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?