HERMANN – Secure seals and gentle handling of fill goods with ultrasonics

Facebook
LinkedIn
Twitter

INTERPACK 2014

[twitter-follow screen_name=’itfoodonline’]


For info please contact +39 0362 244182, or mail to info@itfoodonline.com.

interpack2014 (4)Secure seals and gentle handling of fill goods with ultrasonics

Ultrasonic sealing for packaging. Hermetically tight joints reduce the leaker rate and make the industry sit up and take notice. Moreover, narrow seams save packaging material while cold tools are gentle on sensitive feed material. A closer look at a reliable and efficient alternative for heat sealing.

interpack2014 (3)Sealing is a process of substance-bonded joining of composite material with a thermoplastic layer, with this layer being subject to technical welding conditions. For this reason, ultrasonic welding systems have been successfully implemented for a variety of sealing applications in the field of packaging, such as bags, blisters, pods, and cartons for quite some time now. The benefits speak for themselves:

interpack2014 (2)Short cycle times between 80 and 200 ms
No thermal influences on feed material and packaging material
Quality assurance by means of detailed parameterization
Sealing of product-contaminated surfaces (liquid, powder, paste)
Good hot tack
Saving of packaging material due to narrow seams
Reduced effort for cleaning of sealing tools
Good overall equipment effectiveness (OEE)

interpack2014 (1)Basic ultrasonic principles: Transformation of mechanical vibration into heat
The ultrasonic waves are mechanical sound waves that are transformed into friction heat and thus cause a substance-bonded connection at the molecular level. Friction heat is generated by deformation work, i.e. inner friction within the molecular bond, and through boundary layer friction, i.e. outer friction between the sealing materials. During the ultrasonic process, mechanical vibrations of an ultrasonic frequency are applied to the foil at a specific amplitude, force, and duration. The process parameters depend on foil structure, foil thickness and joint geometry.

From the applied supply voltage, the ultrasonic generator generates high voltage, which oscillates at an ultrasonic frequency. In the converter (sound transducer) the oscillating high voltage is transformed into mechanical longitudinal vibration by means of piezoceramic elements exploiting the reverse piezo effect. This longitudinal vibration, in turn, excites and causes vibration in a mechanical resonance unit (stack). The vibration amplitude generated in the converter can be changed in the booster and the Sonotrode by utilization of different mass ratios, and thus be adjusted to the respective sealing job. The Sonotrode is the vibrating and “cold” sealing tool that directly applies vibration onto the packaging material at the supplied joining force. The frequency represents the number of vibrations per second and ranges between 20 kHz and 35 kHz (20 kHz is equal to 20,000 deformation cycles per second) for packaging materials. The vibration amplitude is equal to half the peak-to-peak amplitude, i.e. from zero value to peak value, and ranges between 5 and 50 µm.

Narrow seams
Ultrasonic seals are slimmer than heat contact seals, but equally tight and strong; and in addition, they save packaging material. Precise focusing of energy to melt the material is ensured by means of the anvil and/or Sonotrode contour and the small contact surface between Sonotrode and anvil. Anvils mostly have a textured
structure with reduced contact surfaces and serve as passive tools; thus they must be mechanically decoupled to prevent from vibrating and ensure optimum utilization. The contact surface of the anvil consequently defines the sealing zone and hence the point of initial melt formation upon sound exposure; from that point, the sealing layer of the packaging material is transformed into a liquid state. After termination of ultrasonics exposure, the sealing layer material solidifies. During this period the pressure is maintained for melt cool-down for a material and machine-based hold time. Then, the packaging materials are firmly connected with each other.

Tight sealing through product-contaminated surfaces
Tight and strong sealing joints are also achieved for product-wetted sealing layers, since the mechanical ultrasonic vibrations literally “knock off” any feed material residues. This significantly reduces the number of leaking packages. Due to the option of parameter adjustment to the sealing process, variations in layer thickness, humidity content, and temperature difference can be detected and compensated within the specified limits. The ultrasonic generator saves all data and allows for quality assessment of the sealing process by means of reference/actual comparison. Consequently, process check complies with highest quality requirements. Since operation is performed with cold tools, packaging and feed material are hardly subjected to thermal loads. The cleaning effort is reduced and sealing tools are not soiled even in case of malfunctions in the filling process, which is a great advantage, particularly for temperature-sensitive feed materials with low melting points, such as chocolate-containing or chocolate-coated products. The use of nonstick coating, such as Teflon strips, is not required.

Short cycle times
Ultrasonic sealing is characterized by high energy efficiency, since energy is only required during actual sound exposure and short cycle times can be realized with sealing times of between 80 and 200 ms. This is particularly beneficial with thick packaging materials or connections between injection molded packaging materials/accessories and foils. In comparison with permanently heated heat contact processes, ultrasonic systems are ready for operation immediately upon switch-on of the ultrasonic generator. By storing the sealing parameters in the memory of the ultrasonic generator, automatic switch-over among parameter sets is possible. Changes become immediately effective upon start of the next sealing process, so that no machine downtime is required. Intelligent adjustment of sealing parameters to ambience-based or process-related changes allows for consistent and uniform sealing quality.

Increased hot tack
With the ultrasonic process, the heat required for melting is only generated inside the seal (sealing layer) within the joining area. The support layer remains nearly completely cold and upon termination of energy input, the heat dissipates faster to the outside due to the temperature difference between the support layer and the sealing layer so that the hot-tack resistance is significantly higher. Hot tack characterizes the strength of the joint immediately after termination of the thermal joining process without cooling. For vertical form, fill and seal (VFFS) machines in particular, high values are important since the seal joints are subjected to heavy loads through filling immediately after sealing.
The principal difference between ultrasonic sealing processes and heat contact processes is the temperature distribution. In heat contact processes the packaging materials are heated up from the outside by means of permanently heated or impulse-heated sealing tools. Since the temperature at the surface of the packaging material is higher than in the sealing area, heat flow into the inside of the joint is maintained after removal of the sealing tools. Initial hot-tack strength is low and a cooling period is required to achieve a strong joint. Moreover, overly high temperatures in the sealing tools will cause thermal damage to the packaging material (shrinkage or burning), material sticking to the sealing tools, or impairment of the feed material.
By applying ultrasonics, these negative factors can be minimized.

 

Leave a Comment

Stay Connected

More Updates