Providing a Buffer To Accommodate for Variation

I recently received this follow-up question regarding an earlier blog entry…
To establish a molding parameter, what is the normal percentage tolerence to be used for the injection pressure and other parameters? Currently my process does not have any tolerence and sometimes this may cause difficulty in troubleshooting which will result short mold and etc.
My Response
In general, a well-established process encounters approximately 10% variation. For this reason, it is critical to ensure you have enough room to adjust your process inputs for this. 
For example, if first stage injection becomes pressure-limited, the machine can no longer maintain the desired injection rate, resulting in an inconsistent fill rate and injection time. This generally leads to unwanted short shots, sinks, and flash on the final part.
To avoid a pressure-limited process, you should always have more pressure available to fill the mold than is actually necessary. This will allow the machine to maintain the ‘injection speed set point’, ensuring the highest possible repeatability.
The problem you may encounter is the fact that many machines actually need an additional buffer to perform properly. For instance, a process may reach a peak pressure of 10,000 psi during first stage fill… yet, if the machine has a maximum setting below 10,500 psi, the process could become pressure limited.
The best way to approach this is to do the following…
1) Establish a good process with significantly more pressure than is necessary.
2) Reduce the maximum injection pressure until it affects the injection time by increasing the 1st stage fill time.
3) Increase this maximum by 10-15% to accomodate for material variablity.
I also recommend you review a few of our related posts… including
dditional Thoughts
If you are running a lot of regrind, or off-spec material, you may want to increase this buffer to as much as 20%. In such a case, it is imperative that you use a short-shot during 1st stage fill.

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