Rubber parts are small, cheap per piece and easy to underestimate. A gasket, an O-ring or an engine mount is often added to an order at the end, quoted from a sketch, and approved on a hardness reading. Then the O-rings arrive with a ridge of flash on the parting line, the hardness check on the finished part disagrees with the certificate, and a batch of mounts separates from its metal plate after a few weeks in service.
Most of these problems are not about the rubber recipe. They come from the process the factory chose, from a size or surface standard nobody named, from where and how hardness was measured, and — for bonded parts — from the metal side of the part. This article covers those points for buyers of moulded seals, O-rings and anti-vibration mounts from Vietnam. How to write the material call-out and its test list, who owns the compound and what a lot test report must show are covered in rubber and plastic parts made in Malaysia; that method applies unchanged in Vietnam, so it is not repeated here.
💬 Have a seal, O-ring or mount you want made in Vietnam? Send us the drawing and the operating conditions and we will help you prepare an enquiry that rubber moulders can quote on the same basis.
Which moulding process is the quotation based on?
Compression, transfer and injection moulding produce the same part at different tooling cost, flash level and consistency, so ask every factory to state the process behind its price.
Rubber is moulded by three main methods. In compression moulding a pre-weighed piece of uncured compound is placed in an open cavity and the press closes on it. In transfer moulding the compound is pushed from a pot through small channels into a closed cavity. In injection moulding it is fed by a screw into a closed mould. The choice changes the mould, the cycle, the amount of flash to remove and how evenly the part is cured.
| Compression | Transfer | Injection | |
|---|---|---|---|
| Mould cost | Lowest | Middle | Highest |
| Flash | Most; needs trimming | Less, but a pad of cured waste per shot | Least, depending on the mould |
| Labour per part | Highest (loading blanks by hand) | Middle | Lowest |
| Suits | Larger, simpler parts; small volumes | Parts with inserts, more complex shapes | High volumes, tight consistency |
| What to ask | How blanks are weighed and checked | How the transfer pad and runners are handled | Cavity count and cycle |
None is “better” in general. A compression mould may be right for a few hundred large gaskets a year; injection may be right for an O-ring you need in large numbers. The point is that two quotations on different processes are not comparable on unit price alone. Ask each moulder to state the process, the cavity count and whether the price includes trimming. Ownership and release of the mould itself are covered in tooling and mould ownership in Vietnam.
O-rings: name the size standard and the surface grade
An O-ring order needs a size standard with its tolerance class and a surface-imperfection grade, otherwise the factory supplies its own nearest size and its own idea of acceptable flash.
O-rings are among the few rubber parts with complete published standards. Use them:
- Size and tolerance. ISO 3601-1 gives inside diameters, cross-sections, tolerances and designation codes for O-rings in fluid power, with two tolerance classes for industrial use, class A being tighter than class B. Many buyers in North America use the SAE AS568 size series, and Japanese drawings often refer to JIS B 2401. Name the one on your drawing; nearly-equal sizes from different series are not interchangeable.
- Surface imperfections. ISO 3601-3 defines surface imperfections on O-rings — flash, offset, parting-line projection, excessive trimming, flow marks, non-fills, indentations — and grades the acceptance limits. Grade N is the general-purpose grade, grade S is for higher-performance applications, and grade CS is the strictest. If your order does not say, you will receive whatever the factory considers normal.
- Existing moulds. Standard sizes are often moulded in tools a factory already has. Ask whether a quoted standard size uses an existing mould; if so, there is no tooling charge, but you do not own the tool and the cavity layout is not yours to approve.
The ISO 3601-1 tables are based on tooling cut for a standard nitrile compound’s shrinkage. A compound that shrinks differently — some fluorocarbon or silicone compounds, for example — may need its own mould to hold the tolerance. If you change material on an existing size, ask whether the factory will use the same tool and what that does to the dimensions.
Flash and trimming: write the limit where it matters
Moulded rubber always has a parting line, so specify how much flash is acceptable and where, and agree the deflashing method, instead of asking for “no flash”.
Flash is the thin film of rubber that escapes where mould halves meet. It is removed by hand trimming, by tumbling, by tear-trim moulds designed to break it off cleanly, or by cryogenic deflashing — chilling the parts until the thin flash becomes brittle and blasting it off while the thicker body stays flexible. Each method leaves a different edge. Over-trimming can be worse than flash: a nick on a sealing surface leaks.
On the drawing:
- Mark the sealing surfaces and the faces where parting-line flash is not acceptable at all.
- Give a maximum flash thickness and projection for the other faces, or reference the ISO 3601-3 grade for O-rings.
- State whether the parting line may be moved by the mould maker to keep it off a sealing face.
- Ask which deflashing method the factory uses, and approve it at first-article stage rather than discovering it on a production lot.
Rubber dimensional tolerances generally follow ISO 3302-1, which recognises that moulded rubber cannot be held like machined metal. If your drawing carries metal-style tolerances on every dimension, expect either a refusal or an acceptance that cannot be met.
💬 Not sure which faces on your seal need a zero-flash note? We work through the drawing with you before it goes to the factory, as part of sourcing from Vietnam. Ask us about your part.
Hardness: on the part, on a slab, and by which method
Hardness measured on a finished part is often not comparable with the value on the certificate, so agree whether hardness is checked on a test slab or on the part, and by which method.
Hardness is the one property every buyer checks, and the one that causes the most needless disputes. A Shore A durometer needs a flat test piece of sufficient thickness. Pressed onto the curved cross-section of a small O-ring or a thin lip, it reads differently from the same compound in a flat slab. Neither reading is wrong; they are different measurements.
Settle three points:
- Where hardness is measured. On a flat slab moulded from the same batch and cured with the parts, on the finished part, or both. Write it on the drawing.
- Which method. ISO 48-4 for durometer (Shore) readings; ISO 48-2:2018 for IRHD, which includes a microtest method designed for thinner and smaller test pieces. For small O-rings, a microhardness reading on the part is often the only meaningful check on the finished item.
- What tolerance applies to which measurement. A tolerance agreed for slab hardness should not be applied unchanged to a reading taken on the curved part.
If your incoming inspection has only a handheld durometer, say so at first-article stage, measure the approved samples with your own instrument and record the result. Your future readings are then compared with your own baseline, not with a number produced by a different method.
Bonded mounts: the metal side decides the failure
A rubber-to-metal part fails at the bond more often than in the rubber, so the metal preparation, the bond test and the load-deflection check need to be specified as carefully as the compound.
Anti-vibration mounts, bushes and bonded seals are made by moulding rubber directly onto metal inserts coated with a primer and adhesive. The bond forms during curing. If the metal surface was not cleaned and prepared properly, or the adhesive was applied unevenly, the part looks perfect and fails in service.
Points to agree:
- Who supplies the metal parts. Inserts may be supplied by you, made by the moulder, or bought by the moulder from a local machine shop or press shop. Each route changes who answers for a corroded or out-of-tolerance insert.
- Surface preparation. Ask for the preparation sequence — degreasing, blasting or phosphating, primer and adhesive — and whether it is recorded per batch. A change of adhesive is a change that needs your approval.
- Bond test. ISO 813:2019 (90-degree peel from a rigid plate) and ISO 814:2017 (two-plate method) are laboratory methods for comparing compounds and adhesive systems. For production, agree a destructive test on sample parts from each lot, where the part is pulled or torn apart and the failure surface examined: a sound bond tears through the rubber, while bare metal or adhesive on the failure surface shows a bonding problem.
- Load-deflection. For a mount, the stiffness matters more than hardness. Specify the load, the deflection range you accept and how often it is checked.
- Corrosion protection of exposed metal after moulding, if the part will be used outdoors or shipped by sea.
Metal inserts bring their own drawing questions; for stamped brackets and plates see sheet metal and stamping in Vietnam, and for turned or milled inserts CNC machining in Vietnam.
Storage, packing and the cure date
Rubber parts age in light, heat and ozone, so ask for packing that protects them and a cure date on every bag, and plan stock rotation from that date.
ISO 2230:2026 gives guidelines for the inspection, record-keeping, packaging and storage of rubber products. The practical points for an order from Vietnam:
- Packing. Opaque, sealed bags or boxes; parts not stretched, folded or hung on hooks; O-rings not tied together with string or wire.
- Labelling. Part number, compound code, lot and cure date on each bag, matching the packing list.
- Transit. Sea freight from a tropical port can mean long periods of heat inside a container. Keep the parts away from sources of ozone and direct sunlight at both ends, and avoid packing them against electrical equipment.
- Rotation. Use the cure date, not the arrival date, for first-in, first-out at your warehouse.
Then build the lot checks — appearance against limit samples, dimensions, hardness by the agreed method — into pre-shipment inspection and AQL, and keep first-article samples from the sample approval stages at both ends.
The enquiry checklist for a rubber part
A rubber quotation is comparable only when every moulder quotes the same process assumptions, size and surface standards, hardness method and test frequency.
| Item | What to state | Where it is covered |
|---|---|---|
| Material call-out | Polymer, hardness, properties with test methods | Malaysia article |
| Process | Ask each factory to state it, with cavity count | This article |
| O-ring size | ISO 3601-1 class, AS568 or JIS B 2401 size | This article |
| Surface | ISO 3601-3 grade, or flash limits by face | This article |
| Hardness | Slab or part, ISO 48-4 or ISO 48-2 | This article |
| Bonded parts | Insert supply, surface preparation, bond and load tests | This article |
| Mould | Ownership, marking, release | Tooling article |
| Commercial lines | Currency, VAT, Incoterm, payment | Quotation comparison |
For plastic profiles and seals made by extrusion rather than moulding, see plastic extrusion in Vietnam.
💬 For each project we find, vet and manage the Vietnamese rubber moulder that fits the part and the volume. Tell us about your seal or mount.
Frequently asked questions
1. Why does my hardness reading not match the certificate?
Most often because the certificate value was measured on a flat slab and yours on the finished part, or by a different method. Agree where and how hardness is measured, and record your own reading on the approved samples as a baseline.
2. Can I order standard O-rings without paying for a mould?
Often, yes. Factories hold moulds for common standard sizes. Name the size standard and tolerance class so you get the size you mean, and accept that you will not own or approve the tool.
3. Is “no flash” a reasonable requirement?
Not as a blanket rule. Every moulded rubber part has a parting line. Mark the faces where flash is unacceptable, give a limit for the rest, and agree the deflashing method at first-article stage.
4. How do I check that a bonded mount will not separate?
Agree a destructive test on samples from each lot, and look at the failure surface: tearing through the rubber indicates a sound bond, bare metal does not. Add a load-deflection check, and treat any change of adhesive or surface preparation as a change needing approval.
5. How long can moulded rubber parts be stored?
It depends on the polymer and the storage conditions. Ask the factory for its guidance by compound, follow the storage guidelines of ISO 2230, and rotate stock by cure date.
Sources: ISO 3601-1:2012 (with Amendment 1:2019); ISO 3601-3; ISO 3302-1; ISO 48-2:2018; ISO 48-4; ISO 813:2019; ISO 814:2017; ISO 2230:2026. Checked on 19 October 2026.
More industrial sourcing guides: Plastic Extrusion in Vietnam: Dies, Dimensions and Regrind · Fasteners and Springs From Vietnam: Classes, Coatings, Lots · Thai Metal and Plastic Parts for Appliances and Machines. All guides by category are on the industrial sourcing page.