pins
begin to seat.
If the operator continues to hold the cycle start pushbuttons after the mold is fully
closed, the pressure column will slide into position under the center of the moving
bolster and the massive hydralock cylinder will begin to expand. At this point, the
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operator can release the cycle start pushbuttons and the press will complete the
molding cycle automatically.
The hydralock cylinder forces the pressure column upward a fraction of an inch
to contact the clamp bolster and then develops hundreds of tons of clamping
force to hold the mold shut against the fluid pressure of liquefied rubber injected
into the mold. The speed and pressure of the hydralock cylinder are adjustable
by on-screen parameters.
When the pressure in the hydralock cylinder reaches the preset value, the PLC
starts the rubber injection stroke. The speed and pressure of the injection
cylinder are also controlled by on-screen parameters. Two (or optionally more)
different speeds and pressures can be defined in the injection stroke, which will
switch automatically base on the injection plunger position.
At the end of the injection stroke, the injection ram can hold an on-screen
settable dwell pressure and time. This dwell cycle can be used to assure that the
rubber has had time to flow to the farthest mold cavities before the injection
pressure is released. The cure timer starts when the injection cycle has
completed.
During the injection cycle or cure time, screen selectable “Bump” options can
activate one or more “bump cycles”. A bump cycle momentarily releases all
pressure on the hydralock cylinder, allowing the rubber to force the mold open a
fraction of an inch. This is done to release air from the mold and allow the rubber
to fill out the extreme corners of the cavity.
Additionally, the OIT screen can be used to program a vacuum time. If the press
is equipped with a vacuum valve on the shop vacuum system or an optional
vacuum pump on the press, this can be plumbed to the mold. Setting a vacuum
time on the screen causes the injection cycle to be delayed for that time period
while the vacuum valve is open to evacuate the mold. The vacuum valve then
remains open until the injection cycle has completed or the vacuum duration has
expired.
The rubber, which has been pre-heated by the screw and the injection process,
now cures in the heated mold. Electric heaters in the top and bottom platens
maintain the mold at the preset curing temperature. The cure timer assures that
the compound has sufficient time to cure at the particular temperature of the
mold. The mold (platen) temperatures and the cure time are programmable on
the OIT. Although it is possible, within narrow limits, to reduce the cure time by
raising the mold and injection temperatures, this can often lead to premature
curing (“burning”) in some areas and under-curing in others. In case of such
defects, reducing the mold temperature, reducing the screw and/or inject speeds
and/or temperatures and lengthening the cure time will often solve the problem.
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At the end of the cure time the following actions will take place automatically:
1. The hydralock cylinder will collapse to give the pressure column clearance
to move.
2. The pressure column will slide out from under the clamp bolster.
3. The mold will open.
After the mold has fully opened and stopped moving, the operator can remove
the cured rubber parts and runner waste from the mold.
The screw can be programmed from the OIT to start after a selectable delay from
the end of the injection cycle or after the clamp opens. This timing should be set
to allow the rubber to be in the inject barrel for a minimal amount of time before
the next cycle begins. Excessive time in the inject barrel can cause fast-curing
compounds to start to cure in the barrel, resulting in “burned” parts.
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If the operator leaves the area of the press
during an auto-cycle, they must turn the Press
Cycle Stop switch to ON. This will not interrupt
the cure cycle, but will prevent the press from
opening automatically at the end of the cycle.
This prevention is both for the safety of any
personnel who may wander into the vicinity of
the press and also to prevent the mold from
cooling excessively if the operator fails to return
and close the mold promptly.
When the
operator returns and turns the Press Cycle Stop
switch to OFF, the press will complete the cycle
normally, or, if the cure timer has already
expired, the press will automatically open. The
mold may also be opened by pressing the Inch
Open pushbutton (except during the cure
cycle).
On the Auxiliary Options screen, the press can also be set to “Inject Transfer
Mode”. This mode allows the use of transfer molds with the benefit of faster
cycle times due to the injection process.
NOTE: The transfer mold must first be modified to accept the injection nozzle by
drilling the top plate.
When the Inject Transfer Mode is on and the Inject Transfer Time is set on the
OIT the basic sequence of events in the injection cycle is modified. After the
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mold is closed and the pressure column moves under the clamp bolster the
hydralock does not expand. Instead, the injection stroke takes place first, filling
the mold transfer pot and overflowing into the mold cavity. After the inject stroke
is complete, the hydralock cylinder expands and continues to expand for the
duration of the Inject Transfer Time parameter set on the screen. This allows
time for the hydralock pressure to force the rubber from the transfer pot into the
mold cavity. After this transfer time has ended, the press auto-cycle completes
as described previously.
As an option, specific molds may be equipped with a “cold runner” system. In
this case, the top heating platen is not used to heat the mold. Instead, the top
half of the mold is often heated by electric elements installed directly into the
mold itself. These can be connected to the Aux heater zone control. A fourth
TCU control zone is required to keep the cold runner plate (above the top half of
the mold) cool enough to prevent the rubber from curing in the runner system.
Molds of this type are more expensive to build but reduce the wastage of rubber
compound in the runners.
In all molds, it is essential that the injection nozzle fit into the bushing in the mold
with a minimum of clearance. This is necessary to prevent liquefied rubber under
the high pressure of injection from building up under the injection tower. Such a
buildup of rubber under high pressure could damage the injection tower or other
press components.
Another precaution that must be observed regarding the potential force of the
injected rubber is that the operator must not begin an injection cycle with cured
parts remaining in the mold. If more rubber is injected into a mold that is already
full of cured rubber, the injection pressure can force the mold open to such an
extent as to damage the pressure column and/or hydralock cylinder.
Some presses may be equipped with fully automatic parts unloading
mechanisms and full guarding. These presses may self-initiate a new cycle after
each previous cycle is complete. In such cases of fully automatic or unattended
operation, the user organization is responsible for establishing additional safety
procedures to prevent injury to personnel or damage to equipment in the event of
any abnormal situations, which may develop or occur.
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