Into solid: Forging
Sharing a close characteristic with plastic compression molding, forging is the method of producing a part by sandwiching a raw metallic piece in between the male and the female molds.
Have you ever seen a blacksmith forging a sword? Well surely the visualization in your head is sufficient enough to help you understand this process in it’s simple and traditional form but I’ll explain it anyway. The desired piece of metal will be heated until it reaches its point of re-crystallization (which means not necessary towards it’s melting point given that it is in a hybrid form between solid and liquid) so that the forger could apply a repeated hammer force to craft the raw piece into the desired shape. In some instances the still glowing piece of metal will be submerged into a cooling liquid to maximize its density. This is known as open-die forging. In a typical large scale manufacturing process however, a series of compression hammer will takes place instead of a regular hammer where it will gently presses certain sections of the metal piece into shape.
The modern industrialized form is where the two part male and female mole takes the stage. The process is the same as the traditional one, except the metal piece will be compressed in between the two halves of the mold (just like a plastic compression process) where a repeated force will be applied downward to force the soft metal into the contour of the desired mold. This is called close die forging or drop forging.
There are also other forms of manufacture, including press forging, which involves a thin metal bar being pressed in series of roller to form thin long shapes. There’s also upset forging, which is used to compress an end of a rod when they’re being held in a mold to produce an end result.
Forging has many advantages when it comes to grain control (determining the density in different sections of the final piece to improve it’s strength) thereby preventing any gaps or metal ‘flaw’ and reduce any excess materials. The drawback however is that even if there are excess part such as a ‘flash’ of the finished product, it cannot be removed manually and relies on machinery to chip or ‘sand’ the undesired part off.