Definition: Hot runner: insulated, hot runner mold, where runners remain molten.   Balance: having uniformity in fill,

In starting up a hot runner, with multiple cavities, one is always interested in a balanced approach to fill. In many cases it is assumed that once one has a Hotrunner tool that all cavities shall fill at the same time and rate. This is a wrong assumption due to the nature of the hot runner system and how it was designed. This is not to say it cannot be this way, as there are systems and designs that facilitate this, such as valve gates, Opti-Flo® and possible other. What is to be discussed is a standard hot runner system purchased off the shelf, or designed without a valve gate or Opti-Flo®.

As has been discussed the startup procedure is to heat the manifold and then the drops. Once the manifold and drops are up to temperature and have soaked for proper time than one can start up the mold, process and produce parts. But the question than is how balanced are we? Why is it that there are occasional short shots? Why do they bounce around from the various cavities?

The test:

To see how balanced your system is it is necessary to run short shots. Hopefully the system can stand this without too much trouble. If not than another method or means must be found. Further one needs to be able to collect the parts and note the cavity number/location of said parts.

The other tools necessary are a scale, a complete set of full parts and part weights, and paper or forms to record part weights is necessary.

The procedure is to have in place the part weights for each cavity and also its location. While producing short shots one must collect multiple shots and note cavities, think at minimum 10 shots. Each shot should be collected and kept separate for documentation later. This should be done after all has reach stabilization in the process, or if a new tool possibly after running 30 to 50 shots. The use of tool maps and locations of cavities is great, along with documentation such as digital pictures.

Observations:

Are all parts the same shortness? Is the fill rate on each shot the same, meaning do the same cavities present the same issue each and every time? Is the process showing signs of stabilizations? What one wants to do is observe the operations and results, let the parts run, do not be surprised when the short shot bounces around, or variation occurs.

The work:

What now has to happen is that the weights must be taken for each cavity and documented. This documentation is now used by the formula: [short shot weight/ full part weight] times 100 which equals percent full. Part weight is used because it is more accurate than a visual looking of the parts, and the parts are a volume, this can be done on multi-cavity tools as well as family tools since percent full is being calculated. If part size is such that 10 parts from said cavity need to be used to get an acceptable weight than run more shots.

The results:

Plot out all the parts and their percent full and compare to rest of the parts for that shot, how do they compare, how does the results from all ten shots compare, is it consistent, both in weight percentage and or shot weight totals shot to shot?

The above while simplified shows what is going on and possibly what can be done next. From a balance view the following should always be the start point of any manifold and drop configuration.

Manifold temperature at Melt temperature

Drops at Melt temperature

If there is an unbalanced system, adjust the manifold temperature up or down and note results, as this temperature effect changes the diameter of the main flow channel in the system. This affects the flow pressure drop within the manifold, have the parts becomes closer or further away from being filled at the same rate? One should adjust so as to get as close as one can by this temperature change.

Once the manifold has been adjusted and stabilized than it comes down to each drop within the system. This is the critical point, as it is hoped for that each drop is identical, and probe heights were all set identical. Here is the problem; do we turn the heat up or down? IF a part is short than all assume the material froze off and that we need to be hotter, but in many cases the heat has to be turned down. Why, is this, because that drop opened first, and when pressure built up in that cavity another drop opened and the flow stopped to this cavity and so forth until all the cavities were full save for the first one to open which by this time had frozen off or the flow front became so entangled in the cavity that it does not flow thus resulting in a short shot.

Thus trial and error must be practiced in these cases, as since the author has experienced both, but knowing where the melt temperature is and what is going on is a big help. But by adjusting the manifold temperature first and then using the drop temperature to balance out the mold one is ahead of the game. To be further ahead one would need to use valve gates and or an Opti-Flow® system.

One does need to know what the system design is and placement of the thermocouple as these would tell what is being measured.

Opti-Flo® is a registered trade mark to Incoe® Corporation; http://www.incoe.com/incoe/skin/homepage.aspx

Engineered Performance
 The Opti-Flo® design, incorporated into select INCOE® manifolds, feature Beaumont's patented MeltFlipper® melt rotation technology. Opti-Flo® manifolds are the first scientifically engineered hot runners available on the market. The technology provides homogeneous melt distribution and balanced filling to all cavities while avoiding invasive and restrictive mixers

The Opti-Flo® manifold systems replace the common practice of creating melt temperature variations in the nozzles to balance cavity filling. The result is improved cavity- to-cavity consistency and product quality.

MeltFlipper® is a registered trade mark to   Beaumont Inc.,

 http://www.beaumontinc.com/

SLSILVEY

26062012