It is well known that molten plastic flows in mold in the form of a fountain during injection molding process. Understanding fountain flow effect is very important for solving common injection molding problems. This article will start from principle and try to explain fountain flow effect of plastic for you.
In injection molding process, although there is a mold temperature machine to heat mold, mold temperature (80~100°) for commonly used thermoplastics is much lower than temperature of molten plastic (above 200°). Therefore, as shown in figure below, when high-temperature molten plastic contacts "cold" mold steel, a cooling layer will quickly form on outer surface, and subsequent plastic will continue to advance through middle of plastic flow under push of screw, eventually filling the entire mold cavity. Such a flow pattern is very similar to that of a "fountain", so it is called fountain flow effect.

 

During filling phase of injection molding (pictured below), plastic enters mold at a relatively fast rate and forms shell. This means that outer surface of part is always formed by plastic in the front of melt in barrel. As shown in image below, if molten plastic in barrel is divided into 10 parts, parts 1 to 7 will form outer surface of part. In cold runner molds, plastic at the tip of nozzle will eventually act as a housing over gate.

 

After entering holding phase, screw continues to push plastic into cavity until gate freezes to compensate for shrinkage of part during cooling. Therefore, plastics of the three parts 8, 9 and 10 in above picture are the last to enter mold cavity in pressure holding stage.
Image in image below takes a molding process from clear color change to yellow. This mold product is just at the overlap of the two colors, and color is completely changed to yellow after this mold. From image below, you can see that gate has a clear transparent skin, and center of gate is yellow plastic, which is also very clearly shown in cross-section of gate. These images clearly show fountain flow effect of plastic in mold.

 

Understanding fountain flow effect of plastic is very helpful to help us understand common injection molding defects, which are embodied in following aspects.

Splay is a common injection molding defect that appears as shiny streaks on part, generally caused by volatiles in melt (as shown in image below). Moisture in the plastic, additives or degradation of plastic, or air mixed in melt can all generate volatiles during injection molding process. If volatiles are present between mold surface and plastic melt, it prevents melt from picking up texture of mold steel, resulting in a glossy or silvery appearance on part surface.

 

As molten plastic enters mold and experiences fountain flow, frozen layer formed on the surface of mold cavity will become thicker and smaller, flow channels will become smaller and smaller (as shown in figure below). Plastic has to pass through a smaller channel and travel a longer distance to reach the end of fill. In order to maintain a constant injection speed of screw, pressure required to push plastic is continuously increased. If required injection pressure reaches maximum pressure machine can provide, actual screw injection speed will slow down. Such a process is called a pressure-limited process, and pressure limitation can lead to poor part filling or part quality problems.

 

A void is a vacuum bubble in a plastic part that usually occurs in areas with thicker walls. If there is not enough packing pressure to pack part, voids can easily form. When plastic is filled into mold cavity, fountain effect will allow plastic near cavity to cool first, and internal melt shrinks from inside to outside, which is easy to form voids. Void can be thought of as an internal cavity that must be filled with additional plastic to prevent it from forming. By looking at fountain flow, you can tell that this plastic will come from melt in front of screw tip.
Bubble
If there is not enough venting in mold, air can become trapped inside part as plastic flows to form shell, forming air bubbles.

 

During injection, molecules of molten plastic always face direction of flow. Once these oriented molecules come into contact with "colder" mold surface, they freeze in this oriented state. During packing phase, inner plastic molecules have not yet solidified, so they begin to relax. This relaxation leads to a loss of molecular orientation in plastic. Therefore, for parts with thicker walls, there is usually a molecularly oriented outer layer and a molecularly unoriented inner layer. The same goes for fiberglass and other fiber-filled materials, where glass or other fiber orientations are present on the skin of the part.