Finding injection moulders in Vietnam is no longer the hard part. Directories list them by the hundred, and any of them will accept your drawing. The trouble starts later: three quotations for the same part that cannot be compared, and a first lot that measures correctly but is rejected because “it doesn’t look right”.
Both problems come from the same place. The drawing — geometry and tolerances — crosses the border accurately. What the drawing does not say travels in emails and phone calls, and arrives as each factory’s own assumption. Four things decide whether a moulded part comes out the way you meant: the actual resin grade and what happens when it is not available, who holds the moulding conditions, where the cosmetic line sits, and how a trial shot becomes approved production.
Mould ownership, temporary import and return of the tool are a separate subject, covered in tooling and mould ownership in Vietnam.
💬 Have a moulder shortlisted but unsure what to send them? Share the drawing and the volumes and we will help you build a package every factory can quote on the same basis.
Why do quotations for the same part not line up?
Because every blank in the enquiry gets filled in by the factory’s own assumption, and each factory assumes something different.
Send “ABS, this geometry, a few thousand a month” to three moulders. One prices the general-purpose grade already running on its machines. Another assumes a flame-retardant grade because the part looks like an enclosure. A third quotes a mould with twice the cavities and a shorter cycle. None of them is being careless — a price cannot be produced without filling the blanks — but the gap between the quotations now reflects different products, not different factories.
Five variables move a moulding quotation more than any other:
| Variable | What to state in the enquiry | What happens if you leave it open |
|---|---|---|
| Resin grade | Resin maker, grade name, and whether substitution is allowed | Each factory prices a different material |
| Cavities | Your required number, or ask each factory to state theirs | Unit prices and tool prices cannot be compared |
| Machine and cycle | Ask for the press tonnage and cavity count behind the cycle time | A short quoted cycle may assume a mould you did not ask for |
| Scrap and regrind | Who carries reject cost; whether regrind is allowed and how much | Sprue and runner losses are hidden in different places |
| Secondary operations | Printing, painting, inserts, assembly, packing spec | One quotation includes work another leaves out |
Normalising the commercial lines — currency, VAT, Incoterm, payment — is a separate step, set out in comparing Vietnamese supplier quotations.
1. Specify the grade, not the resin family
“ABS”, “PP”, “POM”, “PC” or “PA” names a family of materials, not a specification; the maker and grade name, with property ranges where they matter, is what makes two factories mould the same thing.
Within one family there are heat-resistant, flame-retardant, high-flow and plating grades. Flow behaviour changes the moulding conditions, and with them sink marks and weld lines. Usable specification points:
- Maker and grade name. The single most useful line; it fixes most properties at once.
- Flow. Melt mass-flow rate, tested to ISO 1133-1.
- Mechanical properties. Tensile to ISO 527, flexural to ISO 178, Charpy impact to ISO 179-1. ASTM and JIS methods measure similar properties with different specimens and speeds, so do not declare them equivalent without checking.
- Flammability. The UL 94 rating, where the part sits in an electrical product.
- Restricted substances. For the EU, the RoHS Directive 2011/65/EU as amended by Delegated Directive (EU) 2015/863, which added four phthalates and has applied to them since 22 July 2019.
Agree the substitution rule before production, not during it
Much of the resin moulded in Vietnam is imported, and a grade you specified will not always be available locally on the day the factory needs it. Without a rule, the factory will mould with “an equivalent” and may not mention it. The fix is not to forbid substitution but to give it a procedure:
- State, part by part, whether a substitute grade is acceptable at all.
- Name who on your side approves a substitute — engineering, not purchasing alone.
- Require the substitute’s material data sheet with the request. A data sheet describes the grade; a certificate of analysis describes one lot. They are different documents.
- Take restricted-substance compliance as a separate declaration, not as an assumption from the data sheet.
- Say whether regrind may be mixed in and, if so, the maximum share, in writing.
Which documents to request at which stage — data sheets, certificates of analysis, declarations of conformity — is covered in quality documents to request from a Vietnamese parts supplier.
2. Put the moulding conditions on paper
Moulding conditions are developed on the machine and, left alone, live only in the setter’s head; take them as a written condition sheet at first-article approval so a different person can reproduce them.
The sheet should carry the barrel temperatures by zone and the nozzle temperature, mould temperature, injection speed and pressure, hold pressure and time, cooling time, cycle time, resin drying conditions, press tonnage and number of cavities. Drying matters more than it looks: hygroscopic resins such as PA and PC show splay and silver streaks when moisture is not driven off, and the drying record is often the first thing to check when a good part suddenly turns bad.
Ask for cavity numbers to be engraved in a multi-cavity tool. When defects appear, you need to know whether they come from every cavity or one, and without the number nobody can tell.
A local point worth planning for: job changes in Vietnam tend to cluster around the Lunar New Year holiday, and the person who knew how to set your mould may not be the one who restarts it afterwards. A written sheet survives that; memory does not. See planning production around Tết.
Some factories treat their full conditions as know-how. Even then, the drying conditions, mould temperature, cycle time, press and cavity count — what you need to reproduce the part and to trace a defect — are usually obtainable if agreed at the quotation stage rather than requested after a problem.
💬 Need the resin specification and the cosmetic standard written up before you go to quotation? That is part of how we run sourcing from Vietnam — send us the part.
3. Settle cosmetic acceptance with physical limit samples
Dimensions can be argued with a gauge; appearance can only be argued with words, and words drift between English and Vietnamese — so settle the cosmetic line with physical pass and fail samples.
The usual disputes are sink, warp, weld lines, flow marks, flash, gate vestige and splay. Demanding none at all is rarely realistic. The work is deciding how much is acceptable, and where:
- Which faces are cosmetic. Visible after assembly, touched by the user, or hidden — each gets its own standard.
- Viewing conditions. Distance, lighting, angle and time per part. Without them, two inspectors reach two results.
- Where the line sits. A limit sample — a boundary sample, mẫu giới hạn in Vietnamese — showing the worst acceptable condition and the first rejectable one, as physical parts.
| Way of agreeing appearance | How precisely it transfers | Drift across languages | Use during production |
|---|---|---|---|
| Written description only | Wide room for interpretation | High | Every borderline part becomes a discussion |
| Description with photographs | Shared idea, but photographs flatten gloss and texture | Moderate | Borderline parts still stop the line |
| Physical limit samples | Close to unambiguous | Low | The inspector decides at the bench |
Make two sets — pass side and fail side — so the buyer and the factory each keep one. Write the date on them: plastic yellows and deforms with age, and a dispute about whether the sample itself has degraded is avoidable if you have agreed when samples are renewed. The broader sample chain is in sample approval stages before a production run.
4. Gate the move from trial shots to production
A good part from a trial shot proves the mould can make it; it does not prove the process will keep making it, so separate the stages and decide in advance what each one must leave on record.
Factories name the stages differently, but they reduce to trial, correction, first-article approval and first production lot. Automotive programmes use frameworks such as PPAP; outside automotive, asking for them by name tends to slow the conversation rather than help it.
| Stage | What you confirm | Record to keep | If you cannot attend |
|---|---|---|---|
| Trial | Fill, appearance, rough dimensions | Trial conditions, photos of defects | Video of the run plus physical samples sent to you |
| Correction | What was changed on the tool and why | Change record with reasons | Samples from before and after the change |
| First-article approval | Dimensions, appearance and material all match | Dimensional report, condition sheet, resin lot number, retained samples | Third-party witness or a live video session |
| First production lot | Conditions have not drifted | Lot inspection report, defects by cavity | Pre-shipment inspection on terms agreed in advance |
If the condition sheet, the resin lot number and the retained sample all exist from first-article approval, a later defect can be traced to its cause — material, process or tool wear — instead of becoming a guessing match. Watch the first production lot most closely; that is where conditions move. How to set the inspection itself is in pre-shipment inspection and AQL from Vietnam.
💬 If you would rather have someone in Vietnam witness the trials and hold the records, tell us about the part and the timetable.
Frequently asked questions
1. The factory says my resin grade is not available in Vietnam. What should I do?
Do not accept a substitute on the spot. Ask for the proposed grade’s data sheet and compare flow, mechanical properties, flammability rating and restricted-substance status with your specification. If your named approver accepts it, run another trial to recheck appearance and dimensions before production restarts.
2. How many sets of limit samples do I need?
At least one pass-side and one fail-side set each for you and for the factory. Add a set for each additional inspection point, such as a third-party inspector. Date them and agree when they will be renewed.
3. Can I approve first articles without travelling?
Yes, if you replace presence with records: physical samples shipped to you, video of the moulding run, a live session at the press, or a third-party witness. Whatever the method, record in writing what you checked and against which revision of the drawing.
4. Is it reasonable to ask for the full moulding conditions?
You can ask; some factories will share everything and some will share only part. What you need at minimum is what allows reproduction and fault tracing — drying, mould temperature, cycle, press and cavities. Agree the scope in the quotation, not after a defect.
5. Should regrind be allowed?
It depends on the part. Where strength, appearance or regulatory compliance matters, many buyers do not allow it; where cost dominates, a capped share may be acceptable. Either way, write the decision and the maximum share down. If the order says nothing, mixed regrind is not a breach.
Sources: ISO 1133-1, ISO 527, ISO 178, ISO 179-1; UL 94; Directive 2011/65/EU and Delegated Directive (EU) 2015/863. Checked on 17 October 2026.