Mechinopedia For Mechanical Engineers

Casting, that is, pouring a liquid into a mold without pressure, can be done with several plastics. When there is not sufficient justification for making expensive dies, the casting process is generally used.  Open molds into which the liquid resin , mixed with a catalyst or hardener , is poured can be used for most plastics.  Centrifugal casting , using the same methods can be used with several plastics to such as polyesters.  Shell molding is some what similar to casting in that no pressure is used. In this process, powdered resin is placed in a heated mold until a layer fuses to the desired thickness. The unfused resin inside is taken out of the mold and the hollow molding removed. Sometimes the mold is rotated in what is called rotational molding.  Slush molding is similar to shell molding. It employs a thermoplastic-resin  slurry or "slush" which is poured into a preheated mold. The heat causes slurry to set in a viscous layer of the desired wall thickness. Both shel

Blow molding is a process of placing a softened thermoplastic closed-end tube (Parison) and applying air pressure to inflate it. In operation, the heated Parison is continuously extruded , cut to the proper length and sealed at one end, gripped in the two-piece mold and then air is injected to force the plastic against the walls of the water-cooled mold. The parts cools and then the mold is opened and the part is removed.  Production methods range from simple manual operations to relatively complicated automatic ones. Blow molded products include ducts and similar articles. 

Injection molding is the most widely used method of producing parts of  thermoplastic and more recently, thermosetting resins as well. The process resembles the hot chamber die casting of metals : the die, split to allow removal of the solidified product, is kept shut with an appropriate press force and ejectors are provided for removing the molded component. The difference between metals and plastics lies in the supply of the polymer, which is usually fed in a solid form, pellets or powder, through a hopper to a injector screw, a die-end of which is surrounded with heaters that gradually brings the polymer to the required temperature. There the material is softened. The process starts with feeding plastic pellets into the hopper above the heating cylinder of the machine. The resin falls into and is pushed along the heated tube by reciprocating screw (feeder) until a sufficient volume of melted plastic is available at the injection nozzle end. This may take from 10 sec to 5 or 6 min

  Thermoforming sometimes called vacuum forming, is the shaping of hot sheets or strips of thermoplastic materials into a desired shape either by mechanical or pneumatic methods. The sheets of  plastic used in the thermoforming process are produced by either extrusion , calendaring or pressing . Thickness of the sheets that are processed range from 0.125 to 3.2 mm or even greater.  The basic process is quite simple. A piece of plastic sheet is clamed into a frame. The plastic is heated, usually with electric heaters, until it begins to sag. Vacuum , air or mechanical pressure is then applied through small holes in the mold and the plastic is rapidly pulled tightly against the mold creating close profile conformity. The frame is raised, the part is removed and then trimmed in a punch press. The mold may have several shapes, for the same part or for different parts.  In vacuum forming, the sheet is clamped in a stationary frame and the heated sheet is vacuum-drawn into the mold.   A

 A important method of making film and sheet is known as calendaring . In this process the plastic compound (composed of resin, filler, plasticizer and color pigment) is passed between a series of heated rollers. It comes out from the rolls  squeezed into film or sheet. Thickness is controlled by a combination of squeezing and altering the speed of  the finishing rolls. The finished product is cooled by passing through water-cooled rolls. The finished product is cooled by passing through water-cooled rolls. Vinyl floor tile, cellulose acetate sheeting and films are some of the applications. 

Transfer molding, also called extrusion or gate molding , is the process of  forming articles in a closed mold, where the fluid plastic material is conveyed into the mold cavity under pressure from outside of the mold.  The material, often a preheated preform, is placed in a heated transfer pot. As soon as the material is sufficiently softened, the plunger forces the almost fluid plastic through the orifice (sprue) into the closed mold where final cure takes place. Pressures used are 50 to 100% higher than those used for compression molding ; thus better details and higher strengths are possible .  Extrusion Molding :  Extrusion means the continuous flow of material through a die. Extrusion molding is used mostly for production of sheet, tube bar and typical applications in thermoplastic materials. Granulated powder made of such materials as vinyl resins, cellulose derivatives, nylon, polyethylene and polypropylene make up the materials used in the process.  Plastic extrusion machine

Compression molding is essentially a forging process, performed in a heated die that forms a premeasured quality of the polymer. The process is most widely used for the forming of the thermosetting plastics.  The basic procedure for compression molding , consists of  placing a measured amount of powder or a compressed perform (called the charge), into the open mold cavity, closing the mold and then applying heat and pressure through a downward moving die (called the force, plug or core) to the material until it softens and is forced fill the mold cavity. In the closed mold, a chemical reaction or polymerization , that cross-links the polymer chains takes place and the material hardens into the required shape. Heat for polymerization or curing is supplied through the walls of the cavity by steam or electricity.  Molding pressures (hydraulic) may be a low as 0.35 kgf / mm2 (3.5 Mpa) for polyester and epoxy but most thermosets require 1.4 to 4.2 kgf/mm2 (13.8 to 41.4 Mpa) , depending l

Most plastic resins have to be combined, compounded or otherwise chemically treated with processing materials before they are ready for processing. Thorough mixing of ingredients is vital for all classes of  polymers and special equipment has been developed for the purpose. One of the following additions are usually employed.  1. Plasticizers :   Organic Solvents, resin and even water are used as plasticizers . These substances act as  internal lubricants improving flow of  and giving toughness and flexibility to the material. Plasticizers  are also used to prevent crystallization by keeping the chains separated from one another. For example vinyl are generally hard and brittle materials. By adding a plasticizers they can be made soft and flexible. 2. Fillers : Typical fillers which include wood flour, asbestos fiber, glass fiber, cloth fiber, mica, slate powders, may be added in high proportions to many plastics essentially to improve strength, dimensional stability and heat resistan

Thermosetting plastics are made from chains which have been linked together, referred to as cross-linked. These have a three dimensional network of molecules and will not soften when heated. They are practically insoluble, fireproof and usually hard and brittle. These plastics cannot be  reused. EPOXY RESINS : EPOXY RESIN is one of the new comers to the plastic field. It is cured or cross-linked by the addition of a hardener. Epoxies have excellent chemical resistance and electrical insulating properties. Their working temperature is from 150*C to 260*C with fillers additives. Coatings made from these resins combine the properties of  toughness; flexibility, adhesion and chemical resistance to a degree not found in other coating materials. Epoxy adhesives are now finding applications in aircraft, automobiles and in the home. While epoxy adhesive are well known, these plastics are also used as casting for pipe fittings , electrical and other equipment. These are also used in both low-

This is the general name of a large group of plastics usually polyvinyl-chloride (PVC) is the most frequently used of this group. This is a very clear, transparent plastics, easily colored, resistant to most chemicals and very water repellant. It can stand outdoor exposure and is quite abrasion resistant, but it has low tensile strength.  PVC can be extruded as wire insulation, chemical tubing and refrigerator door gaskets. Coating of fabrics of  all kinds for industrial uses (such as tents and tarpaulin-type cover) is a major use of PVC.  Other Thermoplastic types  >>>  click here

This is one of the newer plastics perhaps better known by its trade name of  Derlin . I has been developed as a material for mechanical parts including sprinkler nozzles , handles , gears, housings etc., At present this is widely used for stereo tape catridges, toys , furniture casters and cigarette lighter cases. It has good tensile strength , resistance to temperature (115*C) , low friction characteristics, resistance to most solvent and low moisture absorption.  Other Thermoplastic types  >>>  click here

 The full name of  Acrylic resin is polymethyl methacrylate and it is better known as Acrylic Plexiglass or Lucite. This Beautifully clear, easily shapes plastic is most widely used in sheet form for signs of all kinds. Rods and tubes are cast in glass or metal cylinders or extruded. Acrylics can also be injection molded.  Acrylics will "pipe" light and are used in fiber optics and edge - lighted dials. It is tough but easily scratched. It is now widely used for outdoor signs, contact lenses, brush backs, transparent bowls, drink dispenses, etc.  Other Thermoplastic types  >>>  click here

This is a new plastic material that is probably better known by its trade name Lexan, Merlon and Polycarbafil. It is easy to handle for molding , extrusion and machining . It can be nailed and riveted without cracking.  Polycarbonates , due to their toughness, make excellent safety glass for street lamps, windows and machine guards. They keep their toughness and strength at temperature from - 138 to 120 *C. In thickness over 1.25 mm, they donot support combustion. Polycarbonates are crystal clear or can be prepared to any color. They are used for housings for shavers , power tools, household equipment and blow - molded bottles.  Other Thermoplastic types  >>>  click here

Polystyrene  This is number two in the total volume of resins used. It is a crystal clear, odorless , tasteless plastic with a high gloss. Most grades are quite brittle and unless modified will hold static electricity. Polystyrene has an excellent tensile strength, but it can be used only up to 66 to 90*c. It is easily produced in any form, by most processes, and easily can be joined by the cementing. It is used for bottles, low cost picnic utensils, model knits, signs, toys, etc. in a wide choice of colors. Expanded foam in slabs or beads is a leading use of polystyrene. Polypropylene  Polypropylene is an excellent insulator and is stiffer than polyethylene . It can be molded or extruded into sheet, film or pipe. It is used for automobile accelerator pedals, luggage and hospital equipment. They have unlimited colorability.  Other Thermoplastic types  >>>  click here

Polyethylene leads all plastics in the volume of  resin used each year. They are produced in three types : low, intermediate and high density . All three types have in common the properties of    toughness , resistivity to solvents, alkalies, excellent dielectric properties, good colorability , very low moisture absorption and relatively low cost . The polyethylenes are blow and injection - moulded , extruded into very wide range of products which includes house wares such as bowls , plates, dishpans, paint-brush handles, flexible tubing, bags for packaging vegetables , hardware , squeeze bottles etc., Other Thermoplastic types  >>>  click here

 Nylons is now a generic name, polyamide resins are also sold as Nylatron, Nylafil, Plaskon, and Zytel . Nylon was originally introduced in the market in the form of fibers and fabrics. In recent years , nylon has been successfully molded, extruded, formed into sheet and film and cast to prepare bearings , gears, drawer slides, machine slides and roller besides the many varieties of filaments used in clothing , rope, brush bristles, etc., Its outstanding features are low coefficient of  friction and its resistance to heat, abrasion, and chemicals. It is strong, tough and light weight. It is not recommended for exposure to ultraviolet light, hot water or alcohols, Nylon absorbs water up to 2 % at high humidity , with some expansion resulting.  Other Thermoplastic types  >>>  click here

 The family of  cellulosic comprises a wide variety of materials such as cellulose acetate , cellulose nitrate , cellulose acetate butyrate , cellulose propionate , etc. The cellulosic are prepared from various treatments of wood fibers and cotton.  The cellulosic have good strength, toughness, transparency, surface gloss, chemical resistance and mouldabilty from various grades. cellulose acetate is the most easily worked of all plastics. It can also be made in solid form for molded parts such as toys , cutlery handles , electrical parts , knobs , packaging , etc . In the sheet from cellulose acetate has become the household term " cellophane ". It is also made as an electrical insulating tape. Cellulose nitrate , well known in the trade name of " celluloid " is available in  a wide variety of beautiful colors. Its is still very familiar in spectacle frames, fabric coatings. etc., Other Thermoplastic types >>> click here

Thermoplastics are linear polymers, the molecules of which are synthesized in the shape of  long threads. They undergo no chemical change in the molding operation and is often with the application of heat and harden upon cooling. They can be reshaped while in the softened state and they will reharden ; thus the scrap can be used again. They may become too soft to use at temperatures from 66 to 315*c. Since they become increasingly softer with increase temperature, certain numbers of  the thermoplastic family are liable to permanent distortion under mechanical strain at relatively low temperature. The may flow to an appreciable extent under load at room temperature. They may flow to an appreciable extent under load at room temperature. Most of the plastics in use today are Thermoplastics. 1. Cellulosic  2. Nylons. 3. Polyethylene resins. 4. Polystyrene & Polypropylene. 5. Polycarbonate.  6. Acrylic resin. 7. Acetal ( Poly acetal ). 8. Vinyl plastics.

 The drawbacks of powder metallurgy , which render its use difficult and restricted, ought to be mentioned   alongside its advantages. the drawbacks are : 1. Pure metal powders are very expensive to produce. 2. The size of parts produced its limited because of  the large presses needed to obtain the compressing pressures resulting in increased tool and press costs. 3. The lack of simple methods of  obtaining alloy powders - of steels , bronzes , brasses , etc., 4. The strength properties of  the finished articles are usually lower than those of similar article produced by conventional means.  5. Articles made out of metal powders possess, as a result of their porosity and increased tendency to oxidation throughout the whole body and only on the surface of the article.  6. Powder metallurgy products possess comparatively poor plastic properties (impact strength, elongation). 7. Die design limitations limit the type of shapes that can be produced. 8. Since the dies are very expensive,

 The following are the advantages of powder metallurgy , 1. Although the first cost of metal powder is high , there is no loss of material. The components can be produced clean and bright, ready for use. 2. Perhaps the greatest advantage possessed by the process is the case in which the composition of the  product can be controlled. Besides, the material does not run the risk of contamination with any  unwanted substance. 3. Close dimensional tolerances can be maintained especially if a sizing operation is used. 4. Non metallic substances can be introduced as required and in any proportion to get the  desired  properties. 5. A wide range of properties such as density, porosity and particles size can be obtained for particular  applications. 6. It is possible to unite materials that cannot be alloyed in the usual sense or would not  yield the  desired characteristics if they were joined mechanically. 7. The process is also the advantage in the production of magnetic cores having specia