Recently, I had a customer ask me for more detail regarding why their tooling cost got so much more expensive after a slight print change. The following was my explanation:
When determining the process for a particular part we always have to balance the starting wire (stock) diameter with the upset ratio (the amount of material required for upsetting into the remaining cavity space). For a single die process, the starting wire diameter is usually determined by the smallest diameter (in this case 9mm on the original rev). We then do a calculation of volume to determine if we are able to upset 9mm wire into the cavity required for the finished product. The volume calculation is expressed as the number of diameters that is required to complete the upsetting process. For example, if we determined by volume calculation that 36 mm of 9mm diameter wire are required to fill the cavities then it is expressed as an upset ratio of 4 diameters. The rule of thumb limit for this type of process is 4.5 diameters so therefore we can produce in a single die process. When the smallest diameter drops to 8mm but the volume required for the balance of the part does not change significantly this upset ratio rises drastically. In the example, the upset ratio would go from 4 times to 5.7 times which is well above the acceptable maximum. The reason for a maximum upset ratio lies in a principle called "material flow control". In order to be successful jamming a bunch of material in a cavity it is critical that the flow of material is even, and consistent from the centerline out. If it is not "buckling" occurs and we call this "losing control". When you lose control of the material flow you start to get uneven fill and flashing. By increasing the starting wire size and using the first die in a multi die process for extrusion down to the smallest diameter, we are then able to reduce the upset ratio to an acceptable level. The multi-die machine takes longer to set-up, has more tooling and more expensive tooling (extrude dies take longer to make and require a lengthy polishing process) and may run slightly slower. All of these factors contribute to piece price. I hope I haven't confused you with this. Please feel free to call or have anyone call for further clarification.