Stick Packs in Solid Beverage OEM: Dosing Accuracy, Tear Opening and Moisture
Solid beverage packaging has shifted heavily toward stick packs: single-serve, pocketable and ready to mix, which consumers like and which lets brands build sets flexibly by sachet count. From the production side of solid beverage OEM, however, stick packs are the format that tests process stability hardest, because three difficulties land together. Fine-powder dosing at very small weights, reliable sealing of thin film on a low fill weight, and the surface-area effect of a tiny single serve: one gram in a sachet meets far more air than one gram in a jar, and takes up moisture correspondingly faster. Dose weights swinging high and low, tops that will not tear or that tear right off, and product that cakes into a brick within two months are the three classic failures. This article explains causes and remedies across equipment, film and process parameters.
1. How stick packs differ from jars
Jar filling works in hundreds of grams, so absolute deviation can be relatively generous, sealing is one wide continuous heat seal, and the film is usually a fairly thick laminate. A stick pack is the exact opposite. Fill weight runs from one to ten grams, so deviation limits are squeezed into a tiny absolute value, sealing becomes three narrow seams closed simultaneously, and the film is usually thinner to give the right tear feel. The filling method differs even more fundamentally. Jars typically use volumetric or auger dosing followed by drop-filling, whereas a stick pack has to form, fill, seal and cut within one continuous cycle, with powder travelling through a very narrow chute in a fraction of a second. Powder flowability must therefore match the dosing mechanism closely, because any fluctuation in flow is amplified directly into dose variation. That is why a stick pack project has to consider flowability during formulation, rather than revising the powder after the machine refuses to run steadily.
2. Dosing accuracy and the role of flowability
Three dosing principles dominate stick pack machines. Auger dosing suits powders of moderate or better flow, controlling output by screw speed and revolutions, and is accurate but sensitive to bulk density consistency. Volumetric cup dosing is simple and fast but demands an extremely stable bulk density, otherwise fill weight drifts with density. Vibratory tray dosing suits free-flowing or fragile granular material, controlling output by vibration duration. Whichever is used, the precondition is stable powder flow, which is governed by moisture content, particle shape, fines fraction and anticaking level. The same line can behave differently in the morning and the afternoon, often because changing room humidity alters moisture content and flow. Low-humidity control in a stick pack room is therefore not a comfort measure but a prerequisite for dose stability. The correct sequence is to measure angle of repose and bulk density first, then select a matching dosing principle and screw geometry, and only then tune parameters.
3. Designing the tear opening
A tear that will not open, or that snaps the whole sachet off, is the most immediate consumer complaint. A tear notch is essentially a deliberate stress concentration, and it has to direct crack propagation the right way. Three points matter. First, notch shape and depth: too shallow and it will not tear, too deep and the film may already have failed during filling or transport, so laser or mechanical scoring has to hold depth within a narrow band. Second, film orientation: the tear direction should align with the film's easy-tear axis, and biaxially oriented films differ enormously between machine and cross direction, so this must be confirmed at selection. Third, seal width against notch position: if the notch sits inside the seal, opening rips away the seam and sprays powder. Brand requests for one-pull opening, no spillage and no cut fingers map onto scoring technique, film stiffness and corner rounding, and they have to be confirmed by hand on physical samples rather than judged from a drawing.
4. Moisture barrier: film, seals and headspace
Moisture ingress into a stick pack comes from three directions: film barrier performance, seal integrity and residual headspace air. On film, the usual construction is an outer print layer, a barrier layer and an inner sealant, with metallised or foil layers doing the barrier work. Selection should not stop at the water vapour transmission rate figure, because flexing in transit creates pinholes in the barrier layer and the transmission rate then rises sharply. On sealing, any weakness in one of the three seams becomes a moisture path; weak seals come mainly from low seal temperature, insufficient pressure, or powder contamination in the seal zone. Cleaning and air purge design around the sealing area is therefore critical, because powder left after filling sits directly on the seam line as a built-in leak. On headspace, less air inside means a lower starting point for both moisture uptake and oxidative deterioration.
5. Diagnosing the common defects
Leaking powder is the most serious defect, since a batch-wide occurrence means the whole lot must be repacked. Dose drift splits into two patterns. Large random variation points to dosing mechanism stability, such as screw wear, powder build-up in cups, or level fluctuation in the vibratory pan. A consistent offset usually means bulk density change or a falling hopper level altering the drop. Torn-off tops and ragged notches point to scoring depth and knife condition, and possibly lot-to-lot film variation. One subtler defect is brittle seams: the product cracks open at the seal under low temperature or transport vibration, which is tied to seal temperature running so high that the inner layer is over-melted. That requires a peel strength test recorded during trials, not after-sales feedback.
6. Validation and production control
Validation should cover four hard measures. Dose weight is sampled continuously at a fixed interval and judged on standard deviation as well as mean. Seal strength is measured by pulling across the seam against a lower limit. Seal integrity is confirmed by water immersion, dye penetration or vacuum decay. Barrier performance is confirmed by weight gain and caking under accelerated conditions. Baseline values should be set during sampling and compared batch by batch in production. Routine controls then include per-shift checks of powder moisture and bulk density, first-article confirmation of dose and seal on every batch, scheduled cleaning of the seal zone and drop chute, and film lot recording for traceability. Film lot change is the most common source of trouble in stick pack work, because differences are invisible to the eye while the sealing window and tear behaviour may already have shifted, so every lot change should trigger a seal strength and tear check.
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About Guangzhou Mesen Health Biotechnology Co., Ltd
Guangzhou Mesen Health Biotechnology Co., Ltd is located in the Huashan Science and Technology Park, Huadu District, Guangzhou. The company has focused on OEM and ODM services for the big health industry for nearly 20 years, operating dedicated research, production, sales and management centres, and maintaining long-term strategic partnerships with well-known Chinese medicine colleges and research institutions. Capabilities span pressed candy, gummies, capsules, solid beverages, herbal teas, plant beverages, protein powders, dietary supplements and enzyme jelly - from formulation development through contract manufacturing. For details on minimum order quantities, sampling lead times and quotations, see our manufacturing advantages and product range, or contact us for a tailored proposal.
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