A recent commenter to our blog just e-mailed me this age old question regarding gate sizing…
If a molder is having trouble filling a mold, yet the process is not ‘pressure limited’, would opening the gates solve the problem?
I typically ask for more specifics, but in this case it is more
about the philosophy behind making the change.
Whenever making wholesale changes to the tool steel, you must take a scientific approach.
An example of this approach would be as follows…
1. Verify the melt temperature
2. Verify the mold temperature
3. Check the vents (you can often reduce the tonnage or place a piece of tape on the parting line to test if the venting is adequate)
4. Check the nozzle and hot runner system for any obstructions… or improper sizing. Depending on the application, you can perform a pressure loss study, actually dismantle the components, or try replacing them with different sizes.
5. Perform an in-mold rheology study (the gate may be too small if the mold cannot fill after shear thinning takes place… but, if there is no shear thinning present on the rheology curve… the gate could even be too large!!!) I strongly recommend performing these in-mold rheology curves at both higher and lower melt and mold temperatures as these conditions can influence shear thinning and viscosity.
6. After obtaining all this data, and is the problem does not resolve itself in the process of discovery, you should review the facts with the designers and processors to determine the correct course of action.
As a engineer… I always prefer taking a scientific approach to the resolution to any defect. This prevents rash and costly decisions from being made.