Machine States

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 machine jump.

AUTO CYCLE PREREQUISITES

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 is closed.
Ahh finally we are ready to start the cycle. As we close the door we step into:

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 platen.

• 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
MOLD PROTECT

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 here.

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 parallelism.

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 machine.

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 part.

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
SCREW DECOMPRESSION

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 distance.

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 side.

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.

AUXILIARY MACHINE STATES

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 the ejects.

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 recycle.

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.