Friction testing is used in the packaging industry to measure the slip resistance of a product, with the aim of predicting feeding and running speed on an automatic glueing, erecting, filling or packaging line.
Other industries that test for slip include the paper industry (e.g. for the automatic feeding of envelopes and banknotes) and plastic manufacturers (the frictional properties of packaging films).
A products slip resistance is characterised by its coefficients of friction:
Static coefficient of friction = Fs/N
Dynamic coefficient of friction = Fd/N
Where Fs is the maximum static frictional force and Fd is the average dynamic frictional force. N is the Normal force, ie the force of gravity acting on the sample and test sled.
In practical terms, the static slip relates to the force required to get two resting surfaces moving, dynamic slip is the smaller force that is required to keep the surfaces moving once this initial “inertia” is overcome.
These values are expressed as ratios and do not have units, they are usually quoted as a decimal value between 0 and 1, for example, a surface might have a static slip coefficient of 0.35 and a dynamic slip coefficient of 0.18.
All methods of COF measurement involve preparing a sample into two flat pieces, the samples are placed together and a constant weight is applied to represent the normal force. One piece is fixed, and a gradually increasing force is applied to the second sample until the samples begin to slip against one another (Fd Max).
To measure Static and Dynamic coefficients of friction it is necessary to use a fixed bed instrument. These instruments use a motor to pull a sled across the sample, using a load cell to measure the forces.
Original instruments were converted tensile testers that used a cord to pull the sample, the use of a cord has now been removed from many friction measurement standards due to the uncertainty added by its own elasticity and problems with sample positioning.
The Universal Friction Tester uses mechanical linkages to apply the force and uses automatic sled placement for very accurate sample positioning with variable dwell time before testing.
COF can often be related to the feeding and running attributes of products, for instance, U.V. varnished food cartons have a slip coefficient that is related to the formulation of the U.V. coating its cure and film weight.
Cartons that have a very low static coefficient of friction may have handling difficulties as they will tend to slide apart and are difficult to place into feeding hoppers.
In contrast, products which have a high coefficient of friction will tend to stick together and can be prone to misfeeding due to multiple cartons entering the packaging line.
Different packaging lines will often require products with specific surface frictional profiles to achieve their highest running and feeding speeds, it is only by measuring and specifying these values that a manufacturer can achieve maximum productivity.
COF is primarily influenced by the chemical composition of the surface and its surface profile, in packaging, this is often a coating applied to the packaging. Chemical additives are used to adjust the slip resistance, these additives are often waxes or silicones which change the profile on a molecular level and alter the atomic attraction of the surfaces.
Other important factors that affect COF are test speed, the normal force (mass of the sled), contact area and geometry of the sample, these values are often specified in the test method (ASTM D1894 and ISO 8295).
The Hanatek Universal Friction Tester allows the user to measure and store the full force curve which graphically illustrates the frictional characteristics in addition to providing the static and dynamic COF values.
These force curves give more detailed information about the surface of the product allowing us to better understand how the surface will perform in the production environment.
The graphical analysis shows inconsistencies in the surface and identifies other characteristics such as stiction, an attribute during dynamic slip when the surface “judders”.
The unique strength of the Hanatek Universal Friction Tester is that profiles can be overlaid for comparison, allowing identification of substrate or coating changes that can cause problems with product runnability.
This powerful feature can highlight subtle differences in substrates or coatings that allow the user to fine-tune their product for their production conditions giving optimum feeding, running and packing speeds.
Board stiffness and crease resistance are important measures that indicate how a finished carton will run on an automated glueing, filling or packaging line.
Substrate manufacturers, printers, converters and any manufacturer who fills or packs products in cartons can use this measurement to optimise production.
QA departments use these instruments to check the running attributes of finished cartons prior to conversion and filling, reducing lost production time from slow running or difficult to convert packaging.
Carton manufacturers and designers can use a CBT1 crease and board stiffness tester with a Hanatek Carton Crease Proofer to test different substrate and crease combinations in the laboratory without committing valuable production time.
A printed sheet or roll of carton board is die-cut and creased into a preformed carton board blank.
This blank is then often glued and erected before being filled on an automated packaging line, these processes all interact mechanically with the blank to convert it into a three-dimensional object.
To be most cost-effective, it is important this conversion is performed at the maximum speed possible without causing misfeeds and blockages in the process.
It is also important that the finished material has the required dimensional strength to hold and protect the packaged product.
Board stiffness test and crease resistance testing are important parameters that help determine maximum conversion and packaging speeds, they can also be related to the final dimensional stability of the finished product.
Board stiffness is determined by the physical makeup of the substrate i.e. – its thickness, fibre mix, coating and manufacturing method. It is determined by measuring the resistance of a cut sample to a force applied through a pre-determined angle.
Crease resistance is a similar measure of resistance which is made across a preformed crease in the carton blank.
The method for stiffness measurement is outlined in several international standards. These standards describe cutting a sample of pre-determined size either from a sheet of virgin material or from a pre-formed carton blank.
The sample is gripped in jaws and rotated through a set angle, and the force transmitted through the sample is measured. This force is quoted in grams for comparative measurements or g/cm2 -an absolute value for the tested substrate. Some instruments may also quote the value in mN or mN/m2.
The Hanatek CBT1 crease and stiffness tester is a low-cost manual instrument for testing board stiffness and crease resistance, the sample is clamped into the test jaws and the sample is rotated manually. The instrument displays the force measurement in grams and unlike other similar instruments on the market, it automatically calculates the crease to board stiffness ratio. If multiple tests are conducted, batch statistics are displayed. These can be printed for documented traceability.
For improved ease of use, a fully automated carton force analyser (CFA) should be considered, the instrument clamps the sample in electronically operated jaws and rotates the sample automatically at a pre-determined speed with pre-set dwell times.
The use of these operator-independent test methods vastly improves the repeatability of the results. The CFA also graphically displays the developing forces which gives added information on the physical changes that take place during bending.
The key to manufacturing fast running packaging lies in producing carton blanks with certain stiffness/crease ratios as described in BS6965
Plastic films are often used to encapsulate, protect and preserve products that are sold to consumers or industry. The film is used as a two-way barrier to stop product leaking out and also external contaminants from migrating in.
The effectiveness of the film as a barrier is related to its chemical composition and also its thickness.
Films which are below a specified thickness may fail physically- bursting, splitting or leaking, they will also be less effective at stopping the migration of oxygen and contaminants that can lead to product spoilage.
Packaging developers and product manufacturers measure and specify the thickness of the film to ensure the robustness of the packaging and the functionality of the barrier.
Various test methods are used in this specification including burst testing, tear testing and various film migration tests. These tests, including thickness measurement, are repeated as part of Quality Assurance inspection throughout the manufacture of a product.
In contrast to this, film manufacturers need to ensure that a film, whilst conforming to a customers specification is also manufactured in the most cost-effective manner possible. By measuring and manufacturing within tight tolerances the amount of raw materials used can be significantly reduced, leading to improvements in productivity.
The Hanatek Precision Thickness Gauge (FT3) can also measure the thickness of paper, metallic foils, carton board, corrugated board, tissue, textile and any other packaging substrate.
Thickness is a key parameter for many manufacturers as it affects the functionality, quality and cost of finished products.
The FT3 thickness gauge allows product to be manufactured within tight tolerances targeted right in the middle of a customer’s specification.
The FT3 is uses a precision LVDT displacement sensor with highly stable electronics to produce a very sensitive transducer system.
Measurement Parameters-The key to repeatability and accuracy is to tightly control all the parameters that can lead to measurement uncertainty.
The parameters that affect thickness measurement are;
To allow manufacturers in the same industry sector to measure using the same conditions, these key measurement parameters are often specified by international standards (ASTM/ISO etc.).
External effects such as temperature change and vibration will affect the measurement, the FT3 instrument is designed to reduce the impact of these.
The FT3 has a multi-point calibration routine to reduce/eliminate the effect of non-linearity in the LVDT sensor.
The 0.01micron resolution instrument includes routines to reduce the effects of short-term thermo-electronic drift of <0.1 microns.
Many materials including plastic film, plastics, and rubber may crush or deform dependent on the pressure applied during testing and the momentum of the presser foot.
Textiles require high pressures during testing to remove wrinkles and airgaps.
Tissue thickness testing demands a light measurement pressure and slow presser foot speed so the paper fibres are measured without being crushed and flattened.
Tissue thickness testing demands a light measurement pressure and slow presser foot speed so the paper fibres are measured without being crushed and flattened.
It is clear that different industries have adopted measurement parameters that give meaningful results for their own application.
FT3 measurement forces are applied using fixed masses; each instrument is supplied ready to apply the exact force specified in the relevant international standard.
The FT3-V variable pressure thickness gauge can be useful if a sample needs to be tested to more than one standard, it can also be used to determine the compressibility of samples under different conditions.
The measurement head size and profile is usually determined by the ISO Standard for the material.
The speed of the measurement head can also have an effect on thickness measurement. A fast-moving head will impact a sample with more energy and may deform or dent the material.
The measurement velocity of the FT3 head can be set by the user from 1 to 5mm/sec. This speed is sometimes specified in international standards.
The FT3 family of instruments have a number of features that reduce the impact of a human operator.
the main parameters are fully controlled by the instrument; measurement pressure, speed and dwell time.
Mistakes are often made in transcribing results and calculating statistics, these functions are performed automatically by the instrument.
Results can also be output via RS232 to SPC programs, lab networks or MS Excel.
Securely time and date stamped labels with statistical results can be produced throughout a batch run and attached to job cards or retained samples. This traceable control gives the customer confidence that the whole batch of product is within specification and allows manufacturers to issue certificates of conformance.
The FT3 family of instruments comply with a variety of ISO & ASTM standards for many different applications including:
Bespoke cutter die - 1 or 2 of the same shape
Bespoke cutter die - More than 5 different size shapes or additional punched holes within shapes.
