When to Switch from Silicone RTV Vacuum Casting to Steel LSR Tooling

Stay on RTV vacuum casting until labor-per-part and dimensional scatter force you onto steel. Do not switch because a slide deck said “production.”

RTV casting (silicone mold, vacuum, hand pour of a part compound) and steel LSR (hot cavity, cold runner, clamp, platinum liquid silicone) are different factories. The switch is a process change: chemistry, shrink, flash, capital, and MOQ. Treat it as a date on a map, not as a promotion.

What you are leaving, and what you are buying

Leaving RTV means leaving a tool that stretches off undercuts, a near-zero shrink platinum jacket (if that is what you used), and a labor model where a technician is the press. Tin-cure jackets are cheaper and less stable. Neither is an LSR cycle.

Buying steel LSR means heat-cured platinum silicone, production shrink, a metal shutoff, a flash spec you can hold, and a tool class (typically 104 or 103 for a first production tool; 105 if someone is pretending). You also buy design freeze. Steel does not enjoy a weekly CAD tweak.

There is a missing middle people skip: aluminum 1-cavity LSR. If you jump RTV → steel, you are asking the most expensive tool to teach you LSR. Only do that when geometry, grade, and volume are already boring.

Four signals that RTV is done

Check these in order. Volume is last on purpose.

1. The part compound is no longer a stand-in.

If the product needs an LSR grade (medical, self-lubricating, conductive, UL, a specific compression set), every RTV-cast PU or generic silicone part is a lie. Switch process *now*, even at low quantity, via soft LSR tooling. Steel can wait. Chemistry cannot.

2. Scatter is eating the fit.

RTV parts wander with pour mass, vacuum technique, jacket wear, and (for tin) aging of the mold. When your drawing tolerances assume a process capability RTV does not have, more craftsmen will not help. LSR steel (or aluminum first) is a capability upgrade.

3. Labor per good part is the business.

Time the cell: mix, degas, pour, cure, demold, trim, post-work. If that stack already dominates material, scaling RTV is hiring. A steel 2- or 4-cavity does not have to be “cheap per tool” to win. It has to beat that labor curve.

4. Volume is real, and the CAD is frozen.

Only now does annual quantity matter. Tens of thousands of a frozen LSR part is steel. Hundreds may still be aluminum. Tens may still be RTV *if* chemistry and scatter allow.

If (1) or (2) is true and volume is tiny, you still leave RTV. You just do not have to buy Class 103 to do it.

Four signals you should not switch yet

• The lip, the undercut, or the brand logo is still in argument. Steel will be recut. Recuts on hardened cavities are the expensive kind.
• You have never shot the real LSR in *any* metal tool. You do not know flash, gate, or shrink. Buy aluminum or a serious printed insert first.
• Demand is a kickstarter spike, then silence. A Class 103 tool with an 8-cavity plate is how leftover steel becomes furniture.
• The “RTV problem” is inhibition on a dirty SLA master. That is a master-prep problem. Switching to steel will not un-inhibit a print, and it will not teach you RTV.

A crossover you can sketch without fake prices

Pick a horizon (12 months is enough). Add:

Stay RTV: jacket replacements + labor hours × lots + scrap from scatter + freight of bulky hand-packed parts.

Go aluminum LSR: 1-cavity aluminum + tryout + LSR pail policy + trim (hopefully less) + limited tool life (recut or replace).

Go steel LSR: class-based tool + cavitation you can feed + setup fees at your real batch size + amortization until the product dies.

Switch when RTV’s labor+scrap line crosses aluminum, then steel when aluminum’s maintenance+cycle line crosses steel’s amortization. If you cannot sketch those crossings with *drivers* (hours, cavities, expected shots, scrap %), you do not have a switch date. You have a feeling.

Cavitation belongs on the steel side only. An 8-cavity is not the switch. It is an option after the switch, when the press would otherwise run too many hours.

Geometry that fights the switch

Some parts are RTV because the shape is hostile to steel: deep undercuts with no slide budget, glove-mold pulls, one-piece skins. LSR steel will need slides, pick-outs, or a redesign. Price those slides *before* you declare RTV dead. A slide that is free on a whiteboard is a six-week delay in steel.

Conversely, thin flash-prone fins and high-volume gaskets are hostile to RTV. They want metal shutoffs. Do not keep casting them because the first samples looked romantic.

The sentence that should trigger the PO

When you can say all of this without hedging, cut steel:

“Grade is locked. CAD is frozen including parting and gates. Aluminum (or equivalent) LSR samples met flash, hardness after post-cure, and the fit. Twelve-month volume will feed at least a 1-cavity steel tool without silly batch sizes. Undercuts have a slide or a redesign.”

If any clause is false, you are not switching to steel. You are either staying on RTV, or you are buying aluminum.

What not to buy: a steel multi-cavity quoted against RTV sample photos. Those photos never saw a hot runnerless LSR shot. The tool will educate you at production prices.

FAQ

Is annual volume the first reason to leave RTV casting?

No. Check chemistry, dimensional scatter, and labor per good part first. Volume is last on purpose. If the product needs a real LSR grade, every RTV stand-in is already a lie, even at tiny quantity.

When do I leave RTV without buying Class 103 steel?

When chemistry or capability is already wrong but CAD, flash, and shrink are not boring yet. Switch process through aluminum or a serious printed LSR insert. Steel can wait. Skipping that middle asks the most expensive tool to teach you LSR.

Why shouldn't a Kickstarter spike trigger an 8-cavity 103?

Demand that vanishes leaves leftover steel as furniture. Also stay off steel while the lip or logo is still in argument, and while you have never shot the real LSR in any metal tool. Inhibition on a dirty SLA master is a master-prep problem, not a reason to cut steel.

What geometry fights the switch even when labor looks ugly?

Deep undercuts with no slide budget, glove-mold pulls, and one-piece skins. Price slides or a redesign before you declare RTV dead. Conversely, thin flash-prone fins and high-volume gaskets want metal shutoffs; do not keep casting them because first samples looked romantic.

What sentence should trigger the steel PO?

Grade locked, CAD frozen including parting and gates, aluminum (or equivalent) LSR samples met flash, hardness after post-cure, and fit, twelve-month volume will feed at least 1-cavity steel without silly batches, and undercuts have a slide or a redesign. If any clause is false, stay on RTV or buy aluminum.