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The injection molding process occurs cyclically. Typical cycle times range from 10 to 100 seconds and are controlled by the cooling time of the thermoplastic or the currying time of the thermosetting plastic. The plastic resin in the form of pellets or powder is fed from the hopper and melted. In a reciprocating screw type injection molding machine, the screw rotates forward and fills the mold with melt, holds the melt under high pressure, and adds more melt to compensate for the contraction due to cooling and solidification of the polymer. This is called the hold time. Eventually the gate freezes, isolating the mold from the injection unit, the melt cools and solidifies. Next the screw begins to rotate and more melt is generated for the next shot. In the soak time the screw is stationary and the polymer melts by heat conduction from the barrel to the polymer. The solidified part is then ejected and the mold closes for the next shot.
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Step #1 – The uncured rubber is fed into the machine in the form of a continuous strip. |
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Step #2 – The uncured rubber is worked and warmed by
an auger screw in a temperature controlled barrel.
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Step #3 – As the rubber stock accumulates in the front of the screw, the screw is forced backwards. When the screw has moved back a specified amount, the machine is ready to make a shot. |
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Step #4 – With the mold held closed under hydraulic pressure, the screw is pushed forward. This forces the rubber into the mold, similar to the action of a hypodermic syringe. |
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Step #5 – While the rubber cures in the heated mold, the screw turns again to refill. |
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Step #6 – The mold opens and the part can be removed. The machine is ready to make the next shot, as soon as the mold closes. |
Raw Materials
Most raw materials can be used. The resin is in pellets before processing.
- Acrylonitrile-Butadiene-Styrene ABS
- Nylon PA
- Polycarbonate PC
- Polypropylene PP
- Polystyrene GPPS
Tooling
Because of the high pressures involved in this process, steel tooling is preferable. Aluminum may be used for low volumes and prototyping.
Cost
Injection molding offers the lowest piece prices available, but tooling prices are generally the highest.
Advantages
- High production rates
- Design flexibility
- Repeatability within tolerances
- Can process a wide range of materials
- Relatively low labor
- Little to no finishing of parts
- Minimum scrap losses
Disadvantages
- High initial equipment investment
- High startup and running costs possible
- Part must be designed for effective molding
- Accurate cost prediction for molding job is difficult
Applications
Containers, household goods, auto components, electronic parts, flower pots.
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