Engineering Answers / Tooling & Cost
Two quotes for "the same" silicone tool can differ by a factor of three. Almost all of that gap sits in four decisions — and only one of them is about the part.
A silicone mold price is set mainly by cavity count, steel grade and tool life, the area that needs mirror EDM or polishing, and the number of slides and shut-off faces. Tolerance and surface finish are priced per surface, not per part. Part size matters far less than most buyers assume, because a small part with a sealing bead and a mirror finish costs more to tool than a large plain one.
A silicone tool quote looks like one number. It is actually the sum of four decisions made before any steel is cut, plus a tail of smaller line items.
The reason two suppliers can quote the same drawing at 2× apart is almost never dishonesty. One of them has assumed a four-cavity tool with a mirror finish on two faces and one slide; the other has assumed a single cavity, a standard finish and a parting line in a different place.
1. Cavity count. This is the largest single line. Tool cost scales faster than linearly with cavities, because every added cavity multiplies the fitting, spotting and CMM work while the base and the clamping system grow too. A four-cavity tool is not four times a single-cavity tool — it is more, once the added alignment and clamp balance are priced in.
2. Steel grade and tool life. A pre-hardened NAK80 block at HRC 30–34 machines quickly and suits programmes of tens of thousands of parts. A hardened S136 or H13 insert at HRC 48–54 costs more to rough, heat-treat and finish, and delivers 100,000 to 500,000+ shots depending on compound and care. Buying the harder tool for a 20,000-unit programme is buying life you will not use.
3. The area that needs mirror EDM or polishing. This is where quotes diverge most on parts that look identical. A sealing bead, an optical face or a transparent part needs a cavity surface with no EDM recast layer and no polishing direction. That work is priced by area and by how inaccessible the area is — a deep rib with a mirror finish can cost more than the whole rest of the cavity.
4. Slides and shut-off faces. Every slide adds machining, a wear surface, a maintenance item and one more place where silicone can flash. Every shut-off face has to close to within a few microns. A design that avoids a slide and puts the parting line on a non-functional face is cheaper to tool and cheaper to run.
| Cost line | What it covers | How it moves |
|---|---|---|
| DFM and tool design | Parting line, feed, ejection, cooling layout | Fixed, and the cheapest place to remove cost |
| Mold base and clamping | Standard frame, alignment, clamp balance | Grows with cavity count |
| Cavity machining | CNC high-speed milling, then EDM | Grows with detail and hardness |
| Mirror EDM and polish | Functional and optical surfaces | Priced by area and accessibility |
| Fitting and spotting | Closing every shut-off face | Priced by face count, not part size |
| Trial shots and first article | Setup, sampling, CMM report | Fixed per tool, per trial round |
| Cold runner system | Reduces waste on LSR | Optional, pays back on volume |
The cold runner line deserves a note because buyers often strike it out first. On an LSR part it removes cured sprue from every shot, which is material that cannot be reground. The tool costs more and the programme costs less — on high-shot-count parts, the material saved every cycle typically outweighs the added tooling investment.
Part size on its own. A 90 mm plain gasket is a simple cavity. A 12 mm sealing bead with a mirror finish and a 0.05 mm tolerance is not. Buyers consistently expect the large part to be the expensive tool and are surprised the other way round.
Number of dimensions on the drawing. Only the toleranced, functional ones cost money. A drawing with two hundred general-tolerance dimensions is not harder to tool than one with twenty.
Material hardness of the part. A 70 Shore A part is not harder to mold than a 30 Shore A part in any way that shows up in tool cost. It shows up in ejection and in the demolding design.
Ask four questions and the comparison becomes real:
If a quote cannot answer all four in writing, it is not a comparable quote — it is a number. The gap between two numbers is not a discount; it is a difference in what is being built.
Because they move in opposite directions. More cavities means a more expensive tool and a cheaper part, and the crossover point depends on annual volume, not on the drawing. Presenting them as one blended price hides the decision the buyer actually has to make.
Spend on the shut-off faces and on the surfaces that touch a person. Those are the two areas where a cheap tool produces a defect the customer will feel.
Save on cavity count at launch, on steel hardness below the volume you actually have, and on finish everywhere that is not cosmetic or functional. Then revisit all three when the volume is real — a second tool built for volume is a normal, healthy step, not an admission that the first one was wrong.
Usually it is not more expensive — it is built to a different standard. Silicone finds a gap under 0.005 mm, so every shut-off face has to close to a tolerance that a plastic tool can leave looser. The extra cost is in the fitting and spotting of those faces and in the CMM verification, not in the block of steel. Where a plastic tool can run with a visible witness line, a silicone tool has to be flashed-free by design.
Only if the failure mode is wear. A tool built from pre-hardened NAK80 at HRC 30–34 machines faster and costs less; a tool built from S136 or H13 hardened to HRC 48–54 costs more and lasts longer in shots. If the programme is 30,000 units, the pre-hardened tool is the cheaper answer over its whole life. If it is 500,000 units with an abrasive filler in the compound, the hardened tool pays back before the first rebuild.
Move the parting line onto a non-functional face and remove one slide. Those two changes usually cut more from the quote than any negotiation, because both remove fitting and shut-off work. Reducing cavity count lowers the tool price but raises the unit price, so it is a cash-flow decision rather than a saving, and it should be made with the annual volume in front of you.