Mold height adjustment, also called die height adjustment, moves the rear platen and the toggle along the tie bars of a toggle injection molding machine, so that the toggle locks straight just as the mold halves are pressed together with the right force. It is set every time a mold of a different thickness is fitted, and on a toggle machine it is also how the clamping force is set. The adjustment is made by threaded adjusting nuts, the adjustment bushings in the drawing, on the end of each tie bar behind the rear platen.
Why a toggle machine needs mold height adjustment
A toggle has a fixed stroke: from fully folded to fully straight, it always moves the moving platen the same distance relative to the rear platen. Molds come in different thicknesses, so the whole toggle has to be moved forward or back until its closed position matches the mold. The data sheet gives the range as the minimum and maximum mold height.
Once the mold halves touch, the toggle still has a short way to go before it is straight. Forcing it through that last distance stretches the tie bars, and the stretch is the clamping force. So the adjustment does two things at once:
- it sets the rear platen position to suit the mold thickness;
- it sets how far the bars stretch when the toggle locks, and therefore the clamping force.
The clamping force guide explains how much force a mold needs. Direct hydraulic clamps, with a single large cylinder, do not need this adjustment, because the cylinder can stop anywhere in its stroke and the force is set by the pressure. Two-platen machines lock the moving platen to the tie bars at whatever position the mold needs.
How the mechanism works
- Each tie bar has a threaded end behind the rear platen. An adjusting nut, the adjustment bushing, runs on that thread and bears against the rear platen.
- On machines with powered adjustment, each nut usually has gear teeth around its outside, and all four are driven together by a central gear ring or a chain, turned by a hydraulic motor or an electric gear motor.
- Turning the nuts screws the rear platen, and the toggle and moving platen with it, forward or back along the bars.
- A sensor, often a proximity sensor counting gear teeth, or an encoder, tells the controller how far the nuts have turned.
Smaller and older machines may have manual adjustment, with the nuts turned through the same gear by a hand wrench. Either way the four nuts must always turn by the same amount: if one slips relative to the others, the rear platen tilts and the bars no longer share the load evenly.
Automatic mold height adjustment
Many current controllers have an automatic function: the machine closes on the mold and moves the adjustment step by step until the set clamping force is reached. How it judges the force, from the clamp's position and pressure or from strain sensors on the tie bars, depends on the machine. Some machines also correct it during production. This matters because the mold grows slightly as it heats up: on a toggle machine a thicker mold means more tie bar stretch, so the clamping force rises after start-up unless the adjustment is corrected.
Signs of trouble
- The adjustment is slow, stalls, or the motor runs while the rear platen does not move: a slipping gear, a sheared key or a broken chain.
- The automatic adjustment cannot reach the set force, or the force drifts more during production than it used to.
- Flash on one corner, or a platen parallelism error, after an adjustment: the nuts may be out of step.
- Knocking or play at the rear platen when the clamp locks, from worn threads in the adjusting nuts.
- Adjustment alarms or wrong position readings from the counting sensor.
Typical causes are dry or dirty threads and gears, adjusting while the toggle is locked under load, and worn adjusting nut threads.
Maintenance and replacement
- Lubricate the threads, the nuts and the gear or chain as the machine manual specifies, and keep dirt off them.
- Adjust only with the clamp open or unloaded, as the controller normally enforces. Never force the adjustment against a locked toggle.
- Check the chain tension or gear mesh, and that all four nuts have turned by the same amount: paint marks on the nuts make this easy to see.
To replace adjusting nuts or adjustment bushings, the rear platen has to be supported and the nuts removed and refitted in exact relation to each other, so that the rear platen comes back square to the tie bars. Mark every nut and the gear before dismantling, follow the manufacturer's procedure, and measure parallelism and clamping force afterwards. If the tie bar threads under the nuts are damaged, see our tie bar maintenance and replacement guide.
What to send for a replacement quote
Send the machine brand, model and serial number; the tie bar diameter and the thread size, pitch and hand; the adjusting nut's outside diameter and length; the number of gear teeth and the gear module or pitch if the nut is geared; and photos of the old part. Request a quote, or see the tie bars guide and the tie bar nuts page for the rest of the assembly.
Updated by the IMSPARES technical team.
