Rehman 
The short answer
A thermos is an assembly, not a moulding. The injection-moulded parts are the outer shell, the lid and stopper, the handle, the base ring and the pour spout. The insulating element itself — a vacuum stainless steel vessel or a glass refill — is a separate component bought from a different supplier.
The most expensive feature in the entire tool is the screw thread on the lid and neck. A thread cannot simply be moulded and pulled straight out. It needs either an unscrewing mechanism in the mould, a collapsible core, or a split across the parting line — and each choice changes the tooling cost, the cycle time and how the finished part looks.
Anything that touches the liquid — lid, stopper, spout, inner cap — must be virgin food-grade resin with food-grade colourant. The outer shell does not, which is where sensible cost saving actually lives.
Which parts of a thermos are injection moulded?
Outer shell or body. The visible part, usually PP or ABS. Cosmetic, so finish, gloss and colour consistency matter. Often carries moulded-in ribbing for grip or a recess for a printed label.
Lid and stopper. The functional heart of the product. Carries the screw thread, the seal seating, and often a push-button or flip mechanism. Food-contact. This is the part that determines whether the flask leaks, and therefore whether it gets returned.
Pour spout or inner cap. Food-contact, needs a clean sealing face and no flash anywhere near the seal.
Handle. Either moulded into the shell or as a separate clip-on part. If separate, it has to survive the flask being carried full, which is a real load case.
Base ring or foot. Protects the bottom, adds grip, hides the assembly joint. Often a softer material.
What is not moulded: the vacuum steel liner, the glass refill, and the silicone or rubber sealing gaskets. Gaskets are a bought-in component unless you are running two-shot tooling, which is uncommon at Pakistani housewares volumes.
Why the screw thread drives the tooling cost
This is the part first-time housewares brands never budget for, so it is worth understanding before you ask for a quote.
A moulded screw thread is an undercut. The part cannot be pushed straight out of the mould, because the thread is in the way. There are three ways around it and each has a cost:
Unscrewing mechanism. A motor or rack-and-pinion rotates the core to spin the part off the thread. Cleanest possible thread with no visible seam, and by far the most expensive tooling option. It also lengthens the cycle, because unscrewing takes time on every shot.
Collapsible core. The core shrinks inwards to release the thread. Good for internal threads, expensive, and the segments need maintenance because they are moving parts inside the tool.
Splitting the thread across the parting line. Cheapest by far — the mould opens sideways and the thread comes out in two halves. The trade-off is a visible witness line running down through the thread, and if the halves are not perfectly aligned the lid feels rough to turn or does not seal evenly.
For a low-cost domestic flask, a split thread is usually the right economic choice. For a premium product where the customer turns the lid every day, the unscrewing tool pays for itself in returns avoided. Decide this at DFM, because it is a tooling architecture decision, not a detail.
Food-grade requirements — what changes and what does not
The rule is simple: anything that can touch the drink must be food grade, and anything that cannot does not need to be. Applying that correctly is where the cost saving is.
Virgin resin, no regrind, on food-contact parts. Lid, stopper, spout, inner cap. You cannot certify what was in reground material, so it is not used here.
Food-grade colour masterbatch on those same parts. The pigment has to be approved too. This is not automatic and is worth asking about specifically.
The outer shell can use standard grade and can generally contain regrind, because it never touches the contents. That is a legitimate saving on the largest part in the assembly.
Certificates come from the resin supplier, not the moulder. For domestic sale keep the food-grade certificate on file; for export, buyers routinely ask for FDA or EU 10/2011 compliance statements on both resin and masterbatch. Ask for these before tooling is cut, not after your first export order is audited.
Hot liquid matters. A flask holds near-boiling tea. Confirm the grade is rated for the temperature it will actually see, not just that it is food safe at room temperature.
Sealing, leaking and the details that cause returns
Almost every thermos return traces back to one of four things:
Flash on the sealing face. A thin ridge of excess plastic where the mould halves meet. On a sealing surface it holds the gasket open by a fraction of a millimetre and the flask weeps. It signals a worn or poorly maintained tool.
Inconsistent thread pitch or depth. The lid feels tight on one unit and loose on the next. Usually a symptom of an unstable process rather than a bad tool design — shot weight drifting batch to batch.
Gasket groove dimensions. Too shallow and the gasket is over-compressed and takes a permanent set; too deep and it never seals. This is a tooling dimension, so get it right before the steel is cut.
Warp on the neck. A round opening that has gone slightly oval will not seal against a round gasket no matter how good the gasket is. Thick-to-thin transitions near the neck are the usual cause.
The practical test before accepting a first article: fill it with hot water, seal it, invert it, leave it an hour. Then repeat after fifty open-close cycles. A flask that seals once and fails after a week is worse than one that fails immediately, because it fails at the customer.
Volume, cavities and season
Housewares is a volume business with thin per-unit margins, so cavity count decides whether the numbers work.
Small parts want multi-cavity tooling. Lids, spouts and base rings are exactly the parts where a 4 or 8-cavity tool transforms per-piece cost. The shell, being large, usually stays single or two-cavity.
Consider a family mould for the small parts. If the spout, inner cap and base ring are always assembled together in the same ratio, one family tool produces a matched set every shot — one setup, one tooling cost, and no risk of running out of one component while holding stock of another.
Season matters in Pakistan. Flask demand rises with the school year and through winter, which is the opposite of the cooler season. If your moulder also runs cooler bodies, their busy window is summer and their quiet window is exactly when you want flask production — which usually means better pricing and faster delivery if you plan for it.
Colour is a stock decision. Housewares sells on colour range, but every extra colour is a machine changeover and a purge. Fewer colours in longer runs is materially cheaper than many colours in short runs, and that is worth deciding before the season rather than midway through it.
Bottom line
Treat the flask as an assembly of moulded parts plus a bought-in vacuum vessel and gaskets, and quote it that way. Vague requests for a price on a thermos produce vague answers.
Settle the thread strategy at DFM. Split thread for a value product, unscrewing tool for a premium one — it is the biggest line in the tooling budget and the biggest driver of whether the lid feels good in the hand.
Specify food-grade virgin resin and food-grade masterbatch on every part that touches liquid, get certificates on file before tooling, and take the saving on the outer shell instead.
Rehman Industry moulds thermos and water-bottle bodies, lids and closures in Gujranwala, with tooling built and maintained in our own tool room. Send the design or a sample along with your annual volume and colour range, and we will come back within a business day with a thread recommendation, tooling cost and per-piece pricing at volume tiers.
Frequently asked questions
Which plastic is used for thermos and water bottle bodies?
Outer shells are usually polypropylene or ABS — PP for durability and lower cost, ABS where a higher gloss finish is wanted. Lids, stoppers and spouts must be moulded in virgin food-grade resin with food-grade colourant because they contact the drink. The outer shell does not touch the contents, so it can use standard grade material, which is where most of the legitimate cost saving sits.
Why is the screw thread the most expensive part of a bottle mould?
A thread is an undercut, so the part cannot be ejected straight out of the mould. Releasing it needs either an unscrewing mechanism that rotates the core, a collapsible core, or splitting the thread across the parting line. Unscrewing gives the cleanest thread and costs the most; a split thread is far cheaper but leaves a visible witness line down the thread and can feel rough to turn.
Do all parts of a thermos need to be food grade?
No. Only the parts that can contact the liquid — lid, stopper, pour spout and inner cap — need virgin food-grade resin and food-grade colour masterbatch. The outer shell never touches the contents and can use standard grade material, including regrind. Certificates come from the resin supplier and should be obtained before tooling is cut, particularly for export.
What causes a thermos to leak?
Most leaks trace to four causes: flash on the sealing face holding the gasket open, inconsistent thread pitch between batches, a gasket groove that is too shallow or too deep, or a neck that has warped slightly oval so a round gasket cannot seal against it. All four are tooling or process problems rather than gasket problems, so test a first article by filling it with hot water and inverting it after fifty open-close cycles.
Is a family mould worth it for thermos components?
Usually yes for the small parts. If the spout, inner cap and base ring are always assembled in the same ratio, one family tool produces a matched set every shot — a single setup, a single tooling cost, and no risk of running short of one component while holding excess stock of another. The outer shell is normally kept on its own tool because it is much larger.
What we do at Rehman Industry
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