DETAIL ABOUT PVC EXTRUSION
PVC EXTRUSION
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 pipe that hardens during cooling
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REGID PVC PIPES
PVC COMPOUNDINGEssential Compounding Chemicals used with PVC Resin
• PVC Resin
o Suspension Grade o Paste Grade o Copolymer • Primary Plasticiser • Secondary Plasticiser • Stabilisers o Heat Stabilisers o Light Stabilisers • Lubricants • Fillers • Pigments • Special Additives PVC Resin There are 4 types of PVC Resin grouped by polymerization method 1. Suspension Grade PVC 2. Emulsion Grade PVC 3. Bulk Polymerised PVC 4. Copolymer PVC DETAILS OF PVC RESIN
TYPES DETAILS OF PVC RESIN
Suspension Grade PVC: The most widely prevalent type, Suspension grade PVC is made by polymerizing droplets of Vinyl Chloride monomer suspended in water. When Polymerisation is complete, the slurry is centrifuged and the PVC cake is gently dried by special heating systems so as not to subject the unstabilised resin to heat degradation. Particle size of the resin ranges from 50-250 microns and have porous popcorn like structures which readily absorbs Plasticisers. The structure of the PVC particles can be modified by selecting suitable suspending agents and Polymerisation Catalyst. Less porous types are extensively used for the high volume Rigid or Unplasticised PVC applications like PVC Pipes, Windows, Sidings, Ductings. Suspension grades of a coarser particle size and very porous structures absorb large quantities of Plasticiser forming a dryblend at temperatures as low as 80oC.The more porous types are used in Plasticised applications like Cables, Footwear, Soft Calendered Sheeting and Films etc. Emulsion Grade PVC: Emulsion Polymerised PVC is what Paste Grade Resin is and this is almost exclusively used for Plastisols. Paste grade resin is a very fine particle size PVC produced by spray drying an Emulsion of PVC in Water very much like how milk powder is produced. Paste grade resin needs much more energy to produce and is considerably costlier than Suspension resin. The paste grade resin carries the emulsifying chemicals and catalysts with it. It is therefore less pure than Suspension Polymerized or Bulk Polymerized PVC. The Electrical properties of Paste grade resin plastisols are therefore much poorer than Suspension Resin Compounds. Clarity is poorer than Suspension or Bulk PVC. Paste grade resin is compact in structure, and does not absorb much Plasticiser at room temperatures. Temperatures in excess of 160-180oC are needed to drive the plasticer into the Resin during curing. Paste grade Resin is extensively used for Cushion Vinyl Floorings of wide widths. Different layers of specially formulated pastes are coated either on a suitable substrate (Direct Coating) or on Release Paper (Transfer coating). The layers are fused continuously in long ovens and rolled up after release paper is stripped off. The rolled good flooring can have a tough semitransparent wear layer over printed and foamed layers which are sitting on top of highly filled base coats to build up the thickness. Many extremely attractive and rich effects are possible and these represent the higher end of Vinyl Flooring. Bulk Polymerised PVC: Bulk Polymerisation gives the purest form of PVC resin as no emulsifying or suspending agents are used. They are mainly used in transparent applications. They are mainly made available in the lower K value groups, as Unplasticised PVC Foils for Blister Packaging and other Calendered/Extruded Transparent films are best processed from lower K Value grades. Refinements in Suspension resin technology has edged out Bulk PVC in the recent past. Copolymer PVC: Vinyl Chloride is copolymerized with comonomers like Vinyl acetate give a range of resins with unique properties. PVAc or Copolymer of Vinyl Chloride and Vinyl acetate is the most important. The good solubility in solvents of PVAc makes it the prime choice for Vinyl Printing Inks and solvent cements. There is a very special application of PVAc in Floor tiling and it is the resin of choice for Vinyl Asbestos tiles. The Resin is actually a binder rather than the main ingredient. With Copolymer Resin it is possible to manufacture floor tiles with Fillers like Asbestos and Calcium Carbonate accounting for as much as 84% with the Copolymer and other compounding additives as low as 16%. Such high levels are not possible with Suspension resin as its melt viscosity is much higher and cannot coat and encapsulate such high levels of inert filler. Special callendering trains are required for Vinyl asbestos tiles. However with Asbestos falling out favour, such products have slowly died out. K Value PVC Resins are classified by their K-Value, an indicator of the molecular weight and degree of polymerization. • K70-75 are high K value resins which gives best mechanical properties but are more difficult to process. They need more plasticizer for same softness. High performance Cable insulations in Suspension resin and tough coatings for Conveyor belts, Industrial Flooring and similar high end applications in Paste grade are some popular application. It is the costliest. • K65-68 are medium K value resin which are the most popular. They have a good balance of Mechanical properties and processibility. UPVC (Unplasticised or Rigid PVC) is made from the less porous grades while Plasticised Applications are best made from the more porous grades. There is a lot of grade choice as they cater to the Majority of PVC applications. Because of its sheer volume this family of PVC resins are priced the lowest. • K58-60 are low K-value ranges. Mechanical properties are lowest, but processing is easiest. Many difficult to process applications like injection moulding, blow moulding and Clear Calendered packaging film are made from the lower K value ranges. Prices are higher than Medium K Value Resins. • K50-55 are special resins which are tailor made for some demanding applications. Interesting ones are Battery Separator Resins and Blending resins used along withPaste Grade resin to reduce costs. Processing is easiest. As PVC is 56% Chlorine, it is one of the few Polymers which are self extinguishing, as Chlorine is a strong Flame inhibitor. COMPOUNDING
STEP INVOLVES IN COMPOUNDING
RESIN STABILIZERS FILLER PLASTIZIER PIGMENT LUBRICANT All of them are accordingly doses to a mixer,were they are heated at certain temp (75-90approx c). Then they are cool to the room temperature. Then after extrusion process take place to get proper dimension for finished products. Pipes,Profile,article,etc are extruded. The Extrusion Process
The Extrusion Process
In the plastics extrusion process, raw thermoplasticmaterial, or resin, is gravity fed from a top mounted hopper into the barrel of an extruder. Additives, such as stabilizers and UV inhibitors, in either liquid or pellet form are often used and can be introduced into the resin below arriving at the hopper. The process has much in common with plastics injection molding though differs in that the process is usually continual. While injection molding can offer many similar profiles in continuous lengths, usually with added reinforcing, the finished product is pulled out of a die instead of extruding the fluid resin through a die.
As the material enters the feed throat near the rear of the barrel it comes in contact with the screw. The rotating screw forces the plastic resin forward into the barrel that is heated to the desired melt temperature depending on the resin. In most processes, a heating profile is set for the barrel utilizing three or more independent PID (proportional-integral-derivative controller) controlledheat zones that gradually increase the temperature of the barrel from the rear where the resin has entered to the front. This allows the plastic resin to melt gradually as it is pushed through the barrel and lowers the risk of overheating which may cause degradation in the polymer.
At the front of the barrel, the resin leaves the screw and travels through a reinforced screen to remove any contaminants. A breaker plate generally reinforces screens because the pressure at this point can exceed 5000 psi (34 MPa).
After passing through the breaker plate resin enters the die. The die is what gives the final product its profile or shape 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 would produce a product with unwanted stresses at certain points in the profile. These stresses can cause warping upon cooling. Almost any shape imaginable can be created so long as it is a continuous profile.
The product must now be cooled which is usually achieved by pulling the extrudate through a water bath. Plastics are excellent thermal insulators and are therefore very difficult to cool quickly. Compared with steel, plastic conducts its heat away 2000 times more slowly. In a tube or pipe extrusion line, a sealed water bath utilizes a carefully controlled vacuum to keep the newly formed and still molten tube or pipe from collapsing. A set of cooling rollers is generally used in the sheet extrusion process to cool sheet as it exits the extruder.
Sometimes, on the same line, a secondary process may occur before the product has finished its run. In the manufacture of adhesive tape, a second extruder melts adhesive and applies this to the plastic sheet while it’s still hot. Once the product has cooled, it can be spooled, or cut into lengths for later use.
Plastic extruders are also extensively used to prepare recycled plastic waste and/or raw materials after cleaning, sorting and/or blending into filaments suitable for blending into the resin pellet stock used by the plastics industry at large.
Why PVC???
Uses and Application Of PVCUses & Benefits
Vinyl is versatile: it can be as rigid as industrial pipes, as pliable as plastic wrap, and as thin and flexible as wallcovering. It can also be completely clear or matched to any color desired.
Building and Construction
About three-quarters of all vinyl produced goes into long-lasting building and construction applications. Life-cycle studies show PVC/vinyl is effective in protecting the environment, in terms of low greenhouse gas emissions and conservation of resources and energy.
Because it is strong and resistant to moisture and abrasion, vinyl is ideal for cladding, windows, roofing, fencing, decking, wallcoverings, and flooring. Vinyl does not corrode like some building materials, does not require frequent painting and can be cleaned with mild cleaning products.
Packaging
Because it is durable, dependable and light weight, flexible PVC helps packaging do its job to maintain the integrity of the products inside, including medicines. Clear vinyl is used in tamper-resistant over-the-counter medications and shrinkwrap for consumer products. Rigid vinyl film is used in blister and clamshell packaging to protect medicines, personal care products and other household goods.
Healthcare
Vinyl plays a critical safety role in dispensing life-saving medicine through IV bags and medical tubing. The advent of the PVC blood-collection bag was a significant breakthrough because blood bags are flexible and unbreakable, enhancing the development of ambulatory medicine and serving as the foundation for modern blood banks.
Household Products
PVC’s affordability, durability and water resistance make it ideal for rain coats, boots and shower curtains
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