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Optimization of Holding Time for Hot Runners and Valve Gates

For cold runner molds, the pack and hold times are optimized by conducting a gate freeze study (or gate seal study) where the part weight is recorded as a function of the pack and hold times. When the gate freezes, the part weight remains constant with increasing pack and hold times. A second or so is added to the lowest value of time where the part weight stays constant and this number is taken as the total time for the setting of the pack and hold times. However, in hot runner systems or in valve gated systems the gate area always has molten plastic and therefore the above method does not produce acceptable results since the part weight never plateaus off. I have developed a procedure that has produced acceptable results and I am describing it below. To understand the concept, some background is necessary.

The Cosmetic Process Window: The ‘Injection Molding Handbook’ written by Donald Rosato in the mid-1970s is considered as one of the bibles for Injection Molding. A very important concept called ‘Molding Area Diagram’ or MAD was introduced in this book. See Figure 1 that is a scanned copy of the diagram from the handbook. It shows the area on the Ram Pressure vs Mold Temperture graph where acceptable parts can be molded. I believe that this was one of the most important concepts that were introduced but unfortunately over the years has been forgotten.

Ref: Injection Molding Handbook by Donald Rosato

The Cosmetic Process Window:
I have taken Mr. Rosato's concept further and developed some related concepts. (Thank You Mr. Rosato). The first one is called the Cosmetic Process Window or CPW. See the figure below. Inside the CPW one can mold parts that are cosmetically acceptable. Dimensions are not considered. Bigger the CPW, the more robust is the final molding process (when dimensions are considered). For procedures on how to develop this window please request it here.

                                  The Cosmetic Process Window.

As I always say (it is common sense too), ‘If you can’t mold parts that are cosmetically acceptable, why try to even bother about molding them dimensionally? Cosmetic Process Window first!!’ If you have a small CPW, forget about process capability -- depending on the tolerances of course!  The CPW typically establishes the pack & hold pressures and melt or mold temperatures. The rest of the process variables are established using the other techniques of Scientific Molding or 6-Step Study.

The Dimensional Process Window:  Once the 6-Step Study is done and the limits of the processes are determined, a DOE is done to correlate the dimensions to the process variables. (Read about DOEs here.) Contour plots now show the acceptable area within the CPW where the dimensions are also acceptable. Drawing a window within these contour plots will give you the Dimensional Process Window or DPW. See the figure below. Naturally, the bigger this window, the more robust the process will be for the given dimension and cavity. For multiple cavities and dimensions, the contours must be overlaid and the final window must be determined.


  The Dimensional Process Window

Procedure to optimize pack and hold times the hot runner and valve gated systems:
Now that you have the idea of the CPW and the DPW, using this concept for valve gated and hot runner molds is very simple!

1. Mold parts at various hold times and pressures and draw a ‘Pass – Fail’ graph as shown in the figure below.


 Figure: The Cosmetic Points for the CPW

2. Find the possible CPWs and now perform a DOE with these factors with the highs and lows you get form the graph.

   
Figure: The Possible Cosmetic Windows

Example of a VIT for a transparent cover. A polorizer was used to demo the difference in the parts more clearly. A visual inspection is also done.

Figure: Visual Inspection template for a transparent cover.

3. The DOE will then show you the DPW and give you an idea of how robust the molding process will be.

 Figure: The Dimensional Process Window

 It work great, give it a shot! Some day I will put up some pictures of some validations I have done.

Regards,
Suhas Kulkarni

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