(The first part was posted in Vol1)

The gate is central to filling a part. As previously stated it regulates the flow of material into the cavity. Further it can be the shear point for the material if in fact it is the smallest area for volume/time.

The other aspect of the gate is pressure regulation during packing. Now in filling the part in a scientific method or other the concentration has been on placing 90 to 99% of the material into the cavity prior to switching over to pack, and or hold dependant on your machine. What is critical in some parts though is how we pack them, or get them to size. In most cases it is the packing of the cavity that results in the final size, dimensions. The importance of packing pressure, gate freeze time and pressure drop through the gate, is critical though in most cases never done.

In some cases though, over-packing at the gate can occur resulting in a product which fails in the field or use. The relationship between having the gate use its filling function but while at the same time freeze or transfer pressure upon pack is in itself a balancing act, especially given the complexity of today's parts and allowable gating positions for cosmetics.

What is meant for example is that in the manufacture of buckets out of HDPE with a gate in the bottom of the bucket/pail that if it was to be over packed it is likely that the bucket/pails fail, especially on impact or in use. This is caused by stress and the amount of material that is pushed into the bucket at this location. It has been found via various studies that the density of the HDPE can change, with the use of pack in the gate area. This change in density is than related to stress / strain at the gate area, ultimately to ESCR and breakage.

In bucket manufacture for DOT testing the principle is to mold a part that meets all size and function but at the lowest possible weight, which may preclude even a gate freeze occurring prior to screw recovery.

In the molding of Polycarbonate or acrylic lenses this same issue can occur but breakage may or may not occur, the real result is a distortion in a lens.

The area of influence is the area around the gate and a ways away from it, dependant of wall thickness, fill rate, and fill rate of pack.  Since material fountain flows into our cavity the area just past the gate becomes thinner initially upon injecting the material into the cavity. As we go out with the flow of material, we lose pressure not only due to wider surface area, but also to thinner cross sectional flow (material cooling off on side walls of mold). Those whom have worked or seen mold filling schematics / pictures well notice the pressure gradients. Thus the highest pressure is closest to the gate area.

When in fact we switch over to pack, it is around the gate area that shall see the highest pressures. This can manifests itself with flash, stress, part breakage, and possibly even wall thickness change. Though when a part is packed it is hope that extremities are seeing the pressure (which they see some) though the majority or a greater portion is seen around the gate area, not necessary a problem.

What to do? From a process point the problem has to be identified, i.e. all the parts are breaking at the gate area or that is where the breaks appear to be starting.  The first thing to figure is it the gate. Produce a short shot, no pack or hold, and pressure on fill is a smooth upward graph (if not there are two possibilities to be discussed in other notes). If the part now being short does not break crack and has impact necessary it can be packing of the part that is causing the issue.

If this is the case than looking at a pressure drop through the gate and gate freeze time should be considered. This is to document what is going on; the solution to the cracking as was just discovered is to prevent over packing of the gate and surrounding areas. This is solved quickly by minimizing pack time, non gate freeze, decrease of pack pressure, no pack time, and various other points.

If for example you need a full part and dimensions are critical, the above mentioned solutions will not work consistently, as packing gives the size we need and when using a non gate freeze process size may become a variable. In these cases than a gate change / design need be incorporated.

One solution would be to use a tab gate design, now the main pressure is into the tab and not the part, and by design the higher pressure is felt in the tab. Another would be to create a larger pressure drop across the gate by increasing the land length. A third method may be to move the gate to a non critical area where stress would not affect the function of the part.  

 SL Silvey