Metal compaction is a fast moving process that begins with the seemingly simple task of moving powder into a cavity. But powders exist in an area between liquid and solid – sharing characteristics of both. Flowing like a liquid under gravitational pull to fill cavities, they exhibit solid characteristics when interparticle friction inhibits the smooth flow that one would expect from an actual liquid. This friction is determined by three key particle surface attributes: area, roughness, and chemistry. Environmental factors (e.g. temperature, humidity) and weight of the bulk also help define powder flow characteristics. For powder metal parts producers, predictable and dependable flow is most critical in two parts of the production process – bulk distribution and die filling.
Bulk Distribution (Bin Flow)
Two major bin flow issues occur most commonly:
Bin-area problems like ratholes (vertical empty cavity above the bin outlet characterized by stagnant material to the sides) and arches (a stable obstruction over the outlet) in the bulk load during dispensing (see picture)
Chute flow impediments, typically caused by inadequate chute angle or surface finish
Flowability variation between different grades/types of powders can mean that what works great for one blend may not for another. But there are four key variables to be aware of for a given powder, which taken together can help equipment designers significantly:
Angle of Repose
The steepest angle at which a sloping surface of a powder is stable
Flow Rate (Hall or Carney method)
See ASTM B 213 for test method
Mass of non-compacted powder vs. volume
Free-Flowing / Non-Free-Flowing
As defined in ASTM B 213
For more information on using these attributes in system design, a great reference is the ASM Handbook Volume 7 – Powder Metal Technologies and Applications. It can be purchased here.
Once transit-level obstacles are overcome and the powder has arrived at the “business area” of the press, filling the cavity evenly and consistently takes center stage. The inability to flow predictably and evenly into die cavities presents several issues, including:
Weight/height control variation in challenging die compaction applications
Inability to achieve weight consistency in multi-cavity tool arrangements
Fill depth challenges due to limited tool daylight
These problems often manifest themselves in reduced output while compensating for the above symptoms of powder flow inconsistency. To compound matters, what worked in bulk is not always a surefire solution for consistent die filling, especially in complex part applications. In fact, die filling is often considered the key bottleneck in getting the highest throughput possible out of a compacting press. For more on how to tackle this part of the powder flow challenge, take a look at the article on the Gasbarre Fluidized Fill Shoe in this month’s newsletter.
Gasbarre Products offers an inexpensive solution to some of the most challenging die cavity powder flow applications with our Fluidized Fill Shoe (FFS) System. FFS technology was jointly developed with Matsys Materials and Manufacturing Systems. This patented technology can greatly improve powder flow characteristics - allowing better fill density distribution, more consistent compacted part height, and faster die filling. In cases where material flow is problematic to the point of not being practical, fluidizing the powder via an FFS can make all the difference. Applications such as thin parts, multi-level tool parts, and multiple cavity parts can now be made viable with fluidized die filling.
The main mechanism for these improvements is achieved by reducing the interparticle friction and transportation friction of the powder by fluidizing it with a very light air or gas pressure and volume. As can be seen in the chart to the right, the benefits are significant in terms of parts consistency and throughput. In this single-cavity, thin-part test with 50 samples, part-to-part thickness variability was decreased by 40% while output increased by 50%. Bottom line - better parts, faster! And the FFS does not care who manufactured the press – it can be fitted to any powder compaction press.
To see an example of the difference a fill shoe can make, take a look at this short (30 sec) video: Fluidized Fill Shoe Video
To learn more about how a fluidized fill shoe can help you, click HERE for the FFS datasheet or call us – we’d be happy to perform powder flow testing with your powder in a FFS, at no charge to you!