In the course of troubleshooting it is
very useful to break the machines cycle down into basic states so that
we can quickly analyze the problem at hand and get our machines back
on line. There are certainly dozens of individual steps that take
place in any cycle but the purpose of this article is to compile those
steps and break them down into 10 easily recognizable steps and a
couple of auxiliary steps (that we will include for those running
cores and insert jobs) that are basic to all molding equipment. This
article is not meant to replace the owner’s manual of your machine.
You will typically find a very thorough step by step description of
both the hydraulic and electrical sequences that apply to that
particular machine. Lets get started.
This article takes into consideration that you currently running a
normal cycle on either a toggle or standard hydraulic clamp with a
single reciprocating screw. That is without sprue break or nozzle
valve function. References to cores are also left out due to the many
different configurations that may be applied depending on the mold.
Between some of the key machine states (ie...MS 3 and MS 4) there are
usually time delays to allow the pressure and or volume of the oil to
“relax” before going to the next machine state to prevent system
shock that could break hoses, shift valving too hard, and make the
We will begin with the items necessary for an auto cycle. Motors are
running and temps are up with acceptable parameters for both the clamp
and the injection halves of the machine dialed into the control.
• The shot size must be made and on some machines the amount of
injection decompression that is dialed in must also be made.
• The clamp must be fully open to its preset limit.
• The eject typically must be retracted to its limit. In some cases
the machine is smart enough to know if you have it all the way forward
it can begin to cycle. And on some machines with potentiometers that
can feed back to the control the exact position it can begin cycle
from any position. But as a rule of thumb for all machines have the
eject back to the retract limit.
• If iniating a cycle the gate must be open .
• There are a few signals that will prevent the machine from going
into cycle that are overlooked. If the machine has a good alarm or
fault system it will let you know what these are. If not you need to
know them and more importantly your back shifts need to know them to
prevent downtime. If the press will not go into an auto cycle check
the following if your press is so equipped.
• Oil level, temperature, and cleanliness indicators. Theses will
send a signal back to your control system saying it is either unsafe
or unwise to run an auto cycle.
• Injection Barrel under or over temp by the allowed percentage.
• The proper sequence of gate and jam bar switches. On most machines
when you open the gate the control has to see a change in the feedback
signal from both the gate and jambar switch. On some machines these
have to be in order and within a certain time period of each other so
the control does not think some one is cheating the safeties. Often
you will find the jam bar adjusted so it does not clear the flag that
lowers to mechanically prevent the forward movement of the clamp if
the gate has been opened. Either it was adjusted and not locked down
properly allowing it to shift position. Or on ratchet jam bars you
usually have to be in the lower part of the ratchet to allow the
switch or switches to change states.
• The purge guard closed switch must indicate that the purge guard
Ahh finally we are ready to start the cycle. As we close the door we
MACHINE STATE - ONE
CLAMP CLOSE FAST
At this point the ejects if forward are retracted. Oil is diverted
through a series of ports to the clamp cylinder on a toggle, to the
traverse cylinders on direct hydraulic machines with outriggers or to
the “jack ram” on hydraulic clamps without outriggers. The volume
and pressure are ramped up to prevent a jerk as the moving platen and
mold moves forward. This is either done by delaying the high volume
pump from coming in for a short period of time to allow the mass to
start to move or to ramp up a proportional directional valve to meter
the oil to the clamp slowly at first increasing the flow until is at
the preset rate. The clamp moves forward at this rate until the clamp
forward slow down limit is actuated.
This is done a number of ways through the use of hard wired limit
switches that actuated by an arm or a cam that is attached to the
moving platen. Or by a linear potentiometer. On a linear potentiometer
the shaft protruding from the housing is attached to the platen and
pulled back and forth to produce a voltage feedback signal typically 0
to 10 volts that is converted to a measurement value. IE......20
inches of stroke = 10 volts and 10 inches of stroke = 5 volts. The
distance to voltage comparison depends on the machine you are working
with. Also there are other configurations such as the “MTS
Temposonic series ONE” that does not have a housing, instead the
shaft has the electronics built on the end of it and the shaft is
pulled through a magnetic field that is attached to the clamp cylinder
• If you do not seem to be slowing down check the feed back signal
from the measuring device. On newer machines also ensure the Mold
Protect Pressure reducing circuit is active during slow down.
MACHINE STATE - TWO
Once the slow down limit for the clamp is reached the pressure is
reduced to a preset limit and the flow is reduced to allow the clamp
to finish closing if there are no obstructions in the mold such as a
part that did not eject. If the mold comes to a stop before reaching
the fully closed limit or expires the amount of time that is allowed
between the time the slowdown is first made and injection starts the
clamp will stop, open back up and an alarm will go off. This timer is
typically accessible by the setup personnel and can typically be set
between 1 and 30 seconds. An exception are machines with a try again
cycle that will allow the machine to open up, go through the eject
cycle and close again.
• If you have changed molds and the clamp stops before closing fully
due to a longer slow down period you may want to start troubleshooting
MACHINE STATE - THREE
CLAMP LOCK UP
When the mold halves are fully closed you will activate a preset limit
indicating the mold is shut and will iniate clamp lock up.
It is important to note that in a many times you will find limit this
is set too far from the actual fully closed point. Too large of a
setting can allow the machine to apply full pressure to the clamp
mechanism before the halves are closed. This can cause slamming of the
mold, mold damage, and erratic cycles.
When MS 3 is active, the hydraulic system will apply the preset
pressure and volume to the clamp cylinder on a toggle, moving the
mechanical linkage into the locked position. On a direct hydraulic
machine you should here a very distinctive “clunk” sound, that is
the prefill shifting. When the prefill shifts, oil is applied to main
ram area, and seals off the tank from the main ram. When enough
pressure is applied to the ram area to activate a pressure switch that
indicates there is enough tonnage to allow the injection cycle to
begin the high volume of oil that was being used to build tonnage
quickly, is reduced to a lower volume and the pressure continues to
build until it reaches the setpoint of full tonnage.
• If you do not hear the prefill piston shift at this time and
cannot build pressure there is a good chance your trouble lies in the
prefill not shifting.
• On a toggle machine the pressure is usually limited to around 1500
psi (check your print). If you have a history of pin breakage, you
should check the toggle over pressure that is being achieved during
lockup. If you have a history of tie bar breakage check clamp
MACHINE STATE - FOUR
INJECTION FORWARD (BOOST)
During this state the clamp has fully closed and enough pressure is
holding the mold closed that the machine believes it can inject the
melt without blowing the mold open. Weather or not it really can
depends upon the required tonnage to hold the mold shut, the pressure
switch setpoint for injection start, full tonnage setpoint, injection
pressure, injection speed, melt temp, the condition of the tool, and
other parameters such as clamp parallelism. The purpose of this
article is not to get into the molding parameters required to make a
good part but to show the different cycles of the basic molding
Once the start injection pressure switch on a direct hydraulic machine
or the crosshead fully forward set point is made on a toggle press and
the delay inject timer has elapsed, the machine will divert oil to the
injection cylinder(s) to move the screw forward. The pressure and
volume of this oil is determined by the setup personnel. Once moving
forward the screw tip should seat off to prevent the backflow of the
melt across the tip. At the point the melt has volumetrically filled
the tool without packing out the part you should transfer to machine
state five either by reaching the transfer position switch (25 LS on
Milacron equipment), reach the cavity pressure transfer point, reach
the hydraulic transfer point, or run out of time on the boost timer.
Once again this is not an article about processing about cycles, but I
think most everyone will agree the position transfer method is most
preferred and provides repeatable results. If a more exact transfer is
desired use cavity transfer. It monitors the pressure actually in the
cavities and allows the process to transfer to low injection at the
most precise point in the filling of the tool.
MACHINE STATE - FIVE
INJECTION PACK and or HOLD
This is probably the most critical state of the part cycle in terms of
repeatability. It is important to realize that you must switch to this
state at precisely the same point of filling out the part every shot,
and ideally either the part be full but not packed out or at maybe
just a little bit short. At this point you do not need a great deal of
volume to finish pushing the melt into the cavities but do need a
consistent pressure. The volume therefore is usually not adjustable by
setup personnel. The oil typically takes the same path to the
injection cylinder(s) at the factory set volume. The amount of
pressure is dependent upon part requirements to finish packing enough
melt into the cavity to bring the weight of the part to the required
limit or to be able to maintain the dimensional criteria of the part.
The last thing is to set the gate up to prevent back flow out of the
MACHINE STATE - SIX
EXTRUDER RUN (COOLING TIME)
During MS 6 the oil is directed to the extruder motor to turn the
screw and “pump” material to the front of the barrel while mixing
any additives such as color into your melt to create a homogenous
melt. This is highly simplified since a lot of newer screw have mixing
heads and the L/D ratio will greatly determine the machines capacity
to accomplish this feat. The oil being directed is almost always
volume controlled and not pressure controlled to maintain the speed of
recovery of the feed screw. What I would like to cover is BACK
PRESSURE. A lot of people think it is produced by the machine, while
in reality it is developed by the screw acting as a pump. While you
are building up material in front of the screw tip and there is no
more room for that material it begins to push back on the front of the
screw. This pushing effect moves the hydraulic injection ram(s) back
since it is attached to the screw. The amount of oil that is being
pushed out of the inject forward side of the ram is reduced and as it
is the force on the front of the screw has to increase to overcome the
pressure that builds up in the ram due to the oil not returning to
tank freely out of the injection ram. To increase the pressure on the
front of the screw, the screw must “pump” the melt with more force
thus increasing the shearing effect on the melt. The pressure is
controlled typically by the same main pressure relief valve that is
used to control injection pressure. On older machines they use
separate pilot heads. On newer machines they use a electronic pilot
head to control the main relief head and just send a signal out
proportional to the amount of the back pressure desired. On some newer
machines they use a separate backpressure relief valve.
If you are reading 150 psi of back pressure and you have a typical 10
to 1 ratio of injection ram surface area to screw diameter surface
area then you have 1500 psi of plastic pressure pushing back on the
screw. If you increase the back pressure to 200 psi the screw has to
obtain 2000 psi of force. Another thing to note is that the point the
machine reads back pressure from is typically the same point the
machine reads injection pressure from.
Also while the screw is running and screw is going back a void is
created at the area on the front of the injection ram. This is
typically filled with exhaust oil from the extruder motor. This is
often a curious circuit since we often think of the hydraulic system
as producing pressure. In this case a vacuum is applied to draw the
oil into the cylinder. On some cartridge valve machines they will use
a 2:1 cartridge (x area twice that of the B area) and use the vacuum
to pull through the X-port of the cartridge valve to actually pull
open the cartridge valve and allow oil to be pulled from the a to the
b port to fill the void.
MACHINE STATE - SEVEN
Once the shot size limit switch is reached the screw will stop
spinning and a factory set amount of oil pressure and volume is
applied to the front of the injection rams to push the screw back and
reduce the pressure on the front of the screw to prevent drool when
the mold opens. Another way to see this is to remember if you are
running 150 psi of back pressure when the screw stops spinning you
will have 1500 psi of plastic pressure at the point in time the screw
stops pumping material. Most of this will go back across the tip to
flight area of the screw (remember the screw tip is not seated at this
time). But there is still a considerable amount of pressure built up
in front of the tip or in the residence area as some molders call it.
There are two ways this is typically controlled either through time or
With time controlled the timer is started either at the point the shot
size is made or when the cooling timer times out and before the mold
opens. The timer controls the time the oil is applied to decompression
circuit. With distance controlled the machine will typically start the
decompression circuit just as it does with time control but will bring
the screw back to a preset limit.
MACHINE STATE - EIGHT
CLAMP BREAKAWAY(Open Slow)
This state will typically start at the end of decompression and the
cooling timer has timed out.
On a hydraulic machine the main ram area of the clamp is depressurized
before the prefill valve is allowed to open. Once it has been
sufficiently depressurized and the prefill is open oil is applied to
the open lands of the clamp ram at a preset flow and pressure to
retract the ram. The oil from booster tube area is directed to the
tank. On a hydraulic machine with outrigger cylinders the oil is
applied directly to the cylinders. There is not usually any open lands
on a main ram that uses outrigger cylinders.
On a toggle machine the oil is applied to the crosshead cylinder to
retract the mechanical linkage at a slow rate.
The oil from the close side of the cylinders is directed back to tank
at this time.
The purpose of breakaway is allow the mold to separate and the part to
be transferred to the desired side. For example most molds opening up
will typically transfer the part to the core side from the cavity
MACHINE STATE - NINE
CLAMP OPEN FAST
The path of oil to open the ram basically remains the same during this
state with the exception of increasing the volume to an operator set
point to increase the velocity of clamp open. Often a machine will
take the oil from the close end of the ram and regenerate the energy
into the volume of oil going to the opening lands which will increase
the speed of clamp open. You must pay close attention to your machines
prints to determine how this maximum speed is obtained to determine
the cause of a slow clamp. For instance you may have a proportional
directional valve for velocity of clamp open oil and use a throttle
valve to control the oil from the pump circuit itself. The throttle
valve will be typically wide open and you will be controlling the
speed with the proportional directional valve.
MACHINE STATE - TEN
CLAMP OPEN SLOWDOWN
The purpose of slowdown is to enable the clamp to slowdown and stop at
a precise position to allow a robot to interface, and to prevent
excessive shock when the mass of the moving platen and moving half of
the mold stops suddenly.
The path of oil as in the two previous steps remain basically the same
with the exception of decreased volume and usually decreased speed.
The regeneration if used in state nine is also dropped off.
Core circuits - If running cores you have set them at some point
before the injection process starts. Your process may require you to
set and or pull them before clamp close, during clamp close, or after
the clamp has fully closed.
Eject forward - If running cores you will have to pull them until the
limit switch is made or the time has expired (if you are running timed
core sequence) before the control will allow eject forward sequence to
initiate. The eject will then go forward until the forward limit is
made. This may happen at any time after the clamp has opened and
passed the minimum limit of clamp open stroke to allow operation of
Eject retract - When the ejects reach the forward limits they will
retract until the retracted limit is reached. Your machine may have an
forward hold time on it. Your process may also have multiple ejects
for the particular process you are running.
Insert molding - If running an insert job the machine will retract the
ejects and sit there until you open and close the gate. At this time
the machine thinks you have placed the insert to be molded around in
the mold (some molds have a device built into them to allow detection
of the insert and will not allow the machine to cycle via a feed back
to the control). Also the insert function is typically disabled if you
are in automatic.
After completion of all machine cycles your machine control will allow
The importance of recognizing these individual steps cannot be
stressed enough. My suggestion is you take a machine in your shop and
get the prints to that machine. Make sure they are the right prints!
You need to use the most common machine in your shop. Go out to the
machine and first of all establish each of the machine cycles as
stated above on your machines bar chart. Your bar chart is probably
already broken down into states. Go around and familiarize yourself
with all the components used for each of the machine states. If this
is your first machine to do this with it should take you about a day.
You need to do this so as you trace out the hydraulic prints you can
visualize where each component is.
Then take the hydraulic print and trace out each of the circuits. A
neat trick is to make ten copies of the hydraulic print and trace them
out, one for each machine state. Your goal here is to be able to go
directly to the components that drive a particular machine state. This
includes knowing how each device is ported and tubed. This step will
take you 3 or 4 days if this is your first machine.
After we fully understand the machine from a hydraulic point we must
finally turn to the electric prints and determine how each of the
devices in the hydraulics are electrically driven. Your goal here
should be to determine where the signal for the device originates from
and how it gets to the device as well as being able to look it up on
the print. This is an additional 2 days.
During this self training period if you do not have a record of the
normal LED operation of your controller you need to do so now.
Understanding the “Wake Up” and “Running” diagnostics of your
controller is as important an knowing how to troubleshoot a machine
that is running.
Once you understand how to breakdown these machine states on a sample
machine, having full understanding of its prints and circuits you can
quickly learn other machines as you need to. When you are done,
training others to quickly learn this method is easy to do. Most
importantly when you have a problem with your press the amount of time
needed to troubleshoot it will be minimized.