During a routine high-speed production run of a popular nutraceutical supplement, a contract manufacturer suddenly faced complete encapsulation line stoppage. Capsules were denting and cracking, fill material was spilling everywhere, and machine jams occurred frequently. The production manager’s frustration peaked as deadlines loomed, downtime increased and yield dropped. The capsule supplier’s technical team arrived onsite to find the root cause wasn’t immediately obvious. Capsule dimensions appeared within specification, equipment seemed properly maintained — yet the problem persisted.
How the capsule manufacturer handles such urgent issues can redefine the supplier-client relationship. Adopting the right approach converts these challenges into opportunities for strengthened partnership through effective resolution. Such scenarios are not uncommon in high-speed capsule filling operations and often lead to a critical question — does the problem originate from the capsule itself, from process-related factors, or from a combination of the two?
Identifying Root Causes
Manufacturers of nutraceutical and pharmaceutical products frequently encounter machinability challenges when using hard shell capsules during encapsulation. These issues are commonly reported to capsule manufacturers either as customer feedback during routine visits or as formal complaints requiring immediate resolution to maintain production schedules. Once reported, investigations are initiated by capsule manufacturers to determine whether the issue originates from capsule quality, external processing conditions, or other contributing factors.
If investigations confirm that the capsule quality is at fault — for example, due to variations in weights, moisture content, or dimensional inconsistencies — the issue can typically be resolved through Corrective and Preventive Actions (CAPA). The CAPA process aims to eliminate the defect, prevent recurrence, and ensure sustainable improvement through process capability analysis. However, encapsulation issues don’t always stem from capsule quality alone. They can be caused by non-capsule-related factors such as equipment setup, tooling condition, environmental factors, fill material characteristics, operator practices, or by a combination of capsule- and non-capsule-related factors.
When both categories contribute to the problem, the issue is described as multifactorial. In such cases, a comprehensive approach that addresses capsule quality and process-related variables is essential for successful and lasting resolution.
To examine these challenges comprehensively, this review is divided into two parts. Part one (this article) focuses exclusively on capsule-related factors that influence machinability — issues originating from the capsule itself and generally falling within the capsule manufacturer’s responsibility. These cases can be addressed through technical assessment and targeted CAPAs. Part two, to follow, will expand the discussion to non-capsule-related issues and multifactorial cases where capsule characteristics interact with process conditions, equipment, materials or human factors. Together, the two parts aim to provide a practical framework for identifying root causes and improving encapsulation performance.
The capsule manufacturer’s technical service team plays a central role in properly identifying issues and working with the client to correct them. When issues cannot be resolved through machine, material or environmental adjustments, the technical service team gathers appropriate data to report to their manufacturing and quality teams. Through precise diagnostics and close collaboration with the client, they implement CAPA to ensure reliable performance, restore confidence and strengthen partnerships.
Capsule Quality Issues
The machinability of hard shell capsules is influenced by capsule quality issues — such as discrepancies in capsule weight, variations in the cap and body dimensions (external diameters, wall thickness, lengths, dome thickness, and the length of pre-locked empty capsules) and capsule brittleness. These factors can negatively affect machinability at various stages of the capsule filling process (feeding, rectification, separation, filling and closing). They manifest as loose pieces, non-separation, improper seating of the cap or body within machine tooling, tucks, splits, dents, holes, and cracks leading to machine stoppages, loss of fill material and reduced productivity. In addition, such machinability issues may necessitate additional inspection or segregation of affected product, which can increase labor time and associated costs.
Variations in the weights of the cap and body can alter their dimensions and lead to defects. Overly large cap diameters may prevent proper seating in the upper machine segments, while oversized body diameters can cause tucks and splits. If root cause analysis identifies pin design as a source of capsule diameter inconsistencies, the capsule mold pins may need to be redesigned. Thick body walls may hinder the separation of cap and body during encapsulation, whereas thin walls can lead to loose pieces — especially in machines with air transfer systems. Excessive variations in cap and body lengths may also create issues during rectification and closing and insufficient dome thickness in the cap or body can cause dents, holes or cracks. Improper joined length of empty capsules can result in non-separation, or conversely, loose pieces.
Machinability issues due to brittleness are particularly significant for gelatin capsules. This is because water acts as a plasticizer in gelatin, and brittleness tends to occur when capsules lose moisture as a result of storage, transportation, or processing conditions falling outside the labeled specifications. Brittleness can lead to poor joining during capsule closing, pin-hole defects and machine jams, thereby reducing operational efficiency.
To minimize machinability issues and ensure consistent encapsulation performance, suppliers should use properly designed and qualified equipment, along with well-defined manufacturing processes that demonstrate acceptable process capability.
Process Capability
When capsule defects are present and investigation determines that the empty capsules are at fault, the underlying cause may lie in the capsule manufacturing process itself. Therefore, evaluating process capability is essential — following root cause correction and process restart — to confirm process improvement and enable data-driven CAPA to prevent recurrence.
To assess process capability for capsule parameters, a process capability index like Cpk can be calculated from a capability study. This section describes the statistical principles behind process capability evaluation and the importance of measurement system precision for reliable capability estimates using examples. For parameters that follow a normal distribution, Cpk can be estimated for a stable (in control) manufacturing process-one in which both the mean and variation remain consistent, with no out-of-control points.
Normal probability plots and control charts are used to verify data normality and process stability, respectively. Once these pre-requisites are verified, Cpk can be estimated using the process mean, within-subgroup standard deviation, and specification limits as shown below:
CpU = (upper specification limit - mean) / (3 × within-subgroup standard deviation)
CpL = (mean - lower specification limit) / (3 × within-subgroup standard deviation)
Cpk = Min [CpU, CpL]
Acceptance criteria for Cpk:
- Cpk ≥ 1.33: Process is stable and capable. Process capability is adequate.
- 1.00 ≤ Cpk < 1.33: Process capability is moderate.
- Cpk < 1.00: Additional process modification is required to improve capability.
As can be seen from the equation for Cpk, for a process with the target at the center of the specification limits, excessive variation and/or a mean that is not centered can lead to reduced process capability. If the data is not normally distributed, process capability can still be assessed by applying a transformation to approximate normality or using an alternative distribution that better fits the data.
Measurement System Precision
In order for the estimated Cpk to be reliable, data used to assess process capability must be collected using calibrated gauges with acceptable gauge repeatability and reproducibility — commonly referred to as a gauge R&R or measurement system variation.
- Repeatability is the variation in successive measurements of the same characteristic of the same part made by the same person using the same instrument.
- Reproducibility is the difference in the average of measurements made by different people using the same instrument when measuring the same characteristic on the same part.
A gauge R&R study is used to determine the individual sources of variation and to quantify them, providing an understanding of the variation in both the measurement system and the process. There are three study designs for gauge R&R studies: crossed, nested and expanded.
In a crossed design, operators are selected randomly and each of them measures every part. The following example illustrates a crossed gauge R&R study using cap dome thickness (measured in millimeters). Ten capsules were separated into caps and bodies, and three technicians measured the dome thickness of each cap three times using a calibrated gauge. The data were collected randomly and analyzed using the ANOVA method in Minitab software. The gauge details, the technicians’ names and the tolerance for cap dome thickness were recorded.
| Source | Standard deviation (SD, mm) | Study variation (6 x SD, mm) | % Study variation | % Tolerance |
|---|
| Tolerance = 0.15 mm |
| Table 1. Gauge repeatability and reproducibility for cap dome thickness |
| Total gauge R&R | 0.0021377 | 0.0128261 | 18.16 | 8.55 |
| Repeatability | 0.0020116 | 0.0120695 | 17.09 | 8.05 |
| Reproducibility | 0.0007234 | 0.0043402 | 6.15 | 2.89 |
| Operator | 0.0007234 | 0.0043402 | 6.15 | 2.89 |
| Part-to-part | 0.0115755 | 0.0694533 | 98.34 | 46.30 |
| Total variation | 0.0117713 | 0.0706277 | 100.00 | 47.09 |
Table 1 shows the standard deviation, study variation, % study variation, and % tolerance for each source of variation. The % tolerance for gauge R&R (precision-to-tolerance ratio) quantifies the measurement system’s variation relative to the process tolerance and is calculated by dividing the study variation by process tolerance and multiplying by 100. In the context of gauge R&R, this ratio is critical indicator of measurement system adequacy; a precision-to-tolerance ratio of less than 10% is considered acceptable indicating that measurement system variation is sufficiently small compared to the process tolerance.1 If there is excessive variation in the measurement system, the estimated Cpk value will be unreliable. In this example, the precision-to-tolerance ratio for the gauge R&R is 8.55%. As a result, the estimated Cpk can be considered a reliable measure of process capability.
Figure 1 Process capability sixpack report for body wall thickness
Estimation of Process Capability Index
The following example illustrates the estimation of the Cpk for capsule parameters, using body wall thickness (measured in µm) as a representative case. This parameter is critical to ensuring reliable capsule performance, helping to prevent issues such as loose pieces or failure to separate during filling. The estimation follows the methodology outlined previously and involves performing statistical analysis using the normal capability six-pack tool in Minitab, which provides a comprehensive view of process distribution, control and capability.
Verifying the Prerequisites
The normal probability plot for body wall thickness yielded a p-value of 0.052 from the Anderson-Darling test (Figure 1), indicating that the data follow a normal distribution.
The mean and the standard deviation control charts produced from 20 subgroups (each with 10 observations) show no evidence of shifts, drifts, trends or points outside the control limits. This confirms that the process is stable.
Determination and Interpretation of Cpk
The capability histogram confirms that all values lie within the lower specification limit (LSL=110 µm) and upper specification limit (USL = 158 µm) with a process mean close to the target (134 µm). A Cpk value of 1.37 demonstrates that the manufacturing process is capable of consistently producing capsules with body wall thickness within specifications.
If Cpk falls below 1.0 after initial root cause correction, the process requires additional modifications such as: Further centering the process mean toward target or reducing within-subgroup variation through tighter controls. To further validate a Cpk value greater than 1.33, it is recommended to conduct machinability testing, monitoring for any loose pieces or non-separation during the filling process. This step ensures that the statistical capability aligns with real-world performance and capsule reliability at the customer’s site.
Capsule Quality Issues Requiring Redesign of Mold Pins
When a process is deemed incapable of achieving the desired capsule quality, a redesign of mold pins may be required. This option should only be pursued after all reasonable process improvements have been fully evaluated and exhausted. The investment into new pins for hard capsule machines is considerable, as each machine contains a large number of mold pins.
The increasing speed of capsule filling machines continues to raise performance expectations for two-piece capsules. Over the past decade, filling machine output capacities have nearly doubled, driving a need for greater automation in capsule handling. As a result, certain mechanical and handling stresses increasingly test the limits of existing mold pin designs. Many capsule transport systems operate pneumatically, creating turbulent transfer conditions that can lead to loose pieces — a problem for traditional two-piece capsules engineered to separate quickly and easily during filling. During closing, when the cap and body join at high speed under dynamic conditions, precise alignment across millions of capsules becomes critical. Consequently, capsule pin designs that were once sufficient are now encountering new mechanical and operational challenges.
When these challenges persist despite process optimization, a systematic redesign approach is warranted. Continuous improvement methodologies such as DMADV (define, measure, analyze, design, and verify) provide a structured framework. This involves defining objectives, collecting data across materials and equipment, iterative prototype testing, and final qualification under production conditions.
Conclusion
Machinability issues linked to capsule-related factors can disrupt production, increase waste and jeopardize delivery timelines. The technical service team for the capsule supplier plays a crucial role in correctly identifying these issues, communicating them to the capsule manufacturing site, and participating in the investigation and CAPA implementation.
Be on the lookout for part two of this series, which will examine non-capsule-related factors such as equipment setup, tooling design, formulation behavior, environmental conditions and cases where capsule characteristics interact with these process variables to affect performance.
References
- Wheeler, D. (2023, Sept. 11). Using the Precision to Tolerance Ratio: What Does This Ratio Tell Us? Quality Digest. 1-13.
The author gratefully acknowledges Steve Lee and Tom Breshamer for technical input and valuable guidance on aligning the article’s structure with customer-focused problem-solving approaches to encapsulation challenges.