Application Example: How Feeders are used in the Production of Long Fiber Reinforced Thermoplastics
New Technologies: Long Fiber Reinforced Thermoplastics are the buzz of the auto industry in Europe and the US and are one of the most important trends in the plastics industry today.
Long Fiber Reinforced Thermoplastics are a recent development in newer technologies where PP or thermoplastic material is directly compounded with long glass fibers (rovings) and then molded in tne operation. Long Fiber Thermoplastic technologies are the buzz of the auto industry in Europe and the US and are one of the most important trends in the plastics processing industry today. Glass fibers of ½" (12 mm) up to 2" (50 mm) length give much higher stiffness, strength and toughness than the 1/8" (3 mm) fibers that have been used for reinforcement for decades.
Long Fiber Reinforced Thermoplastics (LFT) have excellent mechanical properties and stiffness-to-weight ratio which is of great interest to the automotive industry. These in-line compounding processes for long fiber material offer users more flexibility, as they are able to both compound and process such materials in accordance with their own formulation and also use ready made compounds.
For this process gravimetric feeders are used to feed the main polymer as well as the additives. Short term accuracy is one of the key elements because of the very short residence time in the twin screw compounder. The process requires a 5-8% 2 Sigma deviation at a sample time of 510 sec. at a feed rate of 1.5 kg (3.3 lbs) of irregular additive pellets.
There are currently three technologies of interest in the current market. These are E-LFT, D-LFT and SLFT. E-LFT is the in-line compounding and direct extrusion of long fiber reinforced thermoplastics as profiles or sheets, while D-LFT is the in-line compounding and direct compression molding of long fiber reinforced thermoplastics. S-LFT is the inline compounding and direct injection of long fiber reinforced thermoplastics. All three processes use continuous gravimetric feeders to feed accurately into the continuously operating extruder. Long fiber thermoplastics technology uses special screw configurations for both, single and twin screw extruders.
The base polymer is generally fed with a single screw or vibratory loss-in-weight feeder. The integrated vacuum receiver is responsible for the steady refill of the LWF. One to three additives such as colors, stabilizers, etc. in powder or pellet form are added via smaller loss-in-weight feeders (single or twin screwfeeders, vibratory feeders or Bulk Solids Pump feeders). Edge trimmings and other recycled materials are shredded and fed back into the process at a predetermined ratio using vibratory loss-in-weight feeders.
The glass fiber filaments are pulled into the extruder from continuous glass rovings on bobbins by the rotation of the screw shaft. The throughput of the fiber is volumetric and quite steady. Each fiber stand is monitored by an infrared sensor. The gravimetric throughput of the fiberglass can also be checked by mounting the bobbins on scales and measuring the weight loss over time.
Most LFT end products are produced for the automobile industry. These molded parts include body panels, sound shields, front-end assemblies, structural body parts, truck panels and housings as well as doors, tailgates and fender sections. Development is also strong in other areas of application in the electrical and building industries.
Feeding Systems in LFT-Compounding
Key to a feeder's ability to attain high accuracy over short intervals is the resolution of its weighing device and response of its process controller. As the illustration above shows, lengthy performance timescales permit a given blending accuracy to be attained with a low performance weighing system. However, to achieve the same accuracy in the short interval characteristic of continuous mixing operations, a much higher weighing performance is required.
The new generation of digital process weighing (such as K-Tron's Smart Force Transducer II & III) addresses these concerns, representing an important advancement in feeder performance. Now fully integrated into K-Tron's complete line of gravimetric feeding, batching and metering equipment, the technology sets new standards in process feeding by delivering faster, higher resolution weighing with integrated digital filtering capabilities, especially beneficial in hostile plant environments, low rate feeding, and applications requiring high accuracy over very short intervals.