Extrusion: Plastics extrusion is basically defined as converting plastic power or granules into a continuous uniform melt and funcing this melt through a die which yields a desired shape. Widely used for continuous production of film, sheet, tube, and other profiles; also used in conjunction with blow moulding. Thermoplastic moulding compound is fed from a hopper to a screw pump where it is heated to plasticate then pumped out through the shaping orifice (die) to achieve desired cross section. Production lines require input and takeoff equipment that can be complex. Low tool cost, numerous complex profile shapes possible, very rapid production rates, can apply coatings or jacketing to core materials (Such as wire).Continuous Process, In principle, the plastic raw material is plasticated by means of a screw plastication unit and the molten material is continuously pumped out through a standard orifice (die) in order to take the shape and then the shape is set by cooling/sizing system.
Types: - Screw Pump, Hopper, Barrel, Hopper Throat, Drive System. Usually limited to sections of uniform cross section.
Clamping Force required: FC=APf, A-Projected Area. P-Blow Pressure (Usually in the range of 0.5to1.5MPa). F-Safety factor (25% extra)
Venting: If venting is not provided, the following defects may occur in the moulding: Discoloration, Sink mark, Incomplete filling. Position of the Vent: At the point where flow paths are likely to meet, at the bottom of the projection, At the point of further most from the gate on symmetrical moulding.
Wire Coating/Cable Covering:
1. Unit comprises of
2. Un wind unit (For conductor)
3. Pre treatment unit
4. Wire coating unit
Steps Involved:
1. Wire/conductor is unwound & straightened by Tension.
2. Control Unit.
3. Pre treated to promote adhesion of molten plastic
4. Then passed through the Cross head die of the coating unit
5. Coated wire is then cooled by passing through cooling trough
6. Wound on the winder
Extrusion blow moulding: Different types of Extrusion Blow moulding:
1. Continuous: Single station Method, Twin Station Method, Rotary Table Method.
2. Intermittent: Reciprocating Screw, Ram & Accumulator Head
Reciprocating screw extruder: When making large parts, the parison becomes so large and heavy that it sags under its own weight and it becomes thinner and thinner as it is extruded. Here, the resin is plasticized by the rotating screw and the melt accumulates in front of the screw. The screw retracts to accommodate the melt, and then it is pushed forward by hydraulic means, forcing the melt through the die head to form a parison (see Fig.) At that point, the mould closes; the blow air inflates the parison, forcing the hot plastic towards the mould cavity walls. After sufficient cooling the blow moulded part is ejected. During the blowing and cooling stages, the screw retracts and accumulates another charge. Even if the blowing and cooling times remain the same as in any other process, the overall reduction in cycle time can be significant, making it possible to produce larger objects that could be made with normal extruder of the same size.
Ram accumulator extrusion: The ram accumulator type blow moulding is recommended for larger parts. This type of machine (see Fig) is used to make parts, weighing up to 50 pounds. The main applications are industrial parts, shipping containers, and toys. The size of the extruder is independent of accumulator size, and in some cases more than one extruder is used. The main disadvantage of the system is that the first material to enter the accumulator is the last to leave, (FILO). This makes the process unsuitable for heat-sensitive materials. A variation on ram accumulators that provides first-in, first-out material flow is the accumulator head machine. In this case, melt from the extruder enters the accumulator head from the side and flows around a mandrel; a tubular plunger pushes the entire shot through the die. Parts as large as 300 pounds can been produced by this method.
Accumulator head method: This method also utilises the external accumulator that can be many times larger than the volume of a reciprocating screw cylinder. It is designed to produce containers with capacity of 100 to 400 L. Here, one to four extruders continuously fill the accumulator head with enough plastic for a complete parison shot. The container is annular, that makes it possible to produce parisons with more uniform circumferential wall thickness. After the correct charge has been accumulated, a hydraulic ring piston drives out the melt to form a parison, which is then placed in the mould and blown. The parison is extruded by moving an annular ring plunger downward to displace the material through a die bushing and mandrel. Fig. 4.5 shows the die head accumulator, which can produce large HMW-HDPE containers.
Special requirements for the blow moulding extruder are: Different types of polyethylene with widely differing melt viscosities must be processed. Single screw extruders for PVC must plastic ate powder blends for crystal clear, impact resistant and glossy bottles with melt temperatures close to the decomposition limit, whereby the addition of processing aids is severely limited. The melt must be conveyed against high die resistance, that is, with a high melt pressure at the screw tip. The melt temperature must be kept as low as low as possible to avoid parison elongation. The addition of dyes, stabilisers and varying quantities of reground material must be possible without difficulty. The parison die connected to the extruder into which the melt is fed from the side has the function of deflecting the melt through 900 and forming a parison through annual cross section and a defined wall thickness. Up to a container size of 25-30lit, the continuous parison extrusion can be used. For larger container sizes, the discontinuous extrusion process must be used. The extruded parison is picked up by the closing mould and transported under the blowing station where the moulding is inflated, generally with a pressure of 0.8 to 1 MPa, and thus simultaneously cooled. To reduce the blowing and cooling time (6080 percent of the total cycle time), intermittent blowing is frequently used. This results in an additional cooling effect, which is amplified by blowing on to critical zones, particularly in the base and neck areas. In all blow mould parting line flash is pinched off outwardly. Flash in the case of bottles with symmetric design, can account for 2025% of the material used and for over 50% in case of a symmetric mouldings. The proportion of flash or its rework ability for production can be decisive for the economics of the process. Extrusion blow machines have a variety of ancillary equipment, viz. blowing and ejection stations, flash removal, weighing and density determination, discharge stations, filling, locking, printing and labelling of containers, etc. In mass production line extrusion blow machines are automated, electronically controlled and hydraulically or pneumatically actuated. Extrusion blow moulding is the preferred process for forming containers and other hollow products of all sizes and shapes.
Die head assembly: To facilitate the melt flow and to prevent accumulation of plastic, the surfaces of the die and the core should be well polished. If the surface of the channel is rough, stagnant flow layers of the polymer may form, the material will eventually degrade, causing dark streaks to appear in the parison. Roughness in flow channels may also cause the parison rupture. Streamlining the shape of flow channels also helps to prevent areas of stagnation and welding of the plastic streams after it has flown around the head mandrel. An annular restriction called a "choke" is often placed on the mandrel to increase the head pressure. This choke decreases the cross-sectional area of the annular flow channel and builds up melt back pressure in the weld area just past the flow deflector, upstream from the choke. If a choke is used and flow is not streamlined polymer or hard gels may get trapped in the channel. In addition to ensuring uniform flow conditions in the extrusion head, it is necessary to control the flow leading to the die orifice. A tapered approach angle to the die orifice is recommended for most applications. Each type of material will require a different approach angle. The ratio of wall thickness at the die orifice to the length of die land is a controversial subject, for linear polyethylene, for example, the die length as high as twelve to twenty times the wall thickness is recommended by material suppliers.
Die assembly: Die Assembly.(To convert a Plastic melt into a tubular shape). Torpedo-head or spider type dies (center fed dies) [Used for heat sensitive materials like PVC]. Pinhead dies (side fed dies). [Used for heat stable materials like poly olefins]. The torpedo head dies are generally recommended for the use of heat sensitive material like PVC. The pinhead dies or side fed dies are commonly used for heat stable material such as poly olefins. The side feed dies are generally difficult to centre and they may cause serious problems with the weld line, the spider-type torpedo dies eliminate most of these problems but they are more complicated. Furthermore, the possibility of stationary flow with resultant decomposition is considerably less with a centrally fed than with a right-angle fed extrusion head. For example, side fed extrusion heads with core pins that work satisfactorily with PE usually produces weld lines with PVC. Thus, for the latter plastic, a centrally fed torpedo head is used - it minimises the interruption of material flow and prevents movement of the mandrel under generated high pressure.
Parison: A uniform parison is of paramount importance. Extruder must deliver an extrudate with in close dimensional tolerance. Irregular extruder output causes weight variations from one blow part to the next. Temperature of the parison should be controlled. Parison “draw down” results from gravitational pull on the plastic during extrusion. The bottom becomes heavier while the top thins down. The lower the melt viscosity the greater the drawdown. Drawdown can be reduced by: using polymer grades with long-chain branching, lowering the melt temperature, increasing the extrusion rate. Also, the parison programming can be used to compensate for drawdown. Parison extrudate swell occurs when the melt exits from the parison die.
Parison programming: Programming controls the wall thickness as the parison emerges from the die. The draw bar is attached to a mandrel that forms the inside of the parison. A bushing shapes the outside of the parison. The parison thickness changes with vertical movement of the mandrel. The sequence of mandrel & bushing movements constitute the parison programming. The programming is accomplished electronically, either by mechanical means, with a patch panel, or by a microprocessor.
Die shaping: Used to improve wall distribution of a blow moulding part. Irregularly shaped parts such as gas/fuel tank usually need die shaping. The shaping can be done either on a bushing or on a mandrel depending upon the easy machining. [Usually bushing for diverging die & mandrel for converging die]. Frequently, a combination of programming and die shaping gives the best results. For example, this is the case for a conical – shaped part where shaping is cut on a converging mandrel. The die mandrel moves up to the maximum die gap to extrude thick parison for the corner.
Melt temperature:
High Melt Temperature: Leads to better finish as well as better physical & mechanical properties. May cause parison sag. Requires longer cooling cycle & cause more shrinkage.
Low Melt Temperature: Results in better shaped parison, shorter cooling cycle & less shrinkage. Too low melt temp leads to non-homogenous melt as well as inferior mechanical & physical properties. It is desirable that the parison have the lowest practical melt temp.
Blowing air: Expands the parison tube against the mould walls. Forcing the material to assume the shape of the mould & duplicate surface details. During the expansion phase high volume of air is desirable for the rapid accomplishment of the expanded parison against the mould walls. The maximum volumetric flow rate into the cavity at a low linear velocity can be achieved by making the air inlet orifice as large as possible. The pressure of the blowing air influences the surface details in the moulded item. Some PE containers with heavy walls can be blown and pressed against the mould walls by air pressure as low as 200 to 300 kPa. Large items such as 5 L bottles require still higher air pressure (600 to 800 kPa). The clamping force of the mould platens must be sufficient to withstand the blowing pressure inside the blow moulds. An excess of 2550% of pressure over the calculated value of projected area of the product items blowing pressure is advisable.
Surface imperfection: Die lines & scratches are surface imperfections formed on parison. Cleanliness of the parison die is important in reducing these lines. Chrome plating of the die & Teflon coating may be helpful. Die lines can be minimized by by the actions (Increasing blowing speed, increasing air pressure, increasing mould temp.)That minimizes cooling of the melt until it is pressed against the mould surface.
Pinch of design: To perform this, most moulds use rugged, double angle pinch off. Its land will stand up to the pressure and will provide a clean break point. The 30° provides a slight back pressure and causes the melt to thicken in the weld area. The flash pocket depth is related to the pinch off area and is very important for proper moulding and automatic trimming of the part. The width of the pinch off edge depends on the material to be processed, the wall thickness, the closing speed and the blowing time. For short runs, the pinch off can be made of aluminium, but for long production runs the moulds have inserted pinch off areas made of steel or beryllium copper. Too big a relief angle can hinder the adequate cooling of the edge zones, which in turn could result in dull spots next to the weld line. With too small a relief angle a cooled off seam might be pushed into the mould cavity forming a deep groove along the weld line.
Blow Ratio: The blow ratio is the ratio of the outer diameter of the blown container divided by the outer diameter of the parison. The required die gap can be approximately calculated multiplying the desired wall thickness of a blown bottle by the blow ratio.
Drums or barrels: With this Moulding technique, a hollow, semi-hollow or completely solid ring can be mounded. Solid ring can be produced with good consistency, but hollow & semi-hollow ones are difficult. Preferred material is HMWHDPE.
Two stage process: In this process the injection mounded perform can be produced any time at any place. The perform is to be re-heated before stretching and blowing. In the process the performs are transported through an oven where they rotate continuously to attain even temperature throughout the surface. Usually the threaded area is protected from heating by heat shield. After heating the performs exit from the oven and are allowed to equilibrate. Since the performs are usually heated by IR heaters the temp on the outside skin is higher than the inside skin. During equilibration, the outside skin temperature drops while inside skin temperature rises. Then it is transferred to blowing section the blow moulds closes at the threaded area of the perform and clamped to required tonnage. The stretch rod enters the perform through the perform towards the bottom of the mould. The air is then blown in to the mould up to the required pressure the bottle cools due to contact with the cold mould. The blown air is exhausted, the rod retracts and the moulds open to eject the bottle.
Extruder: Extruder is a machine that accepts solid particles (Powder) or liquid (molten) feed, conveys it through a surrounding barrel by means of a rotating screw, and pumps it, under pressure, through an orifice.
Extruder output: The simplest way to understand the operation of SSE is to mentally unwind the screw into a long, straight channel of decreasing depth. Now the barrel is visualized as a flat metal slab placed above the screw flights at the distance corresponding to the actual gap in the extruder, between the barrel and the screw flights. In this schematic, the screw rotation inside the barrel is equivalent to sliding the metal slab over stationery straight channel at an angle corresponding to the pitch angle of the screw.
APPLICATIONS OF EXTRUSION:
Film: Blown film, Cast film, Co-extruded films, BOF.
Material Used: PP,PVC, LDPE, HDPE, PET, Nylon etc.
Pipe/tube: Material: HDPE, LDPE, LLDPE, PVC etc.
Sheet: Material: HDPE, ABS, HIPS, PC etc.
Monofilament: Material: PP, Nylon etc.
Extrusion Coating/Lamination: Coated Playing Cards, Wrapping and LDPE laminated Woven sacks, Material: LD, PP, HDPE.
Box Strapping: Material: PP, HDPE etc.
Tape/Woven Sack: Material: PP, HDPE.
Wire Coating/Covering: Primary/Secondary insulation, Material: LDPE, PVC, And Nylon.
Profiles (Door and window): Material: PVC
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