What is a servo-valve?

A servo valve is an electro-hydraulic operated valve that electrically controls how hydraulic fluid is supplied to an actuator. Positional movements by an hydraulic cylinder can be set by a varying analogue or digital control signal providing precise pressure, position, velocity and force for the valve. Signal can be also applied to ensure any movement is conditioned by a dampening effect.

What happens if I fit a proportional valve or servo valve with the wrong flow rate on a blow moulding machine?

Moog proportional and servo valves come in a number of different flow rates.

Generally speaking, low flow is used on very small machines while high flow are fitted on larger machines. It is not always obvious if you have fitted one with the incorrect flow rate. Often the error may be indicated by a slight reduction in repeatability.

What does a parison programming system comprises of?

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

What are the typical tests carried out on Moog proportional and Moog servo valves?

All our proportional and servo valve tests are carried out to the original manufacturers published specifications. The Typical tests carried out on a Moog proportional or a Moog servo valve are:

  1. Input Signal/Flow
  2. Flow Linearity and Symmetry
  3. Null bias
  4. Input Signal/ Pressure
  5. Pressure Gain
  6. Internal and External Leakage Stability
  7. Coil resistance and Coil Insulation

What are the typical Parison Programmer faults?

It is very rare that the parison programmer has a genuine fault – its nearly always something else!


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.

Cable damage

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

Hydraulic accumulator

If you are getting sluggish response it is worth checking the hydraulic accumulator.

Virgin-regrind mix

If you are trying to produce a much lighter container than normal you might find that there is a greater variation in weight than you can accept. On the subject of weight variation the biggest culprit is inevitably traced to variations in the virgin-regrind mix.

Moog mould movement servo-valve faults

When these servo-valves go faulty it is normally only the pilot stage that is blocked with dirt.

If you suspect the servo-valve is faulty remember to check that the safety solenoid (if fitted) is energised and that the +15V and –15V supply to the servo-valves electronics are present and correct.

If you find that the servo-valve you are fitting (either new or repaired) does not work look to see that the pilot pressure and pilot return are correctly ported. This may involve moving grub screws from one port to another as these are not always factory set to the machines porting.

Many machine have a separate filter in the pilot pressure line and if this is partially blocked the Moog servo-valve will not work properly.


Parison programming

Is there a surcharge for while you wait servo valve repairs?

We do not apply a surcharge for while you wait servo valve repairs.

What is your average turnaround for a servo valve repair?

We can usually turnaround in about 2 days.

Does Robin Enderby Associates represent Moog?

As of a few years ago we no longer represent Moog. This change had absolutely no effect on our activity of selling Moog products. We still sell and support the older Moog 25 point slider type of Parison programmers. We have a massive stock of complete systems and spare parts for what is considered the most user friendly Parison programmer ever. We have an in-house repair service for servo valves. A typical repair price is £165 with a 24 hour turnaround. Moog no longer supplies valve spares, but we have a large stock of genuine Moog parts and full specification data from the time before the agreement ceased.

How is a servo-valve used in the blow moulding process?

A conventional extrusion blow moulding machine incorporates an annular die gap through which the moulding material is extruded to produce the tubular parison which is subsequently subjected to blow moulding to produce a product which is outputted from the machine.

The die gap can be controlled by a movable member which is displaceable by supply of hydraulic fluid to a cylinder under the control of an electrohydraulic servovalve. The servovalve is under the control of a parison programmer which is set by the machine operator to the desired wall thickness and weight settings and which receives a feedback signal from a transducer indicative of the actual position of the movable member, and which supplies an output signal to the servovalve to effect movement of the movable member to a desired position corresponding to the desired width of the die gap.

Furthermore the parison programmer may be programmed to vary the wall thickness during extrusion of the parison so that the wall thickness of the generally tubular parison varies along its length in the manner required by the form of the final moulded product, that is to take account of features such as the shaped bottom and the necked outlet of a container, for example.