Author Archive | Techceuticals Team
Die Ring Alignment Guide
Stokes Tablet Press Clutch Adjustment – Tech Tip
This is Mike Tousey Techceuticals covering a stokes tablet press twin disc clutch adjustment. Its pretty straightforward. I’m on the back of the machine on a tablet press,, on the fly-wheel side opposite of the hand wheel. What we’re looking at right now is trying to focus on the pin that I’m pointing to here with my screwdriver. And what we want to do with that pin is take that pin and slide it out and up or down. It’s spring loaded and that takes that pin and disengages it. What it’s connecting to is the plate behind it, the pin goes into that plate and locks it into place. So, to adjust it, is to simply to move a slight amount, make a movement up or down to loosen it, go up or right to left. To make it tighter, we;ll go down just little bit. I’m going to turn the hand wheel so you can see it better here. So, now all I have to do is reengage that pin. Now, notice that it doesn’t go all the way in. To get it to go all the way in all you have to do is move your adjustment just a touch, right there, it slides into the plate and I just adjusted it one or two clicks and that’s a significant amount so do just a little bit at a time. Make sure the clutch handle feels like it gets tight and then goes over like a speed bump. It will get tight for one second and then goes into a loose position. And that is the feeling you want. You don’t want a clutch that’s going to be hard to push in, it’s going to be a little bit tight and then it goes into a loose position and when you disengage it, the same way in reverse.
Results of Lubrication Issue - Punch and Die Tooling
Mike Tousey with Techceuticals discusses the importance of lubrication in powder. A lack of lubrication can cause the issues in this video.
Hi this is Mike Tousey here with the Tech Tip of the day. I just recently received a set of tooling from a client that said they don’t understand what happened in their operation. I thought I’d take a moment just to run through this. The report was that the lower punch head had extreme wear but the rest of the tool really didn’t show any problems and it didn’t have any problems with the tablet at all. So it is an interesting challenge here. The upper punch, head flat, the head of the tool, the inside bevel, neck, barrel, tip all have very little wear look in great condition, almost look brand new. The lower punch, so you got the upper and the lower, the lower here, the upper here, and the lower punch looks very much the same as the upper with regard to the tip, the cup, the tip, the barrel, the neck. The inside bevels got a touch of wear slight indication that something was tight. But then when we flip this punch around, we can see some pretty extreme wear on the head the outside bevel of the lower punch.
So what happened? Let’s take a look also at the dye. If we take a look the dye, and we take a look at the dye bore, we can see a little bit of compression or what I would call a wear ring, a compression ring, where its probably a little difficult for you to see in the camera, or for me to hold the dye correctly for you. It’s starting to show that there was a pretty good amount of radio force, so that’s an indication of higher ejection forces. What causes higher ejection forces? Often times we may have a material that flows great, that compresses really well, but the fact that it can’t get out of the dye properly or there is a lot of radio outward force within the dye, that the tablet actually as it comes out it really expands and sometimes it can even comes back apart. This particular customer’s tablet came off great, but we could not call this successful because there’s extreme damage to the lower punch head flap, and that’s simply because the amount of lubricant within the granulation. How it was blended in, we don’t know. If it got added, we don’t know. But we do know that there is a really strong indication here of extreme wear and this will not be related to this machine, or the cams, or this tooling, it’s all related directly to the amount of force it takes to get that tablet out of the dye. Some of our industries were trying to go, customers were trying to go excipiant free, which means that they are taking some of the lubricant out of the powder. This is a very common result when you take the lube out of the powder.
The purpose of a lubricant within the powder is in fact to help the tablet slide out of the dye. Without that a lot of machines actually will cease up and lock up and my guess is if I had been there, if were all there and watch this run, we would have heard this problem. That during the ejection the movement of pushing the tablet up and out of the dye during that stroke force, you actually can hear when granulation is real dry, it actually squeaks or squeals as the tablet is being pushed out. I suspect we would have heard that, it’s quite common, when powders are not blended correctly, when the lubricant is not blended correctly in, or there is the absence of the lubricant. So this is not success. Even though the tablet was good, weight, hardness, thickness, everything was great about the tablet, when you tear up the tooling and probably damage the ejection cam that is not considered success. So you need to go back to the drawing board a little bit, look at the issues, look at how we blended it in, make sure we are using a lubricant. So that’s tech tip of the day, love to have your feedback, thanks very much, this is Mike Tousey.
Coating - Key elements to tablet film coating - These are the must haves!
- Consistent Tablet hardness is step one, good coating is next to impossible without consistent hardness (the wider the range the greater the variation) ….soft tablets erode easily and hard tablets do accept the solution and the coating wants to peal.
- Pre-heating or coating the inside of the pan will prevent “marking”
- Loading the pan in the correct position so tablets do not hit the baffles and anti-slide bars is a must to prevent chipping and breakage.
- Jog or slow tumble to dedust tablets (cores) before spraying. Dust and tablet flash (crowning, extrusions) must be eliminated or it could ruin the entire batch. The defect is picking…whereby dry particles pull coating away from tablet to tablet….so a small problem grows and could ruin the entire batch.
- For most film coatings the product bed temperature must be maintained above 42°C…typically 45°C is a sweet spot. If temperatures are too low the coating will not dry quickly and could result in sticking, if temps are too high the coating will dry too quickly and not adhere to the table surface.
- Start slow: in the beginning the “pan rpm” should be slow, just quick enough to have good even flow (no stalling). This is dependent on tablet size, shape, and hardness. So typical speeds are in the area of 2-4 rpm.
- Spraying the solution: to begin with the guns should be clean, needles are straight and lubricated, seals are clean and not damaged. Check to make sure all guns are spraying within a range of ±10ml. Set guns about 8-10 from the tablet bed surface and make sure the atomizing air provides a spray that will dry on contact with the tablets.
- Tablet / spray exposure: the idea is to have the spray dry immediately after it reaches the tablet. As the tablets tumble and flow they get exposed to a slight mist that dries quickly. If the tablets are exposed to too high of spray rate they might stick to other tablets or to the coating pan surface. Once the tablets are covered they are now protected and not a vulnerable to pan speeds and spray rates. (coatings make the tablet stronger), then the pan rpm can be increased and the spray rates can be increased.
- Once the desired amount of coating is complete the spray and heat are turned off. The pan rpm is reduced to a very slow rotation so the tablets can cool to ambient temp. Unloading hot tablets can result in sticking and moisture buildup.
- Unload carefully, tablets are tougher now but still subject to breakage from poor handling.
- Make sure the coating pan and inlet/ exhaust plenums are CLEAN and DRY.
Die Alignment Guide - Part 2
Additional examples of damage to dies.
Die OD damage 1: Shows a flat spot
Die OD damage 2: shows small nicks, dents, and burrs that will damage the die socket.
Die socket damage: Horizontal lines at the top of the die shows how dies were forced into the die socket crooked. The vertical lines are from the nicks, dents and burrs.
Die Alignment Guide - Part 1
The die alignment guide is designed to assure that the die enters the die socket correctly, and to ensure that the die is free of any nicks or burrs that could damage the die sockets.
How to use the die alignment guide:
1. Make certain the die slides through the guide smoothly; do not force it. If the die hangs up that means the die edge needs to be de-burred before use.
2. Inspect the die socket for prior wear, and record any old damage.
A photo is worth a thousand words: vertical lines in the die socket are from a burr on the die, and horizontal lines are from putting the die in crooked, and then forcing it into place.
3. Use the die alignment guide to direct the die into the socket to prevent it from cocking and damaging the die socket. The die is much harder than the die socket. The die socket will always show evidence of improperly installed dies.
Capping: What's your action plan when capping occurs?
Let’s start with the things we must know to evaluate the root cause(s) of capping.
What is capping? When the top (cap) of the tablet splits or fractures from the body of the tablet. Capping is caused by non-compressible fines particles that migrate when the air is pushed out during compression. The fines collect at the junction of the upper cup edge and the tablet band.
- Tooling root causes:
- Upper punch cup edges curl inward…called J-hooking.
- Is the cup polished to a nice finish?
- Die wear rings…also known as compression rings.
- Punch working length must be accurate, not more than .002” variation within the set.
- A tapered die will help the air escape and improve ejection.
- Make certain all die are installed in the same direction
- Are the head flats uniform within the set? A large head flat provides more dwell and a longer compression cycle than that of a head flat that is small.
- Press setup:
- Punch penetration should be set as high as possible without losing powders from the die. Typically 2.5mm-3mm punch entrance works best.
- Overload: make certain the overload settings are set to the maximum force of the tooling.
- Pre-compression should be set very lightly at first
- Press speed should be within pre-established ranges.
- Weight control:
- Weight variation nets hardness variation. Low weights mean lower compression forces which mean the tablet will be softer. As weights increase so does the hardness. When capping occurs weight variation can be the cause of capping whether there is too much force or not enough force. In other words if a tablet caps when the tablet weight is lower (this means compression force is lower) the ability to hold the powders together fails. When weights are high it is very possible to use too much force which can fracture; breaking the bond along the band and the upper cap.
- Powders characteristics:
- Moisture continent
- Particle size distribution. Small particle have lower compressibility
- Blend uniformity; a poor blend can be the root cause for capping.
OK, so you have capping: where do you begin?
- You need to have confidence the punches and dies are in good shape, polished and they have passed inspection. If the Dies are tapered you want to verify what the taper is…is it on both sides, and is it the same on both sides. If you don’t know the answers to these questions don’t tear the press down (yet). Let’s see what we can and should do next.
- The place we always start when solving defects: “Weight”. Always minimize weigh fluctuations. Any variation in weight can and will affect capping.
- Slow the press down just to see if the capping will improve. If it does this means that the slower speed provides ample dwell and air release time. This means we can make adjustments to punch penetration, and pre-compression which should allow us to increase the press speed. If slowing the speed down did not improve the capping, then the powders may need to be looked at; it probably isn’t the press. I understand that no one wants to say it’s the powder because they think formulation changes may lead to regulatory issues, and it is possible, but not always true.
- Compression force: some granulations need a lot of dwell time and small change can mean the difference between success and failure. A robust formula is what is needed to perform at high speeds otherwise they are very temperamental, meaning any slight change in particle size, and moisture content could mean failure. Start by making a soft tablet with low force, and then make small adjustments to increase hardness. This assures that over-pressing is not the issue.
- Pre-compression force: It is best to start off making a soft tablet with very little pre-compression force. The objective is to start to form a tablet without pushing the small particles outward toward the upper cap.
Is it the Press or the Power?
Friability limits must be less than 1%. It’s commonly thought that friability is a formula related defect, and for the most part this statement is correct. However, tablet friability can be compensated for on a tablet press and in tooling design.
During compression, particles need to bind and lock together. If the powders are not cohesive, or the bond that holds particle together is broken, the result is particle loss; a friability problem.
Step 1: Study the effects of Weight Control. Controlling friability requires accurate control of tablet weight. Set the machine so weights are on target with very little variation. If weights are high this means the tablet will be harder, and low weighs means the tablet will be softer.
Insufficient or non effective binders require higher compression force to hold the particle together. A well granulated formula with a good binder can handle a wider range of compression forces and is less subject to friability failure. If the particles within the formula are friable, high compression forces can and will break particles apart. Lower compression forces may not be enough to bond particles together, again causing friability failure. The bottom line is friability can be caused by both low compression force and high compression forces and this depends on the nature of the raw materials, the process, and the type of binder in use.
Step 2: Study the effects of Dwell Time. Determine if friability is from too much, or too little force. Again, tablet weights must be accurate as described above. Slow the machine down. If friability issues improve then longer dwell is beneficial. However, if there is no improvement or if the tablet becomes more friable then too much dwell time is breaking apart the particles, or what holds the particles together (binder). Some products are very sensitive to the slightest change in pressure/force, this is why weights must be accurate. At some point all particles will reach maximum compressibility. When they reach that point the ability to lock to other particles is broken. If the binder that locks granules together breaks apart from force the result is the same.
Step 3: Study the effects of Tablet Expansion. When a tablet is ejected it expands. If the expansion is too rapid, or too great the particles break apart. A common fix is to add a taper in the die to allow for a more gentle release. Measure the expansion. The taper in the die should be about 70-80% of the expansion. In other words if the tablet expands .010” then the taper should be .007”-.008” at 6mm depth.
Step 4: Study the effects of Air Release. Entrapped air can cause the tablet to break apart. Slowing the press will extend dwell time, and give the air more time to escape. Press speed and the amount of clearance between the upper punch tip and the die also contributes to the air release. For many powders increasing the venting by increasing the taper or increasing the tip/die clearance, slowing the machine down, and using pre-compression correctly all contribute to improved friability.
Step 5: Study the effects of Tooling Condition. Die wear rings can cause friability issues. Make certain dies are inspected and replaced when wear rings are detected. Cup depth – The deeper the cup the more sensitive the tablet is to air entrapment and compressibility. Lettering, bisects, and logos all should be on the upper punch, and it is best to leave the lower punch cup plain if possible. Cup edge land and a compound radius within the cup will also contribute to better friability results. Polished punches and a formula with a well blended lubricant all contribute to the success of a tablet.
Friability is a defect: it has many root causes and for anyone to say it is simply a formula issue is somewhat misleading. Contact me to discuss your friability issues.
Automatic Tablet Weight Control, Reject Systems & Variables
Tablet weight changes can be detected and controlled by monitoring compression force.
A tablet press can be instrumented with a strain gauge that senses and reacts to compression force changes. Compression force fluctuation is the result of slight changes in tablet weights. It is normal and expected that powder blends are not perfectly uniform; this fluctuation is detected by checking tablet weights during compression. When tablet weights change they typically trend, either increasing or decreasing in average tablet weight. As tablet weights increase the compression force increases, and as tablet weights decrease the compression force decreases.
As blended powders are being compressed there is a consistent and measurable force rhythm that is detected using a strain gauge. The strain gauge sends a signal to control the volumetric powder fill level (weight adjustment cam), which controls tablet weights. Thus we have created a loop; the amount of force to compress is really reporting tablet weight. The controls can be set up to react to the compression force changes, and adjust the weights within a range, or stop the press. If the amount of force is too much, or not enough this is directly attributable to tablet weight, provided that the press is being operated correctly and all parts and components are in good operating condition.
When using an automatic weight control device it is important to note that changes to press settings like thickness, speed, force feeders, and pre-compression can be misinterpreted by the force monitor. Proper steps must be followed to prevent improper operation. Then acceptable tablet weight ranges are established so that off weight tablets can be automatically rejected.
Automatic Weight control is based on compression force. Anything that influences compression force will change tablet weight. Starting automatic press controls is much like setting the cruise control on our car; you first get going and once you are satisfied you lock it into cruise. The same must happen with the press; first get it set up the way you want it and then ask it to hold your set point (as close as possible).
Like cars there are many versions of tablet press controls; not all are created equal and it depends on the vintage. The newer the controls the more they link to other machine functions that can influence compression force while maintaining tablet weight. What I mean is; if the machine is now on cruise control and the operator decides to change anything: speed, pre-comp, thickness, punch penetration, feeder speed, and so on….any of these changes influence compression force which the machine will interpret as a need to change the weight. Some machine controls recognize changes to some of these functions and others do not…again there are many levels and ages of controls.
There are many variables that can influence the “signal” that can have a negative impact on true weight control. Consider all of these variables when evaluating a weight control system.
- Machine condition: The Turret, Pressure rolls, roll shafts, and bearing condition have a huge impact on the “signal”.
- EMF: Electromagnetic forces influence some systems differently that others; dedusters, metal detectors, radios, cell phones, material handling equipment can ruin the “signal.”
- Punch length: Punch condition and matching are critical
- Punch tip diameter and condition: set to set variation
- Punch tip relief: set to set variation
- Die condition: use of taper, die wear, point of compression
- Punch tightness and punch socket condition, punch lubrication
- Use of pre-compression
- Punch Penetration at pre & main compression
- Formula, processing, blend uniformity, lubrication, fines, moisture content, Electro static charges
- Temperature & humidity
- Press speed
- Feeder speed
- Scrapper blade condition and installation
- Tail-over-die condition and installation
- Product Head pressure
Traditional methods of checking tablet weights at given intervals does not guarantee that all tablets fall within the required weight (dosage) specification. Blend uniformity creates some of the biggest challenges. Even the best blends can drift in and out of acceptable weight specification if they are not monitored closely.
Automatic weight control systems do not necessarily guarantee accuracy. If there are other variables such as a worn machine, worn tooling, and some electromagnetic forces for ancillary equipment, then verification that controls are accurate become even more important.
The objective is simple; we are making tablets that must be held within an acceptable range. The question is: are the systems you are using providing this guarantee?