Sealing Technology for Sachets: A Comparison of Methods and Parameters

It's the stitch that matters, not the machine

A sachet consists of a sealed seam with contents enclosed within it: The surfaces are small, the seams are narrow, and the contents sit just millimeters from the sealing point. As is typical in the industry, the majority of all sachet complaints stem from seal defects—not from dosing errors or machine downtime. The choice of sealing method and strict adherence to parameter settings therefore determine the scrap rate and complaint costs. The machine specifications for this are outlined in the article on Sachetmaschine.

Method 1: Heat Sealing—the Industry Standard

Two continuously heated sealing jaws press the film layers together; the temperature is maintained at a constant ±5 °C via PT100 sensors. This process accounts for the lion's share of the installed base because it is fast, energy-efficient, and easy to configure.

• Temperature range: 140 to 220 °C, depending on the film laminate
• Seal Period: 0.3 to 1.5 seconds
• Contact pressure: 2 to 6 bar
• Seam strength: 8 to 12 N per 15 mm seam width for standard joints

Limitations of the process: Heat transfers into the product and the film—temperature-sensitive fillings and thin barrier layers require tighter process control or a different process.

Method 2: Pulse Sealing – for heat-sensitive films

In pulse sealing, a resistance wire heats up only during the sealing phase—0.2 to 0.5 seconds of current flow—after which the jaws cool down under pressure before opening. The film is not exposed to sustained heat.

Advantages: clean, narrow seams on pure PE and PP films and on temperature-sensitive products. Disadvantages: lower cycle rate (50 to 120 bags/min., depending on the system) and wear on the hot-wire elements. Typical applications include sample sachets and specialty films in small batches.

Method 3: Ultrasonic Sealing – Seals Through Product Residue

A converter transforms electrical energy into mechanical vibration (20 to 40 kHz); the sonotrode remains cool, and heat is generated solely by internal friction within the sealing layer. The key advantage: The seam remains airtight even when product residues—such as powder, grease, or liquid droplets—are present in the sealing zone. This is precisely the weak point of heat sealing.

• Seam strength: 8 to 15 N per 15 mm, highly reproducible
• Seam width: limited to about 10 mm
• Investment: significantly higher than the heat-sealing process; justified by lower scrap rates for products prone to contamination

Main applications: Pharmaceutical sachets, wet wipes, and sterile products—where contamination through the seam is likely and a leaky seam is costly.

Method 4: Hot Glue—The Special Case

Apply hot melt (melting temperature of 120 to 180 °C) to one side of the substrate, then press. This process seals film combinations that do not have a thermally compatible sealing layer and remains a niche application in the sachet sector—particularly noteworthy for composites containing paper or cardboard.

The Trio of Parameters and Their Enemies

Regardless of the process, three factors determine the seam: temperature, pressure, and time. During operation, three disruptive factors come into play:

1. Foreign matter in the seam: Residues of the filled product between the film layers drastically reduce seam strength. Solutions: Suction at the filling tube, precise dosing timing, and tube flushing.
2. Film fluctuations: Tolerances in thickness and seal coat thickness shift the process window—incoming inspection of each batch is less expensive than scrapping an entire coat.
3. Wear on the sealing jaws: Burnt residue and unevenly worn baking surfaces distribute the pressure unevenly—the seam shows lighter streaks. Cleaning after each layer and changing the baking sheet at regular intervals are part of the scheduled Maintenance.

Checking Seams: Three Methods

• Punching test per layer: Paperclip-like test specimens from ongoing production, manually subjected to tensile stress—the simplest form of ongoing inspection.
• Seam Strength Test: Tensile test using test clamps; result in N per 15 mm seam width; target values are typically specified for each format in accordance with industry standards.
• Leak Test: Internal pressure or vacuum testing of the finished sachet—a mandatory part of the release testing for pharmaceutical and liquid products, often integrated with the production line’s testing technology (Metal Detector and Re-weighing Check).

Frequently Asked Questions

What is the correct sealing temperature?

There is no universal temperature—the process window depends on the film laminate. Manufacturer specifications range from 140 to 220 °C; the optimal setting is verified for each film batch using a die-cut sample and saved in the recipe.

When is ultrasonic sealing a good option?

When product ingress into the seam is likely (powders, pastes, moist products) or when very thin films have a narrow heat-seal window. For clean powders with standard film, heat sealing remains the cost-effective standard solution.

How wide should a sachet seal be?

Typically 5 to 10 mm; narrower for small formats, wider for liquid products. Narrow seams save on film, while wide seams accommodate tolerances—the design should be discussed with the film supplier during the format planning phase.

How much does scrap caused by sewing defects cost?

Every leaky bag is a defective bag plus the cost of handling a complaint; the total cost is calculated as the number of units × defect rate × cost per bag plus recall costs. Once this calculation is done, it either justifies or refutes any investment in better sealing and inspection technology.

Related pages: Vertical Filling and Sealing Machine | Stitch-sewing packaging machine | Metal Detector and Re-weighing Check | VFFS Guide

Request technical specifications and a quote for your production line. Please specify the product, bag size, and target output (bags/min.) – Request Technical Specifications and a Quote.