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Addition directly into the process

Feeding systems for the production of hot melt adhesives
Addition directly into the process

The production of solvent-free hot melt adhesives is a continuous process. In most cases a twin-screw compounder or co-kneader is used to obtain a homogenous melt. To achieve the required product quality, it is necessary to guar-antee high feeding accuracy of the additives. K-Tron offers various feeding systems for precise addition of components into the extrusion process.

Alfred Häuptli

Hot melt adhesives are solvent-free. They are characteristically solid at temperatures below 82 °C, melt into low-viscosity fluids above this point and set rapidly upon cooling. The development of hot melt adhesive technology was inspired by the previous use of molten wax for bonding. When this method no longer satisfied performance needs, 100 % thermoplastic systems were introduced. Today, there are many different hot melt adhesives available in the marketplace, the most common being those used for hot melt, pres-sure-sensitive adhesive applications:
  • Ethylene vinyl acetate (EVA) copolymers, compatible with paraffin – the original hot melt
  • Styrene-isoprene-styrene (SIS) copolymers
  • Styrene-butadiene-styrene (SBS) copolymers
  • Ethylene ethyl acrylate copolymers (EEA)
  • Polyurethane reactive (PUR)
These polymers alone do not normally exhibit the full range of performance characteristics required for an end product. A variety of additives, such as resins, waxes, antioxidants, plasticisers and other materials, are thus added to the adhesive formulation to enhance the polymer effect. These adhesives are meanwhile established across a broad spectrum of manufacturing industries, where they can be applied in small bond points to eliminate the use of mechanical fasteners like staples, screws, rivets, clips, snaps, nails or stitching.
Extrusion process
Hot melt production is a continuous process. In most cases a twin-screw compounder or co-kneader is used to obtain a homogenous melt. The elastomers are metered individually or as a premix into the main inlet of the extruder by means of a gravimetric feeder. After mixing/plasticising and compounding the various ingredients, the resins (solid or liquid) and plasticisers/oils are also added to the extruder gravimetrically. The liquid can be injected at several locations along the extruder for larger quantities, or according to the process, with the help of multiple kneading and homogenising stages. The typical throughput ranges from 100 to 1000 kg/h. Depending on the recipe and number of ingredients, the average feed rates/ratios are as follows: elastomers 30 to 50 %, resins 20 to 40 %, plasticisers/oils 10 to 40 %, fillers 1 to 10 %, pigments 0.1 to 3 % and stabilisers: 0.1 to 3 %.
Owing to the high throughput of the extruder, tough performance requirements are specified for the various types of hot melt with regard to the working load, joint stress, service temperature range, lifetime, etc. The residence time of 10 to 20 s in the extruder mixing zone tends to be very short. To achieve the required quality of the end product, it is necessary to guarantee a consistently high feeding accuracy of the additive feed of ±0.2 to 0.5 % in 2 s over 30 consecutive samples with a sampling time of 10 s per sample.
Typical feeders
K-Tron offers various feeding systems for precise addition of the specific components into the extrusion process. The K-ML-KV2 and K-ML-KV3 vibratory feeders are specially designed for gentle handling of resin. The K2-ML-S60 and K4G-L-BS60 single-screw feeders can be easily adapted to the characteristics of medium to high-flow powders and granules, e.g. elastomers, SIS, resin, etc. The K2-ML-T35 and K4G-ML- KT20 twin-screw feeders are ideal for feeding fine, low-flow powders, e.g. stabilisers, fillers, pigments, etc. The K-ML-P liquid loss-in-weight feeders provide the perfect feeding system for a wide variety of liquids with different viscosities, temperatures, etc. Choosing the optimum refill system is crucial to ensure proper and reliable feeding performance of loss-in-weight feeders. K-Tron vacuum conveyors represent an excellent solution for efficient refilling of these feeders with solids of any kind. Thanks to the dual cyclone technology of the M-Series receivers, the majority of free-flowing materials can be conveyed without the need for filters. P-Series vacuum hopper loaders with an integrated filter or refill hoppers with an extracting device and a shut-off valve, Big Bag discharge station or Bag dump station are required for sticky, solid additives such as Irganox, Tinuvin, CaCO3, stabilisers, etc.
Control system
The loss-in-weight feeders feed the ingredients in preset proportions, either defined on the operator interface or downloaded from the host computer via the line controller (KSL) to the locally mounted control module (KCM). All process relevant parameters are stored in a separate recipe for each feeder. The feeder-mounted K-Tron Control Module (KCM) combines the function of motor drive and feeder control in a compact package. In this example, the K-Tron Smart Line Controller (KSL) acts as a smart interface for up to eight feeders in one line. The modular K2 continuous loss-in-weight feeders with integrated refill system guarantee high short-term accuracy with a large operating turndown. The advanced Smart Force Transducer (SFT) digital weighing technology provides the key. High-resolution weighing and powerful, on-board, digital signal processing ensures precision feeding even at short intervals or in vibration-prone plant environments. The SmartConnex control technology is renowned for long-term stability, good repeatability and consistent feeding quality.
Hall F1.0, Booth A4
cpp 432

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