Creating a molding window on the floor is a fairly standard procedure practiced time and time again using basic parameters.
But what if you could use FEA to further optimize that window?
We are going to show some of the differences between creating a molding window on the floor vs. creating a molding window using FEA.
Creating a Molding Window on the Floor
When an operator sets up a process for a specific part on the floor, the process is typically created using the following parameters:
Establish
· A reasonable 1st stage fill time
· Best time to switchover to 2nd stage
· A reasonable pressure to pack during 2nd stage
· Length of time to pack
· Length of time to hold (cool)
But how is each of the above mentioned parameters set on the floor (without FEA)?
Here are some typical methods:
A reasonable 1st stage fill time: The operator might typically fill the part quickly (as quick as possible) without any visual effects of burning or venting issues. This parameter may sometimes be specified by the material supplier.
Best time to switchover to 2nd stage: After increasing the shot size little by little, the operator visually sees how full the part is and estimates a time to switch over to 2nd stage. This time will vary from operator to operator.
A reasonable pressure to pack during 2nd stage: This is a big unknown and will vary tremendously from operator to operator. The pack pressure is usually kept as low as possible without any visual evidence of voids or sink marks. This parameter may sometimes be specified by the material supplier.
Length of time to pack: More times than not, the operator will weigh the part until the part weight levels off. This would typically determine when the pack time should end.
Length of time to hold (cool): The part is usually cooled until it can be ejected without any visual defects from either ejector pin deformation or warpage.
The only floor method that uses reasonable data is the length of time to pack which uses part weight measurements.
Creating Molding Window using FEA
We will now look at how FEA will determine a process window on those same parameters utilizing analytical data.
1st stage fill time: FEA data can take the fill time and provide shear rates through the gate orifices coupled with flow front temperatures. A fill time that is too short may cause too much shear in the gate or cause the flow front temperature to rise too much. Conversely, a fill time that is too long may cause the flow front to drop and cool too much. Acceptable shear rates and temperature changes are dictated by the material manufacturer.
Time to switchover to 2nd stage: An analysis can show exactly what percentage of the part is filled at specific times. In order to prevent flashing (switching over too late) or premature switchover (hesitation in flow front), the analyst would run several iterations identifying which point would be the best to prevent both scenarios from happening.
Pack Pressure during 2nd stage: The analysis can provide data in the form of shear stress during pack. When packing, the material must be packed into the cavity in a proportion relative to the rate the cavity is cooling. Typically, the pack pressure should be profiled to decrease over the length of time that it takes for the part to cool. The shear stress data will show how much the material is being stressed as the part is being packed while cooling during second stage. The pack pressure must be low enough to prevent excessive stress. The material manufacturer specifies the maximum stress allowed by a specific material.
Length of time to pack: Analytically, the pack will be held until the gate freezes off. This can be determined by utilizing the material manufacturer's no-flow or freeze temperature.
Length of time to hold (cool): This is determined by using the material manufacturers stated ejection temperature. The part is cooled until it is entirely below the ejection temperature.
The difference between the methods used on the floor and FEA differ drastically.
A molding window created by an operator has very few pieces of hard data to set up a molding window. Oftentimes, a molding window set by an operator will be extremely small due to missing data. A molding window which is too small can lead to loss in part quality.
Because FEA utilizes both analytical and empirical data, it is extremely accurate and effective when creating a molding window. Most times, FEA creates a molding window is so large that it allows the operator to fine tune the process to their liking yet still remain within the window.
In conclusion, an analyst using FEA has the ability to create a definitive molding window using hard data. When compared to the method used on the floor, it stands to reason that FEA is the best way to create a window based on analytical data in conjunction with the material supplier specifications.
A molding window created with FEA will typically produce parts of higher quality, higher dimensional stability along with shorter cycle times.
Contact Bozilla Corporation for your FEA and injection molding troubleshooting needs and please visit our website at www.BozillaCorporation.com.
