Welcome!

Although this seems strange, it is not uncommon. Since changing the injection rate causes a difference in the amount of material entering the mold during fill, this might occur when molding a part where the highest pressure losses occur near the end of fill. You might see a drop in pressure if the part thickness increases, and a rise in pressure when the thickness decreases at the end of fill.

In most cases, you will see a smooth in-mold rheology curve.

-Andy

This can be done using any calculator, the two basic equations are as follows:

(ºC x 1.8) + 32 = ºF

(ºF – 32) / (1.8) = ºC

Therefor:

(250ºC x 1.8) + 32 = 482ºF

Adversely:

(482ºF – 32) / 1.8 = 250

One way to remember this relationship is 1 degree Celsius equals 1.8 degrees Fahrenheit plus 32 (since water freezes at 32ºF rather than 0ºC).

-Andy

Today, I moved a mold from one machine to another. The peak injection pressure was 1450 bar with a 0.6 sec. cycle time. On the second machine, I can only get 1.03 sec. injection time at 900 bar. I cannot fill the mold even though the maximum available pressure is 1800 bar.

It is likely that your new machine lacks the injection speed in cubic millimeters per second to fill the mold.

The best way to approach this is to first determine the ‘Shot Volume Factor’  for your two machines (see Understanding Shot Volume Factors… for more about this). This factor converts the movement of the screw into the volume it displaces (this is expressed in 

mm^3/s).

How long does process stabilization take, from set-up to first good parts?

With the explosion of SMED, Single Minute Exchange of Dies, there are many companies who have successfully reduced their changeover time to a matter of minutes or even seconds. Unfortunately, the startup time is not always as fast.

Generally speaking, most processes take around 5-10 minutes to stabilize. This typically occurs when the flow length and the ability to fill the mold is not significantly affected by the temperature of specific mold components. You can still shorten this time to stabilization by taking steps like the following:

• Pre-conditioning the tool temperature before installing it into the machine
• Pre-heat the hot runner system before installing it into the machine
• Use insulation between the mold and the machine platens
• Purge regularly when the machine is idle to help stabilize the melt temperature

High speed molding, and thin wall molding processes where you are processing with a much tighter window, stabilization can take up to 2 hours. When such a situation takes place, it is important to document the variations that take place and develop a time schedule to adjust for this. For example, if the cores take an hour to stabilize, then you might be have to use one 2nd stage packing pressure and 1st stage speed for the first 15 minutes, and then drop them down each 15 minutes until the process stabilizes.

Always document the process using the process outputs. This will help you better determine when the process achieves stabilization.

For more on this, I recommend: Process Inputs vs. Process Outputs

-Andy

Although we have training courses dedicated to this topic, I will try to give a brief overview here: