Parison Fault Finding ¦ Trouble shooting

 

Parison ProblemsTypical Faults

Before doing anything ask yourself this question – Was the machine running normally when the problem occurred?

  1. If you can’t get a straight YES then there is a good chance that the fault is man-made.
  2. Have you changed any hydraulic parts, altered any wiring, changed any hoses, adjusted ANYTHING?
  3. If the machine was running normally when the parison programming system went wrong then read on……

Parison programming system

A parison programming system comprises of:

  • The programming unit (Moog, Hunkar, Femit, Maco Breeze Profiloc PR30 etc..)
  • A servo-actuator – the fancy name for the hydraulic cylinder- servo valve – LVDT (sometimes called DCDT)
  • A hydraulic supply
  • If it is an accumulator head or recip-screw machine then these will also be a long-stroke transducer measuring the shot position.

 

Servo Valve Failure

Servo valves are high on the list of being the cause of problems, partly because they are dirt sensitive, and partly because so many companies have inadequate maintenance standards.

  1. Always change filter elements when the dirt alarm switches or the pop up shows. The longer you leave it the nearer you are to machine Armageddon.
  2. If you have more than 5 machines buy a servo valve tester (our’s come with the best instructions!) Plug it in the valve and the cylinder should move.
  3. If the cylinder moves then there is every chance that the servo valve is OK This simple test only shows the valve is basically functional, it won’t show some of the more subtle faults the valve can have.
  4. If you get no movement then the valve is probably faulty, but at this point you had better check the pressure is present at the “P” port.
  5. Some servo valves can be accidentally fitted 180 degrees out of position (Moog 62, 76 and 760 series for example). If the hoses have been off for any reason check they are the correct way round.
  6. If the pressure is on the correct port of the servo valve and the cylinder rod is not seized (very unlikely) then the servo valve is probably faulty.

If the cylinder moves with the test box then:

  1. Check the LVDT (DCDT) transducer signal is getting to the programmer. The voltage should change when you move the cylinder with the test box.
  2. Actually these transducers don’t usually go faulty – most failure is due to physical damage

Cable

Cable damage is common. Cables melt when they touch the head or extruder barrel. Wires often get pulled out of plugs.

Remember- it’s very rare that the parison programmer has a genuine fault – its nearly always something else!

Cards

  1. Resist the temptation to fault find by swapping circuit cards. Very often a different card will be set up differently so it might seem to behave ‘differently’ and make you think you’re on to something- you’re not – and are in danger of chasing a ‘red herring’.
  2. Some cards might be linked differently so plugging them in another unit can ‘blow up’ the card. You now have two units not working!

Hydraulic Pressure

The actual hydraulic pressure is not very important. Some machines (e.g. Hayssen) operate the parison programmer at below 1000 psi (70 bar) most machines run at 1500 psi (105 bar). If you are getting sluggish response it’s worth checking the hydraulic accumulator.

Sometimes when you run a new job for the first time you may find that the programmer doesn’t seem to do what you want, and you then suspect it may be faulty. This isn’t necessarily the case. It could be that the plastic pressure acting on this larger die (pin) is too great for the cylinder to control – if this is the case you won’t be able to get the weight down low enough.

Weight variation

  1. If you’re trying to produce a much lighter container than normal you might find that there is a greater variation in weight than you can accept – there are modifications that can be made to optimize the programmer’s performance for thin walled bottles.
  2. On the subject of weight variation the biggest culprit is inevitably traced to variations in the virgin-regrind mix. Try running a few hours on virgin only – if this solves the problem then it’s not the parison programmer.
  3. Another way of eliminating the programmer from the cause of weight variation is to lock the die to a fixed gap. How you do this is dependant on the head design. On most Kautex, Bekum and Battenfeld-Fisher machines there are settable limit stops on the parison control cylinder.
  4. If you unplug the servo valve, the cylinder will drift until it reaches one of the limit stops. All you have to do is to set the stop to the position that will hold the die gap at a setting that produces the correct weight moulding. Obviously you will be making parts without the benefit of a parison programmer but it will prove if the programmer is the cause of weight variation !

Beware

IF the parison programmer IS faulty and it’s an old model it’s very likely that the manufacturer will tell you it’s obsolete and un-repairable. Of course this might be true, and it might also be that this is how the manufacturer gets you to buy a new ‘box’. Phone some of the independent repairers (us) and get a second opinion. You might even pick up a refurbished unit – what’s the point of putting a brand new parison programmer on a 20 year old machine?