Two-plate vs three-plate mold

The short answer

A two-plate mold opens at one parting line. The runner, the gate and the part all lie on that line, so the melt enters the part from its edge, and the runner comes out attached to the part. A three-plate mold adds a floating runner stripper plate and opens at two parting lines. The runner sits on its own plane above the cavity, the gate can enter anywhere on the face of the part, and the runner is torn away from the part as the tool opens. Two-plate is the default. Three-plate is chosen when the gate position or automatic degating is worth the extra tool cost.

How a two-plate mold works

The fixed half (A plate, cavity) and the moving half (B plate, core) meet at a single parting line. The sprue feeds a runner cut into that parting line, and the runner feeds each cavity through a gate at the edge of the part. When the tool opens, the part, runner and sprue stay together on the moving half and are ejected as one shot. The component list is the one in injection mold explained, with components: clamp plates, A and B plates, support plate, ejector system and cooling.

• Gate types that suit it. Edge (side) gates, fan and tab gates for flat parts, and submarine or tunnel gates, which run under the parting line and shear off as the part ejects, giving automatic degating in a two-plate tool.
• What it is good at. The simplest construction, the lowest tool cost, the shortest mold height and the fastest cycle. Most parts, and most multi-cavity tools for small parts, are two-plate.
• Where it runs out. The gate has to be at the perimeter. A deep cup, a cap or a large cover that needs to fill from the centre, or a part that needs several gates across its face, cannot be gated well from the edge.

How a three-plate mold works

A third plate, the runner stripper plate, sits between the top clamp plate and the A plate. The runner is cut between the stripper plate and the A plate, on a different plane from the part, and drops into each cavity through a small pin-point gate on the face of the part. The opening sequence is controlled by puller bolts and latch locks, in three steps:

1. The A plate separates from the runner stripper plate. The pin-point gates break, leaving the runner behind.
2. The runner stripper plate moves off the top clamp plate and strips the runner off its sucker pins, so it falls clear, or is picked by a sprue picker.
3. The main parting line opens and the ejector system pushes the finished parts off the core, already degated.

• What it is good at. Centre gating of round and cup-shaped parts, several gates across a large part to shorten flow length and control weld lines, balanced feeding of multi-cavity tools where edge gates would be uneven, and automatic separation of runner and part without a sub gate.
• What it costs. An extra plate, longer guide pins, puller bolts and latches, more machining, and more open height on the press to let three plates separate. The runner is longer, so each shot carries more scrap or regrind, and the opening stroke adds cycle time.
• Limits of the pin-point gate. The gate is deliberately small so it breaks cleanly. That raises shear on the melt, which can be a problem for shear-sensitive or heavily filled grades, and it limits how much packing pressure reaches thick sections.

Side by side

	Two-plate mold	Three-plate mold
Parting lines	One	Two (runner plane and part plane)
Gate position	Edge of the part, or submarine under the parting line	Anywhere on the part face, pin-point
Degating	Manual or robot trimming, automatic only with a submarine gate	Automatic, the gate breaks as the tool opens
Runner scrap	Shorter runner	Longer runner, more regrind per shot
Tool cost and build time	Lowest	Higher, extra plate and opening mechanism
Cycle time	Shortest	Longer opening stroke
Press requirement	Standard open height	More open height (daylight)
Maintenance	Simple	Latches, puller bolts and sucker pins to maintain
Typical parts	Housings, brackets, flat and box parts, most small multi-cavity parts	Caps, closures, cups, round covers, large parts needing several gates

Where a hot runner fits

A hot runner removes the runner altogether: a heated manifold and nozzles carry molten plastic straight to gates on the part face, so it offers the gating freedom of a three-plate tool with no runner scrap and a shorter cycle. It costs more to build and needs a temperature controller, so it earns its place at higher volumes, with expensive resins where regrind is not acceptable, or where cosmetic gate vestige matters. The trade-off between cold runner, hot runner, thermal gate and valve gate is covered in runners in injection molds.

How to choose

• Start with two-plate if the part can be gated from its edge without weld lines on a cosmetic face or a flow length the material cannot fill. It is the cheapest tool and the fastest cycle.
• Use a submarine gate in a two-plate tool if the only reason for a three-plate tool is automatic degating and the gate can reach the part through a side wall.
• Move to three-plate when the part must fill from its centre or from several points on its face, and volume does not justify a hot runner.
• Move to a hot runner when volume is high, runner scrap or regrind is costly, or cycle time drives the piece price.

The choice shows up directly in a tooling quote, as the extra plate and opening mechanism, the runner weight per shot, and the cycle time used for the piece price. What drives injection molding cost in India explains how those lines turn into a number.

What Trumould builds

The tool room builds two-plate, three-plate, hot runner, multi-cavity, family and stack tools, 1 to 32 cavities as standard, inside a 600 by 900 mm platen envelope. See injection molds and tooling. Send a model with the quantity you expect, and the quote states which construction is proposed and why, including where a cheaper tool would do the job.