Toggle crosshead of an injection molding machine

The toggle crosshead is the block on the end of the clamping cylinder's piston rod that drives the toggle mechanism of an injection molding machine. It moves in a straight line along the center of the machine, between the upper and lower toggles, and pushes or pulls both of them at once through the short connecting links. When the crosshead moves forward, the links straighten and the mold closes; when it moves back, they fold and the mold opens. In the drawing it is the block on the cylinder rod, between the two toggles.

How the crosshead works

The clamping cylinder is mounted on the rear platen, and its piston rod is joined to the crosshead by a thread, a nut or a flange, depending on the design. Above and below the axis the crosshead has bores for the pins of the short links, each running in connecting rod bushings. Because one block drives both toggles, they fold and straighten symmetrically, which keeps the moving platen square.

On many machines the crosshead slides on guide rods or in a guide fixed to the rear platen. The guide keeps it moving straight and takes side forces off the cylinder rod, so the rod only has to push and pull. The guides and the crosshead's own bushings need lubrication like the rest of the toggle: see toggle lubrication.

The relation between crosshead travel and platen travel is not constant. Mid-stroke, a short crosshead movement moves the platen a long way; close to lock-up, a long crosshead movement moves the platen very little, with a very large force. That is why the clamp speed and position settings near lock-up are so sensitive, and why errors in the crosshead and its links show up as uneven locking.

Variants

There is no standard crosshead. Its shape and size depend on the toggle design and the machine's clamping force: a five-point double toggle has a central block with pivots for the short links of the upper and lower toggle, often in pairs on either side. Other toggle layouts use differently shaped crossheads.

The toggle link and crosshead parts we supply are made of 42CrMo or 40Cr alloy steel at HRC 28–35, with precision-bored bushing seats and oil-retaining bronze or self-lubricating bushings, for Haitian MA, MA II and MA III machines.

Signs of a worn crosshead

• A knock at each change of direction, at the start of closing or opening: often a loose rod connection, or worn bushings at the short link pins.
• The crosshead tilts or wobbles as it moves, and the upper and lower toggles do not lock at the same moment.
• Uneven tie bar stretch or flash on one side of the mold.
• Scoring on the guide rods, or play in the crosshead guides.
• Oil leaking at the clamping cylinder's rod seal, or scoring on the rod: a sign of side load from a crosshead out of line. See rod seals.
• Cracks at the pin bosses or around the rod connection.

The usual causes are lubrication failure, a rod connection that has worked loose, worn short-link bushings left in service, and overload.

Inspecting the crosshead

1. Stop the machine, lock it out and release the hydraulic pressure, including any accumulator.
2. Check the connection between the piston rod and the crosshead: look for fretting marks, a loose lock nut or a gap. If you also watch it while the clamp moves, do so at slow speed and only from outside the closed guards. Any movement between rod and crosshead means the connection must be reset.
3. With the clamp depressurized, check the play at the short link pins and at the crosshead guides with a pry bar and a dial indicator.
4. Inspect the guide rods for scoring and check that lubricant reaches every point.
5. Clean the crosshead and look for cracks around the bores and the rod connection.

Replacing the crosshead

The crosshead is heavy, and it holds the short links in place. Remove the mold and support the toggle links and the crosshead with lifting gear before removing the short link pins, then disconnect the crosshead from the piston rod as the machine manual describes. When fitting the new crosshead:

• Fit new bushings in it and in the short links, and inspect the toggle pins.
• Check that the crosshead lines up with the cylinder rod, so the rod is not pushed sideways.
• Tighten and lock the rod connection to the torque in the manual.
• Run the lubrication until fresh lubricant appears at every point, cycle the clamp slowly at low pressure, and check that both toggles lock together before resetting the clamping force.

What to send for a replacement quote

• The machine brand, model, clamping force and serial number.
• Photos of the crosshead from several sides, and of the rod connection.
• The main dimensions: the diameter and spacing of the short link pin bores, the rod connection (thread size or flange bolt pattern), the guide bores if there are any, and the overall size.
• The OEM part number, if you have it.

See the toggle mechanism guide for how the crosshead works with the links, and the toggle links page for the links it drives.