Molding pressure is the force used to inject molten plastic into a mold in injection molding. The pressure must be sufficient to fill the mold and avoid short shots. The pressure affects flow rate, temperature, and cooling time. The pressure is regulated by a gauge and can be programmed.
Molding pressure typically refers to a measurement of force within an injection molding machine. In injection molding, plastic resins are melted and pressed into a steel or aluminum mold, making small plastic parts. Mold pressure determines how hard the molten resin is forced through the machine and injected into the waiting mold.
Solid plastic items can be manufactured using a process called injection molding. An injection molding machine begins with a large hopper into which small pellets of plastic, called resin, are loaded. The pellets are then heated to a certain temperature and melted. Inside the machine, molding pressure determines how quickly material is passed by a large rotating screw or ram and injected into the mold. A die is typically a block of steel or aluminum that is hollowed into the reverse shape of the item to be molded. Then, when the molten plastic is injected into the mold and subsequently cooled, it has conformed to the shape and created the part.
The amount of molding pressure should be sufficient to completely fill the mold, avoiding so-called short shots or plastic parts that have not fully formed. If this happens, you need to increase the molding pressure. Once the mold is filled, the pressure is released and remains level as the parts cool. Once the plastic has cooled, the mold is opened and the part is ejected from the machine.
Resin producers can assist machine operators in the complex task of determining the optimum pressure for the plastic in the machine, but in practice, the exact pressure is tested by trial and error. Molding pressure affects speed, or flow rate, which is another variable, along with the temperature and time required to cool the plastic. Once the variables for optimum performance have been established, an injection molding machine can be set to these levels for future runs, with minor adjustments required from time to time throughout the life of the machine and mold.
The molding pressure on an injection molding machine is regulated by a pressure gauge and can be programmed to a certain pressure rate. The least amount of pressure needed to fill the mold should be used. This means that the least amount of energy will be consumed.
In the United States, pressure is measured in pounds per square inch (psi). In many other countries, pressure is measured in bars. One bar is equal to 100 kilopascals, which is roughly the measure of the pressure of the atmosphere at sea level.
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