Generally speaking,PFA GranulesThe steps of production (polymerization), compounding of the PFA particles and processing of the PFA particles are performed separately in this order. For processors, compounding is often very complex, to avoid this complexity. On the other hand, because to meet the requirements of the end product, it is necessary to pay more and more efforts to obtain the right mixture for each specific use. The processor must at least understand the language of the compounding machine, because otherwise he will not be able to estimate the effect of ingredients and process on product performance. Moreover, the processor must be able to speak the application language of the compounding machine in order to translate the needs of his customers into the requirements to be met by the compounding. This represents the first step in acquiring the necessary skills. In this way, the processor acquires the tools necessary to begin internal self-compounding.

PFA pellet manufacturers have larger production facilities (world class). In contrast to the usual consumption, these facilities produce not only pure universal PFA particles, but also more PFA particles with a special molecular structure. The molecular structure is "comb-shaped molecules". Depending on the molecular structure, the same molecular weight can exhibit significantly different zero shear viscosity values. The viscosity at low shear rates can vary in several orders of magnitude and affect the extended processing of the melt coming out of the die.

Modern catalytic technology can produce customized molecules in large-scale production facilities, which is new. Therefore, there are now a large number of special products. However, day-to-day operations require much greater diversity to meet diverse special requirements. But it's not just expensive specialty products that are needed. Depending on the application, these products can be produced by incorporating polymeric fillers, and the desired end product properties can be obtained at a reasonable price with minimal satisfaction. These methods are well known to many processors who mix the ingredients while dosing them into the extruder. This is the first step on the road to direct crowding out.

Compounding at an independent compounding plant represents an intermediate step. They have the skills to work with processors and end-users to develop formulations for a particular use. This is often the most economical solution for many small and medium extrusion and injection molding operations. Large processing companies can also consider internal self-compounding. The more specialized and larger the operation, the better the economy of direct extrusion. In addition to the process, quality and cost are also directly under the control of the processing company. The cost savings for internal self-compounding is about 10%, while direct extrusion is about 30%. Each case is different, and only separate calculations will provide accurate results.

Requirements on the machine

Compounding can be done on a single screw or multi-screw extruder. However, except in special cases, single screw extruders are limited to simple compounding operations. The co-rotating twin screw extruder has many advantages by using intermeshing self-sweeping screws and a combined structure. For each machining job, a customized sleeve and screw shape can be assembled. Close intermeshing co-rotating twin screw extruders have been successfully applied to a variety of PFA particles, except for PVC-U. In addition, there are a variety of special machines in the field of direct extrusion is not very important. Planetary roller extruders occupy a special position. It combines the good dispersion caused by severe deformation of the tooth mesh with unique heat dissipation.

Hitherto, the cost is much higher with the same amount of processing when twin screws are used instead of single screws. Because the amount of processing of the twin screw increases with power, the relationship improves with increasing size. Since the early days of twin screw technology, modular systems with short sleeve sections have been made. The use of a modular system to handle as many processing operations as possible has made these systems expensive. At the K2004 exhibition, there was a first attempt to accept limited flexibility, with the aim of achieving more favorable costs. In addition to 4D standard sleeve lengths for small and medium gauge, 6D and 8D sleeves have advantages. For single-use systems like those commonly encountered in direct extrusion, longer sleeves can be used, and in extreme cases even one-piece sleeves can be used. In a counter-rotating twin-screw extruder, a one-piece long sleeve is standard. The screw can also be made in one piece, but this is only practical when the machining task is known and the wear occurring is low.

Addition of the ingredients to the hopper and system, either separately or premixed, in a so-called split feed technique is typical for direct extrusion. The PFA particles and all solid ingredients enter through the sleeve port and they account for 15-30% of the processing volume of the system. Immediately after melting, 30-60% of the processed amount of fillers and reinforcements are added with the help of one or more side feeding devices. Because these materials contain a large amount of gas, venting is required. This is followed by wetting and fusion with the PFA particles. Fusion represents a typical compounding operation: mixing, shearing, and elongating. Generally, volatile components must be removed by vacuum. The final processing step is to build up pressure for filtration and discharge by creating a shaped die. Here, the twin screw has shortcomings. For this purpose, gear pumps and occasionally even single-screw extruders are used to build up the pressure. The deeper the twin screw thread, the smaller the pressure accumulation capacity. All other compounding systems except the counter-rotating twin-screw extruder also require a pressure buildup device to discharge the melt against the die.

Many details common to mixers must be noted. Taking the addition of fine or coarse powder and its effect in the feeding section as an example, this problem will be more clearly shown: the coarse packing contains less gas and can be discharged upstream. Lightweight fillers with small particle sizes contain large amounts of gas. If the gas is purged upstream, it can impede the addition and interfere with the delivery. As the exhaust proceeds downstream, the small particles begin to melt in the kneading module to form a sort of sintered mass, so that the extrusion can be carried out in a dust-free manner with a high throughput.

In mixing, the screw shape has an impact on success. The melting is carried out by means of a kneading block. Here, a local high pressure occurs, which presses the mass into the so-called secondary mass, which usually does not break up again. A special mixing section is used for mixing. The special kneading block combination ensures that the melt has a certain shear rate and elongation rate when mixing the various PFA particles and the dispersive mixing additives. This is generally true that the efficiency of mixing and shearing in the screw flow path is about two orders of magnitude lower than that of the specialized mixing and shearing section. This is where the skills of the mixing business come in handy.

Addition of various ingredients

Important cost factors are associated with the addition of ingredients to the mixer feed and subsequent side feed devices. The main part of the technique can also be found here. In fact, when dosing, each component behaves differently. Only round and large particles can be dosed simply. Most additives are small and dusty, in the form of small plates or needles, and require special knowledge of handling. For storage, transportation, filling and emptying of the weighing feeder, there are few good solutions, and too many are insufficient. At every industry exhibition, the types of complex bulk material handling systems and PFA manufacturing companies and continuous progress are a surprise. The differential weighing feeder is extremely accurate for weighing solid and liquid materials. Depending on the bulk material, the discharge system has a single screw or twin screw, spiral, vibrating feeder and pump. For color-changing, easy-to-clean or quick-change hoppers find a place to go.

Accuracy is the highest priority when weighing individual ingredients. Differential weighing feeders and so-called formal weighing, which can also be defined as batch mixers, meet this requirement. Differential weighing feeders require a separate weighing system for each component, resulting in a much higher investment than a central weighing box where each component is dosed. With a central weighing box, you can save up to about half the cost. Premixing is recommended when recipe ingredients must be added in very small amounts. If the individual stations are arranged along the mixing screw, the premixing may take place continuously, or if dosing to the batch mixer occurs, the premixing may take place discontinuously. A variety of small amounts of various ingredients are often mixed in a mixing container and fed in batches.

The main ingredients are added initially, followed by lesser amounts of ingredients such as flame retardants, colorants and stabilizers. Finally, minimal amounts of ingredients are added, which immediately bind to form a mixture. In this arrangement, the weighing feeder is placed on both sides of the mixing screw in a space-saving manner. The mixing screw directly conveys to the left into the feeding mouth. By using the mixing screw, the system is converted from dosing without premix to dosing with premix, which allows the complex process to be significantly stabilized. Complaints about the height of the dosing station are often heard. The dosing station with premix can also be located on the side of the extruder. By means of the auger, the mixture is conveyed to the extruder without separation.

It is advantageous to add fillers and reinforcements to the subsequent melt via a side feed mechanism because the PFA particles can be melted first and then the solids are added to the partially filled flow channel. In order to avoid additional lumps, the addition should be carried out virtually without pressure. Today, feeding is done in two steps. The weighing feeder conveys the filler and reinforcement to the feeding port of the side feeding device, and the side feeding device will feed the partially filled flow channel volume-measured. When the feeding is changed from weight measurement to volume measurement, the operator's inattention will cause the hopper with no observation point above the feeding port of the side feeding screw to be blocked. In fact, the discharge device on the weigh feeder should feed the twin-screw mixer directly. In addition to better process control, this also means less investment, even if the differential weighing feeder is a little more expensive. The system will also be lower, and height is another cost factor.

control of the system

The control element of the system is a gear pump. It operates at a constant speed, provides a certain flow of material, and accumulates the pressure required to discharge the melt from the die. A steady pressure is maintained at the gear pump inlet or between the screw head and the screen changer. The limit time for the metering change to flow through the screw requires a waiting period. The change in screw speed causes an instantaneous reaction, but then the screw idles and the system does not react normally. When restarted, the mixer can be stepped up to full speed in order to prevent the almost idle screw from running at low process speeds.

Flexibility of Formulation

Internal self-compounding has the same degree of freedom in terms of formulation from a custom compounding plant. Naturally, labor costs must be considered. One advantage is that the developer can focus entirely on his products, customers and requirements. Developers of external compounding plants often have a wealth of knowledge from multiple applications, but not necessarily depth. This is especially true for less expensive methods. It is recommended to work closely with all suppliers of raw materials and additives to avoid "ivory tower" type of development. Personal experience shows that in one problem situation, a solution is not found until about 25 suppliers are visited. Only PFA pellet manufacturers possess highly specialized skills and sufficient knowledge of all details and interrelationships.

Inspection of incoming materials

The supplier has the most knowledge and the best opportunity to inspect the product. It is recommended that production control data in the form of factory certificates be required for each shipment. At the time of shipment, proof of the type of PFA pellets is mandatory. The possibility of confusion at the supplier or at the processor's own plant should not be underestimated. The processor's own quality control department should identify the type of PFA particles and determine the typical value of the product to confirm the uniformity of the supplier's production. An inexpensive test method is sufficient for this purpose. The data provided by the supplier and the internally determined data should be compared for stability by statistical process control techniques.

Contamination during transportation (especially in tankers) is not uncommon and must be checked. It is strongly recommended to use the arrival detection screen.

Storage costs

When purchasing ready-to-use mixes, care must be taken to minimize delivery and delivery times. To ensure availability, a large warehouse must be stocked. Such stocking always becomes obsolete because a specification change or a small improvement in product performance requires a correction to the mix. If standard materials can be used, costs and delivery times can be reduced. This also means that the latest technology for additives can be provided while maintaining the advantage in flexibility.

Addition of recycled PFA materials

There will be recycled PFA material in every production operation. With the right machine technology, PFA recycled material can be reintroduced and melted. Fusion is sometimes difficult, especially for high viscosity compounds. When the scrap is crushed, there will always be fines that can be recovered when the internal mixing is carried out.

In general, it is possible and economical to fuse the material recycled by PFA in a certain amount. In general, reintroduction does not pose a problem because the ingredients are better distributed in the second processing operation. Product decomposition due to repeated melting is rarely seen.

The Cost of Quality Assurance

The quality of production must be verified, which involves a certain cost. For example, all ingredients must be inspected and tested at the time of receipt. On the other hand, the detection of individual components is simpler than the detection of complexes, which may be problematic, but does not allow the cause to be specified. The test equipment must be targeted for identification. The quality control tests performed by the manufacturer should also be referenced (see section "Inspection of incoming materials").

There must also be verification of process stability and reliability. Statistical process control techniques are useful in this regard. A grading system helps to determine the stability of production. Direct extrusion can save processing steps. It is often advantageous for only single heating operations, preventing reheating steps. Direct extrusion is absolutely necessary where it is estimated that there is a difficulty in the decomposition of PFA particles or in the melting of highly filled PFA particles. The elimination of a melting step enables the PFA particles, which may be decomposed by pre-heating or difficult to melt at the first location, to be combined with the reinforcement. There are almost always cost savings. Direct extrusion is particularly economical at high processing rates. However, the additional expenditure related to formulation and quality assurance must be borne in mind.

This article comes from:www.tflpfa.com

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