Platen parallelism of an injection molding machine

Platen parallelism is how parallel the mold mounting faces of the moving platen and the stationary platen are to each other. On an injection molding machine it is checked by measuring the distance between the two faces near each corner: if the distances differ, the platens are not parallel and the mold closes on one side before the other. Good parallelism, with the clamp open and under clamping force, is what lets the mold close evenly and the four tie bars share the load.

Why platen parallelism matters

• Flash and part quality. If one side of the mold closes first, the clamping force is not spread evenly across the parting line, and the side that is held less tightly flashes.
• Mold wear. The mold's guide pins, bushings and parting line surfaces take loads they were not designed for, and wear or galling follows.
• Tie bar loading. The bars nearest the side that closes first stretch more, so they carry more than their share of the force and fatigue faster.
• Machine wear. The toggle pins and bushings, the tie bar bushings and the nuts are all loaded unevenly.

What causes a parallelism error

• Worn tie bar bushings, which let the moving platen drop or tilt.
• A worn or badly adjusted moving platen support.
• Wear in the toggle pins and bushings, which pushes the platen off line: see the toggle guide.
• Tie bar nuts or mold height adjusting nuts that are not set evenly, or one nut that has turned: see tie bar nuts and mold height adjustment.
• A machine that is not level, or a frame twisted by an uneven floor: see the leveling procedure.
• Damage on the platen faces, or a platen bent by an overload.

How to measure platen parallelism

The exact method, measuring positions and limits are in the machine manual or the standard the manufacturer works to. A general procedure:

1. Remove the mold, clean both platen faces and remove any burrs, so dirt does not affect the readings.
2. Check that the machine is level.
3. Move the moving platen to a set opening, often near the minimum mold height. Switch off and lock out the machine before reaching between the platens.
4. Measure the distance between the two faces at four points near the corners, each the same distance in from the edges, with an inside micrometer or a measuring bar and dial indicator.
5. Repeat at a second opening, for example near the maximum mold height. If the error changes along the stroke, look at the platen guiding rather than the nuts.
6. The difference between the largest and smallest readings is the parallelism error over the distance between the measuring points. Compare it with the manufacturer's limit.

These readings are taken without clamping force. To check the platens under load, clamp on parallel blocks of known thickness, or on a mold, and measure again as the manual describes. Tie bar strain sensors, where fitted, show whether the four bars share the force evenly, and pressure-indicating film between the mold halves shows where the parting line is touching.

Record the results each time. A slow change over the months is the earliest warning of bushing or toggle wear.

How a parallelism error is corrected

1. Find and fix the cause first: replace worn tie bar bushings or toggle pins and bushings, and reset the platen support. Adjusting nuts to make up for worn parts only hides the wear for a while.
2. Relevel the machine if it has moved.
3. Only then adjust the tie bar nuts or the individual mold height nuts, one at a time and in small steps, following the manufacturer's procedure, and measure again after each step.

Because nut adjustments change how the clamping force is shared, this work is usually done by the manufacturer's service team or an experienced technician. Our tie bar maintenance and replacement guide explains how uneven loading damages the tie bars.

When to check it

There is no single interval that suits every machine. Check parallelism when the machine is installed or moved, after work on the clamp unit or the tie bars, when a mold that used to run clean starts to flash on one side, and as part of preventive maintenance at the intervals the manual recommends.

What to send for replacement parts

When the measurements point to worn parts, send the machine brand, model and serial number, the part and its dimensions, and photos. Parts that commonly restore parallelism are tie bar bushings, adjustment bushings and the toggle bushings. Request a quote, and see the platens guide for the other platen features.