St. Marys Carbon offers a unique solution to our customers called Pressed-to-Size. This is a technology that only a very few carbon companies globally are able to properly execute. Pressed-to-Size allows us to develop cost-effective, low or high-volume components efficiently and with the high level of precision and quality that are required for a variety of applications. Below gives a brief explanation of how pressed-to-size works; however please contact us to learn more and see how pressed-to-size could be utilized for your components.
Most Engineered Materials are compounded with Raw Materials of a prescribed Chemistry, Purity, Particle Size Distribution, and Particle Morphology. This may include a temporary/Green Binder and/or a permanent Binder or Binder precursor.
After Batching, the powders and/or liquids will be processed through a combination of Mixing, Drying/Cooling, Milling, Blending, and/or controlled Agglomeration. This process may include temporary solvents and/or addition of a Die Aid or Lubricant. The result is a material ready for compaction.
There are four general forms of compaction: Extrusion, Wet Bag Isostatic Pressing, Dry Bag Isostatic Pressing, and Uniaxial Compaction.
- Extrusion generally starts with the compound at an elevated temperature to create a paste like consistency which is then forced through an extrusion die driven by either a hydraulic piston or electro-mechanical screw. Defined shape can be imparted on the extruded product ID or OD; however this is limited to only axial features.
- Isostatic pressing uses an optional Core Pin and an elastomeric “bag” (e.g., polyurethane) which is filled with the compounded powder, sealed at the top, may be placed in a perforated support cage, and then subjected to external press from either a gas such as air (Dry Bag Isostatic Pressing) or a liquid fluid such as water (Wet Bag Isostatic Pressing). The water generally contains additives to prevent corrosion of the Isostatic Press. This methodology allows for Net Shape features to be formed on the ID but not the OD (although the OD may be near net shape as is often the case in Dry Bag Isostatic Pressing).
- The last method is Uniaxial Pressing which utilizes a Hydraulic or Mechanical Press to compact the compounded powder deposited in the Die Cavity. There may be multiple Top and/or Bottom Punches and the Die may have horizontal features. During the compaction other components can be incorporated into the compact such as wires when molding a brush. The tool-set may also incorporate a Core Rod to form the part ID and horizontal ID Features. Technology has now advanced to even mold helical features.
In addition to pressing technique, we must also discuss the four general types of compacts: Blank/Block/Billet, Near Net Shape, Net Shape, and Hybrid.
- Blank/Block/Billet – This type of compact has generic dimensions (e.g., Inside Diameter/Outside Diameter/Thickness or Length/Width/Thickness) to support low volume, multipurpose and diverse dimensional requirements. The dimensions are either determined by market standards or requirements in the case of “Blank Sales” or by current business needs in the case of Blanks for internal use to produce finished components. These type of parts are useful to keep in inventory to provide rapid delivery of machined parts.
- Net Shape – When a part is high volume (generally ≥ 1,000 parts per year) and has features with tolerances that are within capability, unique molding tools can be designed and fabricated to yield the desired feature after Bake or Graphitization (e.g., Sintering). This methodology provides the most cost effective component as it minimizes material cost and eliminates machining cost. These molding tools are both Customer and Part Number specific.
- Near Net Shape – When a part is high volume (generally ≥ 1,000 parts per year) but has dimensional requirements that exceed Net Shape capabilities, unique molding tools can be designed and fabricated to minimize machining steps and stock removal thus providing a more cost effective component. These molding tools are in most cases both Customer and Part Number specific.
- Hybrid – These are components which have some features with tolerances within the Net Shape capability in addition to features which are either beyond the capability of Net Shape tolerance and/or are geometrically not conducive to the molding method of choice (e.g., Uniaxial Pressing). Unique molding tools can be designed and fabricated to both produce some features to Net Shape while other features are produced Near Net Shape to minimize machining steps and stock removal thus providing the most cost effective component
In Uniaxial Molding it is useful to segregate features that are in the Direction of Mold (DM) and features that are in the Opposite Direction of Mold (ODM). The DM Features are formed by the horizontal features of the Top and Bottom Punches, the Core Rod, and Die and make up the Part Thickness. A general rule for tolerance capability for these DM features are 3 to 5% of the feature Thickness. The ODM Features are formed by the vertical features of the Top and Bottom Punches, the Core Rod, and Die and make up the Part OD and ID or Width and Length. A general rule for tolerance capability for these ODM features is 1 to 3% of the feature Diameter or Length/Width.
Another consideration for a Near Net Shape or Net Shape part is the part surface and how it will be affected by subsequent processing such as Graphitization and Metal Impregnation. These potential issues need to be reviewed for each specific Application.