Injection piston rod of an injection molding machine

The injection piston rod is the hardened, chrome-plated steel rod that joins the injection piston to the moving part of the injection unit and carries the injection force. In the drawing it is item 5: it runs from the injection cylinder (4) on the fixed plate to the moving plate, where the lock nut (6) holds it, and the moving plate carries the screw drive along the guide columns. Because the rod also slides through the cylinder's rod seals on every stroke, its surface condition decides whether the cylinder stays oil-tight.

Where the piston rod sits

One end of the rod is fixed to the piston inside the injection cylinder, usually with a piston nut. The rod leaves the cylinder through the gland, where the rod seals and wiper are fitted, and its other end is threaded into or through the moving plate and locked with a lock nut. On single in-line injection units the rod connects instead to the screw drive behind the barrel; the injection cylinder page compares the two designs, and the injection unit guide shows how they fit in the cycle.

How it works

The rod transmits the force of the oil on the piston to the moving plate, and with it to the screw. During injection and hold it carries the full injection force; during decompression and back pressure it carries smaller loads in either direction. The load changes direction and size every cycle, which is why the threads and the lock nut must be tight and in good condition.

The rod is made to resist three things:

• Load: an alloy steel core, hardened and tempered, that carries the force without bending.
• Wear: a hard chrome surface that the rod seals and the guide ring in the gland slide on.
• Corrosion: the chrome layer protects the steel from rust when the rod is extended and exposed.

The injection piston rod we supply is 42CrMo alloy steel with a hard chrome plated surface, listed at HRC 55–60.

Signs of wear or damage

• Oil on the rod or dripping from the gland, often the first sign of a damaged rod surface or worn rod seals.
• Scratches or score lines along the rod, usually from dirt carried past the wiper or from a worn guide ring letting the rod touch metal.
• Pitting, rust spots or flaking chrome, often after a long stop in a damp place with the rod extended, or from water in the oil.
• A bent rod: uneven seal wear on one side, a rod that does not run true, or a cylinder that binds.
• A loose rod end: knocking at the start of injection or when the movement reverses, or a lock nut that has moved.
• Damaged threads at either end.

Inspecting the piston rod

1. Release all hydraulic pressure, lock off the machine and clean the rod.
2. Look along the full length of the exposed rod under good light. Run a fingernail across any mark: if it catches, the mark will cut seals.
3. Check the lock nut and the rod end for movement, and look for fretting marks around the nut.
4. With the rod removed, measure its diameter at several points along its length and around it with a micrometer, and compare with the original size and with the gland.
5. Check straightness by supporting the rod near its ends, for example on V-blocks, and turning it under a dial indicator. Compare the result with the limits in the machine manual or drawing.
6. Check the threads at both ends.

Repair or replacement

Light surface damage on a straight rod can sometimes be repaired by grinding off the chrome, re-chroming and grinding back to the original diameter. It is worth considering on large rods; on smaller ones a new rod is often the simpler choice. A bent rod, cracked threads or deep scoring mean replacement.

When you fit a new rod:

1. Fit new rod seals, wiper and guide rings at the same time; see rod seals and cylinder seal replacement.
2. Protect the chrome surface while handling it, and cover the threads when sliding it through the seals.
3. Fit the piston and its nut, and the lock nut at the moving plate, tightened to the torque in the machine manual. The piston rod nut page covers the nut at the piston, and the lock nut we supply is made for guide columns, shafts and piston rods.
4. Check that the moving plate runs freely on the guide columns before applying pressure, so the new rod is not bent by a misaligned plate.

Making the rod last

• Keep the hydraulic oil clean and the wiper seal in good condition.
• Replace worn guide column bushings before they let the plate load the rod sideways.
• Wipe the exposed rod and protect it during long stops.
• Check the lock nut at planned maintenance.

What to send for a replacement quote

1. The machine brand and model, and the injection unit size.
2. The rod diameter and overall length.
3. The thread size and length at both ends, and the shape of the piston end.
4. The cylinder bore, and whether the unit has one or two injection cylinders.
5. Photos of the rod and its ends, and the OEM part number or a drawing if you have one.

Request a quote with these details.