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Injection molding

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    Injection molding is a process used to form products from plastic. The process

    requires a mold, clamping component, injection unit, and some sort of plastic. As time has

    advanced so has injection molding by developing new techniques and new products to aid

    in the manufacturing of the injection molded parts.

    Injection molding was used as early as the 1860’s. It can be used to form many

    different products. Whether the products are small, large, complex, or simple they can be

    produced. Injection molding has derived from metal die casting. However, the polymer

    can’t just be poured into a mold, it has to be forced into the mold cavity. The polymer is

    forced into the mold and pressure is held on it to avoid shrinkage in the mold cavity as it

    cools. Injection molding is capable of producing a large number of parts with very high

    precision. All thermoplastics except polytetraflouroethylene (PTFE), polyamides, and

    some aromatic polyesters can be used by the injection molding machine. Some

    thermosetting plastics can also be used.

    The typical fabrication process can be done by one of two different types of

    injection molding equipment. Either a plunger, or reciprocating screw type machine can

    be used. The process starts by melting the polymer resin. Once the resin is melted, a mold

    is placed in the clamping unit. The clamping unit is to hold the mold together.

    The plunger or reciprocating screw then force the polymer resin into the mold. In the

    plunger operated machine, the plunger is hydraulically operated. This forces the plastic

    through a heated area, where it is then spread into a thin layer by the torpedo. Then the

    melt comes to the nozzle and is injected into the mold. The reciprocating screw rotates,

    this moves the polymer resin forward for injection. As the screw rotates it acts to melt,

    mix, and pump the polymer to prepare it for injection. The reciprocating screw machine is

    the most widely used of the two machines. Once the polymer resin is injected into the

    mold cavity, the mold is allowed to cool. The mold has a gate, which limits back flow and

    directs the flow of the melt into the mold cavity. Once the mold has cooled and the

    polymer has solidified the mold can be removed and the part can be ejected. When the

    gate freezes, the screw begins to rotate again and the part is ejected. This completes the

    cycle time. Cycle times range due to the amount of time the polymer needs to cure or

    solidify. This is called the hold time.

    Some advantages of injection molding are high production rates, design flexibility,

    low tolerances, can process wide range of materials, low labor, little or no finishing, and

    scrap is held to a minimum. However, some disadvantages are high startup and running

    costs, part must be designed for effective molding, accurate cost prediction is difficult, and

    The high tooling costs come from the molds being built to a high level of precision.

    The molds are usually constructed of hardened tool steel, and aluminum or other soft

    metals when tooling life is not an issue. Tooling costs can range from $5,000 to

    $100,000. However, there are some parts that can not be formed by any other method of

    processing except injection molding. These parts typically become feasible around 1,000

    pieces. To go with the high tooling costs there are a large number of variables that go

    along with it. Injection molding machines may require special plant services that other

    As technology advances so must the industry to keep up production. One way

    injection molding is keeping up is by becoming automated. Usually, operators are placing

    parts into molds, and then taking the parts out. Now, robotic devices are being used to

    place inserts before molding and remove parts after molding as well as a host of other

    operations as well. Not only does the robotics speed up the process, but makes it much

    Another way industry is trying to keep up with technology is by using computer

    software. The software is called “Mold Adviser,” which is a mold design and analysis

    package that can be used to help speed up operations while reducing tooling costs. Using

    the past standard operation of designing molds a company could easily waste six to twelve

    weeks and anywhere from $30,000 to $40,000 on fixing a mold that has a problem with

    filling correctly. The new software will detect these problems up front before production

    begins. It is used with a CAD program, and is really divided into two parts. The Part

    The Part Advisor can test the parts design and analyze the part to see if it will fill

    correctly. Once the part has been analyzed, users begin the Mold Advisor. The Mold

    Advisor will help setup gates and runner systems, predict plastic flow, shot size, and cycle

    time. Users of the new software have reported that the software is easily learned and user

    Injection molded products include TV, VCR, CD’s, CD players and computer

    housings. So, one can see the impact that the products of injection molding have on our

    every day life. Injection molding has been used since the 1860’s. Products of all sizes and

    shape have been produced. However, with advancing technology and the high cost of

    molds it will be interesting to see how much further injection molding will advance and

    Works Cited:

    Harris, Laura. “Injection Molding.” 30 Nov 1999.

    “Injection Molding.” 30 Nov 1999.

    Mapleston, Peter. “New approaches show promise in large-part molding.”
    Modern Plastics Nov 1999.

    Snyder, Merle R. “Automation is gaining in vertical infection molding machine
    market.” Modern Plastics Aug 1996.

    Taylor, Gerry. “One-two punch knocks out injection-molding problems.
    Machine Design 17 June 1999: 80.

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