Definition: 1. see diaphragm gate 2. a gate that forms a complete thin web across the opening of a part, used in the molding of annular and tubular parts. 3. A circumferential fan gate

The disc gate is typically used to manufacture round and or tubular parts that need to be round and or free of weld lines. Material is introduced in the center of the disc (round runner) and it than flows radically out to the gate area which than feeds the part.

The three areas for the disc gate design.

1-     How do we originally get material into our disc?

2-     What thickness do we make the disc area, which shape?

3-     How do we design the actual gate area to the part?

Getting material to the disc gate is done in all ways. If a large part we would direct gate via the sprue into our disc, this could be cold or with a heated sprue bushing. If we had multiple parts in the tool we may first use a cold runner in a 3-plate tool design. If we use this method than we actually have 2 gates, one from the drop of the 3 plate and another gating into our part. The third method would be that of using hot runner drops to the disc, again 2 gates in that we have one at the runner/disc and another at the disc / part.

The thickness of the disc is dependant on many factors, such as volume, what we wish to do, cooling rate of material, capacity of machine to be used and even cosmetics. In some books a suggested starting thickness is a minimum 1.25 times the thickness of the part wall.  What is desired with this disc area is that the material as it exits the first gate area/sprue is evenly dispersed and flowing as a uniform front to the actual gate with the part. We wish to avoid the worming or jetting, also pressure loss, and unevenness of flow at the final point of our disc and that is the gate to the part. Sometimes this disc area can be tapered to feed the actual gate.

For example if our disc is too thin we could be freezing off and choking the flow of material to the part thus preventing the proper packing and filling of the actual part. If too thick we may have jetting and or flow into our part on one side first prior to the other or cold areas within our disc thus preventing flow into our part. In such cases the gate area may have an extremely long land to build up pressure and than flow uniformity to solve an issue with jetting into the part. In other cases there is no disc at all but a runner which feeds a ring runner from which the material now flows into the gate to the part. This method also uses a minimum of material, thus being a cost saver.

The gate into the part is as all gates should be designed. It has a height, width and land area, but in our case the width is the inside circumference (diameter) of the disc to part interface. The only two areas we can change are the height, and land. Ideally the land should always be 1 mm or less thus we are only allowed to play with the height. Various sources use different methods for the height of the gate itself. Some use extremely small or thin heights such as 0.1mm to 0.15mm (0.004 to 0.006 in.), while others state it should start at about 0.75 of the height that would be used as a tab gate for the part. (for example if the wall was 2.5mm than a tab gate would be 1mm(0.4 of 2.5mm) and the disc gate would be .75mm (.75 or 1mm)) This is dependant on the part thickness itself and how the gate is actually tied to the part.

In some cases the gate may impinge into the side wall of the part, such as a tube thus there is a flow against the wall giving the flow front that is desired. While in other cases we may actually use an under lap gate should the part be flat, or the ID is critical on a tube or cylinder. What is important for cosmetics is impinging our flow front. 

In most cases the disc gate is machined out of the part, but punching out if thinner is sometimes the case.

Another gate that is similar in almost all design is the ring gate which would go on the outside of the tubular and or circular part.

The importance with these types of gates is to establish which area is freezing first, and how that affects your finished part.

SL Silvey

Silveys Plastic Consulting

360-882-3183