Clamp Force: RTV Jacket Boxes vs HCR Presses

Clamping force on a silicone compression press is not a thermoplastic tonnage factor, and it is not a copy of an LSR injection bar chart. That formula belongs on millable HCR in a heated press — a molded-*part* quote.

On this site you are closing a mold box: two-part platinum or tin jackets, keys, a mother mold, straps or bolts. The analog of clamp is “will the split stay shut while you dump or pour,” not “will 250 tons hold a biscuit.” Importing “2–5 tons per square inch because that is what we use for nylon” will bruise an aluminum HCR land — and it will also crush RTV keys if you G-clamp a floppy 15A skin like a steel shoe.

The honest HCR estimate is F = A × P × S. The honest RTV estimate is: projected split that still sees hydrostatic head from the dump, times a small pressure, times enough rigidity that keys seat. If the jacket is soft and the box is open-back, no strap number saves a 0.5–1.5 mm edge sag.

Projected area is a shadow — on the press and on the box

HCR (parts OEM): CAD, look along the ram. Sum the silhouette of every cavity, plus transfer pots, runners, and overflow still sitting on pressure. Do not include the entire mold plate. A tall boot with a small footprint needs less clamp than a thin, wide gasket of the same mass. Deep cores add side pressure; that is a core-shift problem, not extra A unless the wall projects on the parting plane.

RTV (this site): The “A” that matters is the split face that will leak if it opens: keys, joggles, the land around each cavity. A 12-cavity gummy tray has a large plan area and almost no *process* pressure compared with a press. The dump is centimetres of chocolate or pectin, not hundreds of bar. What opens the split is a warped jacket, a missing mother mold, or straps parked on one edge so the far keys gape.

If you ignore a family of large overflow pockets still on pressure (HCR), the defect is a press that looked big enough on a napkin and flashes every outer cavity. If you ignore jacket rigidity (RTV), the defect is outer-row dumps 3–6% heavy versus center because the tray sagged.

Where P comes from (without inventing a constant)

On HCR, P is measured or taken from a process the compound will actually run: a cavity-pressure sensor, press force divided by known A on a similar tool, or the supplier’s compression molding-pressure band as a start. Transfer pot pressure is not cavity pressure. A single “silicone is 150 kgf/cm²” rule is fake. So is copying a LIM expansion ceiling (some notes mention up to a few hundred bar on liquid in a hot closed tool) onto a biscuit.

Thick HCR sections and high cavity temperatures raise expansion as the rubber heats. Multi-cavity tools with mixed thicknesses will see mixed P; the land still has to hold the worst cavity.

On RTV, P is closer to hydrostatic head of the cast plus whatever you pull in vacuum. A 40 mm resin pour is not a 400 bar LIM event. Over-clamping a 10A–20A skin brinells keys the way extra S on a soft aluminum HCR land brinells the prototype tool.

Platinum HCR versus peroxide HCR changes chemistry and gas, not the HCR formula. Vacuum compression changes whether air is in the way; it does not cancel P. Vacuum on an RTV box removes air from the pour; it does not replace a seated key.

Multi-cavity layout is a clamp-quality problem

Tonnage is a scalar. The press delivers it through platens that bend.

• Balance the HCR layout about the center of the ram. Eight cavities in a row on one side of a 200-ton press is a tilt.
• Tiny O-rings in a 64-cavity plate can have a tiny total A and still need a press that *covers the plate* and stays flat. Tie-bar spacing and mold-base footprint belong in the same sentence as F.
• Extra S on a soft aluminum land is how you brinell a prototype and then chase flash you created.

On RTV trays, the same physics is jacket bend:

• Balance straps or bolts around the box, not three clamps on the pour side.
• A rigid mother (plaster, resin, printed shell) is the platen analog.
• Bump / vacuum sequences on a press close at low force, then go to full force. On RTV: seat keys, then vacuum/pour, then hold. Do not crush keys “to be safe.”

If you ignore platen deflection, the defect is HCR flash on the corners and an over-compressed center that is out of ISO 3302-1 on thickness. If you ignore mother-mold stiffness, the defect is RTV flash at the far keys and a thick outer row.

Worked structure (not a fake number)

HCR: (1) Export A for one cavity. (2) Multiply by cavity count; add pressurized overflow/pot. (3) Choose P from measured or supplier data for this compound and thickness. Convert so F comes out in kN or tons. (4) Multiply by S (state it: 1.15 is a choice you can defend). (5) Compare to press rating and land stress. (6) Check layout symmetry.

RTV analog: (1) Split area that must stay shut. (2) Head of the dump/pour. (3) Jacket stiffness and key count. (4) Strap/bolt pattern that does not tilt the box. (5) Mother mold if Shore is 10A–20A or the tray is large and wet.

Write the assumptions on the quote. A second shop with a different P is just using a different guess.

RUUIPON Mold Structure Decision: Flat back? → open-back, straps optional. Else two-part / plug. Large wet casting or soft Shore? → mother mold required — clamp without a jacket is theatre.

Common mix-up: quoting press tons for a mold box

A lot of RFQs ask “how many tons” on a platinum gummy tray. In the shop that usually means someone pasted an HCR clamp worksheet onto a pour job.

What to ask when you're buying

• Is this HCR compression (parts) or an RTV box (cavity you dump into)?
• If HCR: show A, P, S, and land stress — not only “250 tons.”
• If RTV: jacket type, key map, strap/bolt pattern, mother mold yes/no?
• Vacuum bump (press) or vacuum degas (pour) — which, and does it replace clamp? (It does not.)
• ISO 3302-1 on the *article* (dump or part), not on the clamp number?

Worked example

You're closing a 12-cavity gummy tray, 180 × 120 mm, 12 mm 15A platinum skin, resin jacket, about 8,000 dumps/season. Seat six keys, bolt a rigid jacket on a pattern that covers the plan, vacuum the mix, dump. Do not size a 150-ton press. If the SKU is instead a 16-cavity HCR gasket, run F = A × P × S on the *parts* OEM and check platen coverage.

We would not recommend importing nylon tons/in² onto HCR — or onto an RTV box

We would not recommend using a LIM expansion-pressure figure as P for a biscuit, extra S on a soft aluminum land, or three G-clamps on one edge of a 12-cavity tray. We would not stay at bump force through an HCR cure and call the vacuum gauge a clamp.

Typical shop values

Reference values only. Confirm material-specific limits against the supplier’s technical data and agree project acceptance criteria before production.

| Item | Typical shop value | | --- | --- | | HCR safety margin S | Often about 1.1–1.25 — a stated choice, not a material property | | LSR leak band (contrast) | About 0.005 mm at a steel land | | RTV cavity Shore | 10A–20A with a rigid jacket | | Loaded-tray edge sag if jacket is late | About 0.5–1.5 mm | | Outer-row gummy vs center if tray sags | Edge often 3–6% heavy | | ISO 3302-1 | M2 common on the rubber article |

Copy this into a clamp RFQ

• Process: HCR compression / transfer (parts) or RTV two-part box (this site)
• Cavity count, CAD for projected area *or* split/key map
• Compound family; vacuum bump vs degas
• Ask the HCR shop to show A, P, S, and land stress
• Ask the RTV shop for jacket, keys, and strap/bolt pattern

Send the tray layout or the HCR silhouette and name the object. We'll size a jacket and keys, not a mystery tonnage. Custom RTV molds.

Related: 1 vs 8 cavity piece price, core shift, what tooling means.

FAQ

Can I use thermoplastic tons-per-square-inch factors for silicone compression? No. Those factors are tied to plastic fill-and-pack. HCR sees a different pressure history, including expansion during heat and cure. Use projected area times a pressure that belongs to this compound and tool, then add a stated safety margin. Do not paste the same tons onto an RTV box.

Do I multiply projected area by cavity count only, or include overflow? On HCR, include every area still sitting on pressure: cavities, transfer pots, runners, and overflow that has not dumped to atmosphere. Do not include the whole plate. On RTV, include the split that will leak; rigidity of the jacket matters more than a second decimal on A.

Is the 400 bar figure from LSR injection guides the right P for compression? That order of pressure appears in some LIM notes as a cavity-expansion ceiling for liquid silicone. Compression HCR is not that process. Using 400 bar blindly on a biscuit tool will usually overstate clamp and still tell you nothing about land stress. It is meaningless on a gummy tray.

Why can a press with enough tons still flash a 16-cavity plate? Tilt and bend. Force through an off-center layout or a thin plate opens the far lands. Flash is then a parallelism problem. The RTV cousin is a warped jacket and straps on one edge.

Does vacuum assist change the clamp calculation? Vacuum removes air so the charge or pour fills the well; it does not remove rubber or hydrostatic pressure after close. Size HCR F for the high-force cure step. Seat RTV keys before you pull. If you stay at bump force through an HCR cure, you will flash with a perfect vacuum gauge.