Injection, Blow, or Rotational Moulding: Which Process for Which Part?

Injection moulding, blow moulding and rotational moulding produce very different kinds of plastic parts, and the right process depends on a number of factors, including part geometry, required properties, annual volumes and economics. 

Injection moulding is best suited for solid, precise, high-volume parts. Blow moulding produces hollow containers like bottles and tanks. Rotational moulding is for large, hollow, lower-volume products where the part is too big or too complex for blow moulding to handle economically. Identify your manufacturing process first, and everything else (tool design, material choice, supplier shortlist) follows naturally.

This guide walks through the three processes side-by-side, the kinds of parts each is suited to, the volume and cost implications of each, and how to work out which one fits your project.

Injection moulding

Injection moulding is the most common plastic manufacturing process in the UK. Molten plastic is injected under high pressure into a closed metal tool, cooled, and ejected as a finished solid part. The process produces highly repeatable, dimensionally-accurate parts at cycle times ranging from a few seconds (small packaging components) to a minute or more (large technical parts).

Best for

• Solid or semi-solid parts with complex geometry
• Tight dimensional tolerances (typically ±0.05mm on critical features)
• High annual volumes (from 1,000 upwards; sustained runs of millions are routine)
• Cosmetic parts where surface finish straight from the tool matters
• Technical parts in engineering polymers, medical grades, and reinforced materials

Not suited to

• Hollow parts above about 500ml internal volume (impractical compared to blow moulding)
• Very low annual volumes below 500 units, unless bridge tooling is used
• Parts over roughly 1m in any dimension (machine and tool size become prohibitive)

Typical examples include medical device housings, automotive interior trim, electrical enclosures, packaging closures, and and wide variety of everyday consumer products like toothbrush handles or power tool casings, remote controls or mobile phone covers. 

If the part is solid, complex and produced in volume, injection moulding is almost always the right answer. To explore UK and Irish suppliers, see the Injection Moulding Companies category.

Blow Moulding

Blow moulding produces hollow plastic parts by inflating a heated tube of plastic (the parison) inside a closed mould with compressed air. As the parison expands to fill the mould cavity, it takes on the shape of the inside of the mould. Once cooled, the mould opens and the finished hollow part is ejected.

There are three main variants. Extrusion blow moulding is the workhorse for everyday bottles, jerrycans and industrial containers. Injection blow moulding produces smaller, more dimensionally-precise containers like pharmaceutical bottles. Stretch blow moulding is how PET drinks bottles are made at high speed and in huge volume.

Best for

• Hollow containers from 50ml up to around 200 litres
• High volume production (bottles are often moulded at 1,000+ units per hour)
• Parts where a thin, uniform wall thickness is required
• Common materials: HDPE, PP, PET, PVC, PC

Not suited to

• Parts with tight tolerances on the internal dimension (the inside wall is not mould-controlled)
• Complex internal features or inserts
• Very large hollow parts above around 200 litres (rotational moulding takes over)
• Structural parts where uniform wall thickness is required across complex geometry

Typical examples: detergent bottles, oil cans, IBC containers, fuel tanks for outdoor power equipment, and pharmaceutical packaging. If the part is hollow with a relatively simple shape and is produced in meaningful volume, blow moulding is usually the economic choice.

Rotational Moulding

Rotational moulding (also called rotomoulding) takes a measured volume of plastic powder and places it inside a hollow metal mould. The mould is then rotated simultaneously on two axes inside an oven. The powder melts and coats the inside of the mould evenly, creating a large hollow part with a uniform wall thickness. Once cooled, the mould opens and the finished part is removed.

The process is inherently slower than blow moulding (cycle times measured in tens of minutes rather than seconds) but allows far larger parts, more complex geometries, and stronger wall sections than any other hollow-part process.

Best for

• Large hollow parts from 50 litres up to several thousand litres
• Low to medium volumes (100 to 10,000 units per year)
• Strong, stress-free wall sections (uniform thickness without molecular orientation)
• Complex hollow shapes with multiple openings or integral handles
• Outdoor and industrial applications (typically polyethylene, UV-stabilised)

Not suited to

• High-volume production (cycle times are too long)
• Tight dimensional tolerance requirements
• Fine surface detail or high-gloss cosmetic finishes
• Small parts under about 1 litre internal volume

Typical examples: water storage tanks, agricultural feed hoppers, industrial chemical containers, children’s outdoor play equipment, kayaks, and road safety barriers. If the part is large, hollow, and produced in modest volumes, rotational moulding is almost always the right answer. 

The Rotational Moulding Companies category lists UK & Irish specialists.

Side-by-side Comparison

When the choice is not obvious (larger bottles, smaller industrial containers, hollow products in the medium-volume range), this comparison often settles the decision:

When None of the Three is Right

The three processes above cover most UK plastic part production, but PlastikCity’s Source a Moulder section covers many more specialist processes beyond them: compression and thermoset moulding, polyurethane (PU), rubber, rigid PVC, LSR, structural foam, FRP, machining and fabrication, and more. A few of the more common edge cases:

• Thin-walled thermoplastic containers and packaging trays: often better suited to thermoforming or vacuum forming. Lower tool costs than injection moulding, lower volumes than blow moulding.
• Very low-volume parts (under 500 units per year): additive manufacturing (3D printing) or specialised low volume injection moulding using aluminium tooling are often more cost-effective.
• Complex technical parts combining rigid and soft-touch sections: two-shot or overmoulding (a specialised form of injection moulding) is typically required rather than one of the three standalone processes.

Not Sure Which Process Fits Your Part?

If your part could plausibly work in two or three different processes, or if none of the common processes feels like a clean match, get in touch with the PlastikCity team. We can look at what your part needs to do and point you toward the right process and the right category of moulder before you start requesting quotes. It saves time at both ends: you don’t end up sending the wrong brief to the wrong manufacturer, and moulders don’t end up quoting work they aren’t best-suited for.

A Three-question Decision

When the part is genuinely ambiguous, three questions normally settle the matter:

• Is the part solid or hollow? Solid means injection moulding. Hollow means either blow or rotational.
• How big is it? A hollow part up to about 50 litres is typically blow moulding territory. Above that, rotational moulding is almost always the answer. Injection moulding typically handles solid parts between a few mm, to 1 meter.
• What is the annual volume? Sustained high volumes favour blow or injection. Low to medium volumes shift the economics toward rotational for hollow parts, or low volume moulding specialists for solid ones.

If the three-question test produces a clear answer, you can move straight to supplier shortlisting. If two processes are plausibly competing for the same part, request quotes from both and compare the piece-part economics at your actual annual volume. Moulders are usually happy to quote alternative processes where they have the capability.

Why use PlastikCity to Find Your Next Manufacturing Partner

Whichever process fits your part, sourcing a capable UK or Irish manufacturer usually means getting three comparable quotes from suppliers who genuinely have the equipment and experience for the specific process and volumes for your project. PlastikCity’s Source a Moulder section exists for this step. You submit one specification, it reaches the vetted moulders whose capabilities match your requirements, and they quote back directly.

Every moulder listed on PlastikCity has been vetted before being accepted onto the platform, covering financial stability, quality accreditations, and capability fit for the category they’re listed in. Partner reputation is visible through buyer feedback.

PlastikCity has been established for over twelve years and now brings together more than 260 vetted Partner companies across 400+ categories covering every part of the UK and Irish plastics supply chain. It is free for buyers, and PlastikCity takes no commission on awarded contracts.