A factory quotes from your files and builds from your files. Anything you leave unwritten gets decided by a production engineer you have never met, optimizing for his cycle time and whatever resin is already on his floor. That is not dishonesty. It is what happens when a document leaves a question open. A manufacturing specification exists to close every question that would otherwise be answered by someone whose incentives are not yours.

Good specs stay unambiguous when read by a non-native English speaker who has no idea what the product is for. Dimensioned drawings, tables, and annotated photographs travel that way. Prose does not.

What a Complete Spec Package Contains

A quotable, buildable package is a set of controlled documents, not a folder of exports. At minimum:

  • 2D drawings for every custom part, each with a revision number and date field.
  • Neutral 3D geometry (STEP) matched to the drawing revision.
  • A bill of materials naming every purchased component with manufacturer part numbers, not descriptions.
  • Material and finish specifications, including color targets and gloss levels.
  • Assembly instructions with torque values, adhesive cure conditions, and orientation photos.
  • An inspection and test plan stating what gets measured, how often, and by what method.
  • Packaging drawings, carton markings, and label artwork with placement dimensions.

The set as a whole is usually called the manufacturing data package, and its completeness is the single biggest predictor of whether your first production run matches your samples.

Drawings and Tolerances: Where Cost Hides

Send the 3D model, but never alone. It carries nominal geometry and nothing else — no tolerance, no surface requirement, no note about which face is cosmetic. A factory that gets only a STEP file builds to its house standard, commonly ±0.15 mm on small molded features and looser on long dimensions.

Tolerance every dimension that matters and explicitly release the ones that do not. A drawing where every dimension is tight raises your quote by twenty to forty percent, because the supplier prices inspection on all of it. Identify the six or eight critical dimensions, mark them, and let a general tolerance block cover the rest. If your parts must fit together across a stack of interfaces, run the stack-up arithmetic before you publish the numbers rather than discovering the problem at first assembly.

Use geometric callouts where fit depends on form and orientation, not just size. Position, flatness, and profile communicate intent that a plus-or-minus number cannot, and the concepts in GD&T basics travel well because the symbols are international. Everything a print must carry, from datum scheme to revision block, is laid out in what a manufacturing drawing must include.

Materials: Trade Name, Grade, and Supplier

"ABS" is not a material specification. There are hundreds of grades, and the cheap ones crack at your snap fits. Write the resin as manufacturer plus grade — for example, a named PC/ABS grade from a specific compounder — and add the property that actually drove the choice: notched Izod impact above a stated value, UL flammability rating, UV stabilization, food-contact compliance.

Then decide, deliberately, whether substitution is allowed. The honest position is that equivalents exist and blanket bans get quietly ignored. Write it as a process instead: any material change requires written approval and a sample lot before production, with a certificate of analysis on the substitute. Factories swap material because it saves them real money, and the countermeasures worth building into the agreement are covered in stopping a factory from swapping in cheaper material.

Cosmetics and the Golden Sample

Appearance is where written specs fail hardest, because tolerance language does not describe a sink mark. Handle it in three layers. First, classify surfaces: Grade A faces the user and gets the strict standard, Grade B is visible in use, Grade C is hidden. Second, define defect limits per grade in a table — maximum count and size of specks, scratches, flow lines, and knit lines, with a viewing distance and lighting condition stated. Third, approve physical golden samples, signed and sealed, with one set at the factory and one in your possession.

Color needs its own line: a measured target with a delta-E limit, the substrate named, and texture given as a supplier chip code rather than a word like "matte."

Acceptance Criteria the Factory Can Be Held To

A spec without acceptance criteria is a suggestion. State the sampling plan by name and level, with defined limits for critical, major, and minor defects. Then define each class for your product, because "major" means nothing until you say a missing gasket is major and a scuff on a hidden rib is minor. How the sampling math maps to your payment gates is in AQL inspection explained.

Add functional tests with pass criteria and equipment: drop height and orientation, hipot voltage and leakage limit, actuation force range, cycle life sample size. Require the data submitted, not a stamp. And require a first-article report before mass production, so a dimensional argument happens on ten parts rather than ten thousand.

Packaging, Labeling, and Change Control

Packaging is a spec, not an afterthought. Give inner carton counts, master carton dimensions and maximum weight, foam or pulp insert drawings, tape pattern, and the exact shipping mark artwork with placement. Freight is priced by cube, so an insert that shaves half an inch pays back across every container. The retail side of the same problem is covered in product packaging design.

Finally, write the change rule into the spec itself: no design, material, sub-supplier, tooling, or process change without a written engineering change order approved by you, and every shipment marked with the revision it was built to. Without that clause, drift is invisible until a field failure forces you to reconstruct which lot changed. The discipline that keeps revisions traceable on your side is described in the engineering change order process.

Get the Package Right Before the First PO

Projects House builds production-ready data packages for US clients through a global engineering and manufacturing network — drawings, tolerance schemes, material and cosmetic specs, and inspection plans written so an overseas factory has nothing left to guess. Send your current files and target volume through our contact form and we will tell you what is missing before it costs you a run.