Platens of an injection molding machine

The platens are the heavy cast iron plates that hold the two halves of the mold and pass the clamping force through it. A toggle injection molding machine has three: the stationary platen at the injection side, which carries the fixed mold half; the moving platen, which carries the moving half and travels to open and close the mold; and the rear platen behind the toggle. The drawing above shows a platen from the front and from the side, with its numbered features: tie bar bores, T-slots, tapped holes, the center bore and locating ring, ejector holes and toggle lugs.

How the platens work in the cycle

1. Closing. The toggle pushes the moving platen along the tie bars until the moving mold half meets the fixed half.
2. Clamping. The toggle locks, and the clamping force passes from the rear platen through the toggle into the moving platen, through the mold and into the stationary platen. The tie bars carry it back from the stationary platen to the rear platen.
3. Injection. The nozzle reaches through the center bore of the stationary platen to the mold's sprue bushing and injects the melt.
4. Opening and ejection. The moving platen moves back, and the ejector behind it pushes rods through holes in the platen to the mold's ejector plate, which pushes the part out.

The mold faces of the stationary and moving platens must stay flat and parallel through all of this. Under clamping force each platen bends slightly, because the mold presses on its middle while the tie bars hold its corners. Platen design keeps that bending small and even, so the mold closes evenly across its whole parting line.

The parts of a platen

Platen

Each platen is a large casting of gray or ductile iron, machined on the mold face, in the tie bar bores and at the toggle mounting. We supply stationary, moving and rear platens for Haitian MA, MA II and MA III series machines. The differences between the three, and how the moving platen is guided and supported, are explained in moving and stationary platens.

Tie bar bores and bushings

The four tie bars pass through bores at the corners. In the moving platen each bore has a bushing, so the platen slides along the bars without wearing the bar or the casting. Worn bushings let the platen drop and tilt: see tie bar bushings and the tie bars guide.

T-slots

Slots across the mold face that take the T-bolts or T-nuts of the mold clamps. Their lips carry the full force of each clamp bolt, so they crack or spread if bolts are overtightened or clamps are set badly. See mold clamps and T-slots.

Tapped holes

Threaded holes in a set pattern, used to bolt the mold or its clamps directly to the platen. The pattern is shown on the machine's platen drawing, and a stripped thread can often be repaired with a thread insert. Tapped holes are covered with the T-slots on the same mold clamps page.

Center bore and locating ring

On the stationary platen, the mold's locating ring fits in the center bore and lines the sprue bushing up with the nozzle. On the moving platen, the center bore lets the central ejector rod through. See locating ring.

Ejector holes

Extra holes in the moving platen, in a pattern around the center, for ejector rods that push the mold's ejector plate at several points. The ejector guide explains how the machine's ejector drives the mold.

Toggle lugs

Lugs on the back of the moving and rear platens where the toggle links are pinned. They carry the full clamping force through the toggle pins, and a crack at the root of a lug is serious. See the toggle guide and toggle pins.

Platen parallelism

The mold faces of the moving and stationary platens must be parallel, both with the clamp open and under full clamping force. Worn tie bar bushings, toggle wear, a worn moving platen support and unevenly set tie bar nuts all show up as a parallelism error, and the result is flash on one side and uneven wear on the mold. How to measure and correct it is explained in platen parallelism.

Common problems and their symptoms

• Damaged mold face: dents, raised burrs or weld spatter stop the mold from sitting flat. The mold rocks when clamped, flashes or goes out of line.
• Worn T-slots and stripped tapped holes: clamps that will not tighten, cracked slot lips, and molds that shift during production.
• Moving platen sag: the moving half sits lower than the fixed half and the mold's guide pins wear. The tie bar bushings are worn or the platen support needs adjusting.
• Parallelism error: flash on one side, uneven parting line wear, and tie bars carrying unequal loads.
• Cracks: found most often at the tie bar bores, the toggle lugs or around the center bore, typically after an overload or long running at high clamping force on small molds. A cracked platen must be assessed by the manufacturer or a specialist before the machine runs again.
• Ejector rods binding: rods that scrape or stick in the platen holes, from a bent rod or a burr in the hole.

What to send for a replacement quote

Platens are large, machine-specific castings, so a quote starts from the machine itself:

1. The machine brand, model and serial number, and which platen you need: stationary, moving or rear.
2. The reason for replacement (crack, face damage, worn bores), with photos of the damage and of the whole platen.
3. The main dimensions if you can take them: face size and thickness, tie bar bore diameter and spacing, T-slot and tapped hole pattern, center bore, and ejector hole pattern.
4. The OEM part number or the platen drawing, if you have it.

For smaller items such as locating rings and mold clamping plates, a photo with the main dimensions is usually enough. Request a quote or browse our platen parts.

Maintenance that protects the platens

• Clean the mold faces at every mold change and remove raised burrs with a flat honing stone. Never use a grinder on a platen face.
• Keep the faces lightly oiled against rust when no mold is fitted.
• Use enough clamps, set them correctly, and tighten their bolts to the values in the manual.
• Avoid clamping small molds at full force; check the manufacturer's limits.
• Lubricate the tie bar bushings and the moving platen support as the manual specifies.
• Measure platen parallelism regularly and after any work on the clamp unit. The Haitian MA maintenance checklist shows where these checks fit into a maintenance plan.