What to watch for when ordering a 3D printed prototype from your own design

Watch four things above all when ordering a 3D printed prototype: its material can behave differently from the final part, the model needs correct units and marked critical dimensions, an FDM surface shows visible layers, and the lead time counts only from when the deposit is received. For a series, printing starts only after the prototype is approved.

This is based on how custom 3D printing works with us and on our terms and conditions (T&C), which we describe as worded, not as legal advice. The Slovak text of the terms is the binding one, the English version is an informative translation (T&C 13.5). The examples are illustrative situations, not real jobs. What to send with an enquiry in general is covered in the article on what to send for a quote.

Ten pitfalls in one table

A prototype material is not the final material

A PLA prototype verifies shape and dimensions, not behaviour under heat. According to the filament maker Prusa Polymers, for its branded filaments, PLA withstands 55 °C, PETG 68 °C and ASA 93 °C. The manufacturer rates PLA as not prone to warping, while ASA can warp on large parts.

Example: a customer orders a holder for a car cabin in PLA because it is cheap and quick. The shape fits, but after a summer in the sun the part deforms: according to measurements, the air in a parked car reaches roughly 50 to 60 °C, in worse cases more, which is at or above the PLA limit. Had the customer written “for a car cabin” in the enquiry, we would have suggested ASA, or PETG for a spot out of direct sun, with PLA used only to check the shape.

Under our terms, the customer should state mechanical, thermal and chemical requirements in the enquiry (T&C 7.3). More in our articles on materials for heat and load and on choosing a material.

Units, file and critical dimensions

STL carries no units. A model designed in inches and read as millimetres comes out 25.4 times smaller, and the other way round 25.4 times larger (Protolabs Network). Give the overall dimensions of the part in the enquiry, and we can compare them with the model during the check. The model must also be watertight: slicers repair most errors automatically, but not all, and the “make solid” function almost always works but loses resolution (Prusa Knowledge Base). For STEP versus STL, see our article on modelling software.

Mark the dimensions that must fit. For parts that mate, allow a clearance of roughly 0.3 to 0.5 mm: Prusa gives 0.3 mm as a starting point, a university facility in Zurich gives 0.5 mm, and both figures are fine-tuned with a test print. Vertical holes print smaller than modelled, so in practice a critical hole is printed undersize and drilled out (Protolabs Network). Shrinkage and warping depend on the material and the size of the part.

Tolerances are agreed in advance and form part of the quote. FDM has limits on accuracy; if you need a tolerance in hundredths of a millimetre, look at CNC machining (comparison).

Surface: layers, supports and the visible face

FDM deposits plastic in layers, so they stay visible (0.08 to 0.3 mm with us). A face printed directly on the bed is smoother, a face above supports rougher. Under our terms, visible layers, minor surface imperfections, support marks and shade differences between spools are not defects (T&C 7.1). For a consumer this applies if they expressly agreed to it when confirming the quote (T&C 7.5).

Example: an enclosure has a logo on a face that ends up above supports during printing, and it will be rougher. If you say so in advance, the part can be rotated or split in two and glued, though on small parts the joint is more visible. Assembly, gluing and finishing are charged separately from €10 per hour (price list).

Test pieces, iterations and the step to a series

Prusa Knowledge Base advises allowing for test prints and modelling parametrically, so that a dimension changes with one number rather than by editing a mesh. For an assembly, print only the critical section first, for example the mating seat. Modelling costs from €15 per hour and is charged only after you approve it (T&C 4.2); the minimum order is €10 excluding shipping. If you have no model or yours is faulty, see when to hand modelling to a specialist.

For series production or technically demanding parts, we start printing only after you approve the prototype or the first pieces by e-mail, and properties that match the approved prototype cannot be complained about later (T&C 3.5). Our recommendation, not the wording of the terms: approve the prototype in the same material and settings the series will use.

Example: a display enclosure is approved in PLA, and a run of 100 will be made in ASA because the enclosure will be outdoors. According to the manufacturer ASA warps more than PLA, so the shape may differ only in the series, and the approved prototype is not a good reference. The sequence prototype, pilot batch, tooling is described in our comparison with traditional manufacturing, and for the series, small-batch production helps.

Deposit, lead time, delivery and confidentiality

• Deposit: 60% of the price including delivery in advance, the remaining 40% before dispatch. Printing starts once the deposit is received (T&C 5.1 to 5.3).
• Lead time: a prototype usually takes 2 to 5 working days to make. The exact date is in the quote and runs from the day after the deposit is received (T&C 6.1).
• Delivery: personal collection in Bratislava or courier, the price is in the quote. Check the parcel on receipt. Business customers must report obvious mechanical defects or a mismatch in the number of pieces in writing within 48 hours of receipt (T&C 8.1). This does not apply to consumers, whose rights are covered by the complaints section.
• Cancelling after printing has started: for a business the deposit is non-refundable; a consumer cancelling for their own reasons pays the reasonably incurred cost of material and machine time (T&C 9.5, 9.6).
• Confidentiality: we use your files only to fulfil the order, do not publish them or pass them to third parties, and delete them 6 months after delivery, earlier on request (T&C 4.5, 10.2). We may publish a photo of the part as a reference unless you object; for confidential company development prototypes, objection is assumed (T&C 10.3). If you need a separate confidentiality agreement, say so in the enquiry. The customer is responsible for the rights to the model (T&C 4.4).

An error in your own model: who is responsible

The customer is responsible for the correctness, completeness and technical soundness of the model, dimensions and drawings (T&C 4.1). A product that matches the supplied model is not defective merely because the supplied materials were flawed (T&C 8.5). If a hidden defect in the model appears during production that could not have been detected in advance, we stop work and agree the next steps (T&C 4.3). That is why the basic check before quoting is free.

Towards a business customer we are not liable for strength, heat resistance or suitability of the part for its intended purpose (T&C 7.4), so put your requirements in the enquiry. Towards a consumer we are liable for conformity with the contract, including suitability for a purpose they communicated and we agreed to. Without a separate written agreement, 3D printed parts are not intended for use where their failure could endanger life, health or property (T&C 11.1).

Example: a model has a 0.5 mm wall that cracks during assembly. With a 0.4 mm nozzle one perimeter corresponds to a wall of about 0.45 mm, so it is a single thin ribbon of plastic. The part matches the model, so it is not defective. A free check could have caught such a wall, and in the next round an edited model will thicken it.

Checklist before you send the order

1. I have stated the purpose of the prototype, the environment, temperature, load and the visible face.
2. I know whether a prototype in a cheaper material will verify only the shape or also the behaviour.
3. The model is watertight, the units are right, and I have attached overall dimensions and both STEP and STL.
4. I have marked critical dimensions and clearances and allow for a test piece.
5. I know how many rounds of changes I plan and who will edit the model.
6. For a series, I approve the prototype in the series material.
7. My confidentiality requirements and wishes about photos are in the enquiry.
8. I understand the 60% deposit, the lead time counted from its receipt, and delivery.
9. I know I am responsible for the correctness of the model.

Send a model and expect a quote within 24 hours

If you need custom 3D printing including a prototype, send a model or at least a dimensioned sketch via the contact page. We usually send a quote with the lead time within 24 hours, and the basic printability check is free. More about prototypes is on the 3D printing of prototypes page, and the order flow is in How it works.