Navigating Turret Bore Bushing Material Selection in Consideration of Difficult Formulations

David Risinger and Jeffrey Butler from Scheu & Kniss are seasoned experts in tablet press turret design. In this Q&A, they talk about the critical role of material science when choosing turret bore bushings and optimizing the tablet manufacturing process.

When the bores in a tablet press turret wear, bushings are often the solution manufacturers turn to avoid interruptions in production or the costs associated with a new replacement turret. Material selection for these bushings goes beyond just technical decisions; there should also be consideration of how operational effectiveness might be impacted by the compounds used in the tablet formulation. The choice of materials affects everything from the durability of the equipment to the integrity of the finished tablet.

What is the role of material choice in preventing adverse chemical reactions in a finished tablet?

David Risinger (D.R.): Selecting the right materials is crucial to avoid chemical reactions that could compromise the tablet’s integrity. While bronze bushings typically perform well with most standard tablet formulations, they do not fare well with certain chemicals. Ammonium compounds, such as ammonium chloride and ammonium bicarbonate, may react negatively with bronze, potentially leading to contamination or damage. This requires the consideration of alternative materials for different formulations to ensure product safety and equipment longevity.

What is the impact of corrosive formulations on bushing material degradation?

D.R.: Imagine using a corrosive chloride-based formulation. If the turret in your tablet press is made of cast iron components, it would rapidly deteriorate due to the corrosive nature of the formulation. This could disrupt production significantly, leading to frequent downtime for repairs and replacements.

Switching to a material like the aerospace-grade Inconel, a nickel chromium-based superalloy known for its resistance to extreme environmental conditions, can significantly enhance the longevity and reliability of your turret and tooling. A material like Inconel offers superior resistance to corrosion and is particularly well-suited for environments where harsh chemicals are processed.

What are some design challenges manufacturers may encounter with abrasive formulations?

D.R.: Formulations with increased abrasiveness or chemical reactivity may require the use of specialized materials to prevent rapid wear and tear on the machinery. For instance, nutraceuticals containing abrasive compounds might need harder bushings to minimize equipment degradation. A good approach involves a collaboration between the tablet manufacturer and the turret providers. The turret provider should have thorough consultations with the tablet manufacturer to determine the best materials, such as hardened stainless steel or ceramics, based on the formulation’s specific properties.

Can you elaborate on the process of evaluating and selecting bushing materials for abrasive tablet formulations?

D.R.: When a special application is needed, we work with our metallurgist to identify a selection of materials. However, it is important to note that the first line of defense when trying to mitigate wear issues caused by abrasive formulations is to control the material itself. Poor dust collection, faulty dust seals, and poor lubrication are the main causes of turret and tooling failure, which lead to the need for installing bushings in a turret. Material selection is a balance of failure points. A long-wearing bushing could cause the tooling to wear prematurely. In these applications, a semi-hardened, field-replaceable bushing may be the better answer.

What obstacles do you encounter when retrofitting older equipment with new bushings?

D.R.: There are many limitations to repairing a turret through the use of bushings. The first is the design of the turret. Some turrets can’t be 100% bored and bushed because there is not enough area between each bore to accommodate an adjacent bushing, and one bore would intersect the other. The next challenge is wear in the die bores. The alignment of the punches and dies is paramount for effective equipment operation. Therefore, each bore in a segmented turret (the upper, lower, and die table) must be machined to very close location tolerances. In the case of a non-segmented turret, all repairs are predicated on the die bore locations. If the die bores are worn, the accuracy of the repair is in jeopardy and a new turret is usually the better solution for the project.

How can manufacturers effectively evaluate the lifecycle costs of different bushing materials?

D.R.: Most tableting operations measure costs based on tablets produced or yield and downtime or production loss. Bushings have very little to do with yield, but they have a lot to do with downtime. Manufacturers can assess the lifecycle costs of bushing materials by examining their impact on production efficiency, particularly how they affect downtime, which directly influences overall operational costs. Choosing a material that best suits the application can help to reduce cleaning due to corrosion or scarring. By prioritizing materials that minimize downtime and maintenance, manufacturers can significantly enhance operational efficiency and cost-effectiveness.

What are the key maintenance practices for enhancing the lifespan of bushings in tablet press turrets?

D.R.: Here are three important maintenance procedures to consider:

1. Prevent Powder Fines Migration: Implement regular cleaning to remove powder fines and use air filtration systems to prevent these small particles from accumulating in turret mechanisms, which can result in increased wear and tear.
2. Lubrication Practices: Use the correct lubricant and follow a strict lubrication schedule. Proper lubrication reduces friction, protects the mechanical components, and should be applied in the right quantity to avoid attracting dust.
3. Careful Tooling Installation: Precision in the installation of tooling components like dies and punches is so important. One could say that 60% of turret bore damage is from careless tooling installation. Train technicians to handle and install punches and die accurately to avoid misalignment.

How do the material properties of bushings impact Wash In Place (WIP) systems?

Jeffrey Butler (J.B.): WIP equipment presents challenges to machine designers because the machine components are exposed to water-based solutions that can cause corrosion. In most cases, stainless steel is the material of choice as a solution to these challenges. In theory, stainless steel sounds like the logical choice when using the WIP machine, but in fact, stainless steel causes several adverse issues.

The number one problem is that stainless is not an ideal material where friction and heat are applied to it. Product migration is nearly impossible to stop and usually ends up on the punch barrels. The product “fines” are then introduced onto the punch barrel causing tight punches, which increases friction and heat. Over time this causes the punch bores to begin to score, and the punches to seize. This can cause substantial damage to cams and tooling. The solution is a hardened bronze bushing that is more naturally lubricating and runs better in these WIP environments, thus reducing the risk of damage, repairs, and downtime.

How do material properties like thermal expansion affect the selection and design of bushings for tablet presses?

D.R.: Some consideration should be given to this dynamic in the area of clearances and tolerances. In high-speed or high-compression tableting, heat is generated. The materials chosen and the geometric cross-section of the turret will dictate where the heat dissipates and what effects it has on tooling and turret specifications, including bushing material.

What are future innovations in tablet press turret bore bushing materials?

J.B.: At Scheu & Kniss, we are exploring new materials that offer better lubricity and reduced wear, which can significantly improve the operational efficiency of tablet presses.

We are working toward a new development in bushing design that will add a great deal of flexibility and increase production to the compression process.

What are some market changes you are anticipating as well as developing technologies?

J.B.: With advancements in 3D printing and nano-coatings, the potential to develop components that are not only more durable but also specifically engineered to interact optimally with particular formulations is on the horizon. Such technologies could lead to highly customized solutions that enhance production efficiency and product quality which could affect product lines.

As tablet manufacturers strive to enhance product quality and operational efficiency, the strategic selection of materials will remain an important part of industry practices. The ongoing dialogue between material scientists, engineers, and regulatory bodies ensures that the evolution of tablet manufacturing technologies continues to meet the highest standards of safety and efficacy.