Mold clamps and T-slots for injection molding

Mold clamps for injection molding are steel clamping plates that hold each half of the mold against its platen. Each clamp is pulled down by a bolt that runs into a T-slot or a tapped hole in the platen; its front end presses on the flange of the mold's clamping plate and its back end rests on a support, the heel. The clamps stop the mold from moving while it opens, closes and ejects. The clamping force that keeps the mold shut during injection comes from the machine's clamp unit, not from the mold clamps.

T-slots and tapped holes

The platen face is machined with T-slots, highlighted in the drawing, and a pattern of tapped holes:

• A T-slot takes a T-bolt or T-nut that can slide along it, so the clamp can be placed where the mold needs it.
• Tapped holes take a bolt or stud directly, at fixed positions shown on the platen drawing.

Many platens have both. The lips of a T-slot carry the full force of the clamp bolt, so they can crack or spread if a bolt is overtightened, if a T-nut sits only partly in the slot, or if it is placed too close to the open end of the slot.

How a mold clamp works

A strap clamp is a lever with three points: the mold flange at the front, the bolt in the middle and the heel at the back. The bolt pulls the clamp down, and its force is shared between the mold and the heel:

• The closer the bolt is to the mold, the larger the share of its force that presses on the mold.
• The heel should be the same height as the mold flange or slightly higher, so the clamp is level or tilts slightly toward the mold. A clamp that tilts the other way presses on its edge and holds the mold poorly.

Types of mold clamps

• Plain strap clamps, used with a separate heel block or support bolt.
• Step clamps with matching step blocks, so one clamp suits flanges of different heights.
• Adjustable clamps with a built-in heel screw.
• Mold clamping plates made to suit a particular machine or mold, such as the mold clamping plates and locating rings we supply for Haitian MA and MA II series machines.
• Quick mold change systems: hydraulic or mechanical clamps fixed to the platen, and magnetic platens. They hold the mold without loose clamps and shorten mold changes, but need molds with suitable clamping plates.

Clamping a mold correctly

Follow your setting procedure and the machine manual. Good practice is:

1. Clean the platen faces and the mold's clamping plates, and remove any burrs.
2. Hang the mold, guide its locating ring into the platen bore and bring the fixed half flat against the stationary platen.
3. Clamp the fixed half with clamps on opposite sides of the mold, in the number your procedure or the manual specifies. Large and heavy molds need more clamps.
4. Place each bolt as close to the mold as the slot or hole pattern allows, set the heel height, and use bolts and washers of the grade the manual specifies.
5. Screw each bolt deep enough into the tapped hole or T-nut, as the manual specifies, and tighten all the bolts evenly to the specified value.
6. Close the machine slowly at low pressure so the moving platen meets the moving half, then clamp that half the same way.
7. Remove the crane and the mold's safety strap only when both halves are clamped, then open the mold slowly and check that nothing has moved.

Signs of wear or a poor setup

• A mold that moves, slips down or shifts on the platen during production, often seen as mismatch on the part or damaged guide pins.
• Clamps that are bent or cupped, and bolts that are stretched, have damaged threads or keep working loose.
• Cracked or spread T-slot lips, and burrs at the slot edges that stop the mold sitting flat.
• Stripped threads in the tapped holes. A thread insert of the right size and strength can often repair them.
• Clamps that hit the tie bars, the ejector rods or the safety guard: check the clamp size against the platen layout.

What to send for a replacement quote

Send the machine brand and model; the clamp's length, width and thickness and the size of its slot or hole; the platen's T-slot width and tapped hole thread; the height of the mold flange you clamp; and a photo of the old clamp. Request a quote, and see the platens guide and the moving platen page for the platen itself.