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Plastic Injection Molding
Plastic injection molding is the primary process formanufacturing plastic parts. Plastic is known to be a very versatile and economical material that is used in many applications.
Although the tooling is expensive, the costper part is very low.Complex geometries are possible and limited only to mold manufacturability. Your computer monitor, mouse and keyboard are injection molded plastics.njection molding involves taking plastic in the form of pellets or granules and heating this material until a melt is obtained. Then the melt is forced into a split-die chamber/mold where it is allowed to 'cool' into the desired shape.
The mold is then opened and the part is ejected, at which timethe cycle is repeated.
Metal forging creates metal parts by pressing a block of material into a mold die at high pressure. Metal forging is the mechanical deformation of metal when heated to a plastic state. However, more recently, metal forging includes similar operations done in the cold or room temperature state as well.
Powder metallurgy uses sintering process for making various parts out of metal powder. The metal powder is compacted by placing in a closed metal cavity (the die) under pressure. This compacted material is placed in an oven and sintered in a controlled atmosphere at high temperatures and the metal powders coalesce and form a solid. A second pressing operation, repressing, can be done prior to sintering to improve the compaction and the material properties.
The properties of this solid are similar to cast or wrought materials of similar composition. Porosity can be adjusted by the amount of compaction. Usually single pressed products have high tensile strength but low elongation.
Powder metallurgy is useful in making parts that have irregular curves, or recesses that are hard to machine. It is suitable for high volume production with very little wastage of material. Secondary machining is virtually eliminated.
Typical parts that can be made with this process include cams, ratchets, sprockets, pawls, sintered bronze and iron bearings (impregnated with oil) and carbide tool tips.
• Part must be so designed to allow for easy ejection from the die. Sidewalls should be perpendicular; hole axes should be parallel to the direction of opening and closing of the die.
• Holes, even complicated profiles, are permissible in the direction of compressing. The minimum hole diameter is 1.5 mm (0.060 in).
• The wall thickness should be compatible with the process typically 1.5 mm (0.060 in) minimum. Length to thickness ratio can be upto 18 maximum-this is to ensure that tooling is robust. However, wall thicknesses do not have to be uniform, unlike other processes, which offers the designer a great amount of flexibility in designing the parts.
• Undercuts are not acceptable, so designs have to be modified to work around this limitation. Threads for screws cannot be made and have to be machined later.
• Drafts are usually not desirable except for recesses formed by a punch making a blind hole. In such a case a 2-degree draft is recommended. Note that the requirement of no draft is more relaxed compared to other forming processes such as casting, molding etc.
• Tolerances are 0.3 % on dimensions. If repressing is done, the tolerances can be as good as 0.1 %. Repressing, however, increases the cost of the product.
CNC starts with a piece of metal, sometimes called a "billet."
That piece of metal might have been cast, forged, or rolled (squeezed between rollers, sort of a limited forging, only capable of making flat things with straight grain like a board).
It is put into a fairly standard machine tool, that has had position sensing and motors on the control knobs installed. This is basically just a robot machinist. You use a rotating cutting tool to cut away all the metal that isn't your crank. 3D metal etch-a-sketch, with the computer interpolating so the circles come out looking pretty smooth.
WHY USE CNC Well its good at making small numbers of compicated shapes. In fact, they are just the thing to make the molds (called tools and dies) to do your forging in. (As a result, CNC technology has in fact lowered the "tooling" costs associated with forging!) It got its biggest boost from the missle folks. If you only plan to build 30 of something, CNC is just the thing for parts with a complicated shape, like that landing gear strut on that fighter.
Investment casting, often called lost wax casting, is regarded as a precision casting process to fabricate near-net-shaped metal parts from almost any alloy. Although its history lies to a great extent in the production of art, the most common use of investment casting in more recent history has been the production of components requiring complex, often thin-wall castings.
The major impact rapid prototyping processes have had on investment casting is their ability to make high-quality patterns without the cost and lead times associated with fabricating injection mold dies. In addition, a pattern can be fabricated directly from a design engineer's computer-aided design (CAD) solid model.
Now it is possible to fabricate a complex pattern in a matter of hours and provide a casting in a matter of days.
Investment casting is usually required for fabricating complex shapes where other manufacturing processes are too costly and time-consuming. Another advantage of rapid prototyping casting is the low cost of producing castings in small lot sizes.