Thermoplastics And Injection Molding

Thermoplastics and Injection Molding

Thermoplastics are materials that soften or melt when heated and harden when cooled. Thermoplastic polymers consist of long polymer molecules that are not linked to each other. i.e., have no cross-links.Most thermoplastics are high molecular weight polymers whose chains associate through weak van der Waals forces (polyethylene); stronger dipole-dipole interactions and hydrogen bonding (nylon); or even stacking of aromatic rings (polystyrene). Thermoplastic polymers differ from thermosetting polymers (Bakelite; vulcanized rubber) which once formed and cured, can never be remelted and remolded. Many thermoplastic materials are addition polymers; e.g., vinyl chain-growth polymers such as polyethylene, polyvinyl chloride and polypropylene, and others like polystyrene, polyester, acrylics, nylons and cellulosics

Thermoplastics can be melted and re-molded, and thus they can be recycled. This also means that they can be farbricated by processes that use plastic flow. Injection Molding is one such process.

History of Thermoplastics

* 1868 - John Wesley Hyatt injection molds Celluloid billiard balls.

* 1926 - Vinyl or PVC - Walter Semon invented a plasticized PVC.

* 1936 - Acrylic or Polymethyl Methacrylate

* 1937 - Polyurethanes tradenamed Igamid for plastics materials and Perlon for fibers. - Otto Bayer and co-workers discovered and patented the chemistry of polyurethanes

* 1938 - Polystyrene made practical

* 1939 - Nylon and Neoprene considered a replacement for silk and a synthetic rubber respectively Wallace Hume Carothers

* 1942 - Low Density Polyethylene

* 1942 - Unsaturated Polyester also called PET patented by John Rex Whinfield and James Tennant Dickson

* 1946 - James Hendry builds first screw injection molding machine.

* 1951 - High-density polyethylene or HDPE tradenamed Marlex - Paul Hogan and Robert Banks

* 1951 - Polypropylene or PP - Paul Hogan and Robert Banks

* 1970 - Thermoplastic Polyester this includes trademarked Dacron, Mylar, Melinex, Teijin, and Tetoron

* 1978 - Linear Low Density Polyethylene

* 1982 - The JARVIK-7 plastic heart keeps Barney Clark alive.

* 1985 - Liquid Crystal Polymers

* 1985 - Japanese firm introduces all-electric molding machine.

Injection Molding

Injection molding of polymers is a cyclic process of forming plastic into a desired shape. The thermoplastic material is heated to increase flow properties, forced under pressure into a cavity where the material is cooled, and ejected in its final form.

A great advantage of injection molding is that it is an automated process, which can create complex shapes with very short cycle times. Most parts are complete requiring no secondary operation. Injection molding is used to make parts that have intricate features, undercuts, tabs, holes, webbing etc. Injection molding produces components and products which have consistently accurate dimensions and a high quality finish. A wide range of complex forms can be produced which could otherwise only be manufactured in expensive machining processes. Production is fast and the process produces very little waste.

The Machine

The injection unit: Typically, plastic pellets are loaded into a 'feed hopper', a large open bottomed container, that feeds into the injection unit. In very simple terms, the purpose of the injection unit is to melt the polymer, so that the material flows well, and then inject this material at high pressure into the mold.

The injection unit is a heated barrel that contains a rotating, reciprocating plunger, which injects the plastic through a heated cylinder into a mold. Screw injection machine replaces this by the auger design of the screw creates a mixing action when new material is being readied for injection. The plunger or the screw is inside the heating cylinder and, when activated, mixes the plastic well, creating a homogenized blend of material. This is especially useful when colors are being molded or when regrind is being mixed with virgin material. After mixing, the screw stops turning and the entire screw pushes forward, acting like a plunger for injecting material into a mold.. The rotating screw compacts, melts, homogenizes and pumps the molten material to the injection side of the screw. The screw then reciprocates toward the mold injecting the material into the mold under high pressure.

The mold:

The mold is a very complex unit in which the molten plastic material enters through a sprue and is conveyed through the runners to the mold cavities. A mold is fabricated with at least two halves. The molten material enters the cavity through a gate, which seals when the material cools. Once the part is cooled it is ejected with ejector pins that are also part of the mold. The product generally will have little round circles on the back side where the ejector pins pushed it out of the mold.

Molds are designed for specific parts and can have multiple cavities that generate multiple parts per injection cycle. Molds can be removed and replaced from injection molding machines so that many different parts can be made on one machine. Mold design is a very important step in creating the desired molded part. The mold is housed in the clamp unit that opens and closes the mold.

The mold is made by a mold maker (or toolmaker) from metal, usually either steel or aluminum, and precision-machined to form the features of the desired part. Considerable thought is put into the design of molded parts and their moulds, to ensure that the parts will not be trapped in the mould, that the moulds can be completely filled before the molten resin solidifies, to compensate for material shrinkage, and to minimize imperfections in the parts, which can occur due to peculiarities of the process. The cavities are often made using rough machining to remove the major part of the material and then a process called called EDM (Electrical Discharge Machining).

The clamp unit:

The clamp unit opens and