How to Choose Sachet Packaging for Powder Products | Moisture Protection, Seal Strength, and Filling Accuracy

Moisture resistance is critical, so the preferred choice is a laminated structure that includes an aluminum foil (AL) layer, with the water vapor transmission rate (WVTR) kept below 0.1 g/m²·24h to prevent the powder from caking.

For seal strength, the heat-sealing pressure should remain stable at 0.3 MPa or above, ensuring that the sachet will not burst or leak powder during drop testing from height.

For filling accuracy, it is advisable to pair the system with a screw metering scale, keeping the filling error strictly within ±1%.

Moisture Protection

Water Vapor Transmission Rate

Powders with a particle size below 50 microns have a surface area dozens of times larger than that of coarse particles, which makes them extremely sensitive to ambient humidity. In a hot, humid environment of 30°C and 80% RH, exposed whey protein powder can absorb more than 3% of its own weight in moisture within 120 seconds.

The water vapor transmission rate (WVTR) of a packaging film determines how long the product can hold up on the shelf. It is typically measured by the electrolytic sensor method, with units expressed as g/(m²·24h). Under standard test conditions of 38°C and 90% RH, a single-layer 12-micron PET film usually shows a WVTR of around 50 g.

• 12μm PET / 7μm AL / 50μm PE: WVTR below 0.01 g, classified as a high-barrier structure
• 12μm PET / 12μm VMPET / 50μm PE: WVTR between 0.5 and 1.5 g, suitable for most powdered drink products
• 20μm OPP / 20μm CPP: WVTR above 5 g, recommended only for low-sugar sachets with very fast turnover
• 12μm PET / 15μm nylon / 60μm PE: WVTR of about 3 g; moisture protection is fairly average, but puncture resistance exceeds 15 N

In composite structures, the aluminum foil layer (AL) mainly acts as a physical barrier. When foil thickness increases from 6.35 microns to 9 microns, the number of pinholes per square meter drops from around 50 to nearly zero. That microscopic continuity directly blocks the diffusion path of water molecules. Since a water molecule is only 0.4 nanometers in diameter, any gap larger than that can cause permeation to double within 24 hours.

Powders containing active probiotics must maintain a water activity (Aw) below 0.2. Otherwise, under storage at 40°C, the viable bacteria will die off in large numbers. Barrier deterioration in packaging materials is a major concern. As the laminate passes through high-speed packaging equipment, it is stretched, folded, and compressed during heat sealing, and the aluminum layer may develop invisible microcracks. As a result, the moisture barrier performance of the finished pouch may fall 30% to 50% below the lab-tested data for the original rollstock.

• Heat-sealing pressure: set between 0.2 MPa and 0.4 MPa to ensure complete fusion at the seal area
• Heat-sealing temperature: the PE inner layer is usually set between 130°C and 160°C, with fluctuations controlled within ±2°C
• Cooling time: 0.5 seconds of cooling helps prevent micron-level vent holes caused by heat deformation at the seal
• Residual oxygen: oxygen concentration inside high-spec packaging should stay below 2%, working together with the moisture barrier to slow powder oxidation

Data shows that when the relative humidity inside a package rises from 20% to 40%, the oxidation rate increases by 2.5 times. When using metallized film (VMPET), the aluminum layer adhesion, expressed as optical density (OD), should reach 2.2 to 2.8. Only at that level can it block over 99% of UV light and prevent light-induced quality changes that may look like spoilage.

Many matte-finish powder pouches on the market use a structure such as Matte OPP / VMPET / PE. This design looks more premium, but matte OPP provides noticeably lower barrier performance than glossy PET. Under the same conditions, the WVTR of this structure is usually about 15% higher than that of glossy film. For high-value powders priced above RMB 50, that difference can noticeably affect taste after 12 months of storage.

Powder dust generated during filling can contaminate the seal area and create inclusions. Under 200x magnification, these trapped particles force apart the sealing interface and create a capillary effect. Driven by the pressure differential inside the pouch, external moisture can travel through these physical channels far faster than through normal film permeation.

• Tensile strength: both machine and transverse directions should exceed 30 MPa to prevent rupture during transport compression
• Peel strength: interlayer adhesion should be greater than 2.0 N/15 mm to prevent delamination in humid conditions
• Coefficient of friction (COF): the outer layer should be controlled between 0.15 and 0.25 to ensure smooth film feeding
• Compression resistance: the finished pouch should withstand 60 kg of pressure for 1 minute without leaking

For accelerated destructive testing, sample pouches should be placed in a constant temperature and humidity chamber at 40°C and 75% RH. Three months under these conditions is generally equivalent to about 24 months at room temperature. Moisture content should be tested every 15 days. The flatter the moisture uptake curve, the more reliable the lamination process and the more intact the barrier layer.

A standard 100-micron PE bag can still provide weaker moisture protection than a 30-micron film coated with a high-barrier K coating (PVDC). Even though a PVDC coating adds only 2 to 3 g/m², it can improve barrier performance by 5 to 10 times.

• PVDC-coated PET: WVTR can be reduced to about 0.8 g, while remaining transparent and highly protective
• SiOx/AlOx coatings: use plasma deposition to form a micro-thin quartz-like layer on transparent film
• EVOH coextruded film: oxygen barrier drops under high humidity, but moisture protection remains stable in dry environments
• BOPA (nylon): mainly used to improve toughness; because it has a high moisture absorption rate, it must be buried in the middle layer

Multi-Layer Composite Structures

A 12-micron biaxially oriented polyester film (BOPET) typically serves as the outermost layer in a composite structure, allowing it to withstand the instantaneous 180°C heat from sealing jaws without physical distortion. Its tensile strength exceeds 200 MPa, and at a packaging speed of 120 pouches per minute, print registration error can be kept within 0.5 mm, ensuring a barcode scan success rate of 99.9%.

The printed surface is located on the inner side of the outer film, with polyurethane ink coverage of roughly 1.5 to 2.5 g/m². Total residual solvent in the ink layer must stay below 3 mg/m² to prevent volatiles from migrating through the middle layer and causing off-odors in the powder. Ink drying speed must match a laminator line speed of 200 meters per minute to prevent fine blocking marks.

During dry lamination, adhesive coating weight must be stabilized between 2.5 and 3.5 g/m². The adhesive then undergoes a crosslinking reaction in a 45°C curing room for 48 to 72 hours. If peel strength is below 1.5 N/15 mm, the pouch may delaminate during transport at -20°C. Weak interlayer bonding reduces overall mechanical strength and increases the risk of powder leakage.

When a 7-micron pure aluminum foil (AL) is used as the middle barrier layer, WVTR is typically below 0.01 g. Compared with 12-micron metallized film (VMPET), pure aluminum foil provides 100% light blockage. Metallized film generally has an OD value of 2.2 to 2.8, and still allows some visible light transmission under strong illumination, so it is not recommended for highly active probiotic powders with a shelf life longer than 18 months.

For protein powder packaging, adding a 15-micron biaxially oriented nylon layer (BOPA) improves toughness. The nylon layer offers puncture resistance above 15 N, reducing the impact stress caused by powder particles. With this layer added, the pouch breakage rate in a 2-meter free-drop test drops from 15% to below 2%, which supports more stable performance during long-distance transport and multi-layer stacking.

The inner heat-seal layer usually uses 50 to 80 microns of LLDPE, with a melting point around 135°C. Under a heat-sealing pressure of 0.3 MPa, the PE melts and flows to form the seal zone. For seasoning powders prone to dust contamination, adding metallocene components (mPE) to the inner layer allows the seal to fuse tightly even if 5-micron powder particles are trapped in the seal area.

Total thickness variation across the rollstock must be held within ±5%. Uneven thickness can create film tension fluctuations greater than 10 N. Once tension becomes unstable, cutter positioning drifts, and the standard 8 mm seal width can shrink to below 5 mm. Reduced seal contact area makes the pouch more likely to split at the seal edge under pressure.

Heavy-duty powder sachets weighing more than 500 g per pack should use a PET / PE / AL / PE four-layer structure. The added PE middle layer acts as a cushioning adhesive layer and helps disperse concentrated impact stress. This structure can achieve a burst-pressure test value above 80 kPa, allowing it to withstand instantaneous 5G shock loads at the bottom of a shipping container without rupturing.

Powders with high alkaline content can attack ordinary adhesive layers and cause delamination within 3 months. A medium-resistant adhesive designed for chemical resistance should be used instead, with a temperature range from -40°C to 120°C. Although this increases the cost per square meter by 15% to 20%, it ensures chemical stability and prevents packaging components from migrating into the food.

Laser easy-tear technology can improve opening performance. The laser precisely cuts through the outer 12-micron PET layer without damaging the middle aluminum foil layer or the inner seal layer. This non-contact process keeps the average opening force between 3 N and 5 N. Because the barrier layer remains intact, moisture permeation can still be held below 0.01 g.

The outer layer COF should be controlled between 0.15 and 0.25, while the inner layer should stay between 0.20 and 0.30. If the outer surface is too tacky, film feeding will drag and jam. If the inner surface is too slippery, powder splashing during filling becomes severe. Monitoring shows that every 0.05 fluctuation in COF raises automatic line reject rates by 1.5%, which has a clear impact on production efficiency.

For products exported to high-temperature, high-humidity regions such as Southeast Asia, an additional 2 g/m² PVDC high-barrier coating can be applied to the PET film. This improves moisture resistance by five times compared with standard film. Under 38°C and 90% RH, PVDC-coated film can limit the monthly moisture increase inside the pouch to less than 0.02%, helping preserve freshness.

The difference in heat-shrink behavior between layers must be limited to within 2%. If the PET and PE layers shrink unevenly during sealing, the cooled seal can curl severely. The resulting stress may induce micron-scale cracks in the aluminum foil. Moisture can then penetrate through these invisible channels, causing vacuum packs to lose vacuum and visibly swell within 30 days.

Surface resistivity of the composite film should remain between 10⁹ and 10¹¹ ohms. Excessive static causes powder to cling to the inside of the seal area during filling. These adhered particles can reduce seal strength by more than 40%, leading to microleaks at the bottom or side seals. Installing static elimination devices helps maintain smooth film travel and supports continuous output above 3,000 pouches per hour.

Seal Integrity & Contamination Resistance

Powder products with particle sizes between 5 and 50 microns generate dust during filling, which can instantly coat the sealing area. These barely visible fine particles become trapped between the heat-seal layers, forming a non-uniform interlayer about 15 microns thick.

Under 200x magnification, the powder particles at the seal can be seen prying apart the molten plastic molecular chains. This microscopic physical separation creates a capillary effect, increasing moisture permeation from 0.01 g to more than 3 g.

Experimental data shows that when powder contamination at the seal reaches 0.5 mg/cm², seal strength drops sharply from a normal 35 N/15 mm to just 12 N/15 mm. That loss in strength increases breakage rates in subsequent drop testing by five times.

Seal-jaw temperature is typically set between 140°C and 160°C. Testing shows that fluctuation must be limited to ±2°C. If the temperature momentarily drops below 135°C, the inner PE layer cannot fully melt, and it will not properly envelop the dust particles trapped in the seal.

Maintaining seal pressure between 0.3 MPa and 0.5 MPa is fundamental to seal integrity. Insufficient pressure causes incomplete seal bands of uneven width, while excessive pressure squeezes molten PE toward the pouch edges, thinning the base of the seal and making it brittle enough for powder particles to rupture it.

• Seal dwell time: set between 0.5 and 0.8 seconds to ensure sufficient molecular interlocking
• Cooling pressure: 0.2 MPa for 0.3 seconds to prevent thermal shrinkage and micron-scale vent channels
• Seal pattern: serrated seal textures increase local pressure and are better than flat seals at displacing powder from the interface
• Jaw alignment: axial deviation between upper and lower jaws must be kept within 0.1 mm to prevent uneven stress and one-sided leakage

For highly moisture-sensitive powders such as probiotics or freeze-dried coffee, the inner seal layer is often upgraded from ordinary LLDPE to metallocene PE (mPE). This material has better melt flow and can rapidly envelop powder contaminants as small as 5 microns, almost like a liquid.

The seal initiation temperature of metallocene PE is about 15°C lower than that of standard PE, giving it a much wider process window on high-speed packaging lines. Under the same seal-contamination conditions, the sealing pass rate of mPE is usually about 25% higher than that of ordinary PE.

By adding a certain proportion of ionic polymers such as Surlyn, the seal layer can still maintain a peel strength above 20 N even when contaminated by oily powders or strongly alkaline powders.

This type of high-performance inner layer also improves hot tack, meaning the instant seal strength immediately after the jaw opens and before the seal has fully cooled. On high-speed lines running at 150 pouches per minute, hot tack needs to reach at least 1.5 N/15 mm to prevent falling powder from pulling the seal open.

Static electricity is a major contributor to seal contamination. Friction between powder and the inner film surface can generate surface voltages above 5,000 volts.

Keeping the composite film’s surface resistivity between 10⁹ and 10¹¹ ohms helps reduce static attraction. Combined with an online ionizing bar, this can reduce dust coverage in the seal zone by more than 80%, improving seal cleanliness at the source.

The most common method for testing seal integrity is the methylene blue dye test. The finished pouch is placed in a vacuum chamber containing a 0.5% methylene blue solution, vacuum is pulled down to -60 kPa, and held for 3 minutes. After removing and rinsing the pouch, any blue penetration line visible in the seal area is considered a failure.

Seal Strength

Powder Carryover & Static Control

As the filling machine’s rotating shaft drives the dosing piston in a steady reciprocating motion, powder with a particle-size distribution of 15 to 45 microns collides intensely inside the enclosed feed tube. Friction between the powder and the inner wall generates electrostatic potential in the range of 4,000V to 6,000V.

If the dust layer adhering to the laminate surface reaches 20 microns in thickness, molecular motion in the seal area encounters greater resistance, and the polymer chains can no longer fully penetrate and crosslink.

Under these conditions, peel strength in the sealed area can drop from the standard 16N/15mm to around 3N/15mm. When subjected to stacking pressure, such weak seal points can form capillary channels about 0.08 mm wide.

Moisture entering at that rate can reduce viable probiotic counts by two orders of magnitude within 30 days. If the inner liner film has a surface resistivity below 10⁹ ohms/sq, charge dissipation can be completed within 0.05 seconds.

• Ionizing bar voltage should be set between 18kV and 22kV
• Vacuum pressure in the negative-pressure dust collection system should be maintained at -12kPa to -18kPa
• The vertical distance between the discharge tube outlet and the seal line should remain within 15mm to 25mm
• The inner layer of the laminate should contain 0.8% monoglyceride antistatic agent by mass
• The seal jaw surface should use a grid pattern with a depth of 0.3mm and a pitch of 0.6mm
• Step accuracy deviation under servo motor drive should be controlled within 0.02mm

Choose metallocene polyethylene (mPE) with a melt flow rate (MFR) of 2.2g/10min. Its narrow molecular weight distribution gives it excellent hot tack. During the 0.15-second instant after the seal jaw leaves the film, the sealed area must withstand the impact energy created by the fall of 25g of powder.

Under the same temperature conditions, HDPE provides less than 1N/15mm of holding strength, while modified mPE remains stable at above 4.5N/15mm.

The middle layer of the aluminum foil laminate uses 7-micron 1235 alloy foil. Because aluminum foil has high thermal conductivity, heat transfers very quickly. If seal jaw temperature fluctuates by more than ±2°C, it can cause either over-melting or under-melting of the PE layer.

Over-melting leads to material loss at the root of the seal, reducing thickness from 30 microns to below 10 microns. In low-temperature transport at -10°C, these areas lose flexibility and can crack brittly under vibration.

The seal jaws must close within 0.08 seconds after the screw stops rotating. If the delay is shortened to 0.03 seconds, residual powder will remain in the sealing path. Increasing the seal width from 5mm to 8.5mm provides more than 40% additional sealing margin for unavoidable powder inclusion, ensuring that WVTR remains below 0.5g/m²/24h.

• Vacuum chamber test pressure should be calibrated to -90kPa and held for 60 seconds
• Penetrant dye testing should use a 1.0% rhodamine aqueous solution
• The automatic vision inspection system should detect seal-line continuity errors no greater than 0.1mm
• Burst testing (ASTM F1140) should reach a peak pressure of 45kPa to 55kPa
• The peel tester should maintain a constant displacement speed of 50mm/min
• Accelerated stability testing should be carried out for 21 days in a 40°C constant temperature and humidity chamber

The heat-seal pressure sensor records mold closure pressure in real time, with the standard operating range set between 0.45MPa and 0.65MPa.

If the surface flatness deviation of the seal jaw exceeds 0.015mm, uneven pressure distribution can cause over-sealing on one side of the pouch and invisible pinholes on the other. Under an internal pressure of 0.2MPa, such micro-defects can leak as much as 0.5ml of air per hour, enough to cause oxidation-sensitive powders to discolor in a short time.

The metal ions in the molecular chains of Surlyn can spontaneously encapsulate and isolate oil, protein, or other contaminant particles up to 0.015mm thick during heat sealing.

This material offers 120% higher seal strength than ordinary PE and provides a broad processing window from 130°C to 170°C. For powders containing instantized ingredients or trace moisture, this type of resin also prevents delamination at the seal caused by chemical attack.

Workshop humidity must be tightly controlled between 45% and 55% RH. If humidity drops below 30% RH, frictional static on the film surface increases geometrically.

Once powder absorbs moisture and cakes into lumps larger than 50% of the inner film thickness, those agglomerates can physically puncture the heat-seal layer during compression, exposing the aluminum foil layer directly to electrochemical corrosion. A five-layer coextruded barrier film can better balance mechanical strength and seal-layer thickness.

Inner Layer Material Parameters

Under the vibration of the cutter at 120 pouches per minute, the molecular weight distribution of the inner film determines its viscoelastic behavior in the molten state. The branch structure of LLDPE becomes mobile at 135°C. If the branches are too short or unevenly distributed, tear strength at the seal can drop from 35N/cm to 20N/cm.

The density of metallocene polyethylene (mPE) is controlled between 0.918 and 0.925g/cm³. Compared with conventional LLDPE, its seal initiation temperature (SIT) can be reduced by 15 to 20°C. At 115°C, mPE can already achieve a heat-seal strength of 10N/15mm. On high-speed filling lines, this reduces thermal energy consumption by 25% and also minimizes wrinkles caused by heat shrinkage.

Surlyn, an ionic polymer, contains acid groups neutralized by sodium or zinc ions. When the seal surface is contaminated by a 0.01mm layer of coffee oil, the sealing integrity of ordinary PE drops below 60%. Surlyn, however, uses ionic crosslinking to encapsulate the oil molecules and maintain more than 95% seal integrity. Its melt strength at 140°C is five times higher than that of standard PE.

Increasing the inner PE layer thickness from 25 microns to 40 microns improves puncture resistance by 70%. For powders containing sharp crystals, an inner layer thinner than 30 microns can develop pinholes during transport vibration.

Replacing it with a five-layer coextruded PA/PE film raises dart impact strength above 400g, ensuring that 0.5kg powder packs can survive a 2-meter drop without damage.

• Select polyolefin resin with a melt index (MI) between 1.0 and 2.5
• Keep erucamide slip additive content between 300ppm and 500ppm
• The hardness of the antiblocking additive in the inner layer should not exceed Mohs 3
• Interlayer peel strength of the laminate must be greater than 2.5N/15mm
• Variation in the inner layer’s rheological flow rate under molding pressure should remain below 5%
• Seal-layer thinning must be kept within 15% of the original thickness

The coefficient of friction (COF) of the inner film should remain stable between 0.15 and 0.25. If COF exceeds 0.3, sliding resistance against the metal surface of the forming collar increases, and the seal position can shift by 3mm to 5mm. That shift prevents the seal zone from aligning with the center of the heated mold, causing an actual pressure loss of more than 40%.

In aluminum foil laminate structures, the 12-micron PET face layer and 7-micron aluminum foil layer together form a thermal barrier. Heat takes 0.12 seconds to transfer from the seal jaw to the inner PE layer. If the thermal diffusivity of the PE layer is below 0.15 mm²/s, the actual interface temperature never reaches the polymer’s melting point. Under those conditions, line speed is effectively capped at 60 pouches per minute.

• Inner layer transparency should remain above 85% for visual inspection
• Antistatic additive must not migrate to the surface in less than 24 hours
• In peel testing, the failure mode must be substrate breakage, not interface separation
• Hot tack at 110°C must exceed 3.0N/15mm
• The material’s heat of fusion (ΔH) should be controlled between 110J/g and 130J/g
• Carbon buildup on the seal jaws after 10,000 consecutive seals must remain below 0.05mm

To prevent volatile oily components in the powder from damaging the seal, the inner layer must also resist the contents chemically. In accelerated aging at 45°C, standard PE begins to show a decline in seal strength after 14 days. By incorporating 15% acid-modified polyolefin (EAA), the seal’s resistance to acidic or oily powders can be extended to 24 months.

Leak Detection & Compression Testing

On the vacuum chamber, the gauge needle must pass the -90kPa mark. If a powder sachet in the transparent water tank contains a pinhole larger than 15 microns, the air inside will escape under the pressure differential as a continuous stream of 1mm bubbles. The hold time is set at 60 seconds. Even the slightest bubble release indicates that moisture gain during shelf life may exceed 0.5%, causing the powder to cake and deteriorate.

A vacuum decay tester must have a resolution of 0.01Pa/s. This non-destructive method monitors the slight pressure rise inside the vacuum chamber and can detect microleaks as small as 0.5 microliters per minute. In nitrogen-flushed coffee powder packaging, a physical defect larger than 5 microns can drive residual oxygen inside the pouch from 0.8% to over 5.0% within 48 hours.

“In a laboratory environment at -20°C, the flexibility of the laminate drops by 30%. Negative-pressure testing under these conditions can reveal brittle microcracks that are not detectable at room temperature.”

An inflation needle is inserted into the pouch and sealed with a gasket. Air is introduced continuously at 20mbar per second, and the live pressure curve collapses sharply at 52kPa. If the rupture point lies 0.5mm inside the horizontal seal, it indicates that excessive sealing temperature has caused thermal degradation of the polymer chains, leaving that region weaker than the 30-micron base film itself.

When pressure testing cannot pinpoint leakage paths below 0.01mm, rhodamine dye becomes useful. Apply a 1.0% dye solution along the seal edge and leave it for 15 minutes. If the red penetration band observed under the microscope extends across more than 25% of the seal width, the process is judged to have insufficient molecular crosslinking and an extremely high risk of moisture ingress.

• Automatic inflation rate should be set between 15mbar/s and 25mbar/s
• Peak burst pressure must not fall below the 40kPa minimum standard
• The physical coordinates of the rupture point relative to the seal center must be recorded
• Dye penetration depth must remain within the 1.5mm safety threshold
• Microscopic analysis must record the fibrillation length of the peeled seal surface
• Deformation under a static stacking pressure of 0.3MPa must be evaluated

Seal consistency across multiple lanes directly affects the pass rate of the entire lot. Compare finished pouches from Lane 1 and Lane 10. If the burst pressure difference exceeds 10kPa, it indicates that the pressure-balancing mechanism of the multi-lane sealing mold has developed a mechanical deviation greater than 0.02mm. After 2 hours of operation, that deviation can produce localized weak seals caused by uneven thermal expansion.

Finished pouches must also undergo a 1.5-meter drop test covering three orientations: front, back, and seal side. Over 5 consecutive drop cycles using 10 samples, the seal line must show no delamination or leakage. Based on the impulse theorem, a 25g filled powder pouch experiences an impact force at the moment of collision equal to roughly 30 times its own weight.

“Static compression testing simulates the squeezing stress created by 50kg of cargo during logistics. The seal area must show zero displacement after 3 minutes under constant pressure.”

Accelerated aging is carried out in a constant temperature and humidity chamber at 45°C and 75% RH. After 21 days of simulated storage, the measured decline in seal strength must remain within 15%. For active probiotic powders, if barrier performance drops after aging and WVTR rises from 0.5g/m²·day to above 2.0, the rate of probiotic inactivation will increase by threefold.

A 200kg load is applied on top of the carton, and the sachets at the bottom are required to withstand a uniform pressure of 1.5kPa. If the seal material undergoes creep under long-term compression, the 5mm seal width will thin out and stretch.

• 45°C accelerated aging should simulate a 12-month shelf life
• The difference in peel strength before and after aging must be less than 2N
• Deviation in measured WVTR must be controlled within 10%
• The 1.0% dye test must be held for at least 30 minutes
• The change in pouch stiffness after aging must not exceed 20%
• Any interference caused by slip additive bloom at the seal must be recorded

For easily oxidized fruit powders, the online residual-oxygen analyzer should be checked once every hour. A sampling needle extracts gas from inside the pouch and feeds it to the sensor, which reads oxygen concentration to 0.01%. If the value is too high, the investigation should go beyond nitrogen purity and trace back to the temperature distribution across the sealing mold during its 0.15-second closure cycle. Any fluctuation greater than 3°C can create micron-scale oxygen transmission channels across the seal surface.

Filling Accuracy

Factors Affecting Accuracy

When the moisture content of a powder rises from 3% to 5%, its bulk density changes enough to shift filling consistency by 4.5%. In a 500-liter hopper, the compression force acting on the powder at the bottom decreases as the powder level drops. When the material level in the hopper falls from 80% to 20%, the fill weight of each sachet can quietly decrease by 0.2 g.

Moisture and compression are not the only factors. As powder moves rapidly through the feed tube, friction can generate static electricity of up to 2,000 volts. The charged particles repel one another, causing the powder to disperse like a mist as it falls and lodge in the sealing area. Installing an ionizing bar at the seal zone to reduce the voltage to below 200 volts can keep seal-failure rates under 0.1%.

The screw auger, made from 304 stainless steel, must have a surface roughness of Ra 0.8 microns or lower. If the surface has scratches just a few microns deep, powder will cling to them. After 2 hours of operation, the buildup can reach 0.5 mm in thickness. That reduces the effective conveying space of the screw and causes fill weights to fluctuate by 0.15 g to 0.3 g.

Screw smoothness matters, but so does the clearance between the screw and the inner wall of the tube. That gap should be controlled between 0.5 mm and 0.8 mm. If it exceeds 1 mm, powder will begin to leak through on its own. When filling highly free-flowing products such as juice powder, the overfill can exceed 3%.

To keep the screw and related components running reliably, the factory settings must be tightly controlled:

• Main motor speed fixed at 1,500 rpm
• Agitator blade speed in the hopper fixed at 25 rpm
• Powder shut-off valve response time faster than 15 milliseconds
• Thread machining tolerance on the screw within 0.01 mm
• Wear life of the powder-blocking seal longer than 2,000 hours

High-speed camera footage shows that powder takes about 120 milliseconds to fall from the discharge outlet to the bottom of the bag. If the air pressure inside the tube and inside the bag is unbalanced, small vortices form and can blow away about 0.05 g of fine powder. Adding a vent channel on the side of the tube helps recover that lost weight.

Another solution is to use a twin-screw system. The larger screw handles 92% of the fill, while the smaller screw tops up the remaining 8%. The main screw runs at 1,200 rpm, while the fine-feed screw slows to 150 rpm. This combination reduces filling error from the usual ±1% to ±0.2%, saving a significant amount of material.

But even with twin screws, powder flow inside the hopper remains critical. If the angle of repose exceeds 45 degrees, voids are likely to form inside the hopper. Installing an ultrasonic vibration plate that vibrates 18 times per second can break up the powder and keep it flowing more like a liquid, while also increasing filling speed by 20%.

Daily production also requires close monitoring of environmental and material changes:

• Workshop humidity maintained between 45% and 55%
• Variation in average particle size kept within 15 microns
• Antistatic value on the surface of the packaging film kept below 10¹¹ ohms
• Infrared level sensor error in the hopper kept below 5 mm
• Daily zero-point drift of the weighing system kept below 0.01 g
• Air pressure for rejecting off-spec sachets held steady at 0.6 MPa

If volumetric cup filling is used instead of a screw auger, weight accuracy becomes highly dependent on bulk density. When the product is loosely packed, the weight tolerance typically ranges from ±1.5% to ±3%. In pharmaceutical-grade packaging lines, the standard is much tighter. If the standard deviation across 1,000 consecutive sachets increases from 0.05 g to 0.12 g, the defect rate can jump from 0.03% to over 5%.

To prevent defective bags from slipping through, the conveyor should be fitted with an inline checkweigher capable of sampling 200 times per second. When the system detects that the average weight of 5 consecutive sachets has drifted by 0.1 g, the controller sends a command to the servo motor, and the screw rotates an additional 1.2 degrees on the next cycle to automatically compensate.

Performance Comparison of Filling Systems

A 2.5 kW AC servo motor drives the screw auger, and the encoder resolution reaches 23 bits. One full rotation of the screw is divided into 8,388,608 micro-steps, and the control unit sends a pulse command every 0.5 milliseconds. This level of digital control keeps powder displacement tolerance within 0.01 mm.

A single-head screw filler packaging 15 g of drip coffee powder can run at 60 sachets per minute, with a weight deviation of about 0.15 g. A dual-head system splits the job between two screws, one delivering 14 g as a coarse fill and the other topping up 1 g with fine adjustment. This reduces the deviation to 0.05 g, saving about 1.2 kg of raw material per 10,000 sachets compared with a single-head system.

Cup fillers rely on volume rather than weight. The rotary table usually carries 6 to 8 telescopic stainless steel cups, and the capacity is adjusted by changing the overlap height of the upper and lower cup walls. When packing 40-mesh fine granulated sugar, the table rotates at 50 rpm. With a bulk density of 0.85 g/cm³, the cup volume is set at 11.76 ml, which yields exactly 10 g.

The weakness of cup filling is that it cannot handle moisture-sensitive powders well. If ambient humidity climbs from 40% to 75%, the powder absorbs moisture and gets heavier without a visible volume change. A cup that originally dispensed 10 g may begin filling 10.4 g instead. Over an 8-hour shift, the giveaway loss can equal 200 extra sachets.

For high-value protein powder priced at RMB 300 per kilogram, weighing fillers are more economical. The load cell under the machine samples at 4,800 times per second. The weigh hopper first drops in 90% of the target amount, then the last 1.5 g is fed slowly by a small vibrating tray at a rate of 0.1 g per second.

Load cells are sensitive to motor vibration in the workshop. The weighing platform is mounted on 4 high-performance damping springs, which filter out ambient mechanical noise in the 10 Hz to 50 Hz range. With a sensitivity of 0.01 g, the sensor can detect even the 5 mg pressure effect caused by air movement around the weigh pan.

• The surface hardness of 316 stainless steel screws should reach Mohs 5
• Maximum servo system torque should cover 3 times the actual filling resistance
• Temperature drift of the weighing sensor should remain below 0.002% per °C
• The lead tolerance of the cup-adjustment screw should stay within 0.02 mm
• The control system should support storage for 100 different recipes
• The reject mechanism’s solenoid valve should respond in under 8 milliseconds

If powder sits in the hopper for 2 hours, the material at the bottom becomes more compact and can reach a bulk density 12% higher than the powder at the top. If the agitator speed drops below 15 rpm, it cannot break up that compacted layer, and sachet weights will gradually drift upward.

High-quality screws are machined on 5-axis CNC equipment, with pitch deviation under 0.01 mm. Ordinary screws made on standard lathes may have thread spacing errors of 0.1 mm. At high speed, that 0.1 mm variation can create random discharge fluctuations of 0.2 g per sachet.

The clearance between the screw and the outer housing is equally important. If it is left at 1.2 mm, fine particles such as salt can leak 0.5 g into the bag after shutdown. Tightening that clearance to 0.6 mm and pairing it with a pneumatic shut-off valve that responds in 12 milliseconds can cut the leakage to 0.02 g.

• Average roughness of all product-contact parts: Ra 0.4
• Dynamic response bandwidth of the servo drive: 2.5 kHz
• Zero stability of the weighing module over 24 hours: 0.01 g
• Hopper wall angle greater than 65 degrees
• Seals must remain leak-free under 0.4 MPa pressure
• The weighing signal converter should use 24-bit A/D resolution

Extremely fine chemical powders behave almost like smoke. Vacuum-assisted filling can solve this issue. The system creates a negative pressure of 20 Pa inside the bag and pulls the powder in. Although this method is slower, at about 25 sachets per minute, it can reduce dust loss from 2% to 0.3%.

When the infrared level sensor detects that the hopper level has dropped below 30%, the automatic feeding pump must start within 3 seconds. If the level fluctuates between 20% and 80%, the pressure acting on the bottom screw can differ by 1,500 Pa, causing fill-weight variation of about 0.3 g.

Equipment Test Methods

If the manufacturer claims the machine can run at 120 bags per minute, then test it directly at 120. Collect 100 consecutive finished sachets and weigh them one by one on an electronic balance with 0.01 g precision. If the difference between the heaviest and lightest sachet exceeds 0.5 g, the machine’s stability should be questioned.

After recording all 100 weight values, calculate the average. If the target is 10 g and the mean comes out at 10.05 g, the machine is overfilling. Then calculate the standard deviation. If it exceeds 0.15 g, that indicates pulse jitter or missed steps in the servo drive under high-speed operation.

A 5-minute test is not enough to reveal thermal drift. Let the machine run continuously for 2 hours, and pull 10 sachets every 15 minutes for weighing. As the motor and screw heat up, a 10°C temperature rise can expand the stainless steel screw by 0.01 mm. Even that tiny expansion can change the discharge volume, so compare the weights after 2 hours with those at startup.

• During 30 minutes of empty running, bearing temperature rise should not exceed 35°C
• After 120 minutes at full load, average weight drift should stay below 0.1 g
• Servo drive error log should remain at 0
• Air supply pressure fluctuation around 0.6 MPa should stay within 0.02 MPa
• The cutter should cycle 100 times per minute without stringing
• Reject mechanism accuracy must reach 100%

Then test the machine with a dampened powder. Increase the moisture content from 3% to 6% and see whether the machine can compensate automatically. Without a feedback system, the extra density can shift fill weight by 0.2 g. At that point, adjust the correction parameters on the touchscreen and observe how quickly the machine stabilizes.

Testing zero drift on the load cell is simple. Stop feeding and let the weigh pan run empty for 10 minutes. If the display moves from 0.00 to 0.05, the electronics are likely being affected by electromagnetic interference. In production, that interference feeds false signals back to the motor and can waste 0.5% of material.

Turn on the packaging machine next to it and see whether vibration causes the scale to jump. Set the vibration frequency to 30 Hz. A good sensor should be able to filter out the noise algorithmically. If the display starts bouncing like an ECG trace, the machine will be useless in a real workshop environment.

Accelerate the line from 40 bags per minute to 100 and watch whether the powder drop point shifts. Powder takes about 0.15 seconds to fall into the bag, and as the line speed rises, the bag position changes faster too. If the discharge tube no longer aligns with the bag opening, 0.1 g of powder may stick to the seal area. Tear the seal open by hand and check whether powder is trapped inside, and whether the seal strength still exceeds 20 N.

• Repeat positioning accuracy after disassembly and reinstallation of the screw: 0.05 mm
• Level sensor recognition error for black and white materials: 0
• Delay from touchscreen command to cylinder movement: less than 5 milliseconds
• Temperature difference across the four corners of the sealing mold: within 1°C
• Cumulative elongation of the chain drive system: less than 0.2 mm per meter
• Restart recovery time after emergency stop: shortened to 3 seconds

Voltage stability must also be tested. Lower the supply voltage manually from 220V to 200V and observe whether the servo motor slows down. A good drive system should compensate over a wide voltage range. If the voltage drops and the fill weight drops with it, then during peak power usage the factory will end up producing off-spec product.

Finally, compare 5 batches of raw material from different dates. If the D50 particle size of one batch is 80 microns and another is 120 microns, observe the difference in discharged mass for the same screw rotation angle. If the error widens to more than 2%, it means the screw pitch design cannot handle particle variation and the machine is too sensitive to raw-material changes.