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Cooling Time or Temperature?

You may still have to make a few more adjustments to the cooling time and coolant temperature to get the optimal settings, but this will get you close in a very short period of time.

Question: Which do I work on setting first, cooling time or mold temperature when performing initial tool trials?

My Response: The most efficient approach is to first start with a long cooling time and determine the lowest mold temperature which allows you to mold an acceptable part. Once this is complete, you can determine the lowest acceptable cooling time based on part quality and part ejection.
-Andy

Calculating Screw Speed

The following is a formula which can be used to quickly determine the screw speed during setup. This is based on consuming 80% of the cooling time for shot recovery.

Question: Could you provide me with a quick formula my setup techs can use to estimate the final screw speed?
My Response: This is a quick set of calculations for determining the screw speed to consume 80% of the cooling time:
(RTdesired)= (tcool) x (0.8)

(RPMfinal) = (RPMcurrent) x (RTcurrent) รท (RTdesired)

Symbol Key
RTdesired  = Desired Recovery Time
RTcurrent   = Current Recovery Time
RPMfinal  = Final RPM Setting
RPMcurrent  = Current RPM Setting
Additional Thoughts
If you put this into a spreadsheet, then the technician would only have to enter the Current Screw Speed, Current Recovery Time, and Cooling Time.
(RPMfinal) = (RPMcurrent) x (RTcurrent) / (tcool x 0.8)
-Andy

1st Stage with Multi-Cavity

The intent of the short-shot approach is to help ensure that all cavities are short during 1st stage fill. This ensures none of the cavities are packed during fill and therefore receive the same amount of packing in all cavities during 2nd stage.
Question: How do I determine 90-95% fill for a multi-cavity tools? I’m not looking for theory, just a practical procedure.

My Response: There are many opinions on this matter, but I have had great results following the guidelines below:
  1. Turn off 2nd stage packing and mold a short shot with all cavities short
  2. Incrementally decrease the transfer position and mold parts until the first cavity fills completely
  3. Weigh and record the shot weight of all cavities without the sprue or runners.
  4. Multiply that weight by 0.90 and 0.95 to determine the lower and upper limits of your target weight
  5. Adjust the transfer position until you achieve a 1st stage part weight between those limits
-Andy

Part Sticking and Mold Temperature

The documentation of coolant temperature both entering and leaving the mold is critical to good documentation. This reduces the time required to get to first piece approval during mold start-ups. Troubleshooting will also become much easier, faster, and efficient.

Question: We encountered part sticking after the mold ran a while. In this part, this situation is typically caused by a warm mold. We checked the water lines and ensured there was flow. We then re-configured cooling lines and even tried to put a portion of the mold on chiller water, yet the parts were sticking after about 30 minutes each time. In the end, it turned out the problem was air in the thermolator affecting the coolant flow.
My Response: Documenting coolant temperature entering and leaving the mold would significantly reduce the time it took to troubleshoot the problem. If, at the time of part sticking, the technician would first check the coolant temperature since it is already know part sticking was related to the mold temperature. The tech would immediately notice a difference between the current coolant temperatures and those of the documented standards. This difference should trigger the technician to start troubleshooting the thermolator and cooling line configuration to identify the source of the difference.
-Andy

Blocking Mold Cavities

Blocking off mold cavities will quickly cause you to quickly lose the profit margin on your existing production. Just 1 out of 8 cavities will require the mold to operate 12 percent longer to meet the same production goals.

Question: We run many multi-cavity molds and when there is a complication with a cavity, we just block it rather than fix it. We typically block 20-30% of the mold cavities during production. Is this a bad practice?
My Response: It may be necessary to block a particular cavity if the production demands delay the repair for a short while; but it is difficult to justify the blocking of mold cavities as a standard operating procedure.
When you block 25% of your mold cavities the you must run the mold 33% more cycles to achieve the same amount of production. The main reason a molder uses a high cavitation is to increase productivity due to either customer demands or profit margins. Blocking off cavities generally counteracts the benefits of the high cavitation. This most often results in either a break-even or profit-loss for the production run. In today’s tight market, it is difficult to envision such a company remaining competitive over the long term with such a high margin of loss. In any case, it is critical to institute a repair policy to begin returning the tools to proper operating condition. Without such a plan, your facility will never be prepared for growth as it’s production capacity is hindered by the artificially high production runs.
In the case that a molder must block off a cavity, it is critical that they maintain the same process outputs such as fill time and packing pressure. The only process output adjustments will be a reduction in 1st stage fill weight and final part weight due to the reduce number of cavities. In this case, the molder should schedule the mold to be repaired at the earliest convenience.
-Andy

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