Quote
How to get an accurate injection molding quote
A molding quote is only as accurate as what you send with it. This is the RFQ checklist and the reasoning behind each line, written from the quoting side of the desk.
What "accurate" actually means
A rough estimate and a firm quote are different documents, and confusing the two wastes everyone's time. A rough estimate can be given from a picture and a guessed volume: it tells you the order of magnitude, not a number for a purchase order. A firm quote needs geometry an engineer can actually price, because tooling cost is driven by the mechanisms the geometry demands, slides, lifters, cavity count, and piece price is driven by cycle time, which is driven by wall thickness. Neither can be estimated accurately from a photo or a sketch. If a supplier gives you a firm number from a picture, that number is a guess wearing a firm quote's clothes, and it will move once the CAD arrives.
The RFQ checklist
Geometry, as STEP
STL is a mesh: it discards the units and the exact curves the tool has to be cut to, so a quote from an STL alone carries more slack than one from STEP or IGES. If STEP is not available, say so and expect the number to be rougher.
A drawing for anything the model cannot carry
A 3D model shows shape; it does not show which dimensions matter, what finish a surface needs, or what a hole threads into. A PDF drawing with tolerances called out, material and grade specified, and any threads, inserts, or cosmetic requirements marked is what turns a shape into a specification. Without one, the molder guesses which features are critical and either over-quotes the whole part to be safe, or under-quotes and finds out at first article.
Material by grade, not family
"Nylon" is not a specification, PA66-GF30 is. Shrinkage, and therefore the tool dimensions and the tolerance the part can hold, changes with the grade, and a glass-filled grade shrinks and abrades tooling differently from an unfilled one. If the grade is not decided yet, state what the part has to survive, temperature, chemical exposure, a load, a drop, and let the molder propose one, but expect the quote to be conditional on the final grade.
Quantity now and quantity later
Prototype quantity, first production order, and expected annual volume are three different numbers that answer three different questions: prototype quantity decides whether a tool is needed at all, first order decides how the first tool run is planned, and annual volume decides tool class and cavitation, which is what the piece price you are quoted this year is actually built on. A quote given without an annual volume is pricing the tool against the wrong quantity, on purpose, because there is no other number to use.
Which dimensions carry a fit or a function
Calling out a tight tolerance block across a whole drawing does not make the whole part more precise, it makes the whole part more expensive, because every toleranced dimension adds machining time and inspection. Two or three critical dimensions, clearly marked, get a defensible number. An untouched drawing gets priced defensively, against the worst case on every dimension.
Certifications and documentation you need with the parts
A first article inspection report, a dimensional report, material certificates, a process capability study, PPAP: these are agreed before the order because they take engineering time to produce, and time is schedule. Asking for a PPAP submission after the parts have shipped does not get you one.
What happens to an accurate RFQ
Two numbers come back, stated separately: the tooling cost, paid once, and the piece price, paid on every part, plus a lead time to first article. Bundling the two into a single per-part number hides which one you are actually negotiating when you ask for a lower price. A design-for-manufacture review sits between the RFQ and the quote, not after it: an engineer checks wall thickness uniformity, draft on faces parallel to the pull direction, undercuts that would need a slide or lifter, and whether the tolerance called out is achievable in the material named, before the number is committed to. Catching a thin wall or a missing draft angle here costs a comment on a drawing. Catching it after steel is cut costs a tool rework.
An RFQ with complete geometry and a stated volume gets a reply from an engineer within 24 to 48 hours, a price, a lead time, and any design notes that would reduce either. An incomplete RFQ does not get a faster answer, it gets clarifying questions first, which is the same information this checklist asks for, just collected one round later.
What changes the number after it is quoted
A quote is firm against the inputs that produced it. Three things reliably move a firm number once it has been given, and none of them are unreasonable to change mid-program, they are just not free.
| Change | Why it moves the number |
|---|---|
| Material change | Different shrinkage can mean the tool needs adjustment, and piece price moves with resin cost. |
| A tolerance tightened after quoting | Adds machining and inspection time that was not priced into the original number. |
| Volume moving by an order of magnitude | Changes which tool class and cavitation actually make sense, in either direction. |
A supplier who quietly absorbs these on every program either prices everyone else's changes into your job, or does not stay in business.
The numbers behind the quote
These are Trumould's own figures, the same ones stated on the injection molding capabilities page, repeated here because they answer the questions an RFQ checklist raises.
- Minimum order quantity. No contractual MOQ. The practical floor is that a production tool rarely pays for itself under roughly 500 pieces, which is where CNC machining or 3D printing usually wins instead. A quote for 50 parts on a production tool is not wrong, it is quoting the wrong process for the volume.
- Lead time to first article. New tooling runs roughly 4 to 8 weeks from PO to first article, by tool class and complexity. A part running on tooling already owned can turn around in as little as 3 working days. The gap between the two is almost entirely the tool build, not the molding.
- Tolerance. Molded plastic tolerance follows DIN 16742, which sets bands by material rather than one number. An unfilled, low-shrink resin such as ABS, PC, or POM holds roughly ±0.2 mm at 100 mm nominal; a high-shrink resin such as PP loosens to roughly ±0.5 mm at the same size; critical, CMM-verified features can be held to ±0.05 mm where the design genuinely needs it. The full tolerance group table is on the capabilities page.
Common mistakes that cost a round of quoting
- Sending a rendering instead of CAD and expecting a firm number. A picture communicates intent, not geometry a machinist can cut to.
- Leaving out annual volume and being surprised the piece price looks high. A price quoted against a small first order, with no annual volume stated, prices the tool for that first order alone.
- Tolerancing the whole drawing because it seems safer. It is not safer, it is more expensive, and it does not tell the molder which two or three dimensions actually matter.
- Treating a rough estimate as a number to build a business case on. A rough estimate is directionally useful and nothing more; the business case should wait for the number that comes back from real geometry.
Questions
Frequently asked
The questions buyers ask before sending an RFQ. Each answer comes from the sections above.
Ask an engineerWhat is the difference between a rough estimate and a firm injection molding quote?
A rough estimate can be given from a picture and a guessed volume and tells you the order of magnitude only. A firm quote needs geometry a machinist can price, usually a STEP file, because tooling cost follows the mechanisms the shape demands and piece price follows cycle time, which follows wall thickness. Neither can be estimated accurately without the actual model.
Why does an injection molding quote ask for STEP files instead of STL?
STL is a mesh approximation that discards units and exact curves. STEP or IGES carry the true solid geometry the tool is cut to, so a quote built from STEP carries less uncertainty than one built from STL. If only STL is available, expect a rougher number.
How long does an injection molding quote take?
An RFQ with complete geometry and a stated volume gets a reply within 24 to 48 hours: a price, a lead time, and any design notes that would reduce either. An incomplete RFQ gets clarifying questions first rather than a faster answer.
What is the minimum order quantity for injection molding?
There is no contractual minimum order quantity. The practical floor is that a production tool rarely pays for itself under roughly 500 pieces; below that, CNC machining or 3D printing is usually quoted instead because no tool has to be paid for.
What can change an injection molding quote after it has been given?
Three changes reliably move a firm number: a material change, because shrinkage and resin cost differ by grade; a tolerance tightened after the fact, because it adds machining and inspection that was not priced; and a volume that moves by an order of magnitude, because it changes which tool class and cavitation make sense.
More
Related guides
What drives injection molding part cost
Tooling cost against piece price, and the volume crossover that decides which process wins.
Injection molding capabilities
Tooling options, materials, tolerances, finishes, and lead times, in full.
Injection molding design guide
Wall thickness, draft, ribs, bosses, gates, and ejector pins.
Send an RFQ against this checklist
Geometry as STEP, a drawing with the critical dimensions marked, and your volumes. We come back within 24 to 48 hours with a price, a lead time, and any design notes that would reduce either.