Hydraulic pressure gauge of an injection molding machine

A hydraulic pressure gauge is the dial instrument that shows the system pressure on an injection molding machine, usually mounted on or near the pump or the valve manifold. It is a mechanical gauge: a Bourdon tube inside the case tries to straighten as the pressure rises, and a small movement turns the needle. Most gauges on hydraulic machines have a glycerin-filled case, highlighted in the drawing, because the liquid damps the needle against pump pulsation and vibration. The gauge needs no power, so it is the simplest independent check of what the controller display says.

How a hydraulic pressure gauge works

The drawing shows the gauge from the front and with the dial removed. Its numbered parts are:

1. Glycerin-filled case: the sealed case and window. The glycerin damps the needle, and the plug on top closes the filling hole.
2. Dial: the printed scale, usually in MPa or bar (1 MPa = 10 bar).
3. Needle: sits on the pinion shaft and points at the pressure.
4. Bourdon tube: a curved, flattened tube closed at its tip. Oil pressure tries to straighten it, and the tip moves in proportion to the pressure.
5. Movement: a link, a toothed sector and a pinion that turn the small movement of the tube tip into the full sweep of the needle.
6. Connection: the threaded socket that screws into the gauge port. Inside the case it anchors the Bourdon tube and carries the oil into it.

Types and mounting

• Direct mount: the connection is at the bottom of the case and the gauge screws straight into a port on the pump, manifold or a gauge line.
• Panel mount: the connection is at the back and a front flange or bracket holds the gauge in a panel, connected by a gauge hose or tube.
• Range: the full-scale value. Choose one above the highest pressure the gauge will see, so the normal working pressure is well inside the scale.
• Accuracy class: the permitted error as a percentage of full scale. A class 1.6 gauge with a 0 to 25 MPa scale can be out by up to 0.4 MPa anywhere on the scale, so a range much higher than you need also reads less precisely.
• Fill liquid: glycerin is the usual fill; silicone oil is used where the temperature varies more widely.

The hydraulic pressure gauge we supply is glycerin-filled, reads 0 to 25 MPa with an accuracy of ±1.6 %, and is available for direct mounting or as a vertical, non-panel 25 MPa Tecsis gauge.

Many machines protect the gauge with a gauge isolating valve, such as a push-button valve that connects it only while pressed, or a damping restrictor in the connection. Both reduce the pressure peaks that reach the Bourdon tube.

Signs of a faulty pressure gauge

• The needle does not return to zero when the system is fully depressurized.
• The reading differs from a calibrated test gauge, or from the controller in a way it did not before.
• The needle sticks, jumps or shakes hard: the movement is damaged or the glycerin has leaked out.
• Glycerin leaks from the case, or has turned cloudy or dark so the dial is hard to read.
• A cracked window, a bent needle, or a needle that has gone past the end of the scale.
• Oil leaking at the connection.

Why gauges fail

• Pressure peaks from valves switching and from the accumulator, and overpressure above the gauge's range, which can bend the Bourdon tube permanently.
• Vibration, especially on gauges mounted directly on the pump.
• Heat from the power unit, which ages the case seal and the fill.
• Mechanical damage during maintenance and mold changes.

Checking and replacing a pressure gauge

1. Stop the pump and release all hydraulic pressure, including the accumulator if fitted. Do not rely on the suspect gauge to show that the pressure is gone.
2. Unscrew the gauge with a spanner on the flats of the connection, never by turning the case.
3. To check a gauge, fit a calibrated test gauge at the same point and compare the readings at several pressures, or send the gauge for calibration.
4. Fit the new gauge with the right seal for its thread: a sealing washer for parallel threads, or a suitable thread sealant for tapered threads.
5. If the filling plug has a vent, follow the gauge maker's instructions: some gauges must be vented after fitting so pressure inside the case does not affect the reading.
6. Pressurize slowly and check for leaks.

The pressure test in our article on troubleshooting hydraulic pressure loss uses a calibrated gauge to tell a pump or relief valve problem from a fault further down the circuit. For the electronic sensor that the controller reads, see the pressure transducer.

What to send for a replacement quote

1. The range and units printed on the dial, and the accuracy class.
2. The dial size, and whether the connection is at the bottom or the back.
3. The connection thread size and type.
4. Direct or panel mounting, and the flange dimensions for a panel gauge.
5. The fill liquid, and photos of the dial and the connection.
6. The machine brand and model.

Request a quote, and see the hydraulic system guide for where the gauge sits in the circuit.