Although many specifications are listed on the technical data sheet (TDS), the diameter of glue sticks is the most important specification for a successful glue gun application. Most glue guns are designed to accept glue sticks with certain diameters. Although tolerance is permitted for the glue sticks, e.g. 11-12 mm, a consistent diameter is always desired for long term processing. Glue sticks with a larger diameter may not fit properly and may not be smoothly inserted into the gun. Conversely, a smaller diameter will be too loose upon triggering the stick. An accurate diameter of glue stick will provide a consistent adhesive output without damaging the glue gun. An inconsistent output, or bead size, can also result in poor bond strength.
All the polymeric materials impart a certain degree of shrinkage upon cooling from high temperatures to low temperatures. When hot melt adhesives are made in a mixer at elevated temperatures, the polymeric molecular chains are fully extended and orientated. Upon cooling at the holding tank before extrusion, the polymeric molecular chains can relax and coil back to a certain extent. When the adhesive is extruded through a fixed sized Teflon tube, the diameter of the glue stick is basically fixed. When the adhesive leaves the Teflon tube, it is immediately immersed and quenched in an icy water bath. The diameter of adhesive shrinks slightly due to thermal contraction. To compensate for this size change, the diameter of the Teflon tube should be slightly larger than the targeting size depending on the degree of shrinkage of glue sticks.
Certainly, the percent of shrinkage of each hot melt formulation is not the same due to their difference in molecular structures, weight ratios of polymer/tackifier, and viscoelasticity. Theoretically, more viscoelastic content and more tackifiers in the adhesive formulation will require more time for molecular chains to relax back to the random coil state. The residual stress stored in the adhesive matrix will gradually cause some dimension change. Indeed, it is very difficult to maintain the same diameter for different formulations based on a standard process condition. Fortunately, size deviation resulting from molecular motion is not as significant as that caused by process condition.
The distinct diameter variation on glue sticks is mainly caused by the cooling-stretching process in the water bath after the molten adhesive is extruded out from the Teflon tube. An inconsistent pumping rate for adhesive output, and pulling line speed for the adhesive stick, will significantly cause the diameter change of glue sticks. In order to precisely control the diameter of glue sticks, a very accurate volumetric gear pump is required. Additionally, the pulling speed for glue sticks before guillotine cutting should be perfectly consistent according to the output rate of the volumetric gear pump.
To formulate hot melt adhesives that possess desired processability and adhesion performance is not a difficult job. However, to make glue sticks with very precise and consistent diameters requires a sophisticated production and stick forming system plus substantial trial and error efforts.
For more information call or email Pierce Covert,
Glue Machinery Corporation