Capsules represent a widely used oral dosage form for delivering medicines and supplements, serving to mask unpleasant tastes and odors while providing a shell that protects the contents until digestion. These specialized containers are generally classified into two main types: hard-shelled capsules, which consist of two pre-fabricated, telescoping pieces, and softgel capsules, which are single, hermetically sealed units. The manufacturing processes for these two forms are distinct, reflecting the different physical properties and fill materials they are designed to accommodate. Understanding the production methods reveals the engineering and chemical control necessary to create reliable encapsulation.
Essential Capsule Shell Materials
The primary material for most capsule shells is gelatin, a protein derived from collagen found in animal skin and bone, often sourced from bovine or porcine origins. Gelatin dissolves predictably in the stomach. Hard gelatin capsules typically contain about 12 to 16 percent water, which maintains the shell’s flexibility and integrity.
A popular vegetarian alternative is Hypromellose (HPMC), synthesized from plant cellulose. HPMC capsules benefit moisture-sensitive contents because their lower equilibrium moisture content (4 to 6 percent) minimizes water migration into the fill material. Both gelatin and HPMC shells incorporate colorants and opacifiers, such as titanium dioxide, to protect light-sensitive contents and distinguish products. Softgels require higher concentrations of plasticizers, like glycerin or sorbitol, added to the gelatin formula to provide the characteristic pliability and softness of the one-piece shell.
Steps in Hard-Shell Capsule Production
The manufacture of two-piece hard capsules occurs in two distinct, highly automated phases: creating the empty shells and then filling them with the active ingredients. Shell production begins with preparing a concentrated solution of gelatin or HPMC in demineralized water, which is heated and transferred to the dipping section of the capsule machine. This viscous solution may contain up to 40 percent gelatin by weight.
The shell-forming process uses pairs of precision-machined stainless steel pins, which are dipped into the liquid polymer solution. The temperature of the pins is below the gelling point, causing a thin film to solidify on the surface. The pins are then slowly rotated and moved through a drying furnace, where blasts of temperature- and humidity-controlled air remove excess moisture, hardening the film into the capsule shell.
Once dried, the cap and body halves are stripped from the pins, trimmed to their exact length, and then automatically joined together. These empty, two-piece capsules are shipped to manufacturers for the second phase: filling.
High-speed automated filling machines orient the capsules, separate the cap from the body, and fill the body with the specified dose of powder, pellets, or granules. The filling is achieved using precise mechanisms, such as tamping pins or dosing disks, which compress the powder mixture into a plug and then eject it into the capsule body. After the contents are accurately dispensed, the cap is replaced over the body, and the two pieces are locked together by pressure. Modern capsule fillers can process tens of thousands of capsules per hour.
The Softgel Encapsulation Process
Softgel capsules are produced using the highly integrated rotary die method, which forms, fills, and seals the capsule simultaneously. This process requires the fill material to be a liquid, suspension, or semi-solid with specific viscosity and stability characteristics. The shell material, a highly plasticized gelatin solution, is cast onto cooling drums to create two continuous, uniform ribbons of film.
These two gelatin ribbons are fed between rotating die rollers. The rollers have carved pockets that determine the final size and shape of the softgel. As the ribbons converge, an injection wedge positioned between the rollers introduces the liquid fill material.
The injection of the fill material causes the gelatin ribbons to swell into the die pockets. Pressure from the die rollers, combined with heat from the wedge, seals the two halves of the ribbon around the fill material. The die system then cuts the filled and sealed capsule from the excess ribbon material.
The newly formed softgels immediately undergo post-encapsulation steps to achieve final firmness and stability. The capsules are first transferred to drying tumblers to remove surface moisture, then moved to drying tunnels where controlled humidity and temperature stabilize the shell. This drying prevents sticking and ensures the shell achieves the necessary structural integrity for handling.
Quality Checks and Product Stability
Quality control is applied at multiple stages to ensure the finished capsules are safe and deliver the intended dosage.
- Weight uniformity testing confirms the amount of encapsulated material is consistent across the batch. This is achieved by assessing the mass variation for softgels or the content uniformity for hard capsules.
- Dissolution testing verifies the capsule shell ruptures and releases its contents within a specified time frame when exposed to simulated body fluids. For immediate-release products, this time is typically set to 30 minutes, ensuring the active ingredient becomes available for absorption.
- Physical stability checks, including monitoring moisture content and performing disintegration tests, predict the product’s shelf life and determine appropriate storage conditions.
To enhance the security and integrity of hard capsules, an optional process called banding or sealing may be applied after filling. This technique involves applying a narrow band of gelatin around the junction where the cap meets the body. This additional seal prevents tampering and improves the capsule’s resistance to moisture vapor penetration.