The effects of joining Polypropylene Strapping.
Polypropylene strapping tape, was welded with a standard semi-automatic strapping machine. The effect of the welding temperature, surface profile of the sealing tool and the strap properties (draw ratio and embossing pattern) on the microstructure and the joint strength was studied. A near optimal temperature range was identified. The draw ratio of the strapping was found to influence the weld efficiency.
Polypropylene strapping with high tensile strength is commonly used for strapping many products ranging from light cardboard packs to heavy loads such as palletised boxes, cartons, timber. Welding or heat sealing has long been used to seal plastic strapping by using high temperature tools. Despite the widespread use of the technique, the influence of the welding process parameters on the properties of the joint have not been studied. Plastic Strapping is produced by extrusion followed by drawing at moderate temperatures to achieve high molecular orientation.
For reducing fibrillation and to improve the weldability, the tape is embossed after the drawing stage by means of textured hot rolls.
The strapping cycle comprises feeding, tensioning and sealing of the tape around the pack. Thermal welding is the more common sealing process of polypropylene tape, although in recent years friction welding is increasingly used to good effect.
The plastic strapping welding process involves four steps.
i -- the heating blade is positioned between the tapes.
ii -- the sealing block moves upwards, slightly pressing both surfaces against the blade.
iii -- after a fixed heating time the heating blade retracts.
iv -- the sealing block moves to squeeze the melted surfaces together and to cut the unused tape. After a cooling period the welded strap is released.
Obtaining a strong strap weld is an important part of good strap performance. In this study, polypropylene straps having different draw ratios and different types of embossing patterns, were welded with a strapping machine. The effect of the welding temperature and the sealing block surface profile on the morphology and failure is presented.
The material used in this study was polypropylene strapping having a cross section of 12 X 0.6 mm. All the straps were produced from a polypropylene homopolymer, with a Melt Flow Index about 4 g/600s at 230[degrees]C/21.6 N). The straps were drawn in an oven at 95[degrees]C at draw ratios of 5:1 to 9:1, . A sample of the extrudate from which the straps were drawn was included in the testing program. Strapping of unknown draw ratio, was included in this study to evaluate the effect of different types of embossing.
The following tests and equipment were used to characterize the strapping:
* density measurement by the column gradient method;
* microscopical observation and birefringence measurement, on cross-sections cut along the drawing direction, with a Zeiss Universal polarizing microscope equipped with an Ehringhaus quartz compensator;
* scanning electron microscopy to observe the embossed surface.
* determination of the shrinkage on annealing at 130[degrees]C and 150[degrees]C in an air circulating temperature controlled oven;
* determination of the tensile strength and elongation at break using a JJ type T 5002 tensile testing machine at a rate of 200 mm/min. For eachstrap, a minimum of seven specimens were tested. The effective cross-sectional area of the tape was determined from the weight and the density of a measured length of strap.
The strapping was welded using a semi-automatic strapping machine at temperatures between 340[degrees]C and 480[degrees]C. The heating, pressing and cooling times were constant and pre-set by the machine manufacturer. The complete welding cycle was 1.4 s. These welds were made using the sealing block, with a serrated profile, provided with the machine. The joints had a width of 12mm and a length of 30mm.
For investigating the effect of the block profile and welding pressure, some welds were produced with tape at 400[degrees]C using a flat block and a block of greater height The effect of misalignment on seal strength was determined on strapping welded at 400[degrees]C by displacing of the strapping maximum misalignment allowed by the machine of 2 mm.
The microstructure of the welds was analyzed by microscopy. The mechanical behavior were studied by means of shear and peel testing, performed at the same rate (200mm/mn) as was used for the strapping. For each type of weld, seven specimens were tested and the average value and the standard deviation of the breaking load and elongation at break evaluated.
Properties of the Straps
The density and birefringence of the undrawn extrudate was 905 kg. and respectively.
(Birefringence is formally defined as the double refraction of light in a transparent, molecularly ordered material, which is manifested by the existence of orientation-dependent differences in refractive index. Many transparent solids are optically isotropic, meaning that the index of refraction is equal in all directions throughout the crystalline lattice. Examples of isotropic solids are glass, table salt, many polymers, and a wide variety of both organic and inorganic compounds.
The birefringence of the extrudate is very low in comparison to that of the strapping. indicating the strong effect of the drawing operation on molecular orientation. The increase in birefringence with the draw ratio is greater for the lower draw ratios. The birefringence tends to level off above the draw ratio of 8:1.
The tensile strength of the straps increases monotonically with the draw ratio up to a draw ratio of 8:1 and then begins to level off in a manner similar to the birefringence. The strain at break decreases with the draw ratio in a manner that is nearly a mirror image of the tensile strength vs. draw ratio curve. The large decrease in the density of strap was caused by voiding and splitting within the structure, suggesting that the strength improvement from increasing the draw ratio is reaching a limit.
The shrinkage increases with the draw ratio. The values at 150[degrees]C are particularly relevant from the user's point of view, as they indicate the tendency of the strap to contract near the weld zone. The high shrinkage (between 32% and 41%) exhibited by the straps at this temperature suggests that, to prevent a poor seal by contraction at the welding stage, the depth of the heated zone should be kept to a minimum and that the tape ends should be tightly gripped.
WELDING BEHAVIOR OF THE TAPES
Effect of the Welding Tool Temperature
The welding temperature has a marked effect on the morphology, strength and failure behavior of welds. Most of the welds have maximum strength and ductility at a welding temperature of 430[degrees]C. The mechanical behavior is closely related to the morphology of the welds. When the heating tool temperature is too low (below 400[degrees]C for most tapes) the welds show voids and splits at the interface with the unmelted material. Low temperature and low amount of melt prevented complete filling of the gaps between the mating surfaces, resulting in a poor weld. For the same welding temperatures, the welds made onstrap showed higher splitting than the others tapes. This behavior is caused by the deeper embossing of the strap requiring more melt to fill the gaps at the weld zone.
The fracture path of the welded tapes depends on the weld morphology and can help to track the thermal history of the welds. The welds made at too low a temperature broke at one of the boundaries between the melted and unmelted material where the splits occurred.
The increase in welding temperature up to 430[degrees]C (460[degrees]C resulted in a reduction of the flaws and in an increase in the shear strength and ductility of the welds. Flow lines and swirls were observed at the widest regions; at the thinner zones the material oriented in the axial direction of the tape.
The excessive heating of the tape at tool temperatures of 460[degrees]C or higher changed the morphology of the welds. The microstructure was spherulitic for some tapes and splitting at the interface was greater than in the previously referred welds. These features probably resulted from a combined effect of degradation of the polymer by excessive heating and by increasing contraction upon cooling of the spherulitic structure.
The welds made in the correct temperature range were free from flaws and had a good adhesion at the interface. The fracture path in such welds moved periodically along both sides of the weld boundary.
The use of a temperature above optimum value resulted in a characteristic failure behavior of the welded tape. The fracture initiated at one end of the weld zone, where the molten material accumulated during the joining stage, and propagated to the base material.
The peel test was less effective than the shear test in assessing weld quality. The peel strength for strapping is almost unaffected by the welding temperature; the shear strength passes through an optimum range and then decreases. This result correlates well with the failure behavior of the welds in the peel test. Except in samples welded at the lowest temperature, which fractured through the joint interface, the other samples failed through the base material This behavior may be explained by the reduced interfibrilar strength arising from the high molecular orientation of the tape, making the drawn tape more susceptible to crack propagation along the axial direction than the unoriented material in the weld.
Effect of Welding Tool Geometry and Tape Misalignment
The modifications of the surface pattern of the pressing block (flattening of the surface and increase in the height to increase the welding pressure) affected the weld morphology. However, it had only a marginal effect on the shear strength The use of the flat block for the tape with deeper embossing resulted in more uniform welds and in an apparent decrease in the dispersion of the results.
As expected, the tape misalignment reduced the joint shear strength In the machine used, the maximum misalignment reduced the strength by 4%.
Effect of the Embossing Depth
The embossing depth seems to affect the weld morphology; a shallow embossing promotes the formation of welds with more uniform thickness and a higher orientation level.
Effect of Strapping Orientation
The draw ratio of the tapes influences its welding behavior. Strapping with higher orientation produced thinner welds (less material was left at the weld zone), with more splits and voids than those with less orientation. The increase in orientation increases the melting temperature However, for the range of draw ratios used in this paper, the expected difference in melting temperature is not sufficient for explaining the observed thickness reduction. The increase in stiffness with the draw ratio favored the squeezing out of the melt from the weld region, and is likely the main cause of the observed thickness reduction. The high shrinkage and the high stiffness of the more oriented tapes are also the likely reasons for the higher incidence of voids in the weld zone.
The joint efficiency defined as the ratio between the strengths of the welded and unwelded tapes, decreases with the draw ratio up to the draw ratio of 8:1 ( Thus, the strength improvement achieved by drawing the tapes at higher ratios is lost on welding.
The following conclusions can be drawn from this study of the welding behavior of polypropylene tapes with different draw ratios:
1 - The welding temperature has a strong influence on properties of the welds. The optimum welding tool temperature is about 430[degree]C for most of the tapes, under 1.4 seconds welding cycle.
2 - In general, the microstructure of the welds is very fine and has a much lower orientation than the tapes. Welding temperatures of 460[degree]C or higher produce coarser textures and favor the occurrence of voids and splits at the joint interface.
3 - The welding temperature influences the fracture path of the welds. Below the optimum temperature the fracture path runs along one of the boundaries of the weld zone. At the optimum range the fractured path alternates adjacent to the interface. Above the optimum temperature this path generally moves away from the weld zone.
4 - The original orientation of the tapes influences the morphology and strength of the welds. Increase in orientation reduces the thickness of the weld zone and favors the formation of voids. The welding efficiency decreases with increasing orientation of the tapes.