The FEP Injection Molding Process Step by Step Guide

FEP, or fluorinated ethylene propylene, is a melt-processable fluororesin that is highly resistant to extreme temperatures and stress cracking. It is often used in industrial settings due to its unique properties and versatility. One of the most common ways to process FEP is through injection molding, a process where molten material is injected into an injection mold to form the desired shape.

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The principle of injection molding is that the resin is heated and sheared in the barrel of the injection molding machine to melt into a flowable melt, and then the melt is forced to flow into the closed mold cavity under the thrust of the plunger or screw to form the product. After cooling into a solid, the mold is opened to remove the product. The key to the molding process is heat transfer and melt flow under pressure, which are related to the following factors: resin viscosity, thermal stability, heat conduction, crystallinity and moisture content. The above factors vary with different resin varieties, and are also related to different grades and quality index control of the same variety of resin.

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The barrel, spiral or reciprocating screw, nozzle and mold of the injection molding machine used for FEP injection molding, which are in contact with high-temperature melt, must be made of corrosion-resistant alloys.

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The structure of the screw is divided into a feeding section, a filtering section and a metering section according to different functions.

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1. Melt preparation period: As the screw rotates, the resin material is heated and melted, and the melt is pushed along the spiral blade to the end of the screw. The nozzle is opened and closed with the help of a mechanical valve, and the melt begins to accumulate. The accumulated melt applies back pressure to the rotating screw until enough melt is accumulated to prepare the next batch of melt. At this time, the screw stops rotating.

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2. Melt injection period: The nozzle is opened, and the screw acts as a plunger and moves forward axially. At this time, no rotation occurs. This thrust injects the melt accumulated at the end of the screw into the mold through the nozzle. A check valve is installed at the end of the screw to prevent it from flowing back into the screw.

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3. Mold filling heat exchange holding period: After the mold is filled with melt, the screw pressure must be maintained for a short period of time to compensate for the volume shrinkage caused by cooling of the melt in the mold.

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4. Cooling and unloading period: After the holding period ends, the mold remains in a closed state until the injection part cools to the unloading temperature, and then the next operation cycle begins. The screw rotates again and the preparation period begins.

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