How to Specify Tolerances on an Injection Molding RFQ

What Is a Tolerance Note on an Injection Molding RFQ?

A tolerance note on an injection molding RFQ tells the quote which dimensions must be held and which can stay on the published commercial band. The CAD file carries nominal geometry, and acceptance lives on the drawing or RFQ notes. Without that note, review fills a default that may not match your assembly.

Open the published table on NetProto commercial injection molding tolerance table when you need the size-class bands. Open when tight tolerances are worth it on molded parts when you want the exception list. For the standard versus tight scope choice, read standard vs tight on a quote. This page only covers how to write the RFQ field that sits next to a CAD upload.

The note is a scope statement, not a quality slogan. Hold tight and precision part do not tell a reviewer which snap, seal, or alignment pin must be inspected. A useful note names the feature, points at a balloon or face, and says whether the rest of the envelope follows the commercial table.

A blank tolerance field is still a decision: it means commercial. If your assembly needs a tighter mate, that mate is invisible until you write it. If your print already carries a drawing standard, name that standard so the quote and the drawing do not argue later.

Think of the RFQ note as the inspection plan in one paragraph. The solid shows shape. The note shows which edges you will measure and which edges you will accept on the published band. When those two documents agree, first-article balloons match the tool you released.

Key Facts

1. NetProto follows the commercial injection molding bands on manufacturing standards unless tighter features are approved in writing.
2. The cleared size-class bands are +/-0.10 mm, +/-0.15 mm, +/-0.25 mm, and +/-0.40 mm, tied to SPI commercial practice.
3. CAD is nominal. Critical mates, seals, and snaps belong on a 2D drawing or inspection list.
4. A blanket tight decimal on every edge is not a quality plan. It is an incomplete RFQ.
5. Tighter than the commercial table needs a named face and a reason, then engineering review.
6. Cite the published commercial bands or a named drawing standard. Do not invent unofficial micron targets for the quote field.
7. Upload CAD through the injection molding instant quote platform.

What to Cite Instead of Inventing a Number

Commercial band is published: attribute it, do not reinvent it

When a print is silent, cite the commercial injection molding table on NetProto commercial injection molding tolerance table. That table is the published source for the four size-class bands unless you approve a tighter feature in writing. The cleared pairs, tied to SPI commercial practice, are +/-0.10 mm (0.004 in), +/-0.15 mm (0.006 in), +/-0.25 mm (0.010 in), and +/-0.40 mm (0.016 in). Attribute those figures to that page. Do not invent a fifth band because a metal prototype used a tighter title block.

CNC or metal-prototype title block is not a plastic tolerance

Do not copy a CNC default onto plastic. A machined housing can hold a decimal that a molded snap will not hold on the commercial band. If the metal prototype used a general title-block tolerance, rewrite the title block for molding before you upload. The quote reads the RFQ and the drawing you send now, not the shop that cut the prototype. Plastic shrinks. Metal gets cut. Same decimal, different physics.

CTQ list is snap, seal, datum, alignment, not every edge

Open when tight tolerances are worth it on molded parts for the exception list. That spoke owns which mates deserve extra control. This page does not reprint that list. If a face is on that exception list, name the face here and point at the drawing balloon. If it is not, leave it on the commercial band.

Hold tight is a slogan. Snap A +/-0.15 per manufacturing-standards table is a spec.

If the print already cites a drawing standard, name that standard in notes. Write the standard number and the revision of the print. Reviewers can then match the quote field to the title block instead of guessing which document wins.

A worked example helps more than a slogan. A handheld enclosure has a snap fit to a battery door, a seal groove for a gasket, and a large cosmetic wall. The snap and the seal groove belong on the drawing or inspection list. The cosmetic wall can stay on the commercial table. The RFQ note can read: Non-critical dimensions follow the commercial table on manufacturing standards. Hold snap A and seal groove B to the balloons on drawing 100-revC. That is a complete tolerance note. Every edge to three decimals is not.

Another miss is pasting a prototype inspection report into the quote and calling it a tolerance plan. The report measured a machined or printed stand-in. Molded shrink, gate location, and pack-out will not repeat those numbers. Cite the commercial table for the envelope, then name the two or three mates that the assembly actually uses.

What to tell sourcing on citations: ask them to paste the manufacturing-standards sentence into notes. Ask them not to type an unofficial micron target because a prior shop used it. If a buyer wants a drawing standard you already released, name that standard and attach the print. Do not let the quote field become a second, conflicting title block.

CAD Versus the Drawing

STEP carries nominal shape. The note carries what gets inspected

The solid sets volume, shutoff, and moldability. Reviewers use the CAD to see walls, bosses, and how the tool will open. They cannot see which edge is a critical mate unless you mark it.

The drawing or notes set which features are accepted. A STEP file can start the quote. Tight mates still need callouts. If you are deciding whether paper is required, read when a 2D drawing is required for quoting. A 3D file can start the quote. Tight mates still need callouts.

When CAD rev C and drawing rev B disagree, write which wins

When the solid and the print disagree, write which document wins on that feature. A common miss is a fillet that grew in CAD after the print was released. The quote will read the solid you uploaded. Inspection will use the print you attached. Those two files must share a revision ID.

Worked example continued: the engineer exports housing_100.step from the latest solid. The drawing is still 100-revB and the snap land changed. The RFQ says see drawing. Reviewers will inspect the old snap. Export the matching print or write CAD rev C wins on snap A; drawing 100-revC attached. If the snap land then raises a flag, follow what to do when DFM flags your part.

Resin family in the note. Shrink is not guessed from the solid alone

Name the resin family so shrink is not guessed from the solid alone. Color is not a substitute for family.

The same rule applies when the drawing is tighter than you intended. A leftover title-block default from a metal revision can mark every hole as a critical feature. If those holes are clearance holes, say so. If only two are alignment holes, balloon those two and state that the rest follow the commercial table.

Common mistakes in this split:

• Uploading CAD only and assuming every modeled decimal is an inspection target.
• Attaching a drawing with a title-block tight default and never listing the two faces that actually matter.
• Changing the solid, keeping the old print, and asking why first-article balloons do not match the tool.
• Putting the mate list in email and leaving the quote notes blank. The next reviewer will not see the email.

What to tell sourcing: the first upload can be CAD plus a short note. If mates exist, the package is not complete until the current 2D revision is attached or the balloons are typed into the RFQ. Do not ask them to select a whole-part tight option as a substitute for the print.

RFQ Checklist

Use these numbered steps as the paste block for the quote notes.

1. State that non-critical dimensions follow the commercial table on manufacturing standards.
2. List only mating, seal, snap, or alignment features that must be tighter.
3. Attach or reference the current 2D revision when those mates exist.
4. Name the resin family so shrink is not guessed.
5. Upload CAD through the injection molding instant quote platform.

Use the five steps as a paste block. A complete note looks like this: Envelope and cosmetics follow the commercial table on manufacturing standards. Tight: snap A and datum pin B per drawing 100-revC. Resin: PC-ABS, color to follow. CAD file housing_100_revC.step matches drawing 100-revC.

Worked example, full package: a product manager sends the STEP, the drawing, and a one-line note that says standard tolerances. The buyer later rejects the battery door because the snap sits on the commercial band. That is not a quote error. The snap was never named. The fix is a new note on the same CAD revision, then a re-quote so inspection scope matches the mate.

If you only change notes, keep the same CAD revision ID. If you change the snap geometry, export a new solid and a matching print, then upload again. Reviewers compare records. A silent geometry change under the same filename looks like the same package.

Common mistakes on the checklist:

• Marking every edge tight so the file looks serious. Reviewers will treat it as an incomplete RFQ.
• Leaving the field blank and hoping the title block is enough. Title blocks get missed when they are not called out.
• Naming a resin color but not the family. Shrink depends on the family.
• Uploading an assembly root and expecting the quote to find the one molded housing inside it.
• Writing see email instead of pasting the mate list into the notes field.

What to tell sourcing: ask them to paste the five-line checklist into the quote notes. Ask them not to convert the whole part to a tight selection. If a supplier treats tight as a whole-part upgrade, send them the commercial table and the named mate list instead. The next review should reflect the features you named, not a slogan.

Keep the package on one record. Chat messages and slide decks do not travel with the quote. The notes field does.