Four Numbers That Decide Every In-Mould Cylinder Spec — and Why Bigger Is Rarely Safer

By Luisa Piovanelli — Enerfluid SNC, Brescia

 

There is a pattern I have seen many times across tool builds — in injection moulding, in die casting, in press tooling. An engineer is specifying the hydraulic cylinder that will move a core inside a mould. The cavity plate is already crowded. The cooling lines are in. The ejector pins are placed. The hot-runner manifold has taken its share of the steel. And now the cylinder has to fit into whatever space is left.

The instinct, when the margin feels tight, is to go a size up. A bigger bore means more force. More force feels like more safety margin. It feels like the conservative choice.

In my experience, it is almost never the right one.

A larger bore inside a mould does not just take up more pocket volume. It pushes the cooling circuit into a compromise. It may require more ejector pin adjustments. And if it shifts the mould base to the next standard size, the press required to run the tool becomes more expensive — and the tool becomes harder to sell. Three cost lines affected by one sizing decision made without doing the calculation.

The correct approach, in my view, runs in the opposite direction. Find the smallest cylinder that still meets the force requirement, with an honest safety margin, and design the pocket around that number. The calculation is not difficult. You need four things: force, pocket, stroke, and temperature at the cylinder location. Get those four right and the spec almost always fits without oversizing.

Here is how to work through each one.

Number 1 — Force: Set and Pull Are Two Different Problems

The force question inside a mould is not one number. It is two, and they behave differently.

Set force — what you can calculate:

The load the cylinder has to resist during fill is the polymer pushing back on the core. The formula is:

F = projected core area × injection (nozzle) pressure

Nozzle pressures in plastic injection run roughly 690–1,725 bar (10,000–25,000 psi), depending on the machine screw ratio and the injection pressure setting. Take the core area projected onto the plane perpendicular to cylinder travel, multiply by the nozzle pressure, and you have the force trying to push the core back during fill.

The cylinder’s extend force must be above that — with a margin of at least 10–25% to absorb system losses and pressure spikes. Document this number clearly. The margin is not optional: a cylinder sized without margin works fine on a good day and backs up under pressure on a bad one. Published data shows cores backing up as much as 0.5 mm (0.020 inch) under injection pressure in poorly specified setups, leading to flash, dimensional drift, and the tool leaving the press.

Retract force — what you have to estimate:

This is harder to calculate precisely. After the shot, shrinkage grips the core. The breakaway force to pull the core free depends on the material, the geometry, the draft angle, and the surface condition of the core itself. There is no single formula that covers all cases reliably.

What I can say is this: many sizing errors in the field are not extend failures — they are retract failures. The cylinder sets the core correctly, holds through fill, and then cannot pull free. The result is a stuck core, a tool stoppage, and a conversation about bore changes or part-design modifications that could have been avoided at the specification stage.

So “force” in a mould cylinder spec means two things: how hard you need to push, and how hard you need to pull. Carry both numbers into the supplier conversation.

Number 2 — Pocket: The Space That Already Exists

The pocket is the real constraint. It is not the number in the catalogue marked “compact.” It is the outline cube — including hydraulic port clearance, sensor clearance, and full rod travel — that exists in the tool after everything else is already placed.

This constraint has been getting tighter, not easier. Conformal cooling, now standard in many printed inserts, reaches into steel volumes a drilled line could never access — which means it also occupies steel volume a cylinder might have used. Multi-cavity layouts are increasing the number of in-tool actions per cavity. Every slide, core pull, and collapsible core is competing for the same plate volume that the cooling circuit needs.

The discipline for this situation is not generous — it runs against instinct. Find the smallest cylinder that still satisfies the force requirement. The mould base size is the most visible cost line in a tool budget; the actuator specification is one of the few places a designer still has direct control over that number.

One useful design principle: if the force calculation demands a bore larger than the available pocket allows, the answer is not to compromise the cooling. The answer is to raise the pressure instead of increasing the piston area. This is the role of a pressure intensifier, which I will come to in a moment.

Number 3 — Stroke: From Set to Clear, With Margin for Ejection

The stroke requirement is the distance from the fully-set (extended) core position to the fully-retracted position, measured in the tool. Add enough margin beyond that for the moulded part to clear during ejection — under-stroke and the core fouls the part on the way out.

The stroke decision also settles the single-acting versus double-acting question, which is worth thinking through explicitly rather than defaulting to one configuration.

Single-acting (spring return) saves a port and eliminates a hydraulic channel through the plate. If channel routing is the binding constraint in your plate geometry, this matters. The trade-off: the return spring reduces the effective stroke available, and the retract force is limited to what the spring provides. For applications where pull force is generous relative to shrinkage resistance, single-acting is simpler and cheaper to route.

Double-acting gives full hydraulic force in both directions, which solves the retract problem more reliably. The cost is a second port and a second channel to route past the cooling lines. Choose on the basis of what is actually scarce: if the plate routing is already congested, single-acting simplifies the design. If the retract force is the concern, double-acting is the correct choice.

Number 4 — Temperature: At the Cylinder, Not at the Press Controller

This is the most frequently misread number in the specification, and it causes the most field problems.

What matters is the steel temperature where the cylinder sits, not the process temperature displayed on the machine controller. Those two numbers can be far apart.

In standard plastic injection moulding, the cavity steel typically runs 35–75 °C. FKM (Viton-class fluoroelastomer) seals, the default for cavity-adjacent service, cover continuous operation to roughly 200 °C, with short-term capability into the 230–260 °C range depending on grade. For most injection moulding applications, standard FKM seals are appropriate.

The hot-runner manifold zone is different. Manifold temperatures sit at 150–250 °C, and that is the temperature band where seals degrade if the cylinder is mounted too close without thermal isolation or cylinder cooling.

High-pressure die casting (HPDC) is a different context entirely. Preheated aluminium dies operate at 220–300 °C steady-state, with measured peak temperatures above 450 °C and cycle swings of around 350 °C per shot against molten metal near 700 °C. Cylinder placement, cooling circuit design, and seal selection for an HPDC tool cannot be reasoned about the same way as for plastic injection. This requires explicit discussion at the specification stage, not standard catalogue selection.

Two things worth knowing before specifying:

First, the cylinder’s rated temperature is usually lower than the seal’s rated temperature, because sensors and electronics fail before the seal does. As a published reference point, some cylinder suppliers rate their standard units at 80 °C with inductive sensors fitted and 180 °C without — with external limit switches required above the lower figure. Check the rating of the complete assembly, not just the seal material.

Second, above the FKM service envelope, the choices are a cooled-cylinder variant or moving the cylinder out of the high-heat zone entirely. Neither is difficult to specify if the temperature is identified early. It becomes difficult when the temperature is not measured and the cylinder is specified for a position the seals cannot sustain.

Why Raising Pressure Is Often Better Than Increasing Bore

If the pocket cannot accommodate the bore the force calculation demands, the correct move is not to upsize the mould base to make room. The correct move is to raise the supply pressure using a pressure intensifier.

The principle is straightforward. A pressure intensifier links a large low-pressure piston to a smaller high-pressure piston. Force on the shared shaft is equal — so pressure on the small-piston side rises in proportion to the area ratio, while flow falls in the same proportion. This is the hydraulic equivalent of a gear reduction: the same energy, delivered at higher pressure and lower flow.

In practical terms: a unit with a 25:1 area ratio turns standard shop air at 6 bar into approximately 150 bar of hydraulic oil pressure. At that pressure, a smaller-bore cylinder can generate the same force as a larger-bore cylinder at lower pressure — and it fits a smaller pocket.

There is one boundary to state clearly: an intensifier multiplies pressure, not flow. It delivers adequate holding force for a press or clamp cycle. It is not designed for high-speed continuous cycling. For the core-hold problem — preventing a core from walking under injection pressure — it is the cleaner answer than boring up.

A related consideration: if the injection force on a slide is high relative to the cylinder’s extend force, or if the press hydraulic pressure can drop during fill, a standard cylinder with a check valve may not hold reliably. The correct component in that situation is a locking cylinder — one designed to hold preload mechanically at zero supply pressure. That is a specification decision, not an upgrade. The distinction matters because a locking cylinder and a standard cylinder are sized and sourced differently.

When Hydraulic Is Not the Right Answer

Any honest discussion of in-mould cylinder selection needs to include the cases where a hydraulic cylinder is not the right choice. Getting this right matters for the overall design, and I would rather say so clearly than pretend the question does not come up.

Mechanical cam, angle pin, or heel block — the lowest raw-material cost and well-understood performance for short strokes where the core movement can be tied to platen opening. The constraints are real: the core must move in lockstep with the mould, the heel block and gibs take plate volume, and the interface wears on a maintenance schedule. When the stroke is short, the timing is acceptable, and the slide force is within what the steel contact can hold, this is a valid choice. When the stroke is long, or the core must move independently of mould opening, hydraulics are the better fit.

Pneumatic — genuinely appropriate for low-force applications in clean environments, such as medical device tooling or food-contact applications where any hydraulic oil leak is disqualifying. Shop air at 5–7 bar generates limited force from any compact piston area, and air compresses, so holding force drifts during fill. For cleanroom or food-contact applications where the force requirement is modest, pneumatic is the right trade-off. Saying this explicitly builds trust more than ignoring it.

Electric or servo — the best positional repeatability and controllability, with no fluid to manage. The constraints are capital cost, sensitivity to cavity heat, and the need for a control system capable of managing the servo axes. Right where programmable positioning is the primary requirement and the thermal environment can be managed. Not the default for force-critical applications in hot tooling.

Buying Decisions for a European Tool Build

Three things determine whether the right cylinder is also the right purchase — and all three are easier when the supplier is close to the tool build.

CAD at concept stage. You want a STEP file or native model while the pocket is still being designed around the part, not after the plate is already detailed. A cylinder introduced late into a detailed design either changes the cooling layout or requires a compromise the designer would not have accepted earlier. This is worth specifying explicitly with the supplier at the quote stage.

Lead time mapped to the build schedule. Catalogue items and configured variants — non-standard bore, modified seal grade, alternative mounting — run on different production schedules. Know which your tool date needs, and get confirmation in writing. The tools that come off schedule are more often waiting for a component than for a design decision.

Retrofit constraints. A new tool gives you freedom to specify correctly. Replacing an existing cylinder in a running tool fixes the mounting hole pattern, the port positions, and the stroke length before you start. Design to those constraints from the first sketch — it avoids late-stage surprises and components that technically fit the pocket but require plate rework to install.

Frequently Asked Questions

How do I calculate the force a core-pull cylinder needs to hold during injection?

The inject force is the projected core area multiplied by the nozzle pressure at the injection point. Nozzle pressures in plastic injection typically run between 690 and 1,725 bar (10,000–25,000 psi), depending on the machine and material. Size the cylinder’s extend force above the calculated load with a safety margin of at least 10–25%. Size retract force separately — it has to overcome shrinkage friction on the core, which depends on material, draft angle, and core geometry, and is harder to predict than extend force.

What temperature rating do I need for the seals on a mould cylinder?

Read the temperature at the cylinder location in the tool — not the process setpoint on the machine. For standard plastic injection with the cylinder placed in the cavity steel (35–75 °C), standard FKM (Viton-class) seals are appropriate: continuous rated to approximately 200 °C. If the cylinder is close to a hot-runner manifold zone, or in a die casting tool, the temperature at the cylinder can be substantially higher. Note that the cylinder’s rated temperature is usually lower than the bare seal’s rated temperature, because sensors and electronics fail first — check the complete assembly rating for your configuration.

When does it make sense to use a pressure intensifier inside a mould?

When the pocket available will not accommodate the bore that the force calculation demands at standard line pressure. An intensifier multiplies pressure by the piston area ratio — a 25:1 unit turns 6 bar of shop air into approximately 150 bar of hydraulic oil — which allows a smaller-bore cylinder to generate the same force as a larger-bore cylinder at lower pressure. The limitation: intensifiers multiply pressure, not flow rate. They are appropriate for holding and pressing functions, not for high-speed continuous cycling.

What is the difference between a locking cylinder and a standard hydraulic cylinder in tooling?

A standard cylinder holds position by maintaining hydraulic pressure at the port. A locking cylinder adds a mechanical hold — typically a spring-loaded or wedge-based mechanism — that maintains the set position even if supply pressure is lost or drops during injection. For applications where the press hydraulic pressure can fluctuate during fill, or where the injection force is high relative to the cylinder’s hydraulic hold force, a locking cylinder is the correct specification, not a standard cylinder with a check valve.

Hydraulic cylinder vs. angle pin — how do I decide?

The angle pin (cam or heel block mechanism) is a valid choice when the stroke is short, the timing can follow mould opening, and the slide force is within what the cam preload can hold. Hydraulic is the better choice when the stroke is long, when the core must retract independently of mould opening (for sequential core pull or before the mould opens), when the slide force is too high for a cam to hold reliably, or when you want to recover cycle time by retracting before mould open. The honest answer is that both systems work in their correct application — the error is choosing one for an application that suits the other.

How do I know how long a mould cylinder will last?

Think in cycles, not years. Rated cycle counts in this product category are typically in the millions — three million double strokes is a published reference point for some designs. At a 15-second cycle running two shifts for 250 days a year, three million cycles is approximately two years of operation. Ask the supplier for the rated cycle count at your specific operating pressure and temperature, the recommended re-seal interval, and whether service kits are available and stocked. A cylinder that is cheap to buy but whose service kits have a six-week lead time is more expensive over the tool’s life than a better-specified unit from the start.

Before You Call a Supplier

No supplier can size a mould cylinder accurately without four numbers from the tool. If you bring these four to the first conversation, you will get a faster and more accurate recommendation — and you will avoid the range of errors that come from over-generalised specs.

1. Required force — set force (calculated from projected area × nozzle pressure, with margin) and retract force estimate
2. Available pocket — the real cube in the tool, including port clearance and full rod travel, measured after all other components are placed
3. Stroke — from set position to fully-retracted, plus ejection clearance margin
4. Maximum temperature at the cylinder location — measured in the steel, not read from the machine controller

These four numbers define the specification. Everything else — bore selection, seal grade, single- or double-acting configuration, whether an intensifier is needed — follows from them.

Enerfluid SNC — Brescia, Italy · enerfluidsnc.com