Manufacturing Processes

These cards explain several manufacturing processes.

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Method of Mixing Resins

  1. Mixing polyester resin to use with fiberglass cloth is similar to mixing epoxy adhesives. By mixing two compounds a chemical reaction occurs where the mixture begins to harden. With resin, a catalyst is used to make the compound to begin to harden. The speed that the mixture sets up and hardens at can be changed by the amount of catalyst used. The more catalyst mixed into the resin the faster the batch will set. A catalyst is a substance that starts or speeds up a chemical reaction without affecting the actual reaction.
  2. Pour an amount of resin into a wide mouthed contained. Make sure the container used has a wide enough top opening to allow easy mixing and access to the resin.
  3. Add catalyst MEKP to resin.
  4. Mix the two products together thoroughly. Work quickly to achieve a well blended product, but be careful not to mix too fast and cause air bubbles to form in the resin.
  5. Prepare small batches of mixed resin and catalyst if you think you will have to use a large amount of product that will take more than a few minutes to apply, as once the resin and catalyst are mixed the hardening process begins immediately.

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Method of Extrusion

The extrusion process can be done with the material hot or cold. The plastic is fed in pellet form into the machines hopper. This machine is known as an Extruder. The material is conveyed continuously forward being softened by both friction and heat. The softened plastic is then forced out through a die and directly into cool water where the product solidifies. From here it is conveyed onwards into the take-off rollers, which actually do the pulling of the softened plastic from the die. The die is a metal plate placed at the end of the extruder with a section cut out of its interior, this cutout, and the speed of the take-off rollers, determines the cross-section of the product being manufactured.

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Method of Die Casting

There are two vertical platens. Platens are the plates on the molding machine upon which the molds are mounted. One platen is fixed and the other moves so the die can be opened and closed. Metal is poured into the shot sleeve and then introduced into the mould cavity using a piston. Once the metal has solidified, the die is opened and the cast is removed. The machine and its dies are very expensive, so pressure die casting is only for high-volume production. First, the mold is sprayed with lubricant and closed, this helps control the temperature of the die and assists in the removal of the casting. Molten metal is then shot into the die under high pressure. Once the die is filled the pressure is maintained until the casting has solidified. The die is then opened and the shot is ejected by the ejector pins. Finally, the scrap must be separated from the casting. This scrap is recycled by remelting it. The high-pressure injection leads to a quick fill of the die, which is required so the entire cavity fills before any part of the casting solidifies. In this way, discontinuities are avoided even if the shape requires difficult-to-fill thin sections. This creates the problem of trapped air, because when the mold is filled quickly there is little time for the air to escape. This problem is minimized by including vents along the parting lines.

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Method of Rotational Moulding

Rotational molding is used to create hollow items. Rotation molding systems have a number of parts including molds, oven, cooling chamber and mold spindles. The molds are used to create the part. The quality and finish of the product is related to the quality of the mold being used. The oven is used to heat the part while also rotating the part to form the part desired. The cooling chamber is where the part is placed until it cools. The spindles are mounted to rotate and provide a uniform coat of plastic inside each mold. Polymer (usually in powder form) is loaded into the mold. The mold is heated in an oven while it rotates, until all the polymer has melted and stuck to the mold wall. The hollow part is rotated through two or more axes, turning at different speeds.. The length of time the mold spends in the oven is critical: too long and the polymer will degrade, reducing impact strength. If the mold spends too little time in the oven, the polymer melt may be incomplete. The polymer grains will not have time to fully melt and coalesce on the mold wall, resulting in large bubbles in the polymer. This has an adverse effect on the mechanical properties of the finished product. The mold is cooled by a fan. The polymer must be cooled so that it solidifies. The part will shrink on cooling, coming away from the mold, and allowing easy removal of the part. The cooling rate must be kept within a certain range. Very rapid cooling (for example, water spray) would result in cooling and shrinking at an uncontrolled rate, producing a warped part.

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Method of Vacuum Forming

Vacuum forming is( used to make simple moulds using thin sheets of thermoplastic. A mould is created from wood or epoxy resin and this is placed on the table (platten) of the vacuum forming machine. The sheet plastic is heated until it becomes soft. The table with your mould on is lifted into position and a vacuum is used to draw the plastic over the mould. Vacuum forming only works with thin plastics and moulds with no undercuts.The plastic can then be trimmed to the required shape.

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