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How to choose an injection molding factory: 9 checks before you place the mold order

October 25, 2026

Choosing an injection molding factory is mostly about one question: does this plant already run parts like yours, on presses and molds like the ones your part needs? A plant that does is cheaper to work with even when its quote is not the lowest, because the mold works sooner and the parts stay right on repeat orders. A plant that does not will learn on your mold.

The nine checks below are the ones that separate those two kinds of plant. Most can be answered from documents before you visit. The last three are best checked on the floor.

1. The press range fits your part

Every part needs a clamp force and a shot size. Clamp force keeps the mold closed against melt pressure. A quick estimate is the projected area of all cavities times a pressure factor for the resin, about 2 to 5 tons per square inch depending on resin and wall thickness. Shot size is the weight of all parts plus the runner.

Ask for the press list with brand, clamp force, screw size and year. Then check that your part lands in the middle of the range the plant runs every day. A 250 t part in a plant whose largest press is 280 t will run, but it will run at the limit. A 60 t part in a plant full of 800 t presses will be quoted on an expensive machine.

2. Someone answers for the mold

The mold decides cycle time, scrap and how long the part keeps measuring right. Ask who designs the mold, who cuts the steel and who repairs it. There are three healthy answers:

• An in-house tool room that designs and builds molds.
• A fixed partner tool shop the plant has used for years, with repairs handled by the plant’s own mold technicians.
• Your existing mold, transferred in, with the plant’s technicians checking it before the first run.

The unhealthy answer is a different tool shop for every job, chosen on price. When parts measure wrong, the molder will say it is the mold and the mold maker will say it is the process.

3. Resin is dried and traced

Many resins absorb moisture. PA, PC, PBT and PET must be dried to a set moisture level before molding or parts become brittle, splay or change size. Ask which dryers the plant has, by resin, and how drying time and temperature are recorded.

Then ask how resin lots are traced. A good plant can tell you which resin lot went into a given box of parts. It also has a written rule for regrind: how much, from which parts and whether it is allowed at all on your part.

4. Measuring matches your tolerances

A plant can only hold what it can measure. Calipers and height gauges are enough for many consumer parts. Tight positional tolerances, small features and curved surfaces need a CMM or a vision measuring system. Ask what the plant uses, whether gauges are calibrated on a schedule and who signs the inspection report.

If your drawing has tolerances tighter than ±0.05 mm, ask for a sample inspection report from a similar part the plant already makes, with customer names removed.

5. The sample process is written down

The first shots from a new mold are called T1. They almost never pass every dimension. What matters is how the plant gets from T1 to approved parts. Ask the plant to describe it:

1. Mold design review with you before steel is cut: gate location, parting line, ejector pin marks, cooling.
2. T1 samples with a full dimensional report against your drawing.
3. A list of mold corrections, with which dimension each one fixes.
4. T2 samples and a new report, repeated until the part passes.
5. A process sheet that records the settings that made the approved parts.

A plant that sends T1 parts with no report is asking you to do its inspection.

6. Quotes are built the same way

Two quotes for the same part can differ by a factor of two because they are not quoting the same thing. Before you compare, make sure every plant quoted:

• The same number of cavities.
• The same mold steel and expected mold life in shots.
• Hot runner or cold runner, stated.
• The same number of sample rounds included in the mold price.
• Piece price at the same order quantity, with packing stated.

The biggest gap is usually mold steel and cavities. Our guide to injection mold tooling cost covers how each choice moves the price.

7. The floor matches the documents

Now the visit. Walk the floor during a shift, not on a quiet day. Things to look at:

• Are the presses on the list running, and running parts in the size range you need?
• Are molds stored labeled, cleaned and rust protected?
• Is there a setup sheet at each running press?
• Do parts come off the press into clean, labeled containers?
• Is there a quarantine area for rejected parts, and is it used?

None of this needs an audit form. Ten minutes on the floor tells you whether the documents describe the plant or a plant the sales team would like to have.

8. Change control exists

On repeat orders the risk is not the first run but the fortieth. Someone changes the resin supplier, the cycle time or a mold insert to keep the press running. Ask how a process change is approved and whether you will be told before it happens. The right answer is a written rule that changes affecting your part need your approval, and a record of the last change made for another customer.

9. Export paperwork is routine

If the plant ships to you across a border, check that it issues a commercial invoice, packing list and certificate of origin without help, and that it packs for sea freight: inner bags, fixed carton counts, labeled cartons and pallets that fit a container. A plant that has never shipped overseas can learn, but not on your first order.

Questions that expose a weak plant early

Send these with your RFQ. The answers tell you more than the price does.

• Which of your presses would run this part and at what cycle time?
• Where would you put the gate and why?
• What draft do you need on the textured faces?
• Who owns the mold and how is it released if we move production?
• What is included in the mold price: how many sample rounds and corrections?

A plant that answers with specifics has thought about your part. A plant that answers only with a price has not.

Where a manufacturers’ rep fits

Running these nine checks on a plant you have never visited takes weeks. A manufacturers’ representative does them once, in person, and then matches your part to the plant whose presses and resins fit it. In our case each partner plant holds one lane, a band of press sizes and part types, so your RFQ goes to the plant that runs that work every day. See how the network is built, or send your RFQ and we reply within 24 business hours.

For a broader view of the process itself, the injection moulding article on Wikipedia is a good neutral overview of machines, molds and common defects.

Frequently asked questions

Keep reading

• Injection mold tooling cost: what drives the price and where you can cut it