Extrusion Dies For Plastics And Rubber Pdf Download
Damiane Burgun
This two-day workshop introduces die design principles for all major extrusion processes (sheet, pipe, tubing, blown film, profiles, and coextrusion). Learn how to apply polymer behavior to die design dies for various extrusion processes. Find out when calibration is necessary and how to design it. Explore common design issues to avoid. This course is for dies designers, extruded part designers, process engineers, and tool shop supervisors.
For any extrusion die manufacturer, good die design is a major factor in ensuring extrusion dies will always produce a quality final product. Balanced melt flow across the die ensures uniform exit velocity and output consistency. Expert die design achieves this uniformity in a continuous fashion with minimal pressure drop.
Die design also affects productivity since it impacts on the throughput of the extruder. These parameters directly relate to extruder back pressure and, in turn, product density and screw efficiency. Critical factors include:
All dies are manufactured using a smart, modular design to allow rapid fitting and removal. The special Diamond America swing bolt configuration means that changeover between runs, or disassembly for cleaning or maintenance takes only minutes.
Our decades of innovative design and manufacturing expertise, along with our state-of-the art, cutting-edge technology make us a true leader in providing top-quality solid, semi-hollow, and hollow dies to service multiple industries that use rubber and plastic injection molds to create their products. We have the latest design software and 5th Axis machines on-site, allowing us even further precision and flexibility with your designs.
The rubber extrusion process begins with a rubber compound being fed into an extruder. The material is fed into a feed hopper, which takes the material and feeds it into various flutes in a revolving screw. The screw will begin to carry the rubber forward into the die, with an increase in pressure and temperature occurring as the material gets closer to the die itself.
Once the material reaches the die, the built-up pressure forces the material through the openings, where it will consequently swell to various degrees based on the material compound and hardness. Because of this tendency toward swelling, many extruded parts require plus or minus tolerances on their cross sections. During vulcanization, the extruded rubber will swell or shrink in both its cross section and its length, depending on the type of rubber compound used.
An extrusion die is a precise and specific tool made by cutting an opening through a blank of steel. The shape of the opening will match the finished rubber cross section desired for an extruded part. Once in place, the rubber material will be forced through this die via the pressure that builds up from the revolving screw of the extruder.
Many rubber compounds tend to swell when passing through the extrusion die, causing them to experience an increase in dimensions. Thus, each die is made according to each particular part and material to ensure that all tolerances are met for the finished extruded rubber part.
The closer tolerance classes outlined below should not be specified unless required to do so by the final application and they should be restricted to the critical dimensions outlined by the Association for Rubber Products Manufacturers (ARPM) below. The closer tolerances demanded, the tighter the control must be exercised when the material is being extruded and hence the higher the cost incurred. In general, softer materials and those requiring a post cure will need greater tolerances.
Timco can also cut custom rubber extrusions in a range of lengths, shapes, and rubber compounds to meet various temperature, weather, and technical specifications. Some of them, such as EPDM or neoprene, can be extruded like dense rubber or sponge rubber. Common extruded rubber products include:
Plastics extrusion is a high-volume manufacturing process in which raw plastic is melted and formed into a continuous profile. Extrusion produces items such as pipe/tubing, weatherstripping, fencing, deck railings, window frames, plastic films and sheeting, thermoplastic coatings, and wire insulation.
This process starts by feeding plastic material (pellets, granules, flakes or powders) from a hopper into the barrel of the extruder. The material is gradually melted by the mechanical energy generated by turning screws and by heaters arranged along the barrel. The molten polymer is then forced into a die, which shapes the polymer into a shape that hardens during cooling.[1]
The first precursors to the modern extruder were developed in the early 19th century. In 1820, Thomas Hancock invented a rubber "masticator" designed to reclaim processed rubber scraps, and in 1836 Edwin Chaffee developed a two-roller machine to mix additives into rubber.[2] The first thermoplastic extrusion was in 1935 by Paul Troester and his wife Ashley Gershoff in Hamburg, Germany. Shortly after, Roberto Colombo of LMP developed the first twin screw extruders in Italy.[3]
In the extrusion of plastics, the raw compound material is commonly in the form of nurdles (small beads, often called resin) that are gravity fed from a top mounted hopper into the barrel of the extruder. Additives such as colorants and UV inhibitors (in either liquid or pellet form) are often used and can be mixed into the resin prior to arriving at the hopper. The process has much in common with plastic injection molding from the point of the extruder technology, although it differs in that it is usually a continuous process. While pultrusion can offer many similar profiles in continuous lengths, usually with added reinforcing, this is achieved by pulling the finished product out of a die instead of extruding the polymer melt through a die.
The material enters through the feed throat (an opening near the rear of the barrel) and comes into contact with the screw. The rotating screw (normally turning at e.g. 120 rpm) forces the plastic beads forward into the heated barrel. The desired extrusion temperature is rarely equal to the set temperature of the barrel due to viscous heating and other effects. In most processes, a heating profile is set for the barrel in which three or more independent PID-controlled heater zones gradually increase the temperature of the barrel from the rear (where the plastic enters) to the front. This allows the plastic beads to melt gradually as they are pushed through the barrel and lowers the risk of overheating which may cause polymer degradation.
Extra heat is contributed by the intense pressure and friction taking place inside the barrel. In fact, if an extrusion line is running certain materials fast enough, the heaters can be shut off and the melt temperature maintained by pressure and friction alone inside the barrel. In most extruders, cooling fans are present to keep the temperature below a set value if too much heat is generated. If forced air cooling proves insufficient then cast-in cooling jackets are employed.
At the front of the barrel, the molten plastic leaves the screw and travels through a screen pack to remove any contaminants in the melt. The screens are reinforced by a breaker plate (a thick metal puck with many holes drilled through it) since the pressure at this point can exceed 5,000 psi (34 MPa). The screen pack/breaker plate assembly also serves to create back pressure in the barrel. Back pressure is required for uniform melting and proper mixing of the polymer, and how much pressure is generated can be "tweaked" by varying screen pack composition (the number of screens, their wire weave size, and other parameters). This breaker plate and screen pack combination also eliminates the "rotational memory" of the molten plastic and creates instead, "longitudinal memory".
After passing through the breaker plate, molten plastic enters the die. The die is what gives the final product its profile and must be designed so that the molten plastic evenly flows from a cylindrical profile, to the product's profile shape. Uneven flow at this stage can produce a product with unwanted residual stresses at certain points in the profile which can cause warping upon cooling. A wide variety of shapes can be created, restricted to continuous profiles.
The product must now be cooled, and this is usually achieved by pulling the extrudate through a water bath. Plastics are very good thermal insulators and are therefore difficult to cool quickly. Compared to steel, plastic conducts its heat away 2,000 times more slowly. In a tube or pipe extrusion line, a sealed water bath is acted upon by a carefully controlled vacuum to keep the newly formed and still molten tube or pipe from collapsing. For products such as plastic sheeting, the cooling is achieved by pulling through a set of cooling rolls. For films and very thin sheeting, air cooling can be effective as an initial cooling stage, as in blown film extrusion.
Plastic extruders are also extensively used to reprocess recycled plastic waste or other raw materials after cleaning, sorting and/or blending. This material is commonly extruded into filaments suitable for chopping into the bead or pellet stock to use as a precursor for further processing.
There are five possible zones in a thermoplastic screw. Since terminology is not standardized in the industry, different names may refer to these zones. Different types of polymer will have differing screw designs, some not incorporating all of the possible zones.
Each zone is equipped with one or more thermocouples or RTDs in the barrel wall for temperature control. The "temperature profile" i.e., the temperature of each zone is very important to the quality and characteristics of the final extrudate.
Typical plastic materials that are used in extrusion include but are not limited to: polyethylene (PE), polypropylene, polyacetal, acrylic, nylon (polyamides), polystyrene, polyvinyl chloride (PVC), acrylonitrile butadiene styrene (ABS) and polycarbonate.[4]
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