It is important to avoid issues during an application based upon temperature control. This article will help explain some important tips.
In industrial processes, controlled temperatures are crucial to obtaining the desired end result. Plastic injection molding is a superb example of the importance of maintaining suitable process temperatures. One of today’s most common methods of manufacturing, plastic injection molding is used to make millions of parts and products each year, from small and precise wristwatch gears to large plastic dumpsters. For this reason, it has become an integral part of manufacturing in a range of markets from medical to industrial to automotive.
Maintaining Proper Conditions
Three important process conditions must be correctly mediated to obtain a structurally sound and aesthetically acceptable part from a plastic injection machine: pressure, time and temperature. Plastic injection-molding machines require immense amounts of pressure to properly fill molds. If not enough pressure is utilized, the molded part will not be completely filled, but too much pressure will also have ill effects. Air bubbles, charred plastic and flash induced by blowing out the mold all can be attributed to excessive pressure.
Temperature affects every step of the injection-molding process, and it must be properly managed. Installing a temperature controller on the plastic injection-molding machine is one way to ensure the process is being maintained at the correct temperature. When plastic is introduced into the barrel of the machine, it is heated and mixed. The temperature profile must be set up carefully to ensure that the plastic is thoroughly melted and mixed but not allowed to burn. As shown in figure 1, heating bands are wrapped around the barrel. For a proper temperature profile, the bands closest to the hopper have the lowest temperature. This temperature should increase along the barrel toward the check valve with the highest temperature immediately before the mold. (For more information about Figure 1’s heating bands check out, https://www.powerblanket.com/solutions/process-temperature-control/).
The temperature maintained in the mold is equally as important as the temperature profile in the barrel. The mold is maintained at a lower temperature than the barrel to allow the plastic to cool. This temperature typically is between 150 and 350°F (65 and 17°C) and is controlled by running water or oil lines through the mold or by using electric cartridge heaters. Water cooling lines are used for the lower temperature cooling applications while heated oil lines are used when the mold must be maintained at a temperature above the boiling point of water. Monitoring fluid temperature in the cooling lines with immersion probes is one way to ensure that the lines are maintained at the optimal temperature. Electric cartridge heaters are a clean replacement for heated oil lines, but they can introduce an uneven heat profile around the mold if not thoughtfully placed.
The temperature of the mold also can have a variety of effects on the final product manufactured. A mold temperature that is maintained below the ideal point could result in knit lines, an area behind a protruding part in the mold where the melted plastic comes together again but does not merge into a uniform front. A too cool mold also could result in sinking or a partial product, where the plastic solidifies before it completely fills the mold cavity. A mold that is too hot could result in warping of the part; the trapping of gas around the gate, causing circular streaks; or blistering of the plastic, resulting in raised sections on the surface.
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Speak with an AccuTherm representative today at http://accutherm.com/ to discuss how temperature control will benefit your industry!