Disadvantages of extrusion molding
- Very limited design geometry.
- Tolerances are relatively large due to die swell.
- Very limited range of products can be produced such as seals and hoses.
Advantages
- Mold cost is low.
- Cycle times are short.
- Part cost is low due to simple manufacturing process.
Examples of extrusion molded products
- Gaskets
- Hoses
- Straws
PVC pipe
Extrusion molding is somewhat similar to the injection molding process. The key difference is that extrusion molding produces a long continuous shape rather than a single unit with complex geometry.
- The process starts off by feeding plastic resin in the form of pellets into a hopper. In addition to resin any required pigments or additives are also fed into the hopper.
- The hopper feeds a long heated cylinder called the extruder via a feed throat. A large spinning screw within the cylinder feeds the pellets along the barrel.
- While the pellets pass through the heated barrel the melting temperature is reached and resin becomes molten. At the end of the barrel there is a screen and breaker plate. The purpose of the screen is remove any contaminants from the molten resin. The breaker plate changes way in which the resin flows in order to optimize the direction in which it approaches the die.
- The final step of the process involves the molten resin flowing through the die. It is critical that the die is designed to not only have the required profile to create the final form of the plastic. But to also facilitate even flow of the resin. If the flow is uneven, unwanted stresses within the plastic which can cause warpage during the cooling phase.
- The extruded product is now cooled by pulling it through a water bath. This is an important point of the process as the plastic form can warp if cooled too quickly.
- Some production lines may include a secondary operation such as spooling the finished product or cutting it to required lengths for final use.
Learn more about rotational molding.