Extrusion processors

The designer should be aware of the fact that this is to be considered in designing with extruded products. The designer can exercise little control over this pull back condition except to be guided by the experience of the extrusion processor to indicate which materials are particularly susceptible to this problem and what the recommended wall thicknesses are to minimize the effect. In general, one of the best ways to improve the condition is to slow down the rate of extrusion. As a result, products have a tendency to pull back. They also will be more costly to produce. 

Historically, most extrusion processors have relied on postextrusion flavor application as a means of characteristically... 

A. Whelan and D. Dunning, The Dynisco Extrusion Processor s Handbook , published by the authors for Dynisco, Inc., 1st Edn., 1988. 

The Dynisco Extrusion Processors Handbook, 1st Ed. Whelan, T., Dunning, D., Eds. London School of Polymer Technology, Polytechnic of North London London, 1988. 

It had been generally accepted by most extrusion processors and suppliers that the melt temperature variance at the end of an extruder was negligible. Stationary thermocouples had been immersed in melts, but very limited useful data could be obtained, as probes tended to disturb the melt flow or be damaged. Obtaining the profile with a standard immersion thermocouple required that an operator position the probe manually, plot the position, and so on. Results were not repeatable, or were tentative at best. 

Additives are incorporated into polymers several ways the resin supplier may include them in the raw material, an intermediate compounder may produce additive-enhanced feedstock, or the final extrusion processor may add them directly. When incorporated in final processing, additives maybe introduced individually or as part of a masterbatch, a blended compound containing one or more additives in a polymer carrier. A masterbatch material is incorporated most efficiently when its carrier is compatible both chemically (same polymer) and Theologically (same flow characteristics) with the primary resin. 

Since these benefits appeal to most extrusion processors, grooved barrel sections have become quite popular. Also, the use of grooved barrel sections allows processors to extrude materials that cannot be processed on conventional extruders, e.g., very high molecular weight poiyethyienes and powders. A grooved feed housing is shown in Fig. 7.18. 

Designers and processors to produce products at the lowest cost have unconsciously used the basic concept of the FALLO approach. This approach makes one aware that many steps are involved to be successful, all of which must be coordinated and interrelated. It starts with the design that involves specifying the plastic, and specifying the manufacturing process. The specific process (injection, extrusion, blow molding, thermoforming, and so forth) is an important part of the overall scheme and should not be problematic. 

Subject to the limitations indicated, control over extrusion process products is consistent enough to make a uniform repeatable product once the limitations are accommodated. Here, as in other processing, good communication between the processor and the designer will help make for a successful economical product. 

By extrusion parison control it is possible to minimize the wall thickness variation and the extent of stretching and stretch orientation. These are the province of the processor when the designer is not familiar with BM. Knowledge is required to provide information on what is possible and to select the specific BM process that has the capability to mold the product. The designer should be aware of the possible failure modes and compensate for them in the design. There is little else the designer can do but select the best material and process to make the product. 

These absorb moisture, which then has to be carefully removed before the plastics can be fabricated into acceptable products (2,3). Low concentrations, as specified by the plastic supplier, can be achieved through efficient drying systems and properly handling the dried plastic prior to and during molding, extrusion, etc. (Figs. 7-24 and 7-25). When desired processor can have these hygroscopic plastics properly dried and shipped in sealed containers. 

It is imperative that the processor utilize every advantage available to ensure success since the specification of equipment in an extrusion line also affects resin usage and labor. Thus, it is always more cost effective in the long run to design and install an extrusion line that (1) has a maximum rate capability of at least 25% more than the expected maximum rate, and (2) has a properly engineered line (that might have a higher capital cost) to achieve maximum profitability. 

An extrusion trial was performed at the processor s plant using a 38.1 mm diameter production extruder, a proprietary screw design, and resin that had previously exhibited flow surging and reduced rate. The extruder was equipped with three barrel zone heaters with control thermocouples (labeled Tl, T2, and T3) and two pressure sensors. One pressure sensor was located in the midsection (zone 2) of the barrel (P2) and the other at the end of the barrel near the tip of the screw (P3). Both transducers were positioned over the top of the screw such that a pressure variation due to screw rotation would be observed. 

Foamed PP is a new alternative material for processors of PP or PS thermoformed items for packaging applications. The joint R D programme of Italproducts, Reedy International and PP resin suppliers has made it possible for everybody involved in the packaging industry to install and run an extrusion and thermoforming line capable of producing medium density EPP items. The features of the equipment required are described. ITALPRODUCTS SRL REEDY INTERNATIONAL CORP. 

Germany s Bayer AG has invested in a 1.8 billion plant in Shanghai that will produce the same chemical. U.K.-based medical plastics processor Flexicare Medical Ftd. built a 40,000 square foot injection molding and extrusion factory in Dongguan. 

Control feeding devices to the hopper of primary equipment (injection molding, extrusion, etc.) is important to provide products that meet performance requirements at the lowest cost. Equipment manufacturers have increased the feeding accuracy using different devices such as microprocessor blender/mixer controllers. Also materials are being reduced in size with more uniformity to significantly improve uniformity in melt. Processors can use blenders and other devices mounted on hoppers that target for precise and even distribution of materials. 

The resin manufacturer and the specific resin product code should be established. Processors are very knowledgeable in molding and extrusion techniques but are not well versed in colorant use and resin compatibility. That is the job of the colorist. It may be helpful to contact the resin manufacturer directly for additional information regarding colorant and additive compatibility, especially when designing colorant systems for some of the more exotic engineering resins and alloys. The resin manufacturers may have done research or had experience that may help in selecting the colorant or avoiding certain problematic colorants or additives. 

Another scale-up variable that can be easily controlled is the length to diameter ratio L/D). Scale-up with an equivalent L/D ratio is beneficial in avoiding non-linear scale-up issues, especially at production scale. Residence time distribution (RTD) is a useful term for understanding scale-up of hot-melt extrusion processing. The RTD is used to attempt to quantify the average amount of time a material spends in the processor. The RTD depends on screw speed, screw element design, and material characteristics. The preference during scale-up is to maintain an equivalent ... 

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