The Injection Pressure that is required to push the molten plastic into an injection mold is similar to the force required to push play-doh through a toy extruder. There are many variables that affect the force required to inject molten plastic into a mold.
As the flow advances, the molten plastic begins to cool. The rate of cooling depends on the temperature of the melt as well as the temperature of the mold (depending on the cooling channels as well). As the plastic cools it begins to solidify onto the walls of the sprue, runners, gates, and cavities. The solidification of the plastic narrows the flow path. A higher pressure is required to push the plastic through the narrowed flow paths. The longer the plastic is in contact with the walls, the more solidification occurs. Eventually the thinner sections will “freeze off” and stop the flow. Thinner parts “freeze off” faster than thicker walled parts since less plastic is required to solidify. For this reason, thinner parts can be ejected earlier than thicker parts reducing the overall part cycle time.
Different plastics have different thermal properties and solidify at different rates. Different plastics also have different flow properties and flow at different rates under the same thermal conditions. These resin characteristics also affect the pressure needed to fill the cavity.
Longer flow paths take more time to fill. Since they take more time to fill, the plastic has more time to solidify. Therefore, longer flow paths require more injection pressure to fill. The addition of gates can effectively reduce the flow path length and therefore reduce the required injection pressure. All of these factors make it impossible to predict the pressure that will be required during the molding process without the use of mold flow analysis.
A typical molding machine is capable of generating 20,000 psi of injection pressure. This injection pressure is what drives the molten plastic from the screw through the sprue, runners, gates and then into the cavities that form the parts. When the injection pressure is exceeded by the pressure required by the part and runner system, the part will not fill completely. The list below shows the things that can be done to fix a mold that is exceeding the machine pressure capabilities. Note that all of the options cost time and money to implement!
- Increasing the melt temperature increases the cooling time and part cost.
- Increasing the mold temperature increases the cooling time and part cost.
- Increasing the runner diameter increases scrap rate and plastic mold cost.
- Increasing the part thickness increases material, cooling time, and injection mold cost.
The use of mold flow analysis prior to building the plastic mold will eliminate the need to make these costly changes after the mold is built.