Tension on Rollers

The tensions applied on the substrates are closely monitored . This is done with the help of a load cell. A load cell is able to convert a force exerted on it by means of deforming a strain gauge which in turns measures this deformation applied on it into an electrical signal. This electrical signal varies when different forces are applied because the strain gauge's electrical resistance varies by the degree it's deformed. A strain gauge is a length of conductor arranged in a zigzag pattern in a membrane whose electrical resistance increases the more it's stretched. The voltages are normally in the order of millivolts and require amplification before it can be used to measure anything.

Normal load cells consists of four strain gauges arranged in the Wheatstone configuration, but the one in this context utilises only two strain gauges arranged in a half bridge configuration as shown below and is calibrated as such to indicate the tension in terms of kilograms, as this is a more common unit that technicians are more accustomed to.

According to the machine operators, the normal operating tensions for the Aluminium foil are around 12 kilograms worth of pressure whereas the PET will require something in the order of 9. With the help of the load cell, it is much easier for operation as the right amount of tension is easier to be applied. With the right tension applied on the substrates, the chances of creasing or wrinkling occurring is reduced drastically.

An improvement to the existing system is the implementation of a Tension Roll Transducer. The Tension Roll Transducer is combined with a regular idler roll consisting of 2 load cells. The reasons as to this being recommended in all the machinery are as follows:

• Measures and displays the actual web tension and allows for the control of the tension too.
• Since they are all assembled into one unit, there no assembly required whatsoever.
• Costs less than an idler roll and a load cell bought separately.
• Easy to install and takes up the same space as an ider roll.
• Only one transducer cable hence no cables will cross the machine. 

In addition to that, the bearing on the nip roller were also replaced as it was observed to not have a stable rotation and it was believed to be one of the reasons for the formation of creases on the final laminate.

Eliminating Wrinkles during Lamination

 

Creasing is when one of the substrates is separated or de-laminated from the other due to different extension properties in both films. This leads to the films extending at different rates when a tension is applied on them. This can be eliminated by the correct use of tensions applied, the proper evaporation provided by the ovens or by simply selecting the correct adhesive.

How creasing is caused

When considering the lamination of a printed PET (Polyethylene Terepthalate) film with an aluminium foil along with an PE (Polyethylene) layer on the other end, the product structure are shown below. In addition to that, for a product's web width of 42”, the suggested tensions could be obtained using the formula below.

Suggested Tension = Tension Level * Caliper * Web Width

The results obtained are as follows:

Material	Caliper (inch)	Thickness (%)	Tensile Modulus (psi)	Tension Levels (lbs/inch/mil)	Suggested Tension (#)
PET	0.00048	10.4	500000	0.5 – 1.5	10 to 30
Foil	0.00035	7.6	10000000	0.5 – 1.5	7 to 21
PE	0.00400	82	25000	0.25 – 0.3	40 to 48

For the lamination process to be completed effectively, the strain applied on each of the materials will have to be approximately equal before the lamination process takes place in order to prevent defects such as creasing.

Using the formula,

Stalin = Stress / Modulus of Elasticity = (Tension / Area) / Modulus of Elasticity = Tension / (Area * Modulus of Elasticity)

Since the Modulus of Elasticity for aluminium is about 20 times greater than that of PET, in order for both of them to undergo approximately equal strains, the aluminium will have to be pulled by its highest possible tension suggested and PET should be pulled by its lowest possible tension suggested.

However, another problem is prone to arise when the aluminium foil is pulled with high tension, which is the formation of diagonal wrinkles in the long web lead before it passes through the nip roller. These diagonal wrinkles are a product of cross machine difference in the web's thickness and could get a lot more noticeable with roller deflection or misalignment. The use of a Herringbone Spreader Roll will eliminate the wrinkles on the foil before being fed into the laminating nip roller. In addition to that, the spreader roll will also compensate for any deflections or misalignment that could cause any wrinkles to the foil.



A Herringbone Roller



Dry Lamination Process

This article focuses on the dry laminating process known as dry bond laminating. In this process, a liquid adhesive, normally using ethyl acetate as a solvent, is applied on the substrate to be laminated by means of a gravure cylinder. The gravure cylinder has a doctor blade attached to it to ensure that a uniform amount of adhesive is applied on the substrate.

Once the adhesive is applied on the substrate, it is passed through three ovens each of varying temperature and all consisting of fans. The heat and airflow in the oven evaporates the solvent while the remaining solid compounds of the adhesive is left on the substrate.

The substrate is then joined with the second substrate with the help of a heated compression nip roller. The high temperature and pressure causes the adhesive to flow freely and create an instantaneous bond between the two substrates once it cools.

The Dry Lamination Process

The stress applied by the laminator throughout the whole process needs to be under the right amount of tension (hence the use of many idler rolls) in order to prevent the films from tearing or creasing (or tunnelling) after the completion of the lamination process.

The correct adhesive will have to be selected as well, in order for effective lamination, an adhesive with a surface tension lower than the critical surface tension of the substrate that is to be coated by it. Some preparation processes such as corona treatment or plasma treatment increase the surface tension of substrates, this allows for more efficient lamination.

However, the bonding ability of the adhesive isn't the only factor that needs to be taken into account. If the adhesive could increase the performance of the final product such as improving its barrier properties, that may go a long way.