Rapid Prototyping Options: RTV Vacuum Casting vs. LSR Soft Tooling

Vacuum-cast RTV parts and LSR shots are not interchangeable samples unless you only care about shape.

RTV vacuum casting (a silicone *mold*, usually platinum or tin, pulling a vacuum around -0.095 MPa / 28-29 inHg, then casting a part resin or a casting silicone) is a geometry printer with a rubber tool. LSR soft tooling (a printed or aluminum *injection* cavity, hot, platinum liquid silicone shot under clamp) is a small version of the production process. Using the first to “prove” the second is how fit samples pass and production parts flash, shrink, or miss hardness.

Two different objects called “silicone prototype”

RTV vacuum casting. You make a master. You mold a rubber tool around it. You cast parts into that tool: polyurethane, a casting silicone, sometimes epoxy. Degassing and vacuum help the copy. Cycle is manual. Shrink of *platinum* RTV tools is typically very low (~0.1% or less). The *part* shrink is whatever you poured. Heat history is room temperature plus any exotherm of the casting resin. Flash is a function of how you split the rubber, not of a steel shutoff.

LSR soft tooling. You cut or print a cavity. You heat it. You inject an LSR grade through a cold sprue or a simple cold runner. The part is the production family (platinum, heat-cured, post-cured if specified). Flash is a metal (or printed) shutoff problem. Shrink is the LSR’s heat-cured band, not the RTV tool’s. Hardness, tear, and compression set can be the datasheet you will buy later.

If the prototype is a handheld mockup, RTV is enough. If the prototype is a gasket that must compress in a groove at 50A after post-cure, RTV is a costume.

When RTV vacuum casting is the right sample

Use it when the question is shape, assembly choreography, or a photo.

• Undercut-heavy models you are not ready to slide-steel.
• Fit checks where a 0.1-0.3 mm argument is acceptable.
• Color and form reviews before anyone should cut aluminum.
• Very small quantities where an LSR pail and a press setup would be theatre.
• Parts that will *remain* craft-cast (art, props, some medical models that never go to injection).

Tin-cure RTV tools are cheaper and more forgiving of dirty printed masters. They age and shrink more. Platinum RTV tools hold size and last longer in the rack, and they inhibit on uncured resin prints. Neither chemistry is LSR.

Casting a *silicone* part in an RTV mold can get you near a target Shore A. It does not get you LSR flow, knit, or post-cure shrink. Do not send that part to a life-test and call it production data.

When LSR soft tooling is the right sample

Use it when the question is the process you will live with.

• Flash spec that a customer will measure.
• Gate witness, knit lines, and whether a thin fin fills.
• Shrink into a plastic housing after post-cure.
• True LSR grade: self-lube, medical, transparent, conductive.
• Cycle-time and trim-labor guesses for a later TCO.
• Any regulatory packet that cares about the compound, not the lookalike.

Aluminum 1-cavity is the usual vehicle. Printed LSR inserts are for even shorter loops, with the heat/flash/abrasion limits already discussed in the aluminum-vs-print decision. Soft tooling still needs heaters, a thermocouple, and a clamp the shutoff can survive.

The fork that changes the buy

Ask: what decision will this sample authorize?

A hybrid that wastes money: RTV samples used to freeze CAD, then a steel LSR tool with no aluminum bridge. The CAD freeze never saw heat shrink or flash. The steel pays for that omission.

A hybrid that works: RTV (or even a printed part) to kill form, then 1-cavity aluminum LSR to kill process, then steel if volume exists.

Cost and calendar, as mechanisms

RTV spends on the master, the rubber, vacuum time, and skilled pouring. Piece cost stays high because a person is the machine. Tool life is counted in casts. Good for tens, sometimes low hundreds, then the tool tears or the flash line goes ragged.

LSR soft tooling spends on CNC (or a print), heaters, tryout on a press, and a pail of compound. First parts are slower than RTV. Each extra part is cheaper than a hand cast, until the aluminum shutoff dies or you still have no volume.

Do not pick on “which prototype is cheaper.” Pick on which wrong decision is more expensive: a pretty PU part that green-lights a steel tool, or an LSR shot that takes two more weeks.

A one-afternoon test if you are stuck

Take the worst feature: thinnest wall, worst undercut, or the sealing bead. Make one RTV cast in a convenient Shore A, and one LSR shot in the intended grade off a single-cavity soft tool (even a crude one). Put both in the mating part after the LSR’s post-cure. The one that still fits and still seals is the process you must budget. If they disagree, the RTV sample was a mood board.

RFQ sentence: “State whether the prototype is RTV vacuum-cast (name mold rubber and part compound) or LSR in a printed/aluminum tool (name LSR grade, post-cure, flash spec). Do not treat them as equivalent samples.”

FAQ

Can a vacuum-cast PU part prove LSR flash and shrink?

No. RTV casting copies shape at room temperature with rubber-split flash. LSR soft tooling is a hot cavity, metal shutoff, and heat-cured platinum shrink. Fit samples that pass in PU are how production parts flash, shrink, or miss hardness.

When is RTV vacuum casting the right sample?

When the question is shape, assembly choreography, or a photo: undercut-heavy models you are not ready to slide, fit checks that can stand a small argument, color and form reviews, or very small quantities where an LSR pail would be theatre. Tin tools forgive dirty prints and age more; platinum holds size and inhibits on uncured resin.

When must I spend on aluminum LSR instead of another RTV loop?

When the sample must authorize flash a customer will measure, gate witness, whether a thin fin fills, post-cure shrink into a housing, a true LSR grade, or a regulatory packet that cares about the compound. Handheld mockups do not need that.

Is a silicone part poured in an RTV mold production LSR data?

It can near a target Shore A. It does not give LSR flow, knit, or post-cure shrink. Do not life-test that part and call it production data.

What one-afternoon test settles the fork?

Make one RTV cast of the worst feature in a convenient Shore A and one LSR shot in the intended grade off a crude single cavity. Put both in the mating part after the LSR's post-cure. If they disagree, the RTV sample was a mood board.