Ceramic Material Manufacturing Methods
All ceramics start as a mixture of powdered base material (Zirconia, etc.), binders and stabilizers. This mixture is “formed” into shapes and then fired (sintered) at high temperature to create hard, dense materials. Forming is done using standard processes such as pressing, extruding, injection molding, tape casting or slip casting.
Ceramics can also be machined prior to being fired using standard machine tools in a process known as “green machining.” Green machining is inexpensive because unfired material is soft. However, firing causes ceramics to lose 20% to 40% of their volume; therefore, green machining followed by firing is suitable only for those applications with loose tolerances (~1% of characteristic lengths). In contrast, tight tolerance parts must be machined using high speed, diamond tools after ceramics are fired.
Some of the better known ceramic manufacturing processes combine sintering with forming.
Ceramics are consolidated into dense material by exposing them to 1800°C – 2000°C for days or weeks at a time, depending on the ceramic and process details. The addition of the thermal energy promotes strong bonds between the raw ceramic particles, leading to densification. Green machined, near net shapes or raw stock material can be sintered. Knowledgeable ceramics manufacturers are very adept at accounting for volumetric shrinkage.
Hot pressing combines the forming and firing steps to produce relatively simple geometric shapes. The ceramic powder is simultaneously subjected to sintering temperatures and uniaxial pressure. Simple shapes are generated by placing the raw material in a high temperature die while under load.
Hot Isostatic Pressing (HIP)
Hot isostatic pressing is a uniform pressure assisted method of sintering ceramics into simple and complex shapes. The pressure, usually applied via an inert gas like Argon to prevent reactions, significantly reduces porosity and therefore improves physical properties. Often times, the pressurization process is preceded by evacuating all air to reduce moisture and impurities. In order for the hot isostatic press process to work, the green ceramic must be placed in a gas tight container. An alternative method is to pre-sinter the ceramic to remove porosity at the surfaces. In this way, the ceramic material itself acts as the pressure vessel. Hot isostatic pressing differs from isostatic pressing in that the former applies uniform pressure to the ceramic during sintering.
Chemical Vapor Deposition (CVD)
Chemical vapor deposition is the process of converting gases (called precursors) into solids by continuously depositing monolayers of material onto a heated substrate. This is a thermodynamically driven process, so control of substrate temperature and chamber pressure is critical. Certain ceramic materials, such as Silicon Carbide and Silicon Nitride, can be manufactured using chemical vapor deposition techniques. Shapes are formed using sacrificial targets premachined into the desired shape of the part. Although the resulting material is much more expensive than its conventionally made counterparts, the cost is warranted by applications requiring superior physical properties.
Reaction bonding uses a chemical reaction to bind ceramic powders into a solid form. After forming, the binder is burned off to create a porous preform, and then capillary pressure is used to infiltrate liquefied reactants (different reactants for different ceramics) into the preform at temperatures just above the ceramic melting point. The resulting reaction creates the solid ceramic form. For example, liquefied Si is used in reaction bonded Silicon Carbide. The main disadvantage of reaction bonded ceramics is that it leads to relatively high porosity.
plastic and rubber
Methods of Processing Plastic
There are a variety of methods used to process plastic. Each method has its advantages and disadvantages and are better suited for specific applications. These methods include: injection molding, blow molding, thermoforming, transfer molding, reaction injection molding, compression molding, and extrusion.
The main method used for processing plastic is injection molding. With this process, the plastic is placed into a hopper. The hopper then feeds the plastic into a heated injection unit, where it is pushed through a long chamber with a reciprocating screw. Here, it is softened to a fluid state.
A nozzle is located at the end of the chamber. The fluid plastic is forced through the nozzle into a cold, closed mold. The halves of the mold are held shut with a system of clamps. When the plastic is cooled and solidified, the halves open and the finished product is ejected from the press.
Thermosetting materials usually are not processed with injection molding because they will soften, they harden to an infusible state. If they are processed with injection molding, they need to be moved through the heating chamber quickly so they do not set.
Blow molding is used when the plastic item to be created needs to be hollow. A molten tube is created with blow molding by using compressed air, which blows up the tube and forces it to conform to the chilled mold. Variations of blow molding include injection, injection-stretch, and extrusion blow molding.
With injection blow molding uses a perform, which is taken to a blow mold and filled with compressed air. As a result, it conforms to the interior design of the blow mold. With injection-stretch blow molding, a the plastic is stretched prior to being formed. Otherwise, it is essentially the same as the injection process.
With continuous-extrusion, a molten plastic tube is continuously created. At the appropriate times, the tube is pinched between two mold halves. Then, a needle or a blow pin is inserted into the tube and blows compressed air up the part in order to force it to conform to the mold interior. With accumulator-extrusion, the molten plastic material is gathered in the chamber before it is forced through a die in order to form a tube.
Thermoforming uses a plastic sheet, which is formed with the mold by applying air or through mechanical assistance. The air pressure used can be nearly zero psi, or several hundred psi. At 14 psi, which is equivalent to atmospheric pressure, the pressure is created by evacuating the space between the mold and the sheet. This is known as vacuum forming.
Transfer molding is generally used only for forming thermosetting plastics. It is similar to compression molding because the plastic is cured into an infusible state through pressure and heat. Unlike compression molding, however, transfer molding involves heating the plastic to a point of plasticity prior to being placed into the mold. The mold is then forced closed with a hydraulically operated plunger.
Transfer molding was initially developed as a method for molding intricate products, such as those with many metal inserts or with small, deep holes. This is because compression molding sometimes disturbed the position of the metal inserts and the holes of these types of products. With transfer molding, on the other hand, the liquefied plastic easily flows around the metal parts without causing them to change position.
Reaction Injection Molding
Reaction injection molding, or RIM, is one of the newer processes used in the plastics industry. It differs from liquid casting in that the liquid components are mixed together in a chamber at a lower temperature of only about 75 to 140 degrees Fahrenheit before it is injected into a closed mold. Here, an exothermic reaction occurs. As a result, RIM requires less energy than other injection molding systems. Reinforced RIM, or R-RIM, involves adding materials such as milled or chopped glass fiber in the mixture in order to increase the stiffness.
Compression molding is the most common process used with thermosetting materials and is usually not used for thermoplastics. With this process, the material is squeezed into its desired shape with the help of pressure and heat. Plastic molding powder and other materials are added to the mix in order to create special qualities or to strengthen the final product. When the mold is closed and heated, the material goes through a chemical change that causes it to harden into its desired shape. The amount temperature, amount of pressure, and length of time utilized during the process depends on the desired outcome.
The process of extrusion is usually used to make products such as film, continuous sheeting, tubes, profile shapes, rods, coat wire, filaments, cords, and cables. As with injection molding, dry plastic material is placed into a hopper and fed into a long heating chamber. At the end of the chamber, however, the material is forced out of a small opening or a die in the shape of the desired finished product. As the plastic exits the die, it is placed on a conveyor belt where it is allowed to cool. Blowers are sometimes used to aid in this process, or the product may be immersed in water to help it cool.
|Thailand, Malaysia and Indonesia are the largest producers of natural rubber in the world. Figures from the World Trade organisation posted on www.thailand.com indicate the following worldwide natural rubber production in 1998.
Natural rubber comes from the Havea brasiliensis tree, which grows in tropical regions. They typically reach 20-30 metres in height on rubber plantations, and are able to produce commercial quantities of latex at about 7 years of age, depending on climate and location. Economical life span of a rubber tree is between 10 to 20 years, but may extend past 25 years in the hands of a skilled tapper and bark consumption.
It should be noted that latex is different to tree sap.
Dry Rubber Production
Tapping Rubber Trees
Havea trees are not tapped any more often than once per day, with 2 or 3 days being the norm. In countries such as Thailand, tapping usually takes place in the early hours of the morning, prior to dawn due to the high day time temperatures and the protective clothing worn to protect against snakes etc. Also flow rates are increased due to higher turgor pressures at these times.
A tapper uses a sharp hook shaped knife to shave a thin layer of fresh bark from the tree. This exposes the latex vesicles. The cut is typically done at 25-30° to the horizontal, as this exposes the maximum number of vesicles. The same incision is re-opened the next time (typically the next day) by shaving off a small amount of bark. Virgin bark is exposed first working around in panels. The same area may be exploited again after about 7 years.
Figure 1. Tapping a rubber tree using angular, semi-spiral incisions.
The thickness of the layer is important as too thick a slice will damage the tree and reduce its productivity and life, while too thin a slice will not produce sufficient latex. Bark is removed in a localised area for a period of time, and then a new area is tapped allowing the tree to repair itself.
The latex runs down and is collected in a cup. Each tree usually produces about half a cup of latex per day and is collected later in the day. Latex will flow for approximately 1 to 3 hours after which time the vesicles become plugged with coagulum.
Processing of Natural Rubber
Processing of natural rubber involves the addition of a dilute acid such as formic acid. The coagulated rubber is then rolled to remove excess water.
Figure 2. Rolling the latex into thin sheets.
Then a final rolling is performed using a textured roller and the resultant rubber sheet is dried. Following this, the rubber is ready for export of further processing. This type of natural rubber accounts for about 90% of natural rubber production.
Figure 3. Final rolling of the latex sheets using a textured roller.
Figure 4. The dried sheet of latex.
Natural Rubber Production
Natural rubber is used in a pure form in some applications. In this case, the latex tapped from trees is concentrated using centriguges, removing water and proteinaceous materials. It is then preserved using a chemical such as ammonia.
Applications of Natural Rubber
|The natural rubber is used for making products such as:
• Some medical tubing
• Elastic thread
- explain methods of processing ceramics
- explain processing of natural rubber
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