Preventing Core Shift in Deep, Hollow Silicone Molded Parts During Injection

Core shift is the steel moving. The rubber then freezes the mistake as a wall that is 0.4 mm on one side and 1.6 mm on the other.

Deep hollow LSR parts (boots, bellows, bottle liners, sheaths, long sleeves) hang a slender core in a hot cavity and inject a low-viscosity liquid. LSR does not pack like polycarbonate, but it still has a flow front and a pressure field. If that field is higher on one side of a poorly supported core, the core walks. Elongation of the *part* will not push the core back. By the time the rubber can pull anything, it is already curing.

HCR transfer can shift a mandrel too. RTV pours around a mandrel mostly sag the jacket, not the steel. This article is LSR injection of deep hollows, with a note where compression mandrels differ.

What actually moves the core

A core is a cantilever or a floating pin. Forces:

• Unbalanced fill. One gate, or a family of gates that do not meet as a ring, loads one flank. The front races down the easy gap (the one already opened by the first millimetre of shift) and the thin wall gets thinner. Positive feedback.
• Jetting onto a face. A gate aimed at one OD will bend a long core before the cavity is full.
• Thermal growth. A core at 180 °C, a cavity plate at a different temperature, and a fit that was reamed at 20 °C. The pin is then preloaded or loose before the shot. Loose is shift. Preloaded is wear and a seized pin.
• Injection rate. Fast fill on a long, thin core is a lateral slap. Slow fill on a hot thin wall is scorch. The window is real.
• Vacuum and air circuits inside the core. A collapsing hose or a one-sided air channel is not a structural I-beam.

If you ignore the first millimetre of walk, the defect is a “flow mark” that is actually a wedge-shaped wall and a leak test fail on the thin side.

Support the steel before you tune the process

DFM order:

1. Two-sided support when the geometry allows: a core that seats in both halves, or a spigot into the moving half, beats a long cantilever. A small through-hole in the product that lets the core locate is cheaper than a hero cantilever. If the product cannot have a hole, say so and pay for the next items.
2. Diameter versus length. A 4 mm core that is 60 mm long will walk. Increase root diameter, add ribs on the *steel* (not on the rubber) in the non-cosmetic ID if the product allows, or split the core.
3. Interlocks and taper seats that center as the mold closes, with allowance for hot growth. Cold-fitted precision that binds at temperature is how you break a core off in the part.
4. Split or collapsible cores when the ID has grooves anyway. Shift and demold get solved together.
5. Material of the core. Hardened stainless for LSR, finish specified (mirror IDs vacuum-lock; matte can drag). Aluminum cores on a deep production hollow are a tryout.

Stripper rings and air-eject still need a core that was *centered during fill*. Ejection cannot un-shift a cured wall.

Gating and fill that do not bend the pin

• Ring gates, fan gates, or multiple drops that meet as a front around the core, not a single side gate on a 50 mm boot.
• Cold-runner balance that is mechanical. Heating one cavity to “slow it down” will not recenter a core.
• Fill from the closed end or the open end on purpose. Simulate or short-shot. Guessing is how you cut a second core.
• Vacuum so you are not fighting an air cushion on one flank that looks like a pressure imbalance.

LSR expansion during cure can still move a weakly seated core if the land is open. Clamp and shut-off are part of core control. Flash on one OD is often the sister symptom of shift: that side had more pressure or more gap.

Wall thickness of the rubber is the other lever. A 0.5 mm wall on a long core has no stiffness to “hold” anything and every tenth of a millimetre of walk is a 20% thickness error. A 2 mm wall forgives more. If the product needs a thin, uniform sheath, you buy support and gate discipline, not a hope that 20 Shore A will self-center.

Measurement: catch shift, not only OD

First article on a hollow should include wall map: ultrasonic, cut-up, or CT, at several depths and quadrants. OD within ISO 3302-1 and a crushed ID is a passed CMM on the wrong feature. Mark a clock on the part relative to the gate so you can see if the thin wall always sits opposite drop 1.

Process capability on wall thickness is the core-shift study. If cavity 3 is always thin at 12 o’clock, that core or that drop is the problem. Do not chase it with a global shrink factor.

RFQ notes for deep hollow cores

Process: LSR injection, hollow, core length and diameter, required wall and wall tolerance, whether a locating hole is allowed, gate faces that are forbidden, and whether vacuum and air-eject are assumed. Ask for the core-support concept in the tool design review, not after T1. If you ignore support, T1 will be a wall map and a change order.

FAQ

Can a softer LSR grade reduce core shift because it “flows around” the pin?

Softer grades often fill easier, which can lower the pressure you needed, but they do not recenter a bent core. Shift is steel moving under a pressure difference. Fix support and gate balance. Using 20 Shore A as a centering spring is not a tool design.

Is core shift the same as shrinkage ovality?

No. Shrink can oval a tube after it leaves a centered core; shift is a wedge wall that already existed in the mold. A cut-up that is thin toward the gate (or opposite it) on every shot is shift. Uniform thinness all around is more likely shrink, grade, or temperature.

Will slowing injection always stop a long core from walking?

It can reduce the slap. It can also scorch a thin wall and still leave a thermal or support problem. Use slower fill as a trial after the core is seated and the gates are balanced, not as the only lever.

Do HCR compression mandrels shift the same way?

They can walk if the charge is off-center or the mandrel is loose in the plates, but the fill is a squeeze, not a jet from a cold deck. Locate the mandrel in both halves and place the biscuit so it does not hammer one flank. Do not copy an LSR side-gate fix onto a compression boot without looking at the charge.

How many wall measurements prove a deep hollow is centered?

Enough quadrants and depths to see a wedge. A single OD and a single ID at the mouth will miss a core that pivots. CT or a section at mid-depth plus the closed end is the usual honest map, clocked to the gate.