The 9-Step Checklist I Use Before Hiring a Plastic Injection Molding Manufacturer or CNC Shop

Who this checklist is for

I have been handling custom manufacturing orders for 8 years. I have personally made and documented 7 significant sourcing mistakes, totaling roughly $18,400 in wasted budget. Now I maintain our team's pre-check list. At STIHL, we buy components that depend on plastic injection molding, injection moulding dies, aluminum precision machining, custom CNC machined parts, extension springs, and high tensile springs. This list is for anyone vetting a new supplier for those processes. It has 9 steps. Work them in order.

This is not a theoretical guide. It is the checklist I wish I had in 2017, before my first $3,800 mistake.

Step 1: Define the process before you define the supplier

Do not start with a supplier list. Start with the process. A plastic injection molding manufacturer is not the same as a CNC shop. An injection moulding die is tooling, not a part. Aluminum precision machining is different from aluminum die casting. Extension spring manufacturers and high tensile spring suppliers may overlap, but their testing equipment often does not.

Write down the process, material, tolerance, finish, volume, and inspection requirement. If you cannot fill that out, you are not ready to send an RFQ. You are just shopping.

Step 2: Send a real RFQ package, not a napkin sketch

Include 3D CAD, 2D drawings with GD&T, material spec, surface finish, annual volume, and target lead time. For injection molding, include the intended mold cavitation, gate location preferences, and resin grade. For springs, include load, rate, free length, solid height, and fatigue cycles.

In September 2022, I sent a spring RFQ with only wire diameter and outer diameter. I got three quotes and one 'sure, no problem.' The 'sure' vendor shipped springs that lost tension after 500 cycles. $1,900 wasted. Not ideal.

Step 3: Verify process-specific capability, not just ISO paperwork

Ask for the equipment list and recent sample parts. For a plastic injection molding manufacturer, ask about press tonnage, mold flow analysis, and resin drying. For an injection moulding die, ask who builds the tool and who maintains it. For aluminum precision machining, ask about 5-axis capability, in-process probing, and finishing partners. For custom CNC machined parts, ask about first article inspection and revision control. For an extension spring manufacturer or high tensile spring supplier, ask about load testing, salt spray, and fatigue testing.

Certificates matter. Capability matters more.

Step 4: Ask for the inspection plan before you place the order

This is the step most buyers skip. Ask: what dimensions will you check, how often, and with what equipment? If they say 'we check everything,' that is a red flag. A real plan lists critical dimensions, sample size, frequency, and records.

Material certs, dimensional report, and—critically—first article inspection. In that order. No FAI, no production.

Step 5: Confirm material traceability and certifications

For custom CNC machined parts and aluminum precision machining, request mill certs tied to heat numbers. For high tensile springs, request wire certs and heat treat records. For plastic injection molding, request resin lot numbers and any UL or RoHS documentation.

I am not a metallurgist, so I cannot tell you the exact alloy for every spring application. What I can tell you from procurement is: if the supplier cannot produce traceability within one business day, they probably do not have it.

Step 6: Nail down tooling ownership, maintenance, and transfer

If you are paying for an injection moulding die, you should know who owns it, where it lives, and what it costs to transfer. Get it in writing. Ask for maintenance intervals, spare cavity inserts, and what happens if the tool wears out.

In Q1 2024, a vendor claimed tooling ownership after a project ended. Legal review sorted it out, but it delayed the next production run by 3 weeks (ugh). Now we put tooling ownership in the PO, not in a side email.

Step 7: Run a pilot order and test communication

Before a full production release, place a small pilot. Not a sample—a real order with real paperwork. Watch how they handle questions, changes, and problems. If they are slow to answer during the pilot, they will disappear during a crisis.

On one pilot, we caught a wrong thread on 50 custom CNC machined parts. The supplier fixed it in two days. That responsiveness earned them the next 5,000-piece order. The cheaper shop that ignored emails did not.

Step 8: Check capacity, lead times, and contingency

Ask for their current on-time delivery rate and how they calculate it. Ask what happens if a press goes down or a spring coiler fails. A good supplier will tell you their backup plan. A great supplier will tell you what they do not do.

This is where expertise boundaries matter. The vendor who said, 'high tensile springs at that cycle life are not our strength—here is who does it better,' earned my trust for the aluminum precision machining work they did quote. I would rather work with a specialist who knows their limits than a generalist who overpromises.

Step 9: Get commercial terms in writing

Cover tooling cost, amortization, MOQ, price breaks, setup fees, revision charges, shipping terms, and payment schedule. Hidden costs (tooling trials, fixture changes, expedited freight) add up fast. Also confirm how they handle engineering changes.

Per FTC business guidance, claims like 'zero-defect' or 'aerospace-grade' must be truthful, not misleading, and substantiated. If a supplier makes a bold claim, ask for the data behind it. 'Trust me' is not a quality system.

Common mistakes and notes

Mistake 1: Assuming 'standard' means the same thing to everyone. In my first year, I assumed 'standard' finish meant the same for an aluminum precision machining quote. It did not. Cost me a $600 redo plus a week.

Mistake 2: Saving money on the wrong step. I saved $120 by skipping mold flow analysis on an injection moulding die. The tool ran, but warpage and weld lines showed up on 1,200 parts. Fixing the tool cost $5,600. The 'budget' choice looked smart until we saw the scrap rate.

Mistake 3: Using one supplier for everything. A plastic injection molding manufacturer may also offer CNC, but that does not mean it is their strength. Same for springs. Keep a primary and a backup. Redundancy saved us during a supply chain crisis.

Mistake 4: Skipping the pre-check list. We have caught 47 potential errors using this checklist in the past 18 months. The checklist takes 20 minutes. A missed requirement can cost 3 days of production delay.

If you only remember one thing: verify the process, the inspection plan, and the boundary. The supplier who says 'we do not do that well' is often the one you should keep for what they do do well.