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.
As the name suggests, this process starts by melting plastic, which then is injected into a mold and allowed to cool in order to produce a part. Although thermosetting polymers also are used, thermoplastic polymers — which include acrylic, nylon and polycarbonate among more than 80,000 other polymers — are the most common type of material used for this process. Thermoplastics are substances that become plastic when heat is applied to them, harden upon cooling and can be subjected to the heating-molding-cooling cycle repeatedly. This material characteristic makes thermoplastics versatile and recyclable. By contrast, thermosets such as epoxy resin and silicone are materials that will set permanently upon being heated.
This permanent change is caused by a chemical reaction that links the polymers into a three-dimensional structure, making thermosets well suited for high temperature applications. Although thermosets are commonly stronger than thermoplastics, they are not recyclable and generally are more brittle than thermoplastics.
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.
During the molding cycle, there are several points where the amount of time must be set precisely to achieve the desired result for a molded part. The residence time — the amount of time the molten plastic remains in the barrel of the injection machine — is one such area.
Residence time must be set such that the material inside the barrel is not heated too long nor removed from the barrel too soon. Setting this time properly will ensure that the injection machine will yield desirable results. If the residence time is insufficient, the plastic will not reach the desired temperature and may harden prematurely upon entering the mold cavity, leaving sinking spots where plastic did not completely fill the mold cavity. If the residence time is too long, the plastic may burn inside the barrel before it reaches the mold.
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.
This gradient allows the plastic to gradually heat, ensuring a smooth transition from solid to liquid and minimizing wear on the reciprocating screw. The actual temperature maintained by the heating bands depends on the material being used and the size and shape of the part being manufactured. A common range typically lies between 300 and 800°F (149 and 426°C).
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.
Inadequate or excessive temperatures can leave flaws in the finished plastic product and also can damage the plastic-injection machine itself. As described below, different types of flaws and issues will result depending on which part of the process is maintained at an incorrect temperature. For all following scenarios, it is assumed that the pressure utilized is appropriate for the given injection process.
Temperatures that are too high in the rear zone of the barrel can lead to bridging between the hopper and the barrel throat, in which a cap of plastic solidifies where the material enters the barrel. Additionally, keeping the temperature of the barrel too high can result in burning of the plastic before it reaches the check valve. This burnt plastic will manifest itself as small black pieces in the finished product. Excessive temperatures in the barrel also could lead to degradation of the plastic polymer, which will produce structurally unsound parts. These problems can be solved by purging the burnt or solidified plastic from the machine and then maintaining a suitable temperature profile along the barrel.
During the injection process, temperatures that are too low can lead to flow marks or wavy lines that mar the surface of the manufactured part. Temperatures that are too high can lead to stringing, in which a remnant from the previous shot will be transferred into a subsequent shot.
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.
Properly identifying and controlling the temperatures at each point throughout a plastic injection-molding machine will ensure that the process runs smoothly and the machine’s longevity is not compromised. Knowing the different flaws that can occur in the finished product — and the temperature control errors that create those flaws — will allow for quick troubleshooting and correction of unexpected results, yielding pristine parts while increasing efficiency.