Common Mistakes in RTV Silicone Mold Design

If you are reviewing a custom RTV silicone mold, most recasts are drawing mistakes, not pour mistakes. The usual set: treating elongation as a draft waiver on a future part tool, forgetting that shrink is process-specific, parking a logo on a shut, writing Shore A as if it were a compound, and freezing CAD before a split or a pour direction exists. Each one is cheap to fix in STEP. Each one is a second resin production mother or a second pour in the shop.

This is a diagnostic list for RTV design reviews. It is not a gallery of every flash defect on an LSR steel tool. On this site the path is photos, sketch, or CAD, then a 3D-printed pattern, often a temporary silicone flip, a resin production mother, then batch RTV. The mother is not the pattern. Free DFM. HTV/HCR compression mold tooling is also quoted on RUUIPON.

Treating elongation as a draft waiver

Soft platinum will peel a zero-draft mascot that steel cannot eject. That is a fact about this pour. It is not a license to freeze zero draft on a later HCR gasket or an LSR consumer part. If the article might ever leave a press, put 1-3 degrees on walls that must still read. We will not freeze zero draft because a glove mold peeled.

Forgetting shrink is process-specific

Platinum RTV after a full cure is typically about 0.1-0.3% linear. Tin keeps moving in storage. HCR compression is often about 1.5-2.5%. LSR is often about 2-3%. Those last two bands are a different RFQ. Compensating a platinum mother with an LSR offset is how the candy comes out small and the gasket comes out large. Write the chemistry on the title block.

Putting the logo on the wrong face

A logo on the shut shears. A logo on a face that must peel through a closed letter tears the islands. Recessed brands that have to read often land around 0.3-0.5 mm. Parking the mark on the parting because the render looked centered is a recast against a new mother, not a mix-day tweak.

Specifying Shore A as if it were a material

Shore A is a grade family, not a compound and not a food-contact path. "50A food grade" without platinum vs tin, without 21 CFR 177.2600 extraction and/or LFGB sections 30/31, is a sticker. Soft 10A-20A skins need a jacket / mother-mold support shell on a wet dump. Raising Shore to "fix sag" before you add that shell is the wrong lever.

Freezing CAD before the split and pour exist

A STEP of the candy is not a mold file. Name peel direction, split / plug / core, and whether a support shell belongs. Catalog copy that says CAD, then resin, then RTV - skipping the flip - treats the first hard copy as the production mother. We will not quote that shortened sentence. CNC of a pattern, or a named pour-on-print, stays valid DFM when we write it down.

Common mix-up: one DFM checklist for RTV, LSR, and HCR

One plastic checklist sent to three shops is how you get a pretty poured block when you needed 5,000 compression gaskets. RTV is pattern / flip / mother / batch skin. HCR is flash land and preform. LSR is heat and, often, a cold-runner. Sharing a mascot outline does not share a tool.

Typical shop values

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

Worked example: 40 mm lollipop letter

Same geometry as the design guide, different failure: the buyer froze a one-piece glove on a 40 mm letter with negative-draft counters. Dump one tore the islands. We did not "just pour softer." Free DFM reopened the letter as a two-part. USD 50 covered the CAD edit. After the new CAD lock: pattern, often a flip, new resin production mother, batch RTV. Witness was a sugar dump that released. MOQ 20 of the corrected design, not twenty of the torn glove.

Stay on this page / skip this page

Stay when you are reviewing a drawing before mix day and want the recast-causing mistakes named.

Skip if you need the constructive sequence - design guide. Skip if the miss is a dump bubble in a glossy well - bubbles in the cast. Skip finished-part DFM - a different RFQ - ask us.

We will not freeze zero draft because a glove mold peeled. We will not copy RTV peel rules onto an LSR ejector. We will not treat Shore A as a compound. We will not pour on an unfinished print and call it a production mother.

Related: design guide, how molds are made, contact.

What to send

• Current STEP / 2D with the millimetres that already exist
• Where the last dump failed (tear start, flash, underfill, logo shear)
• Dump material; whether a later part tool is even a maybe
• Who owns shrink on the file today
• Food-contact path if needed: 21 CFR 177.2600 and/or LFGB sections 30/31

Send the miss and the file to [email protected] or contact. We will name which drawing line caused the recast.

FAQ

Is a torn logo a pour mistake or a drawing mistake? If the dry well is complete and the tear starts at a closed letter or a shut mark, it is a drawing mistake. Recasting the same mother copies the same tear.

Can you fix shrink by pouring a second skin harder? No. Shrink is the chemistry and the mother compensation. Shore does not cancel a wrong offset.

We already approved a pretty render. Is that CAD lock? No. CAD lock is millimetres, logo depth, fill line, Shore band, and structure signed. A render is lighting.

Does this list apply to HTV/HCR compression mold tooling? Only as a reminder that the tool is a different RFQ. Flash land and preform are not RTV peel. Do not send that mold-tool RFQ as an HTV product RFQ here, or ask us for a high-volume LSR tool; the finished part carton is a different RFQ - ask us.

What is the cheapest recast? The one you avoid in STEP. After CAD lock, a logo move is a new mother. DFM is free. CAD revision about USD 50 when we edit your 3D. Prototype usually 3-7 business days after the new lock. MOQ 20 per design.