Tablet quality, such as capping or lamination, can be affected by high ejection forces. Today’s Tech tip discusses how to set your maximum ejection force limits and what to do if the ejection forces are too high.
Ejection forces are mainly the product of radial die wall force multiplied times the coefficient of friction between the tablet’s sideband and die wall. Some things to keep in mind that affect ejection force:
- Higher press speeds increase ejection force.
- Higher compaction pressure increases ejection force.
- Larger tablets have higher ejection forces.
- Die bore wear will increase ejection force.
- Die bore filming will increase ejection force.
- Punch retainers that are too tight can increase ejection force.
Ejection Force Limit
To calculate your maximum ejection force limit, first we’re going to find the size of your tablet’s sideband which can be calculated by:
A = P * E
Where,
A = Surface area of the tablet’s sideband
P = Tablet Perimeter
E = Tablet Edge Thickness (sideband height)
For example,
10mm diameter round tablet
Perimeter = 31.416mm (Pi * Diameter, also shown in the title-block of your tablet drawing)
Tablet Thickness = 5mm
Cup Depth = 1mm
Edge thickness (sideband height) = Tablet thickness – 2 * cup depth = 5mm – 2 * 1mm = 3mm
A = 31.416mm (5mm – 2 * 1mm) = 94.248mm^2. This is the area of your tablet’s sideband.
Next, we want to multiply this area by a pressure to get your force limit. Force = Pressure Area. In most cases to avoid capping or lamination, it is best to target between 3 MPa (Megapascals) and 5MPa. This results in targeting a maximum ejection force limit of 5MPa * 94.248mm^2 = 471 Newtons. Adjustments may then be made to this value depending on the robustness of the tablets being produced.
Mitigation
If ejection forces are too high (causing tablet defects or damage to tooling and the ejection cam), then here are some ways to help.
- Lubricant added to the formulation is helpful in reducing friction which can reduce ejection forces. Since lubricants typically also reduce tablet strength and increase disintegration time, it is vitally important to use the minimum amount of lubricant necessary. Optimizing the amount of lubricant to be added in the formulation is done with a Lubricant study. Lubricant studies will measure ejection forces with varying amounts of lubricant % across a range of compaction pressures.
- Add tapers to your die bore.
- If filming is observed in your die bores, then
- Check the lower punch tip’s scraper edge for sharpness and refurbish or replace tools if worn.
- Check if your formulation is heat sensitive. For more information on addressing heat sensitive formulation’s challenges, please contact us.
- Check your punch to die clearances. Contact us for details on optimizing clearances.
- Check die wear. If you see serious ringing (see photos), then replace the dies.
- Check punch retainers to ensure they barely support the weight of the tooling and no more.
- Use the minimum amount of force needed to reach target tablet hardness and thickness.
- Reduce press speed.
Read also: Dwell Time Calculation Formula
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