Proses Pembentukan Plastik Sumber: MANUFACTURING ENGINEERING & TECHNOLOGY Oleh: Serope Kalpakjian

Plastik: Berasal dari kata Yunani (Greek) plastikos yg berarti “mudah untuk dicetak dan dibentuk”. Plastik juga disebut polymer. Polymer merupakan makro-molekul yg terdiri dari banyak monomer yang terikat menjadi suatu rantai molekul yang panjang melalui proses polimerisasi.

Figure 7.2 Basic structure of polymer molecules: (a) ethylene molecule; (b) polyethylene, a linear chain of many ethylene molecule; and (c) molecular structure of various polymers. These are examples of the basic building blocks for plastics.

Proses Pembentukan Plastik Proses pembentukan plastik hampir mirip dgn proses pembentukan logam (casting, molded, extruded, joined). Plastik lebih mudah dibentuk bila dibandingkan dengan logam. Plastik bisa dicairkan dalam temperatur yg jauh lbh rendah dp logam. Penanganan proses lebih mudah dan memerlukan lbh sedikit energi.

Bahan baku pembentukan plastik Pellet/granules: bahan plastik ini berbentuk butiran-butiran. Serbuk (powder): bahan berbentuk serbuk. Pewarna plastik: memberi warna sesuai dgn yg diinginkan. Pellet/granules Pewarna Plastik

Beberapa jenis plastik: 1. Poly Propylene (PP) Polypropylene (PP) is a thermoplastic polymer, used in a wide variety of applications, including packaging, textile, stationery, plastic parts, food containers, woven bags, laboratory equipment, automotive components and many more. Polypropylene is commonly recycled product

Beberapa jenis plastik: 2. Poly Ethylene (PE) HDPE (high density PE): is resistant to many different solvents and has a wide variety of applications, including: Containers, Fuel Tanks for vehicles, Folding tables and chairs, Storage sheds, Plastic bags, Water supply pipes, bottles - suitable for use as refillable bottles etc. and many more. LDPE (low density): is widely used for manufacturing various containers, dispensing bottles, wash bottles, tubing, plastic bags for computer components and various molded laboratory equipment. Its most common use is in plastic bags. PE is thermoplastic polymers.

Beberapa jenis plastik: 3. Poly Vinyl Chloride (PVC) Poly Vinyl Chloride (PVC): is the third most widely used thermoplastic polymer after polyethylene and polypropylene. It is used in clothing and upholstery and tubing, flooring and electrical cable insulation. It is also commonly used in pool toys, sewerage pipes, Electric wires, Pipes, Signs boards (sheets), PVC Ceiling Tiles etc. and many more.

Beberapa jenis plastik: 4. Epoxies, Phenolics, Silicones. Epoxies, Phenolics, Silicones adalah jenis polymer yang masuk kategori thermosetting plastics. Rantai polymer nya jenis cross-linked structure. Memiliki sifat kekerasan, kekuatan, kekakuan dan dimensional stability yg lebih baik. Memiliki sifat getas (brittle).

Polymer Chains Figure 7.5 Schematic illustration of polymer chains. (a) Linear structure – thermoplastics such as acrylics, nylons, polyethylene, and polyvinyl chloride have linear structures. (b) Branched structure, such as in polyethylene. (c) Cross-linked structure – many rubbers or elastomers have this structure, and the vulcanization of rubber produces this structure. (d) Network structure, which is basically highly cross-linked – examples are thermosetting plastics, such as epoxies and phenolics.

Behavior of Polymers Figure 7.6 Behavior of polymers as a function of temperature and (a) degree of crystallinity and (b) cross-linking. The combined elastic and viscous behavior of polymers is known as viscoelasticity.

Amorphous and Crystalline Regions in a Polymer Effects of Crystallinity: Polymer become stiffer, harder, less ductile, more rubbery, more resistant to solvents and heat Figure 7.8 Amorphous and crystalline regions in a polymer. The crystalline region (crystallite) has an orderly arrangement of molecules. The higher the crystallinity, the harder, stiffer, and less ductile the polymer.

Characteristics of Forming and Shaping Processes for Plastics and Composite Materials

Forming and Shaping Processes for Plastics, Elastomers, and Composite Materials Figure 19.1 Outline of forming and shaping processes for plastics, elastomers, and composite materials. (TP = Thermoplastics; TS = Thermoset; E = Elastomer.)

Proses Ekstrusi Plastik Bahan baku bisa berbentuk pellet, granules maupun serbuk. Screw mencampur material, menekannya sepanjang barrel. Campuran material akan dipanaskan dan dicairkan saat melewati barrel (mulai melting zone). Fungsi screw: (1) feeding, (2) melt or transition, (3) pumping.

Extruder Schematic Figure 19.2 (a) Schematic illustration of a typical screw extruder. (b) Geometry of an extruder screw. Complex shapes can be extruded with relatively simple and inexpensive dies.

Extrusion Die Geometries Figure 19.3 Common extrusion die geometries: (a) coat-hanger die for extruding sheet; (b) round die for producing rods; and (c) dies for producing square cross-sections. Note the non-uniform recovery of the part after it exits the die. Source: (a) Encyclopedia of Polymer Science and Engineering (2nd ed.). Copyright © 1985. Reprinted by permission of John Wiley & Sons, Inc.

Extrusion of Tubes Figure 19.4 Extrusion of tubes. (a) Extrusion using a spider die (see also Fig. 15.8) and pressurized air. (b) Coextrusion for producing a bottle.

Production of Plastic Film and Bags Figure 19.5 (a) Schematic illustration of the production of thin film and plastic bags from tube – first produced by an extruder and then blown by air. (b) A blown-film operation. This process is well developed, producing inexpensive and very large quantities of plastic film and shopping bags. Source: Courtesy of Windmoeller & Hoelscher.

Melt-Spinning Process Figure 19.6 The melt-spinning process for producing polymer fibers. The fibers are then used in a variety of applications, including fabrics and as reinforcements for composite materials.

Injection Molding Proses injection molding u/ plastik, prinsipnya sama dgn hot-chamber die casting u/ logam. Bahan baku berupa pellet atau granulle. Tekanan yg diperlukan u/ injection molding berkisar 70 MPa – 200 MPa (10.000 – 30.000 psi) Skema proses bisa dilihat di Fig. 19.7

Injection Molding Figure 19.7 Schematic illustration of injection molding with (a) plunger and (b) reciprocating rotating screw.

Injection Molding Sequence Figure 19.8 Sequence of operations in the injection molding of a part with a reciprocating screw. This process is used widely for numerous consumer and commericial products, such as toys, containers, knobs, and electrical equipment (see Fig. 19.9).

Products Made by Injection Molding Figure 19.9 Typical products made by injection molding, including examples of insert molding. Source: (a) Courtesy of Plainfield Molding, Inc. (b) Courtesy of Rayco Mold and Mfg. LLC.

Mold Features for Injection Molding Figure 19.10 Illustration of mold features for injection molding. (a) Two-plate mold with important features identified. (b) Four parts showing details and the volume of material involved. Source: Courtesy of Tooling Molds West. Inc.

Types of Molds used in Injection Molding Figure 19.11 Types of molds used in injection molding: (a) two-plate mold; (b) three-plate mold; and (c) hot-runner mold.

EPOCH Hip Stem Figure 19.12 The EPOCH hip stem. This design uses a PAEK (polyaryletherketone) layer and bone-ingrowth pad around a cobalt-chrome core in order to maximize bone ingrowth. Source: Courtesy of Zimmer, Inc. Figure 19.13 An EPOCH hip is removed from the mold after an insert injection-molding operation. Source: Courtesy of Zimmer, Inc.

Injection-Molding Machine Figure 19.14 A 2.2-MN (250-ton) injection molding machine. The tonnage is the force applied to keep the dies closed during the injection of molten plastic into the mold cavities and hold it there until the parts are cool and stiff enough to be removed from the die. Source: Courtesy of Cincinnati Milacron, Plastics Machinery Division.

Reaction-Injection Molding Process Disebut RIM karena dlm prosesnya resin (bhn plastik) dicampur dan direaksikan dgn dua atau lebih ‘reactive fluid’, sehingga terjadi reaksi kimia secara cepat di dalam cetakan. Hasilnya berupa thermoset part. Aplikasi proses: pembuatan fender & bumper u/ otomotif, insulator termal u/ kulkas & freezers, water skis, stiffeners. Skema ada di Fig 19.15 berikut.

Reaction-Injection Molding Process Figure 19.15 Schematic illustration of the reaction-injection molding process. Typical parts made are automotive-body panels, water skis, and thermal insulation for refrigerators and freezers.

Blow-Molding Figure 19.16 Schematic illustrations of (a) the extrusion blow-molding process for making plastic beverage bottles; (b) the injection blow-molding process; and (c) a three-station injection molding machine for making plastic bottles.

Rotational Molding Process Umumnya digunakan u/ produk axy-simetric besar (misalnya tangki air/tandon air, bucket, dll). Bahan baku berupa serbuk plastik (powder), yg volume/jumlahnya disesuaikan dgn volume celah yg ada di dlm mold/cetakan. Mold yg sudah berisi serbuk plastik diputar (dua sumbu putar), diberi campuran zat kimia dan dipanaskan dlm oven; sehingga campuran mencair (melt) & terbentuk produk sesuai yg diinginkan.

Rotational Molding Process Figure 9.17 The rotational molding (rotomolding or rotocasting) process. Trash cans, buckets, and plastic footballs can be made by this process.

Thermoforming Process Figure 19.18 Various thermoforming processes for a thermoplastic sheet. These processes commonly are used in making advertising signs, cookie and candy trays, panels for shower stall, and packaging.

Compression Molding Figure 19.19 Types of compression molding – a process similar to forging: (a) positive, (b) semipositive, and (c) flash, which is later trimmed off. (d) Die design for making a compression-molded part with external undercuts.

Transfer Molding Figure 19.20 Sequence of operations in transfer molding for thermosetting plastics. This process is suitable particularly for intricate parts with varying wall thickness.

Processes for Plastics and Electrical Assemblies Figure 19.21 Schematic illustration of (a) casting, (b) potting, and (c) encapsulation processes for plastics and electrical assemblies, where the surrounding plastic serves as a dielectric.

Calendering Figure 19.22 Schematic illustration of calendering. Sheets produced by this process subsequently are used in thermoforming. The process also is used in the production of various elastomer and rubber products.

Motorcycle Components Figure 19.23 Reinforced plastic components for a Honda motorcycle. The parts shown are front and rear forks, rear swing-arm, wheel, and brake disks.

Methods of Reinforcing Plastics Figure 9.2 Schematic illustration of methods of reinforcing plastics (matrix) with (a) particles, (b) short or long fibers or flakes, and (c) continuous fibers. The laminate structures shown in (d) can be produced from layers of continuous fibers or sandwich structures using a foam or honeycomb core (see also Fig. 16.50).

Tapes used in Making Reinforced Plastic Parts (b) Figure 19.24 (a) Manufacturing process for polymer-matrix composite tape. (b) Boron-epoxy prepreg tape. These tapes are then used in making reinforced plastic parts and components with high strength-to-weight ratios, particularly important for aircraft and aerospace applications and sports equipment. Source: (a) Courtesy of T. W. Chou, R. L. McCullough, and R. B. Pipes. (b) Courtesy of Avco Specialty Materials/Textron.

Tape and Tape-Laying System (b) Figure 19.25 (a) Single-ply layup of boron-epoxy tape for the horizontal stabilizer for an F-14 fighter aircraft. (b) A 10-axis computer-numerical-controlled tape-laying system. This machine is capable of laying up 75- and 150-mm (3- and 6-in.) wide tapes on contours of up to +/- 30 degrees and at speeds of up to 0.5m/s (1.7 ft/s). Source: (a) Courtesy of Grumman Aircraft Corporation. (b) Courtesy of The Ingersoll Milling Machine Company.

Production of Fiber-Reinforced Plastic Sheets Figure 19.26 Schematic illustration of the manufacturing process for producing fiber-reinforced plastic sheets. The sheet still is viscous at this stage and later can be shped into various products. Source: After T. W. Chou, R. L. McCullough, and R. B. Pipes.

Vacuum-Bag Forming and Pressure-Bag Forming Figure 19.27 Schematic illustration of (a) vacuum-bag forming, and (b) pressure-bag forming. These processes are used in making discrete reinforced plastic parts. Source: After T. H. Meister.

Open-Mold Processing Figure 19.28 Manual methods of processing reinforced plastics: (a) hand lay-up, and (b) spray lay-up. Note that, even though the process is slow, only one mold is required. The figures show a female mold, but male molds also are used. These methods also are called open-mold processing. (c) A boat hull made by these processes.

Filament-Winding (b) Figure 19.29 (a) Schematic illustration of the filament-winding process; (b) fiberglass being wound over aluminum liners for slide-raft inflation vessels for the Boeing 767 aircraft. The products made by this process have high strength-to-weight ratio and also serve as lightweight pressure vessels. Source: Courtesy of Brunswick Corporation.

Pultrusion (b) Figure 19.30 (a) Schematic illustration of the pultrusion process. (b) Examples of parts made by pultrusion. The major components of fiberglass ladders (used especially by electricians) are made by this process. Unlike aluminum ladders, they are available in different colors but are heavier because of the presence of glass fibers. Source: Courtesy of Strongwell Corporation.

Design Modifications to Minimize Distortion in Plastic Parts Figure 19.31 Examples of design modifications to eliminate or minimize distortion in plastic parts: (a) suggested design changes to minimize distortion; (b) stiffening the bottoms of thin plastic containers by doming – a technique similar to the process used to shape the bottoms of aluminum beverage cans; and (c) design change in a rib to minimize pull-in (sink mark) caused by shrinkage during the cooling of thick sections in molded parts.

Production Characteristics of Molding Methods