31-05-2012, 04:35 PM
Injection molding
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Applications
Injection molding is used to create many things such as wire spools, packaging, bottle caps, automotive dashboards, pocket combs, some musical instruments (and parts of them), one-piece chairs and small tables, storage containers, mechanical parts (including gears), and most other plastic products available today. Injection molding is the most common method of part manufacturing. It is ideal for producing high volumes of the same object.[5] Some advantages of injection molding are high production rates, repeatable high tolerances, the ability to use a wide range of materials, low labor cost, minimal scrap losses, and little need to finish parts after molding. Some disadvantages of this process are expensive equipment investment, potentially high running costs, and the need to design moldable parts.
Examples of polymers best suited for the process
Most polymers, sometimes referred to as resins, may be used, including all thermoplastics, some thermosets, and some elastomers.[6] In 1995 there were approximately 18,000 different materials available for injection molding and that number was increasing at an average rate of 750 per year. The available materials are alloys or blends of previously developed materials meaning that product designers can choose from a vast selection of materials, one that has exactly the right properties. Materials are chosen based on the strength and function required for the final part, but also each material has different parameters for molding that must be taken into account.[7] Common polymers like epoxyand phenolic are examples of thermosetting plastics while nylon, polyethylene, and polystyrene are thermoplastic.[8] Until comparatively recently, plastic springs were not possible, but advances in polymer properties make them quite practical. Among such applications are buckles for anchoring and disconnecting outdoor-equipment webbing.
Equipment
Paper clip mold opened in molding machine; the nozzle is visible at right
Main article: Injection molding machine
Injection molding machines consist of a material hopper, an injection ram or screw-type plunger, and a heating unit.[2] They are also known as presses, they hold the molds in which the components are shaped. Presses are rated by tonnage, which expresses the amount of clamping force that the machine can exert. This force keeps the mold closed during the injection process. Tonnage can vary from less than 5 tons to 6000 tons, with the higher figures used in comparatively few manufacturing operations. The total clamp force needed is determined by the projected area of the part being molded. This projected area is multiplied by a clamp force of from 2 to 8 tons for each square inch of the projected areas. As a rule of thumb, 4 or 5 tons/in2 can be used for most products. If the plastic material is very stiff, it will require more injection pressure to fill the mold, thus more clamp tonnage to hold the mold closed.[9] The required force can also be determined by the material used and the size of the part, larger parts require higher clamping force.[10]
Mold
Mold or die are the common terms used to describe the tooling used to produce plastic parts in molding.
Since molds have been expensive to manufacture, they were usually only used in mass production where thousands of parts were being produced. Typical molds are constructed from hardened steel, pre-hardened steel, aluminum, and/or beryllium-copper alloy. The choice of material to build a mold from is primarily one of economics; in general, steel molds cost more to construct, but their longer lifespan will offset the higher initial cost over a higher number of parts made before wearing out. Pre-hardened steel molds are less wear-resistant and are used for lower volume requirements or larger components. The typical steel hardness is 38–45 on the Rockwell-C scale. Hardened steel molds are heat treated after machining. These are by far the superior in terms of wear resistance and lifespan. Typical hardness ranges between 50 and 60 Rockwell-C (HRC). Aluminum molds can cost substantially less, and, when designed and machined with modern computerized equipment, can be economical for molding tens or even hundreds of thousands of parts. Beryllium copper is used in areas of the mold that require fast heat removal or areas that see the most shear heat generated.[11] The molds can be manufactured either by CNC machining or by using electrical discharge machining processes.
Injection molding die with side pulls
"A" side of die for 25% glass-filled acetal with 2 side pulls.
Close up of removable insert in "A" side.
"B" side of die with side pull actuators.
Insert removed from die.
Mold design
Standard two plates tooling – core and cavity are inserts in a mold base – "family mold" of five different parts
The mold consists of two primary components, the injection mold (A plate) and the ejector mold (B plate). Plastic resin enters the mold through a sprue in the injection mold; the sprue bushing is to seal tightly against the nozzle of the injection barrel of the molding machine and to allow molten plastic to flow from the barrel into the mold, also known as the cavity.[12] The sprue bushing directs the molten plastic to the cavity images through channels that are machined into the faces of the A and B plates. These channels allow plastic to run along them, so they are referred to as runners.[13] The molten plastic flows through the runner and enters one or more specialized gatesand into the cavity[14] geometry to form the desired part.