 |
A Feed System Can Impact Part Quality |
|
A feed system exists to provide a route for polymer to get from the injection molding machine to the cavity, but the feed system design can have a tremendous impact on the quality of the part it is feeding.
There are many types of feed systems. The basic feed system designs can be narrowed down to:
· Cold runner with cold sprue
· Hot runner with cold runner
Each of these types of feed systems is selected based on the tool builder's knowledge and experience in addition to the tool's particular design restrictions.
Once the tool or tool builder dictates the required type of feed system, it is absolutely necessary to balance pressure loss in the feed system and feed system volume. Having a feed system that is too small can create tremendous pressure as well as the potential for premature freeze-off. Having a feed system that is too large can result in increased cycle times and loss of control of the process.
To understand the importance of a properly designed feed system we must understand plastics' attributes. Plastic has a molecular structure similar to that of millions of springs which can expand and contract.
When pressure is applied to the plastic, it compresses. When the plastic is heated, it expands.
There is a balance that must be recognized between designing a feed system and processing the plastic.
The following example portrays why it is critical to design the correct feed system for a specific tool.
Tool Description:
· 2-cavity tool with fairly small-volume cavities.
· 2- gate locations which are spaced generously apart.
· The mold utilizes a hot runner with two hot drops that feed cold runners which each feed the part with a tab gate.
· The volume of the feed system is significant in proportion to the volume of the cavities (2:1).
Using the above information, the hot runner channels were designed liberally to reduce pressure loss in the feed system. The cold runners were devised so that they did not control cycle time (smaller size).
Because the volume in a feed system is proportionally larger than the cavities, there is a considerable amount of material in the feed system.
During processing, the processor fills the part to about 90-95% then switches over to 2nd stage pack. At 2nd stage pack the processor programs the machine to reduce the pressure to approximately 40% of the fill pressure in order to complete the packing stage.
Unfortunately, reality is not the same as the programming.
The intention is to have the 2nd stage process immediately stepped down to 40% of the 1st stage fill pressure. However at the point of switchover, due to the large amount of compressed material in the feed system, the material will need to decompress resulting in a lag in the process which creates a decayed pack profile rather than the desired step profile. This causes the material to complete the filling of the cavity under higher pressure and faster velocities than desired, resulting in a loss of control in the process.
Having a loss of control in the process reduces part quality because the actual process does not resemble the original process set up on the machine. The part may exhibit flashing, burning (due to fast gassing) and any number of other problems.
If the feed system was designed to consider the pressure drop AND compressibility (volume), the processor could have maintained more control resulting in better part quality.
Bozilla Corporation utilizes processing experience, feed system and tool design knowledge, along with analytical methods to address these issues before they reach the floor.
If you wish to know more about feed system optimization, give us a call and let's discuss your options.
Contact Bozilla Corporation for your FEA and injection molding troubleshooting needs. We invite you to visit our website at www.BozillaCorporation.com. |
