Scraper Seals – Design and Use

The advent of scraper seals for tablet presses was a revolutionary change to improve press performance.? Seals allow faster production speeds due to less tool binding, fewer tablet defects from contamination, and longer life of cams, pressure rolls, and tooling.

Seals help keep oil or grease in the turret guides rather than drip down onto the die table and keep formulation from migrating into the guides fouling the tooling or pressure rolls. ?Seals also may improve tablet weight control as they can provide additional anti-flight performance beyond punch retainers.

Note that not all tablet presses are equipped with scraper seals (particularly older models), but virtually any rotary tablet press can be retrofitted to accept seals.? Upgrading an older machine to accept seals is a worthwhile investment.? Contact Natoli Engineering for pricing and options for your tablet press.

The design of scraper seals includes:

·???????? Material

·???????? Hardness

·???????? Casings or unibody construction

·???????? Dimensional sizes and tolerances

·???????? Single-lip or double-lip

Most scraper seals are made from FDA-compliant polyurethane due to their mechanical properties as you need the seal to be flexible and elastic, easy to injection mold, have good dimensional stability, and exhibit a long shelf life.? Other materials are sometimes used, typically for better wear resistance or the seal does not need to be FDA-compliant.

The hardness of the plastic helps determine the elasticity of the material and the wear resistance.? Harder plastics make the seal more difficult to install but can provide a stiffer resistance for punch retention and better wear resistance.? Softer plastics allow for easier installation but less punch retention and service life.? Most scraper seals are rated around 90 Shore Hardness (the higher the number, the harder the material).

Casings are a design feature for some scraper seals and may be integral to the fitting of the seals for some machines.? The plastic inside the casing can be press fit, providing a stiffer response for anti-flight than a seal without a casing.? Scraper seals with casings tend to be more expensive and installation a little more complex.? ?Seals without casings often can be removed or installed easily by hand with no tools.?

A scraper seal’s outside dimensions are sized for a specific press make and model.? The inside diameter should be slightly smaller than the punch barrel diameter to provide an interference fit.? The amount of drag between the scraper seal and the punch is highly dependent on this interference and the seal’s I.D. tolerance.? ???

Seals can be single-lipped or double-lipped, depending on the Press Manufacturer’s seal specifications.? Single-lip seals are prevalent in vintage machine models while most new tablet presses use double-lipped seals.? Double lip seals provide superior retention of oil versus a single lip design.

Change out these seals as needed to prevent loss of oil into the product or when upper punch retention is diminished.? These seals are not meant to stay in for long periods of time like an oil seal.? Change these when leakage reaches an unacceptable level.? Dust cups or bellows can prolong this duration.

Inspect seals for signs of damage at each press setup.? One of the most common set-up mistakes in the field is keeping worn or damaged seals in place past their useful service life.

Notching a scraper seal to allow a keyed punch to pass through the seal with its key is never recommended.? Rather, select the proper key location for proper fit and function.? Contact Natoli Engineering for information regarding key location for your presses. 636-926-8900 or visit us online at https://natoli.com

Todd Martin

7/23/2024