Tie bar distance and mold size

Tie bar distance, also called tie bar spacing, is the clear space between the tie bars of an injection molding machine, given on the data sheet as a horizontal and a vertical figure. A mold is loaded by passing it between the bars, so the tie bar distance is the first limit on how large a mold the machine can take. The tie bars themselves, highlighted in the drawing, are the four steel bars that join the stationary and rear platens and carry the clamping force.

Reading tie bar distance on a data sheet

• Horizontal × vertical. The two figures are the clear distances between the bars across the machine and from top to bottom. On machines with square platens the two are often equal.
• Clear distance or center distance. Data sheets usually give the clear space between the bars. Some drawings give the center-to-center distance instead; the clear distance is then the center distance minus one tie bar diameter. If the sheet does not say which, ask the manufacturer.
• Related figures on the same sheet: tie bar diameter, platen size, minimum and maximum mold height, opening stroke and maximum daylight, locating ring diameter, ejector stroke, and clamping force.

How to check that a mold fits

1. Width between the bars. For loading from above, the mold's width, including anything that sticks out at the sides, must be smaller than the horizontal tie bar distance, with room to lower it on the crane. Its height may then be larger than the vertical distance, as long as it fits on the platens. For loading from the side, it is the other way round.
2. Things that stick out. Water connectors, hot runner connectors, hydraulic core-pull cylinders, slides and lifting eyes all count. They are a common reason why a mold that fits on paper will not go in.
3. Mold thickness. The closed mold must be between the machine's minimum and maximum mold height: see mold height adjustment.
4. Opening stroke. The mold must open far enough for the part and runner to be ejected and removed, whether they fall, are picked by a robot or are taken out by hand.
5. Locating ring. The ring on the mold must match the center bore of the stationary platen: see locating ring.
6. Ejector. The mold's ejector plate must line up with the machine's ejector holes, and the ejector stroke must be enough: see ejector rods.
7. Clamping force. The machine must hold the mold shut against the melt pressure in the cavity over the part's projected area: see clamping force.
8. Weight. The moving half of a heavy mold hangs on the moving platen and its bushings or supports. Check the maximum mold weight the manufacturer allows.

When the mold is much smaller than the tie bar spacing

A mold that fits easily is not automatically a good match. If a small mold is clamped at high force, the force is concentrated in the middle of the platens instead of being spread out toward the tie bars. The platens bend, the mold's parting line can be damaged, and the platens and bars are loaded in a way they were not designed for. Some manufacturers give a minimum mold size, or a reduced clamping force for small molds: check the manual before clamping a small mold at full force.

When the mold is larger than the tie bar spacing

• Use a machine with wider tie bar spacing, which usually means a machine with a higher clamping force.
• On some larger machines one tie bar can be withdrawn to load a wide mold. This is a machine feature with its own procedure, never something to improvise.
• Tie-bar-less machines, which have no bars in the way, are another option for some molds.

Tie bar damage and replacement

Mold changes are also where the tie bars get damaged: a mold that swings on the crane hook and hits a bar leaves a scratch or gouge that can later start a fatigue crack. Fit tie bar protectors and guide the mold in slowly. How to inspect the bars, what damages them and when they need replacing are covered in our tie bar maintenance and replacement guide.

To quote a replacement tie bar, send the machine brand, model and serial number; the bar's overall length and diameter; the length and position of each thread or groove; the thread size and pitch at both ends; and photos. We supply single tie bars and matched tie bar pairs: request a quote with the details. The tie bars guide shows how the bars fit with their nuts and bushings.