Toggle links, also called toggle arms or link rods, are the steel levers of the clamping toggle on an injection molding machine. They join the rear platen to the moving platen. When the clamping cylinder straightens them, they push the moving platen forward, close the mold and lock it with the full clamping force; when it folds them, the mold opens. In the drawing they are highlighted between the rear platen and the moving platen, in an upper and a lower set.
Where the toggle links sit
On a five-point double toggle, each of the two toggles (one above the machine axis, one below) has three links:
- The rear link, pinned to the rear platen. It has a third bore for the short link from the crosshead.
- The front link, pinned to the moving platen and joined to the rear link at the middle pivot, the elbow.
- The short link (also called the connecting link or connecting rod), which joins the crosshead to the rear link.
Every pivot has a toggle pin turning in bushings pressed into the link bores. On many machines a link position is made of two or more plates side by side on the same pin, which spreads the load. Names vary between manufacturers: toggle arm, long link, short link and link rod all appear in parts lists.
How a toggle link multiplies force
The rear and front links act like a knee. When they are folded, a short movement of the crosshead moves the platen a long way, with little force. As they come into line, the platen moves less and less for each millimeter of crosshead travel, and the force it can push with rises in the same proportion. Close to the straight position the ratio becomes very large, which is how a relatively small cylinder locks the mold with the machine's full clamping force.
Once locked, the links are in compression: they push the rear platen and the moving platen apart, while the tie bars that hold the platens together are stretched. The links, pins, platens and tie bars form a closed loop of force, and at the straight position the cylinder needs very little force to keep the links there. The toggle mechanism guide explains how this sets the clamping force through the mold height.
Materials and variants
Links are made of tough alloy steel rather than very hard steel, because they must resist fatigue under a load that is applied and released on every cycle. The toggle arms and link rods we supply are 42CrMo alloy steel, quenched and tempered to HRC 28–35, for Haitian MA, MA II and MA III machines. The wear happens in the bushings, not in the link itself.
The shape and size of the links depend on the machine model and clamping force. Besides the five-point double toggle, some machines use four-point toggles or other link layouts, and the upper and lower links of one machine may be mirror images rather than identical parts. Always match a replacement to its position.
Signs of wear or failure
- A knock at lock-up or when the mold breaks open. This is usually the pins and bushings first, but the link bores suffer if the play is left too long.
- An oval or enlarged bore, or a bushing that has turned or moved sideways in its bore.
- Cracks around the bores or at the narrowest part of the link. They often show first as a fine line in the paint or a rust stain.
- Scoring on the side faces, where links rub against each other or against spacers.
- A bent link, which makes the toggle bind and the upper and lower toggles lock at different moments.
- Uneven tie bar stretch, or flash on one side of the mold.
The usual causes are running with worn bushings, so that the pin hammers the bore; lubrication failure; a clamping force set above the machine's rating; closing into lock-up at high speed; and molds mounted off center, which load one toggle more than the other.
Inspecting toggle links
- Stop the machine, lock it out, and release the hydraulic pressure, including any accumulator.
- Clean the links and look closely around every bore and at the narrow sections for cracks. Where you suspect one, have it confirmed by dye penetrant or magnetic particle inspection.
- With the clamp depressurized, check the play at each joint. Our toggle clamping system article describes the dial indicator method.
- When the pins are out, measure each bore in several directions with a bore gauge to find ovality, and compare with the limits in the machine manual.
- If a link may be bent, compare the distance between its bore centers and its alignment with the matching link on the other toggle.
Replacing toggle links
Toggle links are heavy, and removing one releases the parts it holds. Remove the mold, support the links and the moving platen with lifting gear or blocks as the machine manual describes, and follow its sequence for removing pins.
- Replace links in matching positions as a pair, so that the upper and lower toggles keep the same geometry. The distance between bore centers must match the original.
- Fit new bushings and inspect the pins while the toggle is apart; a new link on a worn pin wears its new bushings quickly.
- Before moving the clamp, run the lubrication until fresh lubricant appears at every joint. Then close slowly at low pressure, check that both toggles lock at the same time, and reset the mold height and clamping force.
What to send for a replacement quote
- The machine brand, model, clamping force and serial number.
- Which link or links (rear, front or short link; upper or lower toggle) and how many.
- Photos of the old link with its main dimensions: distance between bore centers, bore diameters with and without bushings, and plate thickness.
- The OEM part number or the parts list page, if you have it.
For the crosshead and its short links, see the crosshead page.
Updated by the IMSPARES technical team.