Before ordering, ask an injection molding supplier for the exact resin grade, mold steel, cavity count, expected mold life, dimensional capability, sampling process, inspection method, production capacity, lead time, and tooling ownership terms. A mold quoted at $8,000 and another at $15,000 may not be comparable if one uses softer steel, fewer cavities, or a shorter service target. Ask which dimensions will be measured, how often, and with what equipment. For volume programs above 100,000 parts per year, also request maintenance records, lot traceability, process settings, and a written plan for handling rejected parts or mold repairs.
Start with the part itself rather than the supplier’s company presentation. Send the 3D model, 2D drawing, expected annual volume, resin requirement, cosmetic standard, assembly conditions, and operating temperature. A supplier that reviews only the CAD file may miss requirements that cannot be seen in geometry, such as a ±0.05 mm fit, UL 94 flame rating, or exposure to 80°C service temperatures.
That review should lead into manufacturability. Ask the supplier to mark draft angles, thick sections, ribs, bosses, gate location, ejector marks, weld lines, sliders, lifters, and areas likely to distort after cooling. Many molded parts use around 1° to 2° of draft on smooth surfaces, while textured surfaces may need more, depending on depth, resin, and tool finish. A supplier should explain the reason for any requested design change before machining starts.
Wall thickness deserves a separate discussion because molding behavior changes when one area is much thicker than the surrounding section. Thick sections cool more slowly and can form sink marks or internal voids, while thin sections may freeze before the cavity is completely filled. If a housing uses a 2.0 mm wall but has a 5.0 mm boss base, ask whether the boss can be cored out or supported by ribs instead.
Ask the supplier to show the expected gate position, weld-line location, ejector layout, and likely cosmetic marks on the actual 3D model before approving tooling.
Once geometry is reviewed, move to material. “ABS,” “PP,” or “nylon” is not enough for a purchase order because grades within the same polymer family can differ in viscosity, shrinkage, impact performance, reinforcement, flame resistance, UV stability, and price. Ask for the manufacturer name, commercial grade, datasheet, color reference, additive package, and percentage of glass fiber or mineral filler where applicable.
Moisture handling also matters for materials such as nylon, polycarbonate, PET, and some thermoplastic polyesters. Ask how resin is dried, whether dew point is monitored, and how long dried material can remain exposed before molding. A supplier processing a hygroscopic resin without controlled drying can produce splay, bubbles, molecular degradation, or lower mechanical strength even when the finished part looks acceptable.
Material selection leads directly to the mold specification. Request the steel grade for the cavity, core, inserts, sliders, and mold base instead of accepting a description such as “hardened steel mold.” A tool intended for 20,000 parts does not need the same construction as one expected to exceed 500,000 cycles. Glass-filled resin may also justify harder wear surfaces than an unfilled polypropylene part.
| Item to confirm | What to request before ordering |
|---|---|
| Mold material | Exact steel grade for cavity, core, inserts, and wear areas |
| Cavity count | 1, 2, 4, 8, or other confirmed configuration |
| Runner system | Cold runner or hot runner |
| Mold life | Agreed cycle or shot target |
| Cooling | Cooling-channel layout and connection standard |
| Moving features | Number of sliders, lifters, or unscrewing mechanisms |
| Spare parts | Ejector pins, springs, inserts, seals, or other service items |
| Maintenance | Inspection interval and maintenance responsibility |
Cavity count affects cost, output, and consistency. A four-cavity mold running on a 30-second cycle has a theoretical rate of 480 parts per hour: 120 cycles per hour multiplied by four parts. Actual output will be lower after startup, inspection, maintenance, material changes, rejects, and machine downtime. Ask the supplier to quote realistic hourly and monthly capacity rather than theoretical production alone.
Capacity should then be matched to machine size. Request the intended machine tonnage, shot size, screw diameter where relevant, and whether a second machine is available if the primary machine is unavailable. A 2024 production plan based on one fully occupied press may leave little room for an urgent order, mold repair, or seasonal increase. Ask how much of the quoted capacity is already committed to other customers.
Tooling ownership should be documented before the deposit is paid. State whether the mold becomes your property after full payment, where it will be stored, whether storage is charged, and whether it can be moved to another molding company. Also ask who owns replacement inserts, inspection fixtures, electrodes, CAD files, and mold drawings created during the project.
If the relationship may involve long-term production, a Custom tooling and molding supplier should also be able to describe mold maintenance in measurable terms. Ask whether maintenance is scheduled by shot count, operating hours, or inspection condition, and whether the supplier records lubrication, vent cleaning, ejector wear, cooling-channel condition, slider wear, and replacement history. For a mold expected to run 1,000,000 shots, maintenance records can be more useful than a verbal statement that the tool is “maintained regularly.”
Next, define dimensional acceptance. Do not ask whether the factory can make “precise parts.” Mark the dimensions that affect assembly, sealing, alignment, movement, or product function. If a drawing contains 60 dimensions but only six affect fit, tell the supplier which six require closer process and inspection control.
ISO 20457:2018 provides guidance on tolerances for molded plastic parts, but real production capability still depends on resin, geometry, mold construction, shrinkage behavior, conditioning, and measurement method. A ±0.02 mm requirement on a short molded feature may be possible in some applications, while the same tolerance across a 250 mm housing may require a different design or machining step. Ask the supplier to comment on each unusually tight tolerance before tool approval.
Measurement methods should be agreed at the same time. Calipers may be sufficient for a non-sensitive external width but may not be appropriate for a small bore, positional requirement, profile, or flatness check. Ask whether the supplier will use pin gauges, height gauges, optical measurement, CMM inspection, custom fixtures, or functional gauges, and how many samples will be measured during initial approval.
For a first-article inspection, ask whether the report covers every drawing dimension or only selected dimensions, and confirm the sample quantity before the mold trial.
Sampling terms can create unexpected cost if they are vague. Ask how many mold trials are included, how many sample parts will be supplied at each trial, and whether dimensional reports, molding parameters, material certificates, photos, and defect notes are included. Some projects may need 10 to 30 pieces for dimensional review, while assembly, drop, leak, load, or environmental testing can require more.
The agreement should also separate supplier corrections from buyer-requested revisions. If the approved drawing specifies 50.00 ±0.10 mm and the molded part measures 50.35 mm, correction normally belongs to the tooling or process side. If the buyer later changes the target from 50.00 mm to 49.50 mm, that is a design revision and may require paid modification.
After samples pass, ask how the approved molding conditions will be recorded. Typical records include melt temperature, mold temperature, injection speed, transfer position, packing pressure, packing time, cooling time, screw recovery, back pressure, and cycle time. If a 32-second approved cycle later becomes 24 seconds to raise output, part dimensions, shrinkage, internal stress, or appearance may change.
Quality control should be equally specific. Ask what is checked at startup, during the run, and before shipment. A useful plan may require first-piece approval, dimensional checks every 2 hours, appearance inspection every production lot, and final sampling based on an agreed inspection level. The exact frequency should match part risk and volume rather than being copied from another product.
For appearance, provide accepted and rejected reference samples when possible. “No scratches” can be interpreted differently by two inspectors. A better specification states the inspection distance, lighting condition, viewing time, permitted defect size, color tolerance, and which surfaces are customer-facing. In 2026, digital color measurement and controlled lighting are common enough that color-sensitive programs should not depend only on visual judgment.
Traceability comes next because inspection has limited use if rejected production cannot be isolated. Ask whether carton labels or internal records can identify the production date, machine, mold, cavity, material lot, shift, inspection record, and process version. If a defect is later found in 2,000 shipped parts, good lot information may allow the supplier to isolate one production period rather than reviewing several months of output.
Production records should connect to change control. Ask how drawing revisions, resin changes, mold repairs, process adjustments, color changes, and packaging revisions are approved. If revision C replaced revision B in 2025, the factory should have a method that prevents an old drawing, fixture, insert, or inspection sheet from returning to production.
Packaging also needs numbers. Define pieces per tray or bag, pieces per carton, maximum carton weight, surface protection, labels, pallet pattern, and any humidity or dust restrictions. A transparent polycarbonate lens that passes inspection can still arrive with scratches if 200 parts are packed loose in one bag. Painted or plated parts may need individual cells, film, or separators.
Finish by asking for a quotation that separates tooling, molding, resin, setup, secondary operations, inspection, assembly, packaging, freight, and optional work. Compare suppliers only after those items use the same assumptions. A $0.18 unit price based on 200,000 pieces per order cannot be compared fairly with a $0.24 price based on 20,000 pieces.
Request written lead times for DFM, mold design, steel purchase, machining, mold assembly, first trial, modification, approval, and mass production. If tooling requires 5 weeks and customer testing requires another 2 weeks, the realistic schedule is already at least 7 weeks before allowing time for corrections. Written milestones make schedule changes easier to identify than a single promise such as “delivery in about one month.”
Before releasing the purchase order, the supplier should be able to provide exact answers on resin grade, mold construction, expected life, dimensional acceptance, measurement equipment, sampling quantity, process records, capacity, maintenance, ownership, packaging, and lead time. If several answers remain verbal, move them into the drawing, quotation, tooling agreement, inspection specification, or purchase order before the first steel is cut.