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What is the Molding Pressure?

By Kathy Heydasch
Updated May 17, 2024
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Molding pressure typically refers to a measurement of force within an injection molding machine. In injection molding, plastic resins are melted and pushed into a steel or aluminum mold, fabricating small plastic parts. The mold pressure determines with what force the melted resin is forced through the machine and injected into the awaiting mold.

Solid plastic objects can be fabricated using a process called injection molding. An injection molding machine begins with a large hopper into which small plastic pellets, called resin, are loaded. The pellets are then heated to a certain temperature and melted. Inside the machine, molding pressure determines the speed at which material is passed by a large, rotating screw or ram and injected into the mold. A mold is typically a steel or aluminum block which is hollowed out in the reverse shape of the item to be molded. Then when the melted 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 enough to fill the mold completely, preventing what are known as short shots, or plastic parts that are not formed all the way. If this happens, the molding pressure should be increased. Once the mold is full, the pressure is reduced and stays level while the parts cool. Once the plastic cools, the mold is opened and the part is ejected from the machine.

The manufacturers of resin can assist machine operators with the complex task of determining optimum pressure for the plastic in the machine, but practically speaking, exact pressure is tested through trial and error. Molding pressure affects the velocity — or flow rate — which is another variable, along with temperature and time required to cool the plastic. Once the variables for optimum performance are established, an injection molding machine can be set to these levels for future runs, with minor adjustments necessary from time to time throughout the life of the machine and the 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 the least amount of energy will be expended.

In the US, pressure is measured in pounds per square inch (psi). In many other countries, pressure is measured in bars. A bar is equal to 100 kilopascals, or approximately the measurement of the pressure of the atmosphere at sea level.

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