Hot Runner continued
Drops

Having discussed the hot runner systems the last few times, let us now discuss some startup procedures.

Most system suppliers have set procedures for the startup of their manifold systems and drops. Some have the subject on videotape so your operators can watch and learn. The point or key is to let the entire system come up to temperature, in a prescribed order prior to processing the first shot.

Thus the manifold should be started first allowing it to heat for the prescribed period; dependent on size this may be 1/2 hour to 1 hour. Remember that once the temperature is reach on the controller you still need to soak it for a set length of time, again this is based on manufactures recommendations. At the point that we know the manifold is up to temperature the drops should be turned on and allowed to come up to temperature and soak for their time period.

Some will turn the entire system on all at once, and when the system shows the set temperature, begin to process. How many have had to tear down a system because it leaked? The manifold must expand to its position first, than the heats can be applied to the drops. This is after we have the manifold temperature up, so that when the drops expand they will apply the pressure to the manifold thus providing the seal we desire. As stated earlier we use the slow start on the controller to dry out our heating elements whichever type they may be.

What temperature to start at? The melt temperature of our plastic is the start-point for both the manifold and drops. After taking the first few shots than adjustments can be made starting with the manifold temperature first to get the material to flow. If need be, the raising of the drop temperature would than take place and raising all of them at the same rate. Remember that knowing the location of thermocouples and heater placement now gives the operator a better idea of what has to be done, along with whose system is being used. The balancing of the fill of the tool should first be tried though the adjustment of the manifold temperature and lastly through the adjustment of individual drops. Again, the assumptions we make are that the probes are all of identical design and placement. Please understand the manifold layout and how the system is designed, this is key.

Some points to consider for troubleshooting:

1. If one cavity is always short and temperature has been increased to no effect, drop the temperature.      In most cases, the first to start filling is the last to fill. Thus decreasing the temperature now allows it to fill at a later point.

2. Fill pressure has to be high to blow through the gate area. May want to decrease the probe temperature as the length has started to shut down the gate opening.

3. Always get stringing at the gate. Look to the diameter of the gate and placement of the probe in the gate. How is the cooling around the gate area?

4. All the probe temperatures are vastly different. If all probe heights are set the same, look to the manifold temperature. Then if needed look at the layout of the manifold and how was it to be balanced for the application.

Thanks for the time.
 
 

Steven L Silvey
Sr. Technical Service
General Polymers Division
Ashland Chemical