Multiple benefits of multi-tip punches for tablet production
Natoli takes a whistle-stop tour through the benefits and considerations of switching to multi-tip punches for tablet production
Multi-tip punches are a cost effective way to increase tablet output without the need for additional tablet presses, decreasing total press run times and ultimately minimising operating costs. The use of multi-tip punches follows a similar principle as a single tip tool for the rotary tablet press but there are special designs and operating techniques that must be understood for a successful tableting process. Before we jump into the discussion lets first go over the common features that are offered for multi-tip punches.
Two common multi-tip tool configurations for B and D tooling are assembled and solid type tools with both having their advantages. When choosing which configuration best fits your needs, some things to consider are tool type (B or D), tablet size, number of tips and particle/powder characteristics.
Assembled punches consist of a punch body, caps and tips which allows the replacement of tips without having to purchase the whole tool body. The solid design offers easier cleaning, minimised assembly times, reduced rust potential and less risk of cross contamination.
Rotating heads are a common feature for multi-tip punches, and are preferable on lower punches, as it can increase the life of the tool and press parts. This feature provides a two-part punch configuration where the head rotates independently to the punch body. The advantages of the rotating head include extended tool life as it provides a changing contact surface for enhanced head wear versus a single point contact on rollers and cams. Rotating heads also reduce the torsion stress on the lower punch tips as it contacts the compression rollers. As some tablet presses do not offer lower punch guide keyways, a rotating head may be a critical feature when using multi-tip punches. This is of particular concern when compressing very small tablets known as micro or mini-tabs where the lower punch tips are fragile.
This article will point out the critical stages of the rotary tablet press process and describe what impact multi-tip punches have at each step and how to overcome the common challenges found in the tablet compression industry. Case studies will also be provided to address the impact multi-tip punches have on compression and ejection forces.
Discussion
Die filling stage:
Hopper level
Product is initially loaded into the hopper of a tablet press and the powder must have adequate flow properties to successfully enter the feeder system. The amount of product in the hopper will determine the head pressure of the powder and this pressure can impact tablet weight. Keeping a constant head pressure can help minimise weight variation issues. When multi-tip punches are installed on the press the rate of product consumed is significantly increased and requires loading the hopper more often.
Feeder speed
The feeder delivers the powder from the hopper to the die table. The speed of the feeder is set to maintain a constant charge of powder in the feed system and is affected by turret speed, number of turret stations and tablet weight. With the use of multi-tip punches more product will be drawn from the feeder requiring a higher feeder speed than is used for single tip punches.
Die fill process
The lower punch is pulled down from the fill cam to allow overfill of the die cavity. The excess powder is then pushed out when the punch reaches the dosing cam. The lower punch tip to die clearance must be within tolerance to create a partial vacuum to assist in the die filling process. It is important to monitor punch tip wear and perform regular punch maintenance as this can create a number of issues. A worn out tool, or a low quality tool that is not within specification, can cause weight issues and punch binding as materials buildup on the die wall. Multi-tip punches can exacerbate these issues and require special attention to tool quality.
The selection of the proper fill cam is critical for single tip and multi-tip tools. Powder over-blending is a common issue when the fill cam size is excessive and the recycled material re-enters back into the feeder and is potentially over blended from paddle feeders mixing. This over blending phenomenon is potentially minimised when dealing with multi-tip punches as the powder feeder residence time is significantly reduced. As the number of tips are increased on a punch and the appropriate size fill cam is installed the over blending potential is minimised.
Minitab or microtab compression is a common application for multi-tip punches. When tablets are smaller than 4mm in diameter, one should consider reducing the lower tip length. An undercut die is provided with the shortened lower tip length and this may cause catastrophic failure if the incorrect fill cam is installed. Tooling manufactures are very experienced in this modification and working directly with them will ensure proper tool design and tablet press set up.
Pull down cam
Immediately following the dosing cam is either a tail over die or pull down cam. Both are designed to keep the powder in the die cavity before the upper punch enters the die. The pull down cam will pull the lower punch down, allowing the column of powder to drop below the top of the die surface. This prevents powder loss from die spillage caused by high turret speeds and resulting centrifugal force. When utilising multi-tip punch as with shortened lower tip lengths and undercut dies it is imperative that the pull down cam doesn’t pull the lower tip from the die cavity. This modification should be treated similar to that of the fill cam.
Compression stage:
Pre compression
Pre compression is a de-aeration stage. This allows the removal of air and initial consolidation of particles to help with the main compression process. Common tablet issues including capping and low tablet strengths can be resolved by adding a small amount of pre compression. A multi-tip application may also benefit from the use of pre compression but with the increased number of tablets produced, the press speed can be reduced. This reduction in speed increases dwell and total compression time, possibly eliminating the need for pre compression.
Main compression
The final compression force on the rotary tablet press is a result of the amount of powder in the die cavity and the distance between the upper and lower punch tips. For concave punch tips it is the deepest measurable point in the punch face that is critical. This is the difference between the punch working length and overall length. The overall length is the distance between the head flat and top of the cup where the working length is the distance between the head flat and bottom of the cup. It is the working length that is critical in providing a constant displacement between punches under the compression roller. Assuming a concentric pressure roller and punches that are within working length tolerance, the resulting compression force will be similar for each turret station if the powder is uniform in fill. Monitoring the compression force per station with a force transducer allows the operator to determine weight variability. This tool allows the operator to optimise the feeder paddle speed by setting the speed to slowest setting but still producing consistent tablet weights.
Multi-tip punches add complexity to the compression force measuring system as the force transducer is typically installed in the compression roller pin or strain gages are fitted on the compression assembly. For a single tip tool the force measurement is acting on a single compressed tablet but the force reading for a multi-tip tool is related to all of the compressed tablets on that punch. It is of special importance that the multi-tip punch working lengths are within tolerance since these force systems have no way of determining which tip is causing the issue.
Case study - what impact do multi-tip punches have on the compression force?
At the Natoli Institute for Industrial Pharmacy Research & Development located in the Arnold and Marie College of Pharmacy at Long Island University in Brooklyn, New York, a series of experiments were conducted to study the compression force impact from a single tip, two tip and four tip 0.25” (6.35mm) round tool. The tools are illustrated in Fig. 1. A direct compression blend consisting of 25% w/w of APAP and 74.5% SMCC 50 with 0.5% magnesium stearate was used for the study.
A compaction profile study was performed on a bench top rotary tablet press at 25RPM turret speed. Tablets were collected at varying force levels for each tool configuration. Resulting tablets were measured for thickness, weight and breaking force.
Figure 2 depicts the compaction profile for the three tools. It is clear that the compression force increases to achieve the same tablet breaking force when the tips are increased. One might think that since the area of the tool has doubled, it would require twice the force to achieve the same tablet breaking force. This is illustrated in Fig. 3.
Figure 4 below is the compaction profile normalised for the tablet geometry and punch tip area. This gives a clearer picture of the resulting tablet strengths at different applied compaction pressures. If in fact the force is doubled when the tool area is doubled then the plots would superimpose. At the same compaction pressure, the 2 tip tool configuration results in a 19% increase in tablet strength, and another 19% increase in strength from 2 tips to a 4 tip configuration. In this case study the applied pressure or compression force would be less than double to achieve the same tablet strength.
Ejection
After the tablet is compressed the ejection cam will push the lower punch upward to eject the tablet. The ejection force is a combination of the residual radial die wall and coefficient of friction between the tablet belly band surface and the die wall surface. Excessive ejection force levels will cause premature wear to the lower punches, cams and may cause visual striations on the tablet’s belly band. Multi-tip punches will increase the belly band surface from the increased number of tablets which will inherently increase ejection force. Common ways to reduce the ejection force is added levels of powdered lubricant to the formulation (i.e. magnesium stearate), tapered dies and lined dies of different materials.
Case Study continued- What impact do multi-tip punches have on the ejection force?
As previously mentioned, as the number of tips are increased on the punch, the number of tablets ejected also increases and an increase in ejection force is expected. If the contact area is doubled, one would assume the force would be doubled as well. The results are depicted in figure 5.
The results clearly show a significant increase in ejection force with the increased number of tips but it is not doubled or linear. The properties of the powder play a major role in the ejection forces. When lubrication is excessive the ejection force will not increase with increased compression force since the friction is reduced. This profile finds value in the development arena where lubrication can be optimised. Lower levels of lubricant can reduce capping potential, improve tablet strengths and disintegration/dissolution times.
Tablet take off
The final step on the rotary tablet press is the tablet take-off or tablet removal from the lower punch face. Common problems associated with tablet take off are sticking and picking issues. There are many techniques available to remediate these issues including steel composition, punch face coatings, and logo designs, but when dealing with multi-tip punches the punch overall length is of particular importance. Most tablet presses are designed so that the lower punch tip is flush with the top of the die table to ensure a smooth tablet removal process. Some presses are designed to protrude the lower tips at 0.5mm or more. If the overall lengths are not consistent within a set of punches the tablet removal process may have issues with the tips damaging the take off bar or even the leading edge of the feeder. If the punch tip is slightly below the die table surface the tablet might fracture at the take off bar. For multi-tip punches it is favorable to provide a flush tip situation versus a slightly protruded tip since the latter can cause tablet damage from the trailing protruded tips contacting the leading tablets that were removed from the take off bar.
Conclusion
Multi-tip punches offer many advantages including increased tablet output and reduced operating costs but a full understanding of the available options and the impact they have on the rotary tablet press are key to successful manufacturing.
For more information contact Natoli Engineering Company, Inc.