Most “types of injection moulding” articles muddle three different things — moulding processes, mould tooling, and machine designs — and leave engineers more confused than when they arrived. This guide does it differently.
We cover the six injection moulding processes you’ll actually choose between on a real product programme: conventional, 2K (two-shot), insert, over-moulding, gas-assisted, and thin-wall. For each, you’ll see how it works, what it costs, where it fits, and when to walk away.
It’s written by the toolmakers and project engineers at Sino Manufacturing — a British-owned factory in Shenzhen that has been making things better for OEMs across automotive, medical, electronics, and lighting for over 20 years.
The 6 types of injection moulding at a glance
| Type | Best for | Volume sweet spot | Tooling cost | Unit cost | Key trade-off |
|---|---|---|---|---|---|
| Conventional | Single-material parts at any volume | 1k – 10M+ | $$ (baseline) | $ (baseline) | The default; limited to one material |
| 2K (two-shot) | Multi-material or multi-colour parts in one cycle | 50k – 5M | $$$$ (specialist machine + complex tool) | $$ | High tool cost; pays back at volume |
| Insert | Parts with embedded metal threads, contacts, or magnets | 5k – 500k | $$$ | $$$ | Cycle slowed by insert loading |
| Over-moulding | Soft-touch grips, seals, sealed assemblies | 10k – 1M | $$$ | $$ | Two cycles unless run on 2K machine |
| Gas-assisted | Thick-walled or hollow structural parts | 5k – 500k | $$$ | $ (less material) | Process control window is narrow |
| Thin-wall | Packaging, electronics housings, lightweight parts | 100k – 50M+ | $$$$ (high-grade steel, fast cycle) | $ (per part — at volume) | Fast machines required; not for short runs |
Conventional injection moulding (single-shot)
This is the default — and around 80% of the moulded parts in the world are made this way.
A single thermoplastic resin is melted, injected into a steel or aluminium tool, cooled, and ejected as a finished part. It’s the workhorse of plastic component manufacturing because it scales from a few thousand parts to tens of millions on the same tool, with cycle times measured in seconds.
Choose conventional when:
- Your part needs one material and one colour (or you can paint or print after)
- Volumes justify hard tooling but don’t yet justify a specialist process
- Tolerances and cosmetics are achievable with standard process control
Walk away when:
- You need two materials or hardness zones in one part — go to 2K or over-moulding
- Wall thickness is below ~1 mm at scale — go to thin-wall
- Embedded metal hardware is part of the design — go to insert moulding
→ Read more: Advantages of Injection Moulding
2K moulding (two-shot moulding)
2K moulding injects two different plastics into the same tool in two sequential shots, producing a single bonded part in one cycle.
It’s how a toothbrush gets its hard handle and soft grip without glue or assembly. It’s how a car interior trim gets two colours without paint. It’s how a medical device gets a rigid body and a sealing lip without a gasket.
The catch: 2K needs a specialist machine with two injection units and a rotating or indexing platen, plus a tool engineered for both shots. Tooling cost is roughly 2–3× a comparable conventional tool. The maths only works if you’re running the volumes to amortise it — typically 50,000 parts and up.
Where Sino runs 2K well: automotive interior controls, domestic appliance buttons and grips, sealed electronics housings, medical handpieces.
→ Read more: 2K Moulding Concept: when and why to use it
Insert moulding
Insert moulding places a pre-made component — usually a metal threaded insert, electrical contact, magnet, or sub-assembly — into the tool, then shoots plastic around it. The part comes out of the press already assembled.
It’s the right call when:
- You need a strong threaded fastener point in plastic (brass inserts in housings)
- Electrical contacts have to be encapsulated in a connector body
- Magnets, bearings, or shafts need to be permanently bonded into a moulded part
The trade-off is cycle time. Loading inserts — manually or robotically — adds seconds per cycle, and every second matters at volume. Tooling is also more complex because the insert must be located precisely and shielded from melt flow that could damage it.
Where it fits: electronic connectors, motor housings with bearings, automotive sensors, threaded plastic enclosures.
Over-moulding
Over-moulding is the close cousin of 2K, with one key difference: instead of running both shots in one machine cycle, the first part is moulded, removed, and placed into a second tool to be moulded over.
This is how you add:
- A soft TPE grip to a hard ABS tool handle
- A sealing membrane to a rigid housing
- A coloured accent to a moulded enclosure
The advantages over 2K: lower upfront tooling investment and the ability to over-mould onto parts you didn’t make yourself (purchased components, metal substrates). The disadvantage: an extra handling step and the risk of contamination between cycles.
The right pick when: volumes don’t justify a 2K machine, the substrate isn’t plastic, or the design is still evolving and you want flexibility.
Gas-assisted injection moulding
In gas-assisted moulding, plastic is injected to partially fill the tool, then nitrogen gas is injected to push the melt outward against the cavity walls. The result: a part with a hollow core where you’d otherwise have a thick, sink-prone section.
Why bother? Three reasons:
- Lower material cost — hollow sections use less resin
- Better cosmetic surfaces — gas pressure eliminates sink marks over thick ribs and bosses
- Faster cycle times — less plastic to cool
It shines on structural parts with non-uniform wall thickness — TV bezels, garden furniture, automotive handles, structural brackets. It struggles on small or thin-walled parts where there’s no benefit to a hollow core.
The honest caveat: the process window is narrow. Gas timing, pressure, and entry point all need to be dialled in carefully. This is one of the moulding methods where having toolmakers and process engineers in the same building genuinely matters.
Thin-wall injection moulding
Thin-wall moulding is conventional injection moulding pushed to its physical limit — wall thicknesses below 1 mm, sometimes as thin as 0.4 mm, on parts where speed and lightness are everything.
The applications are mostly in two camps:
- Packaging — yoghurt pots, IML containers, single-use cups
- Electronics — laptop housings, phone backs, smart device enclosures
Thin-wall demands:
- High-injection-speed machines (often 1,000+ mm/s)
- Hot runner systems (cold runners freeze before fill is complete)
- Premium tool steel with excellent thermal management
- Tight process control to avoid short shots and warpage
It’s also the only place where unit cost becomes more important than tool cost — these parts run by the millions, and a 0.5-second cycle reduction is worth more than a cheaper tool.
Choosing the right type by industry
| Industry | Most common types | Why |
|---|---|---|
| Automotive | Conventional, 2K, insert, gas-assisted | Cosmetic interior surfaces, multi-material grips, threaded mounting points, structural brackets |
| Medical | Conventional, insert, over-moulding | Tight cosmetics, encapsulated electronics, sealed handpieces, validated single-material runs for regulatory simplicity |
| Electronics | Insert, thin-wall, conventional | Encapsulated contacts, low-mass enclosures, EMI shielding |
| Lighting (commercial & domestic) | Conventional, 2K | Diffusers, reflector housings, two-tone fittings |
| Domestic appliances | Conventional, 2K, over-moulding | Buttons, grips, soft-touch controls, sealed housings |
| Packaging | Thin-wall | Speed and unit cost dominate every other factor |
What actually drives the cost of an injection moulded part
Three cost levers, in order of impact:
- Tool cost — a one-time charge that swings from a few thousand dollars for a simple single-cavity prototype tool to north of $250,000 for a high-cavitation, hot-runner production tool. It’s driven by cavity count, steel grade, complexity, and whether you need 2K, insert, or thin-wall capability.
- Material — the resin you specify often matters more than the process. Glass-filled engineering grades cost 5–10× commodity polypropylene.
- Cycle time — every second of cycle time multiplies across millions of parts. Cooling, ejection, robot load and unload, and gate freeze all live here.
A surprise to most first-time buyers: the cheapest tool rarely produces the cheapest part. A $20,000 tool that runs a 45-second cycle on a poorly cooled steel will cost more per part over a million-unit programme than a $40,000 tool that runs a 28-second cycle. We’ve watched this play out on real programmes — and it’s why we resist the cheapest-quote race.
Frequently asked questions
The six processes engineers choose between are conventional (single-shot), 2K (two-shot), insert, over-moulding, gas-assisted, and thin-wall injection moulding. Other terms you’ll see — hot runner, cold runner, split cavity — describe how the tool or runner system is built, not a separate type of moulding process.
Conventional single-shot injection moulding accounts for the large majority of moulded parts produced globally. It’s the default for any part that needs a single material and a single colour.
2K moulding produces a multi-material part in one machine cycle on a specialist 2K press. Over-moulding produces the same outcome in two cycles on conventional machines, with the first part transferred manually or robotically into a second tool. 2K has a higher tooling cost but lower per-part cost; over-moulding is the right pick when volumes don’t justify the 2K investment.
Start with the part: how many materials, what wall thickness, any embedded metal, what volume. Then work backwards to the process. If you’d like a second opinion, our engineers are happy to look at your CAD and tell you straight.
Talk to a factory that runs all six
Pick the wrong type of injection moulding and you’ll feel it in your tooling cost, your cycle time, or your quality reports for the next five years. Pick the right one and the part runs itself.
We’ve engineered, tooled, and produced parts using all six processes — for OEMs in automotive, medical, electronics, lighting, and packaging — out of our British-owned, ISO 9001:2015-certified factory in Shenzhen. With our new mould repair facility in Querétaro, Mexico, we’re also a credible China+1 partner for buyers nearshoring out of Asia.
Send us your drawings or a description of what you’re trying to make. We’ll sign an NDA, look at it properly, and come back with a process recommendation, a tooling estimate, and an honest read on whether we’re the right fit.
