Injection vs Blow vs Thermoforming vs Rotomolding Cases

The Same Case, Three Prices — Why?

A procurement manager sends one drawing to three suppliers. Same outer dimensions, same wall thickness on paper, same material family. Three quotes come back: $18, $34, and $61 per unit at the same quantity.

The material is rarely the reason. Almost always, the gap comes down to which molding process the quote was built around. Injection molding, blow molding, thermoforming, and rotational molding each impose their own tooling economics, tolerance behavior, and minimum volume — and those constraints, not the resin price, set the floor on your unit cost.

This article compares all four processes on the factors that actually drive a procurement decision: tooling cost, unit cost at volume, wall thickness control, dimensional tolerance, seal integrity, structural complexity, and economic batch size.

The Four Processes in Plain Terms

Injection Molding

Molten plastic is injected under high pressure into a closed steel mold, then cooled and ejected. The mold is machined to the final part geometry, so every detail — latch catches, hinge knuckles, reinforcing ribs, gasket grooves, moulded logos — is formed in a single shot. Because the cavity is a rigid machined steel block, the part that comes out is dimensionally repeatable shot after shot.

Blow Molding

A tube of molten plastic (the parison) is extruded downward, captured in a two-part mold, and inflated with air until it presses against the cavity walls. The result is a hollow, one-piece shell with no seam line around the equator. Wall thickness is controlled by parison programming rather than by a machined cavity, so it is less precise than injection molding and typically thicker.

Thermoforming (Vacuum & Pressure Forming)

A flat plastic sheet is heated until soft, then drawn over or into a mold by vacuum, air pressure, or a matched tool. The sheet's starting thickness and the draw depth determine the finished wall: material thins measurably wherever it stretches, especially down deep vertical walls and radius corners. Tooling is simple — aluminum or composite — and inexpensive.

Rotational Molding (Rotomolding)

Powdered plastic is loaded into a hollow mold, which rotates slowly on two axes inside an oven. The powder melts and coats the entire interior, building a seamless hollow part with uniform wall thickness. Cycle times run in tens of minutes rather than tens of seconds, and the process scales to very large parts that no injection press could accommodate

Head-to-Head Comparison

Factor	Injection	Blow	Thermoforming	Rotational
Tooling cost	High — hardened steel, precision-machined	Medium — simpler two-part mold	Low — aluminum or composite	Low–medium — fabricated aluminum or steel
Unit cost at volume	Lowest once amortized	Low for large hollow parts	Low–medium	Medium — long cycles limit throughput
Wall thickness control	Precise, per-feature	Approximate; thicker at pinch lines	Varies with draw depth; thins in corners	Uniform by nature, but coarse to adjust
Dimensional tolerance	Tight and repeatable	Moderate	Moderate; affected by cooling shrinkage	Loosest of the four
Seal integrity	Can achieve IP67 / IP68 with gasket groove	Possible, but geometry limits gasket design	Difficult; flanges and seams are the weak point	Good on large shells; coarse mating faces
Structural complexity	High — ribs, bosses, snaps, hinges in one shot	Low–medium — mostly simple hollow forms	Low — one open shell per mold; needs secondary assembly	Medium — molded-in inserts possible, fine detail limited
Cycle time	Seconds per part	Tens of seconds	Minutes	Tens of minutes
Economic batch size	High volume; tooling needs amortizing	Medium–high	Low–medium	Low–medium, and size-limited rather than volume-limited

Tolerance figures vary by part size and material, so treat any published range as indicative. The practical rule is the ranking itself: injection molding holds the tightest and most repeatable dimensions; rotomolding holds the loosest.

Where Each Process Wins

Injection molding — when fit and seals matter

Choose injection molding when the case has to close precisely, hold a gasket, latch reliably thousands of times, and protect contents from water and dust. It is also the right answer when the case needs internal structure — foam-retention ribs, divider slots, molded-in mounting bosses — that would otherwise require gluing or assembling separate parts. The trade-off is upfront tooling cost, which only pays back across a real production volume.

Blow molding — large, hollow, one-piece

Blow molding suits big, simple, hollow parts where a seamless skin matters more than precision: large transport shells, tanks, fluid containers, and the outer shells of wheeled trunks. It is economical at large sizes, but the process cannot deliver the fine latch geometry and gasket grooves that a precision case requires.

Thermoforming — trays, inserts, and fast prototypes

Thermoforming's strength is speed and low tooling cost. That makes it ideal for foam-and-tray inserts, equipment inlays, packaging cradles, and prototype shells you need in hand for a design review. It is a poor choice for a primary protective enclosure: the flange where two halves meet is hard to seal, and the walls thin out exactly where an impact is most likely to land.

Rotational molding — very large, rugged, low-volume

Rotomolding excels at items too large for injection tooling — storage tanks, industrial bins, large insulated containers, and heavy-duty transport cases produced in modest quantities. It delivers seamless, stress-free walls with good impact resistance. What it does not deliver is tight tolerances or fine detail, which is why you rarely see a rotomolded case with a precision gasket seal and a compression latch.

Are These the Only Processes? No — But They Cover Most Cases

These four dominate case manufacturing, but they are not the entire landscape. Three other processes appear in specific niches, and each one exists because of a limitation the four above share.

• Structural foam molding. Low-pressure injection with a foaming agent, producing thick, rigid, warp-resistant walls. It is common in large industrial containers and pallets, where stiffness matters more than surface finish or tolerance. Wall sections are heavy and surfaces carry swirl marks, so it is rarely chosen for a case that has to look finished or seal precisely.
• EPS and EPP foam molding. Expanded bead molding for insulated coolers, cold-chain shippers, and medical transport boxes. These are foam boxes rather than hard-shell cases: excellent insulation, limited structural protection, and no meaningful gasket seal.
• Extrusion and welding. Sheet or profile extrusion fabricated into very large tanks and chemical containers. It exists because the part is too large for any mold — not because it delivers better protection.

If the requirement is a precision enclosure where the two halves close squarely, hold an IP-rated gasket, latch repeatedly, and survive impact, the decision realistically sits among the four processes compared above. The others solve size or insulation problems, not precision ones.

Why Precision Cases Are Injection Molded

Walk through what an IP-rated protective case has to do and the process choice stops being a matter of preference.

A gasket only seals if the groove holds tolerance. An O-ring or molded gasket seals because it is compressed by a precise, even gap between two mating faces. That gap is the sum of every dimensional variation in both halves of the case. Injection molding keeps that stack-up tight enough to guarantee compression across the whole perimeter. A process with looser tolerance forces a thicker, softer gasket to compensate — which seals less reliably and degrades faster.

Latching hardware has to be engineered, not added. A compression latch needs a catch molded into one half and a hinge point molded into the other, both positioned accurately enough to engage without rattle or forcing. Blow molding cannot form that detail; thermoforming would require bonding a separate catch onto the shell, creating a joint that can fail.

Impact performance comes from controlled geometry. Drop resistance is not only about wall thickness — it comes from ribs, corner radii, and material distribution designed to spread impact energy. Injection molding is the only one of the four that reliably produces ribs and bosses at specified thickness in a single shot.

RHINOVO manufactures injection-molded protective cases on presses up to 1400T, across a self-owned mold library of more than 200 tools, with engineering-grade reinforced PP as the standard material. Waterproof models are validated to IP67 (IEC 60529), and materials are RoHS compliant. Because tooling already exists for the standard range, minimum order quantity starts at 100 units rather than at the volume an injection process would normally demand.

Choosing for Your Application

Work through these questions in order:

• Does it need to be watertight or dust-tight? If yes, injection molding is effectively the only viable process for the enclosure itself.
• Does it need internal structure? Foam retention, dividers, mounting points, or stackability push you toward injection molding.
• How large is it? Beyond roughly the size of a large wheeled trunk, rotomolding and blow molding become the practical options.
• What is your annual volume? Injection tooling pays back fastest at volume; thermoforming tooling pays back immediately but limits part performance.
• How precise does the fit need to be? If parts must stack, nest, or interface with equipment, tolerance requirements rule out the lower-precision processes.

Process selection is a design decision to make early, not a quote to chase late. A part designed around injection molding cannot simply be re-quoted in another process and expected to perform the same way — the wall sections, tolerances, and features are specific to the process that formed them.

Frequently Asked Questions

Is a rotomolded case stronger than an injection molded one?

Not inherently. Rotomolded parts have seamless, uniform, low-stress walls that resist impact well at large sizes. Injection molded parts use engineered geometry — ribs, radii, and graded thickness — to achieve impact performance in a smaller, thinner package. Which is better depends on size, weight budget, and whether the case needs a precision seal.

Can a thermoformed case be waterproof?

It can be made water-resistant, but achieving a sustained IP rating is difficult. The flange where the two halves meet is a secondary sealing joint rather than a molded gasket groove, and wall thinning at corners reduces structural margin. For moderate splash protection it can work; for immersion ratings it generally cannot.

Which process is cheapest for a run of 500 units?

It depends on part size and complexity. For small to medium cases, thermoforming usually has the lowest total cost at that volume because tooling is inexpensive. For large hollow shells, blow molding or rotomolding are typically cheaper. Injection molding carries higher tooling, so 500 units is often below its efficient volume — unless existing molds are available, in which case the calculation changes completely.

Do blow molded cases seal as well as injection molded ones?

Generally no. Blow molding produces a hollow shell with approximate wall thickness and limited control over the mating faces, which makes it hard to hold the even compression a gasket needs. Blow molded enclosures usually rely on a cover or secondary sealing method rather than a molded-in gasket groove.

The Takeaway

Four processes, four different sets of trade-offs. Injection molding buys precision, seal integrity, and structural complexity in exchange for tooling investment. Blow molding and rotomolding buy size and seamless walls, at the cost of tolerance. Thermoforming buys speed and low tooling cost, at the cost of structural performance and sealing.

If you are specifying a case for instruments, tools, or equipment that must arrive intact and stay dry, the process decision is largely made for you. The remaining questions are geometry, internal layout, and volume — which is where a conversation with a manufacturer is worth more than another round of quotes.

We, Enping Ruixin Plastic Products Co., Ltd accepts OEM and ODM programs from 100 units, including custom foam inserts, molded-in branding, and color matching across the standard range.

Image credits

• Injection molding schematic — Laurens van Lieshout, public domain, via Wikimedia Commons.
• Injection molding machine — Bricetofly, CC0 1.0, via Wikimedia Commons.
• Blow molding schematic — LaurensvanLieshout, CC BY-SA 3.0, via Wikimedia Commons.
• Thermoforming animation — Colagor, public domain, via Wikimedia Commons.
• Rotational molding oven — Jackyzhaoplastic, CC BY-SA 4.0, via Wikimedia Commons.