Injection Moulding vs Blow Moulding vs Rotational Moulding — Which One for Your Product?

How injection moulding works

Plastic resin pellets are fed from a hopper into a heated barrel. A rotating screw conveys, melts and pressurises the plastic. The entire injection unit then drives forward, pressing the nozzle against a closed steel mould. The screw pushes the molten plastic into the mould cavity at high pressure (typically 70–200 MPa). The mould is held closed by a hydraulic or toggle clamp. The plastic cools, solidifies into the shape of the cavity, the mould opens, and ejector pins push the finished part out. The mould closes again and the cycle repeats — typically every 15–60 seconds depending on part size.

The result is a solid part (not hollow unless specifically designed with internal cores) with excellent dimensional accuracy, fine surface detail, and the ability to incorporate features like threads, snap fits, thin walls, bosses, ribs and living hinges — all in a single moulding cycle.

Typical parts: bottle caps, screw closures, automotive interior trim, motorcycle mudguards, electrical switch housings, instrument enclosures, jugs, cups, thermos bodies, air-cooler cabinet components, poultry floor mats, medical components, gears, connectors, clips.

How blow moulding works

A tube of molten plastic called a parison is extruded downward between two open mould halves. The mould closes around the parison, pinching the bottom shut. Compressed air is blown into the parison, inflating it outward against the mould walls — like blowing up a balloon inside a box. The plastic cools against the mould surface, the mould opens, and the hollow part is ejected.

There are two main variants. Extrusion blow moulding (EBM) — the parison is continuously extruded; used for bottles, drums, tanks. Injection blow moulding (IBM) — the parison is first injection-moulded onto a core pin for better neck-finish accuracy; used for pharmaceutical and cosmetic bottles. A third variant, stretch blow moulding, is used for PET water bottles where biaxial stretching improves clarity and strength.

The result is a hollow part with relatively uniform wall thickness. Blow moulding cannot produce the fine internal geometry or tight tolerances that injection moulding can — but it excels at making containers efficiently at high volume.

Typical parts: water bottles, shampoo bottles, pharmaceutical containers, jerry cans, fuel tanks, drums, automotive ducts, hollow toys.

How rotational moulding works

A measured amount of plastic powder (usually polyethylene) is placed inside a hollow steel mould. The mould is closed and moved into an oven where it rotates slowly on two axes simultaneously. The heat melts the powder, and the rotation distributes it evenly across the inner walls of the mould. The mould is then cooled (by air or water) while still rotating, so the plastic solidifies uniformly. The mould opens and the part is removed.

Cycles are long — typically 20–60 minutes per part depending on size and wall thickness. This makes rotomoulding unsuitable for high-volume production. But it excels where no other process can: very large, seamless, stress-free hollow parts with uniform wall thickness and no weld lines.

Typical parts: water storage tanks (500–25,000 litres), septic tanks, playground equipment, kayaks, road barriers, large planters, chemical storage vessels, fuel tanks for trucks.

Side-by-side comparison

Part geometry. Injection moulding: solid or cored-out, complex geometry, thin walls, fine features. Blow moulding: hollow, relatively simple external shape, uniform wall. Rotomoulding: hollow, large, simple geometry, uniform wall, no fine features.

Part size. Injection moulding: typically grams to a few kilograms — small to medium parts. Blow moulding: small (100 ml bottles) to medium (200-litre drums). Rotomoulding: medium to very large (up to 25,000-litre tanks).

Dimensional accuracy. Injection moulding: ±0.05 mm achievable on critical features. Blow moulding: ±0.25 mm typical — less precise. Rotomoulding: ±1 mm or more — the least precise of the three.

Surface finish. Injection moulding: excellent — glossy, textured, detailed. The mould surface transfers directly. Blow moulding: good on the outside (mould-contact surface), rougher on the inside (free-blown surface). Rotomoulding: granular texture on both surfaces (the powder doesn't compact as tightly as injected melt).

Tooling cost. Injection moulding: highest — precision-machined steel cavities. A simple mould might cost PKR 100,000–200,000; a multi-cavity production tool can run into millions. Blow moulding: moderate — moulds are simpler (no cores, simpler parting). Rotomoulding: lowest — moulds can be fabricated from aluminium or sheet steel because the process uses no pressure.

Per-part cost at volume. Injection moulding: lowest for small-to-medium solid parts — cycle times are short (15–60 seconds) and multi-cavity tools multiply output. Blow moulding: competitive for hollow containers at volume. Rotomoulding: highest per part — long cycle times (20–60 minutes) limit throughput.

Materials. Injection moulding: widest range — PP, PE, ABS, PS, PVC, Nylon, PC, POM, TPE, filled compounds, glass-filled, and engineering resins. Blow moulding: mainly PE, PP, PET, PVC. Rotomoulding: mainly polyethylene (LLDPE, HDPE, XLPE) — material choice is limited because the plastic must melt from powder form.

Volumes. Injection moulding: sweet spot is 1,000 to millions of parts. Economically impractical below ~500 parts (tooling cost per part is too high). Blow moulding: sweet spot is 10,000+ parts per run for bottles/containers. Rotomoulding: sweet spot is 50–5,000 parts — explicitly a low-to-medium volume process.

When injection moulding is the right choice

Choose injection moulding when your part is solid (not hollow), when you need tight tolerances (snap fits, threads, mating surfaces), when you need a high-quality surface finish, when you're running medium to high volumes (1,000+ parts), when you need complex geometry (ribs, bosses, undercuts, living hinges, inserts), or when you need a wide material selection (engineering plastics, glass-filled, flame-retardant, food-safe, medical-grade).

Most manufactured plastic parts worldwide are injection moulded. If you're unsure which process to use, injection moulding is statistically the most likely answer.

At Rehman Industry in Gujranwala, we injection-mould everything from small precision caps to large air-cooler housings, automotive tail-lamp covers to poultry floor mats — covering the full range of what the process can do.

When blow moulding is the right choice

Choose blow moulding when your part is a hollow container (bottle, tank, drum, duct), when the wall thickness is relatively uniform, when you don't need tight internal geometry (no threads, no snap fits, no fine features inside), and when you're running high volumes of the same container shape.

Blow moulding is the default process for the beverage, chemical and packaging industries. If your product holds liquid or gas and is cylindrical or near-cylindrical, blow moulding is almost certainly the right process.

When rotomoulding is the right choice

Choose rotomoulding when your part is very large (water tanks, kayaks, playground equipment), when you need a seamless, stress-free hollow part, when your volumes are low (50–5,000 parts — tooling costs justify this), and when polyethylene is an acceptable material.

Rotomoulding is the only practical process for very large hollow parts (>1 metre) at low volumes. The low tooling cost makes it viable for short runs that would be uneconomical in injection or blow moulding.

Can one manufacturer do all three?

Rarely. The three processes use completely different machines, different tooling, and different operator skillsets. Most manufacturers specialise in one. Rehman Industry specialises in injection moulding — the most versatile and widely-used of the three. If your project turns out to need blow or rotational moulding, we'll tell you honestly and can often refer you to a trusted Pakistani manufacturer who does.

That honest assessment is part of what we offer. No point moulding a water tank on an injection press — it's the wrong tool for the job. But for the vast majority of plastic parts that businesses need (solid, precise, repeatable, at volume), injection moulding is it.

Decision flowchart

Is your part hollow? If no → injection moulding. If yes →

Is it a small-to-medium container (bottles, drums)? If yes → blow moulding. If no →

Is it very large (tanks, structures, >1m)? If yes → rotomoulding. If no →

Is it hollow but needs fine internal features or tight tolerances? If yes → injection moulding with internal cores. If no → blow moulding or rotomoulding depending on volume.

Still unsure? Send us a sketch or description. We'll tell you which process fits — honestly, even if the answer isn't injection moulding.

Bottom line

Injection moulding, blow moulding and rotational moulding each have a sweet spot defined by part geometry, volume and cost. Picking the wrong one wastes money — picking the right one feels effortless. For the majority of precision plastic parts that Pakistani businesses need, injection moulding is the answer. That's what Rehman Industry has done for twenty years.

Need help deciding? Send us your part details — drawing, sample or just a description. We'll advise on process, material, tooling and cost. Usually within a business day.

Going deeper on a specific pairing

This page compares all three processes side by side. If you have already narrowed the choice to two, these head-to-head guides go further on the trade-offs that actually decide it.

[Blow moulding vs rotational moulding](/articles/blow-moulding-vs-rotational-moulding) — the two processes that both make hollow parts, and why they almost never compete for the same job.

[Injection moulding vs rotational moulding](/articles/injection-moulding-vs-rotational-moulding) — precision and speed against cheap tooling and large parts, including where the cost crossover actually sits.