So what is forging anyway? If you know anything about history, you may be imagining a smithy with a hammer and an anvil, pounding away on a piece of hot metal. Forging has been used for centuries—ever since humans started working with metals. Although a lot has changed since those days, the basic characteristics remain the same today.

 

Forging is the plastic deformation of material into the desired shape. See the sidebar “Materials in Forging” for examples of what materials are used in forging in orthopaedics. Today, this deformation of material usually involves heat and force via a furnace and a press. Although this article won’t discuss the types of furnaces and presses available, it is important to know that they are varied and are used to control the different process variables of forging. 

 

 

Gross. Gross-shaped forging usually has a lot of excess stock on a part and requires all surfaces to be machined. Sometimes the forging doesn’t even remotely resemble the end product. This shape is chosen when there are many part codes and relatively low volume for each code. It allows the benefit of the volume of all the codes into one forging part number. For example, there may be 10 different part codes in a part family. Some of these sizes can be grouped together so that one forging can be used for multiple sizes. Other benefits include allowing one type of machining fixture and setup, as well as inventorying only one forging part code.

 

Near-Net. Near-net forgings have minimal stock on a part and closely resemble the final product. Operations such as machining, grinding, and polishing usually follow forging, but for the most part, the process removes less than 0.01 in. of material per surface. In this scenario, there are multiple part codes with a forged part for each code. Higher volumes make more sense for this case, such as greater than 100 pieces per part code, because the manufacturer can amortize the cost of the tooling over a greater number of parts.

 

 

 

 

 

 

 

 

 

The basic forging process for a titanium part with two blows is shown in Figure 1. Raw material is cut into small billets on a saw or lathe and is then heated. Then the press is loaded with dies that are lubed to prevent the parts from sticking to the impression of the die. The first forging blow occurs, and the parts are trimmed and air cooled. During surface preparation, the parts are blasted, chemically milled to remove the oxidized surface, and inspected for any defects. Upon the completion of inspection, the parts continue back to the forging press for the final blow. Surface prep is repeated, and the parts are heat-treat annealed. Upon completion, the excess flash must be removed via mechanical trimming or milling. The final step is a nondestructive test—100% fluorescent penetrant inspection (FPI)—along with any required dimensional or mechanical tests. For example, the FPI test is an industry standard for net and near-net shaped forgings. Some customer specifications require this type of inspection. Other mechnical tests, such as tensile, elongation, or hardness, are conducted because the material’s physical properties have been changed during the heating and forging process. Depending on the part being forged, there are always some dimensional checks that are conducted to ensure that the product meets the customer’s requirements. 

 

Critical factors must be controlled to ensure the desired outcome. These factors include the following:Appropriate controls must be in place to ensure that parts are processed according to predetermined plans established in the validation protocol. The cycle as indicated in Figure 1 can be repeated depending on the complexity of the forging. A net-shaped forging may require multiple blows to achieve the desired shape. Simpler designs may only require one or two blows.

 

 

The last, but certainly not least, point to discuss is the commercialization of forgings in the medical industry. It is important to understand how forging can be used in a complex and changing environment such as the medical device industry. Before starting a forging project, some inputs are required and some important considerations must be addressed.

 

 

It is also important to address tooling for forging, because it can become an impediment if costs are too high. Tooling charges can consist of the raw material for the dies, electrodes and cutters, engineering development time (models, prints, and simulations), and machine time to create the impressions in the die. Costs for this can range from $4000 per part code (for simple parts) to as much as $10,000 per part code (for more complex parts). Typically, this is a one-time charge, and the refreshing of the dies when the impression wears out is included in the initial cost. 

 

Lean manufacturing has had a significant effect on forging in the orthopaedics industry. Lean principles have allowed lead times to drop to as little as two weeks for some forged products. Other programs such as vendor-managed inventory and Kanbans (efficient pull systems) can allow for even shorter lead times (days instead of weeks). Other benefits of these programs include having a small amount of controlled inventory, an increase in product quality, and a decrease in the risk associated with running products through traditional manufacturing programs. Working with customers to remove waste from the product value stream can allow for overall decreased costs. Forging is ideally suited for the application of lean manufacturing principles and these types of programs.