Ce diaporama a bien été signalé.
Nous utilisons votre profil LinkedIn et vos données d’activité pour vous proposer des publicités personnalisées et pertinentes. Vous pouvez changer vos préférences de publicités à tout moment.

Plastics manufacturing processes

2 920 vues

Publié le

Plastics manufacturing processes

Publié dans : Design
  • Soyez le premier à commenter

Plastics manufacturing processes

  1. 1. ASSIGNMENT-3 PLASTICS: •The production of plastics can be roughly divided into four categories: 1. Acquiring the raw material or monomer. 2. Synthesizing the basic polymer. 3. Compounding the polymer into a material that can be used for fabrication. 4. Molding or shaping the plastic into its final form. Raw Materials: Historically, resins derived from vegetable matter were used to produce most plastics. This included such materials as cellulose (from cotton), furfural (from oat hulls), oils (from seeds) and various starch derivatives. Today, most plastics are produced from petrochemicals which are widely available and tend to be cheaper than other raw materials. However, the global supply of oil is exhaustible, so researchers are investigating other sources of raw materials, such as coal gasification. Synthesis of the Polymer The first step in plastic manufacturing is polymerization. The two basic methods by which polymerization can occur are addition and condensation reactions. These can occur in the gaseous, liquid and occasionally solid phase. Sometimes the polymer synthesis can take place at the interface of two immiscible liquids in which the monomers are dissolved. Additives Chemical additives can be used in the production of plastics to achieve certain characteristics. These additives include: ° Anti-oxidants to protect the polymer from degradation by ozone or oxygen ° Ultra-violet stabilizers to protect against weathering ° Plasticizers to increase the polymer’s flexibility ° lubricants to reduce friction problems ° pigments to give the plastic colour ° flame retardant
  2. 2. ASSIGNMENT-3 Plastics are often manufactured as composites. This is achieved by adding reinforcements such as glass or carbon fibers to the plastics, increasing their strength and stability. Plastic foam is a different type of composite which combines plastic and gas. An example of this can be seen in styro foam cups which are made of foamed polystyrene. Shaping and Finishing Compression molding is one of the oldest methods used for converting polymers into useful materials. It uses pressure to force the plastic into a certain shape. One half of a two-piece mold is filled with plastic and then the two halves of the mold are brought together and the plastic is melted under high pressure Methods used for shaping plastics are *EXTRUSION Plastics: Any, especially high density polythene; polystyrene and polyvinyl chloride; all synthetic fibres Tooling cost: Moderate Production volume: High but restricted to minimum order lengths Uses : Anything with a constant cross section: fibres; tubing; pipes; sheets; films; cable sheathing; profiles e.g. curtain rails or window frames
  3. 3. ASSIGNMENT-3 Tolerances: Standard industry tolerances usually provide adequate precision for most applications •Angular tolerances will be ± 1 to 2 degrees •Flatness tolerance across a profile is ± .004 per inch of width •Extrusions will be straight within 0.0125 inch per foot of length •The approximate twist tolerance will be .5 degrees per foot *INJECTION MOLDING Material is introduced into the injection moulding machine via a Hopper. Theinjection moulding machine consists of aheated barrel equipped with a reciprocatingscrew (driven by a hydraulic or electric motor), which feeds the molten polymer into a temperature controlled split mould via a channel system of gates and runners. • The screw melts (plasticizes) (plasticizes) the polymer, polymer,and also acts as a ram during the injection phase. The screw action also provides additional heating by virtue of the shearing action on the polymer. • The polymer is injected into a mould tool that defines the shape of the molded part
  4. 4. ASSIGNMENT-3 Plastics: Commonly all thermoplastics Marks: The plastic enters the mould through what is known as a gate which leaves a 'sprue' which is then broken off but leaves a slightly rough, often circular area; there are sometimes also smooth circular marks left by the ejector pins used to help release the warm moulding from the mould; mould lines are sometimes also visible. Tooling cost: High Production volume: High Uses : Precision technique capable of complicated shapes: e.g. medical components; cheap products produced in very large numbers: Lego; plastic cutlery; machine housings; washing-up bowl
  5. 5. ASSIGNMENT-3 Process comparison: * Typical values shown first (Feasible values in parentheses)

×