Thermosetting Plastic

THERMOSETTING PLASTICS:

          Thermoset, or thermosetting, plastics are synthetic materials that strengthen during being heated, but cannot be successfully remolded or reheated after their initial heat-forming. This is in contrast to thermoplastics, which soften when heated and harden and strengthen after cooling. Thermoplastics can be heated, shaped and cooled as often as necessary without causing a chemical change, while thermosetting plastics will burn when heated after the initial molding. Additionally, thermoplastics tend to be easier to mold than thermosetting plastics, which also take a longer time to produce (due to the time it takes to cure the heated material).
          Thermosetting plastics, however, have a number of advantages. Unlike thermoplastics, they retain their strength and shape even when heated. This makes thermosetting plastics well-suited to the production of permanent components and large, solid shapes. Additionally, these components have excellent strength attributes (although they are brittle), and will not become weaker when the temperature increases.
          Thermoset plastic products are typically produced by heating liquid or powder within a mold, allowing the material to cure into its hardened form. These products can be removed from the mold even without allowing it to cool. The reaction used to produce thermosetting plastic products is not always the result of heating, and is sometimes performed by chemical interaction between specialized materials. Typical types of thermosetting plastics are epoxies, polyesters, silicones and phenolics. Vulcanized rubber is also an excellent example of a thermosetting plastic; anyone who has ever driven an automobile can attest to the properties of a superheated tire—it burns but does not mold into a new shape.
          Each type of thermosetting plastic has a unique set of properties. Epoxies, for example, exhibit elasticity and exceptional chemical resistance, and are relatively easy to cure. Phenolics, while fairly simple to mold, are brittle, strong and hard. Because of their wide range of characteristics, thermosetting plastics find use in an extensive variety of applications, from electrical insulators to car bodies.

Injection Moulding

INJECTION MOULDING:

          This is the most common method of producing parts made of plastic. The process includes the injection or forcing of heated molten plastic into a mold which is in the form of the part to be made. Upon cooling and solidification, the part is ejected and the process continues. The injection molding process is capable of producing an infinite variety ofm part designs containing an equally infinite variety of details such as threads, springs, and hinges, and all in a single molding operation. 

          A plastic is defined as any natural or synthetic polymer that has a high molecular weight. There are two types of plastics, thermoplastics and thermosets. Thermosets will undergo a chemical reaction when heated and once formed cannot be resoftened. The thermoplastics, once cooled, can be ground up and reheated repeatedly. Thus, the thermoplastics are used primarily in injection molding.

There are four major elements that influence the process. They are:
• the molder
• the material
• the injection machine
• the mold

          Of these four, the injection machine and the mold are the most varied and mechanically diverse. Most injection machines have three platens. Newer models use just two platens and may be electrically operated as opposed to the traditional hydraulic models. They can range in size from table top models to some the size of a small house. Most function horizontally, but there are vertical models in use. All injection machines are built around an injection system and a clamping system.
          The injection system mechanism may be of the reciprocating screw type or, less frequently, the two-stage screw type. Also included is a hopper, a heated injection barrel encasing the screw, a hydraulic motor, and an injection cylinder. The system’s function is to heat the thermoplastic to the proper viscosity and inject it into the mold. As the resin enters the injection barrel, it is moved forward by the rotation of the screw. As this movement occurs, the resin is melted by frictional heat and supplementary heating of the barrel encasing the screw. The screw has three distinct zones which further processes the resin prior to actual injection.

          Injection is accomplished through an arrangement of valves and a nozzle, all acted upon by the screw and the hydraulic pump that pushes the resin into the mold. This so-called “packing action” occurs at pressures from 20,000 to 30,000 psi and higher. The temperature of the resin at this time is between 320o and 600o F. The clamping system’s function is to keep the plastic from leaking out or “flashing” at the mold’s parting line. The clamping system consists of a main hydraulic pressure acting on the mold platens and a secondary toggle action to maximize the total clamping pressure.
          The platens are heavy steel blocks that actually hold the mold tightly closed during the injection phase. Most injection machines have three platens. The “stationary” platen has a center hole that receives the injection nozzle and holds the cavity half of the mold. This platen also anchors the machine’s four horizontal tie bars. The “movable” platen holds the core half of the mold. This platen moves back and forth on the tie bars and as the mold opens, the mold’s ejection system of pins and posts expel the finished part. The “rear stationary” platen holds the opposite ends of the tie bars and anchors the whole clamping system. 

          All injection machines have some sort of safety interlock system that prevent access to the molds during the clamping and injection phases when the machine is operating semi-automatically. The operator removes the finished part, closes the door or gate, which sets in motion the next molding cycle. In full automatic operation, finished parts fall into a container, conveyor, or are removed by robot mechanisms.

ADVANTAGES:

1. Injection molding allows for high production output rates.

2. When producing your product you may use inserts within the mold. You may also use fillers for added strength.

3. Close tolerances on small intricate parts is possible with Injection Molding.

4. More than one material may be used at the same time when utilizing co-Injection Molding.

5. There is typically very little post production work required because the parts usually have a very finished look upon ejection.

6. All scrap may be reground to be reused, therefor there is very little waste.

7. Full automation is possible with Injection Molding.

DISADVANTAGES:

1. High set up costs - Moulds etc.

2. Complicated process.

3. can only be used for large quantities due to costs.

Thermoplastic

THERMOPLASTIC:

           A thermoplastic polymer is a type of plastic that changes properties when heated and cooled. Thermoplastics become soft when heat is applied and have a smooth, hard finish when cooled. There are a wide range of available thermoplastic formulas that have been created for many different applications.

          A thermoplastic polymer is made up of long, unlinked polymer molecules, generally with a high molecular weight. Because the molecular chains are unlinked, they rely on other interactions, such as dipole-dipole interactions, aromatic ring stacking, or Van der Waals forces. Thermoplastics generally form a crystalline structure when cooled below a certain temperature, resulting in a smooth surface finish and significant structural strength. Above this temperature, thermoplastics are elastic. As the temperature increases, thermoplastics gradually soften, eventually melting.

          The material properties of a thermoplastic polymer can be adjusted to meet the needs of a specific application by blending the thermoplastic resin with other components. For example, shape memory polymer can be mixed with thermoplastic polymer to create a material that has shape memory characteristics, but retains the basic properties of the thermoplastic. Plasticizers can be added to a thermoplastic polymer to keep the material flexible at lower temperatures. This mixture is often used in plastic automobile body parts to prevent them from cracking during periods of cold temperatures.

        Some of the most commonly found thermoplastic polymers include polyethylene, polypropylene, polyvinyl chloride (PVC), polystyrene, polytetrafluoroethylene (PTFE, commonly known as Teflon), Acrylonitrile butadiene styrene (ABS plastic), and polyamide (commonly known as nylon).

          Because thermoplastics can be melted and reused without any change in material properties, these polymers can be actively recycled. Beverage bottles and household containers with resin identification codes are generally thermoplastic polymers. These containers are ground into chips, melted, refined to remove impurities, and reused as reclaimed material.

Thermit Welding

THERMIT WELDING:

          Thermit welding is an effective, highly mobile, method of joining heavy section steel structures such as rails. Essentially a casting process, the high heat input and metallurgical properties of the Thermit steel make the process ideal for welding high strength, high hardness steels such as those used for modern rails.

          Thermit Welding is a skilled welding process and must not be undertaken by anyone who has not been trained and certificated to use it.
          Detailed operating instructions are provided for each of our processes, but the welding methods all comprise of 6 main elements:

1. A carefully prepared gap must be produced between the two rails, which must then be accurately aligned by means of straightedges to ensure the finished joint is perfectly straight and flat.

2. Pre-formed refractory moulds which are manufactured to accurately fit around the specific rail profile are clamped around the rail gap, and then sealed in position. Equipment for locating the preheating burner and the Thermitt container is then assembled.

3. The weld cavity formed inside the mould is preheated using an oxy fuel gas burner with accurately set gas pressures for a prescribed time. The quality of the finished weld will depend upon the precision of this preheating process.

4. The Thermit® Portion is manufactured to produce steel with metallurgy compatible with the specific type of rail to be welded. On completion of the preheating, the container is fitted to the top of the moulds, the portion is ignited and the subsequent exothermic reaction produces the molten Thermitt Steel. The container incorporates an automatic tapping system enabling the liquid steel - which is at a temperature in excess of 2,500°C - to discharge directly into the weld cavity.

5. The welded joint is allowed to cool for a predetermined time before the excess steel and the mould material is removed from around the top of the rail with the aid of a hydraulic trimming device.

6. When cold the joint is cleaned of all debris, and the rail running surfaces are precision ground the profile. The finished weld must then be inspected before it is passed as ready for service.

 

ADVANTAGES:

1.The heat necessary for welding is obtained  from a chemical reaction and thus no costly power supply is required. Therefore broken parts (rails etc.) can be welded on the site itself.

2.For welding large fractured crankshafts. 

3.For welding broken frames of machines.

4.For building up worn wobblers.

5.For welding sections of castings where size prevents there being cast in one piece.

6.For replacing broken teeth on large gears.

7.Forgings and flame cut sections may be welded  together to make huge parts.

8.For welding new necks to rolling mill rolls and pinions.

9.For welding cables for electrical conductors.

10.For end welding of reinforcing bars to be used in concrete (building) construction. 

LIMITATIONS:

1.Thermit welding is applicable only to ferrous metal parts of heavy sections, i.e., mill housings and heavy rail sections.

2.The process is uneconomical if used to weld cheap metals or light parts.

Loam Moulding

LOAM MOULDING:

          Loam is one type of clay which is made with sand mixed with water to form a thin plastic mixture from which moulds are made. Loam sand also contains ganisters or fire clay. The loam must be sufficiently adhesive so that it can cling to the vertical surface. It always requires special provision to secure adequate ventilation. The object is opened out pores in the otherwise compact, closely knit mass, by artificial means. There are various kinds of organic matter such as chopped straw, and particularly horse manure, is mixed up with the sand, a typical loam sand mixture is given below :

1.	Silica Sand	22  vol.
2.	Clay	5    vol.
3.	Coke	10  percent
4.	Moisture	18-20 vol.

          This applied as plaster to the rough structure of the mould usually made of brick work and the exact shape is given by a rotating sweep around a central spindle. Cast iron plates and bars are used to reinforce the brick work which retains the moulding material. Loam moulds also be prepared by the use of skeleton pattern made of wood. The surfaces of loams are blackened and are dried before being assembled.

          Loam moulds are employed chiefly in the making of large casting for which it would be expensive to use full pattern and ordinary flasks equipment. Objects such as large cylinders, chemical pans, large gears, round bottoms, kettles and other machining parts are produces in the loam moulding.