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Forming and Shaping Plastics

Disiapkan sebagai Materi kuliah PENGETAHUAN MANUFAKTUR

Dipersiapkan oleh: MOERWISMADHI ST. MTRetired POLITEKNIK MANUFAKTUR BANDUNGE-mail : [email protected]

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Characteristics of Forming and ShapingProcesses for Plastics and Composite Materials


Forming and Shaping Processes


Extruder

Figure 18.2 Schematic illustration of a typical extruder. Source: Encyclopedia of Polymer Scienceand Engineering (2nd ed.).


Sheet and Film Extrusion

Figure 18.3 Die geometry (coat-hanger die) for extruding sheet. Source: Encyclopedia of Polymer Science and Engineering (2d ed.).Copyright © 1985. Reprinted by permission of John Wiley & Sons, Inc.Figure 18.4 Schematic illustration of the production of thin film and plastic bags from tube first produced by an extruder and then blown by air. Source: D.C. Miles and J.H. Briston, Polymer Technology, 1979. Reproduced by permission of Chemical Publishing Co., Inc.


Injection Molding

Figure 18.5 Injection molding with (a) plunger, (b) reciprocating rotating screw, (c) a typical part made from an injection molding machine cavity, showing a number of parts made from one shot; note also mold features such as sprues, runners, and gates.


Examples of Injection Molding

Figure 18.6 Typical products made by injection molding, including examples of insert molding. Source: Plainfield Molding Inc.


Injection-Molding Machine

Figure 18.7 A 2.2-MN (250-ton) injection-molding machine. The tonnage is the forceapplied to keep the dies closed during injection of molten plastic into the mold cavities.Source: Courtesy of Cincinnati Milacron, Plastics Machinery Division.


Extrusion Blow Molding (EBM)

Dalam Extrusion Blow Molding (EBM), granulat plastik dilunakkan dan di”extrude” melalui Die menjadi bentuk pipa/selang (a parison). Parison ini kemudian di”tangkap” oleh pasangan Mold yang berpendingin. Setelah “pipa” plastik lunak (parison) ter”pegang” oleh pasangan Mold, pipa lunak dpotong oleh pisau, kemudian Mold dan pipa lunak bergeser kemudian dihampiri oleh Nosel udara tekan, ditiup hingga membentuk produk berongga / botol, kontainer. Setelah bahan plastik mendingin dan mengeras< Mold membuka dan produk plastik di”eject” keluar.

Proses EBM bisa berlangsung kontinyu ataupun terputus-putus. (extrusi parison kontinyu atau intermittent). Mesin EBM dikategorikan sbb: –Continuous Extrusion

Equipment– rotary wheel blow molding

systems – shuttle machinery

–Intermittent Extrusion Machinery– reciprocating screw

machinery – accumulator head

machinery


Extrusion Blow Molding


Injection blow Molding

The process of Injection Blow Molding (IBM) is used for the production of hollow glass and plastic objects in large quantities. In the IBM process, the polymer is injection molded onto a core pin; then the core pin is rotated to a blow molding station to be inflated and cooled. This is the least-used of the three blow molding processes, and is typically used to make small medical and single serve bottles. The process is divided in to three steps: injection, blowing and ejection.

Step 1. InjectionThe injection blow molding machine is based on an extruder barrel and screw assembly which melts the polymer. The molten polymer is fed into a manifold where it is injected through nozzles into a hollow, heated preform mould. The preform mold forms the external shape and is clamped around a mandrel (the core rod) which forms the internal shape of the preform. The preform consists of a fully formed bottle/jar neck with a thick tube of polymer attached, which will form the body.

http://en.wikipedia.org/wiki/Glass

http://en.wikipedia.org/wiki/Thermoplastic

http://en.wikipedia.org/wiki/Bottles

http://en.wikipedia.org/wiki/Polymer

http://en.wikipedia.org/wiki/Molding_(process)

http://en.wikipedia.org/wiki/Mandrel



Step 2. BlowingThe preform mold opens and the core rod is rotated and clamped into the hollow, chilled blow mold. The core rod opens and allows compressed air into the preform, which inflates it to the finished article shape.

Step 3. EjectionAfter a cooling period the blow mold opens and the core rod is rotated to the ejection position. The finished article is stripped off the core rod and leak-tested prior to packing. The preform and blow mold can have many cavities, typically three to sixteen depending on the article size and the required output. There are three sets of core rods, which allow concurrent preform injection, blow molding and ejection.


Injection Blow Molding

Figure 18.9 Schematic illustrations of (a) the blow-molding process for making plastic beverage bottles, and (b) a three-station injection blow-molding machine. Source: Encyclopedia of Polymer Science and Engineering (2d ed.). Copyright ©1985. Reprinted by permission of John Wiley & Sons, Inc.


Injection Blow Molding

Raw Materials• Polyethylene (Low Density) LDPE, LLDPE • Polypropylene PP • Polyethylene - Terephthalate PET • Polyvinyl chloride PVC • Polyethylene (High Density) HDPE

These factors are critical to this process: • Shear & temperature dependent viscosity • Temperature-dependent tensile strength on the pin • Tensile elongation during inflation • Crystallization kinetics on the core pin • Crystallization kinetics during blowing and cooling


STRETCH BLOW MOLDING

The main applications are bottles, jars and other containers. The Injection blow molding process produces bottles of superior visual and dimensional quality compared to extrusion blow molding. The process is ideal for both narrow and wide-mouthed containers and produces them fully finished with no flash. A sign of injection blow molding is the seam where the two halves of the mold meet.

This next picture shows what happens inside the blow mold. The preform is first stretched mechanically with a stretch rod. As the rod travels down low-pressure air of 5 to 25 bar (70 to 350 psi) is introduced blowing a 'bubble'. Once the stretch rod is fully extended, high-pressure air of up to 40 bar (580 psi) blows the expanded bubble into the shape of the blow mold.

In the Stretch Blow Molding (SBM) process, the plastic is first molded into a "preform" using the Injection Molded Process. These preforms are produced with the necks of the bottles, including threads (the "finish") on one end. These preforms are packaged, and fed later (after cooling) into an EBM blow molding machine. In the SBM process, the preforms are heated (typically using infrared heaters) above their glass transition temperature, then blown using high pressure air into bottles using metal blow molds. Usually the preform is stretched with a core rod as part of the process. The stretching of some polymers, such as PET (PolyEthylene Terepthalate) results in strain hardening of the resin, allowing the bottles to resist deforming under the pressures formed by carbonated beverages, which typically approach 60 psi

http://en.wikipedia.org/wiki/Bottle




STRETCH BLOW MOLDING


VACUM MOLDING

The mould closure is done by a peripheric circuit with a high level of vacuum. The resin flux is obtained with a low pressure injection or manually applying the resin inside the mould before its closure; when the closure follows the vacuum is created inside the mould cavity.The more usually applied resins are the polyesters with low viscosity that might be combined with mineral contents.

Vacuum moulding may be used for the production of parts from medium to large dimensions. Allows the creation of parts with both faces with a smooth appearance. But, this method is generally limited to the manufacture of parts with relatively simple shapes. It is recommended for small and medium volumes. Some examples are the production of trays, belting protectors for rotative machines, electricity distribution boxes and scholar chairs

Vacuum moulding works combined with low pressure injection moulding techniques. The particularity of this process consists in break apart the functions of mould closure and resin flux.


Thermoforming Processes

Figure 18.11 Various thermoforming processes for thermoplastic sheet. These processes arecommonly used in making advertising signs, cookie and candy trays, panels for shower stalls, andpackaging.


Compression Molding

Figure 18.12 Types of compression molding, a process similar to forging: (a) positive, (b) semipositive, and (c) flash. The flash in part (c) has to be trimmed off. (d) Die design for making a compression molded part with undercuts.


Transfer Molding

Figure 18.13 Sequence of operations in transfer molding for thermosetting plastics. This processis particularly suitable for intricate parts with varying wall thickness.


Casting, Potting and Encapsulation

Figure 18.14 Schematic illustration of (a) casting, (b) potting, (c) encapsulation of plastics.


Calendering and Examples of Reinforced Plastics

Figure 18.15 Schematic illustration of calendering. Sheets produced by this process are subsequently used in thermoforming.Figure 18.16 Reinforced- plastic components for a Honda motorcycle. The parts shown are front and rear forks, a rear swingarm, a wheel, and brake disks.

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