Break force testing (also called hardness testing) is commonly viewed as a test method suitable only for tablets. However, it also plays an important role in qualifying empty hard capsules and verifying the quality, consistency, and resiliency of finished hard capsules and softgels. In addition, it is a valuable QC and R&D tool.

Some manufacturers offer breaking-force testers that accept both tablets and capsules. Most dual-mode testers, however, can only perform a few types of capsule tests. Other manufacturers offer more versatile dedicated capsule
testers. All tests described in this article used a 400-kilopond (Kp) benchtop softgel breaking-force tester equipped with a grooved tongue [1]. The tester was programmed to perform variable-input tests, which allowed the operator to set key test parameters (e.g., capsule dimensions, depression in millimeters (mm), depression percentage, and force setting).We conducted four types of tests: three depression tests and a burst test. Each test used five randomly selected test samples. To facilitate cleanliness and prevent spills, liquid-filled hard capsules and softgels were enclosed in zip-lock poly bags before burst testing. All empty hard capsules were size 0 unless noted otherwise. The tests, described below, can be administered to either the length or width of the capsule, although for softgels, it’s most informative to test the width. (Measuring the length requires you to apply force on the tip or dome of a softgel and can result in a misleading reading unless you are testing for dome strength or other dome-related characteristic). For dome strength and force-to-lock tests, force was applied only along the length of hard capsules. To obtain accurate and meaningful results, use separate capsule samples for dome-strength and force-to-lock tests.
Related product: 6D(SG) tester for soft gelatine capsules

TYPE OF TESTS

Test type 1
Type 1, the fixed-distance depression test, measures the force required to depress a sample by a specified number of mm. For example, a test sample with a 23.09-mm beginning dimension (D1) requires a force of 1.3 Kp to depress it 3 mm to 20.09 mm, the ending dimension (D2). (The 3-mm setting is the test variable chosen by the operator for this particular test.)

Test type 2
Type 2, the percentage-distance depression test, measures the force required to depress a test sample by a given percentage of a chosen dimension. For example, a 9.74-mm (D1) sample requires 10.8 Kp of force to compress it 25 percent to 7.30 mm (D2). (The 25 percent setting is the selected test variable.)

Test type 3
Type 3, the fixed-force depression test, measures the depression a specified force makes on the sample. For example, a 6 Kp force depresses a 9.81-mm (D1) sample to 8.31 mm (D2). (The 6 Kp force is the selected test variable.)

Test type 4
Type 4, the burst test, is virtually the same as a tablet breaking-force test. The tester first measures the sample’s D1. Next, it measures the force required to burst the sample and the D2 in which the sample failed. For example, a 7.35-mm (D1) test sample requires a force of 15.7 Kp to cause it to burst. The burst event took place at 3.01 mm (D2).

TIME TEST PARAMETER

Full-function capsule testers can incorporate a time feature to selected tests. For example, the fixed-force depression test can apply the selected force for a predetermined period (e.g., 1 minute). Employing the time test parameter allows you to ascertain the capsule’s elasticity characteristics. You can also apply the parameter to fixeddistance and percentage-distance tests.

TEST REPORTS

The test reports are simple to understand, because many of the fields are self-evident. However, a few of the symbols might be a bit unfamiliar, so a brief explanation is warranted. The average of the test results is “sum (Xi)/n.” The highest test result is “Xmax” and the lowest is “Xmin.” The difference between the highest and lowest result is “Xmax-Xmin.” The standard absolute deviation is “Sabs” and the standard relative deviation is “Srel.” The number of test samples is represented by “n” and the number of test results is represented by “0.” The header contains the product identification, the batch number, a description of the test, and the date and time. Typically, a report would also include a graph and a histogram.

DOME STRENGTH

One of the most important and informative breaking force tests is the dome-strength test, which is applied to hard capsule caps and bodies. Dome strength indicates how well the capsule will withstand the filling operation, especially during closing. Generally, capsule domes that cannot withstand 2.0 Kp are likely to dent, fracture, or shatter. Knowing the dome strength of each lot of empty hard capsules can help you reduce waste, improve quality, and maximize production.

We used test type 4 to test cap and body dome strengths. Figure 1 shows a dome test setup. While we used the same test procedure for the cap and the body, we separated the cap from the body to test each independently.We
positioned the samples on the tester with the dome pressing against the tester’s anvil, which is a stationary part attached to the load cell. (Using a grooved tongue makes it easier to align the sample.) Figure 2 and tables 1-3 show results of the dome-strength tests: Figure 2 on a gelatin cap, Table 1 on a gelatin body, Table 2 on an HPMC cap, and Table 3 on an HPMC body. While Figure 2 shows a report from the breaking-force tester, the tables throughout the article simply summarize report data. Note that all the dome strengths are well above the 2.0-Kp threshold, with the gelatin body domes being especially strong and the gelatin caps exhibiting the most consistent resistance.

FORCE-TO-LOCK

Another useful breaking-force test is the force-to-lock test, which determines the force necessary to fully engage the locking rings and close the capsule. This test is particularly useful when you are filling capsules with coarse,
abrasive, or sticky materials. It also helps determine whether the fill weight is too high or whether additional lubricant should be added to facilitate capsule closing. We used test type 1 to measure the force required to fully lock the samples. We employed a micrometer to measure two or three empty random samples to determine the difference in length (in mm) between their prelocked and the fully locked dimensions. We entered the average difference as the test variable. Then we tested a couple of pre-locked samples. (If one or more of the domes become dented, reduce the length of the variable measurement and continue preliminary testing until no evidence of denting remains and the samples fully lock.) Next, we randomly selected samples and began testing. Although no force-to-lock tests were conducted on filled capsules, doing so requires filling sample capsules by hand to the maximum fill weight designated for the product, then closing them just enough to keep the body and cap from coming apart. Enter the difference (in mm) between the pre-locked and the fully locked dimensions and begin testing. (Dents indicate that an incorrect fixed distance has been used, the capsule is overfilled, or the fill material requires additional lubricant.) The proper closed length can be verified for both tests by using a capsule “go/no-go gauge” provided by your capsule supplier. The gauge is a machined die or caliper in which you insert a capsule to determine its length (Figure 3). Tables 4 and 5 show the results of the force-to-lock tests: four on empty gelatin shells and five on empty HPMC shells. The gelatin shells required an average of 0.2 Kp more to lock than the HPMC shells. Both shell types exhibited consistent locking force measurements coupled with low standard and relative deviations.

BURST

The burst test is a breaking-force test useful in determining the quality, consistency, and resiliency of filled hard capsules and softgels. It also helps document drying/curing progress and end points. It is an informative R&D tool for softgels, especially when combined with the other softgel tests described in the next section. To conduct burst testing on liquid-filled hard capsules and softgels, we used test type 4. The photo on page 17 shows a burst test setup. Each product requires a separate, distinctly different test method. For liquid-filled HPMC capsules, we inserted the test sample into a poly bag and positioned it so that force was applied to the width (D1), not the length. Then we recorded the force and the dimension (D2) at the time of the burst. Table 6 shows the results of the burst test on liquid-filled HPMC capsules. The starting dimensions (D1) of the capsule samples were extremely close and consistent. Meanwhile, the force required and the dimensions of the samples (D2) at the time of the burst event were less consistent. For example, samples 1 and 5 each have the same D1, but sample 1 required 15.7 Kp to cause it to fail while sample 5 required 21.9 Kp (40 percent more). If this difference were prevalent throughout a meaningful sample size, it could indicate poor banding or shell quality. For softgels, the goal of burst testing is to ascertain the integrity of the shell’s seal. We performed two separate burst tests, each with its own set of randomly selected samples.We tested the Y axis (length) of one set of samples and the X axis (width) of the other set of samples. (This test is very similar to the burst test for liquid-filled hard capsules.) To test the Y axis, we positioned the sample so that the seal was at the top and bottom or was parallel to the anvil. To test the X axis, we positioned the sample so that the seal faced the anvil or was parallel to the tester’s tongue. Figure 4 shows samples in both positions. Tables 7 and 8 show the results of burst tests on softgels: seven on the Y axis and eight on the X axis. Test report data indicate the Y axis is much stronger than the X axis. The Y axis required an average of 18.3 Kp to cause a burst, while the X axis required a low of 4.4 Kp. Sample 5 in the X axis test is either an anomaly, an indication of poor quality, or a result of the sample shifting during the test.

OTHER SOFTGEL TESTS

We conducted three additional softgel tests (types 1-3), all of which provide force data relevant to the physical characteristics of any given shell formula and, as such, are excellent R&D tools. They can also provide periodic data to document curing and drying and establish relevant end times. Each test requires its own set of randomly selected samples.

During a fixed-distance depression test (type 1), we depressed softgel samples a predetermined fixed distance (2 mm) and determined the force required to do so. See Table 9. The force required was reasonably consistent throughout the samples, with the exception of sample 4, which required approximately half the force of the majority of the test samples. Such a low force measurement suggests sample 4 was either not completely filled or had
a structural abnormality.

During a percentage-distance depression test (type 2), we depressed softgels a predetermined percentage (25 percent) of their D1 to determine the force required. See Table 10. Again, we saw reasonably consistent results, with the exception of sample 3, which required much less force. This low force result indicates that something is amiss with the quality of this particular lot number, especially when we consider the results of the softgel fixeddistance test.

During a fixed-force depression test, we depressed softgels using a predetermined fixed force (6 Kp) to determine inconsistencies of D2. See Table 11. The results showed a standard relative deviation of only 1.3 percent.

CONCLUSION

The 12 tests described above demonstrate the usefulness of conducting breaking-force testing on hard capsules and softgels. They have the potential to provide significant test data that can help you make informed decisions related to product development, quality, and production efficiency. Currently, there are no USP, FDA, or other guidelines related to testing hard capsules or softgels outside of disintegration, dissolution testing, and weight. Force tests
for empty and filled hard capsules require no predetermined test parameters since they are straightforward destructive tests designed to measure critical points of physical failure. Force tests for softgels require test parameters that should be determined by your QA/QC department.