As almost all vibratory mass finishing processes are wet, (they use a controlled amount of water and liquid compound combined with the finishing media) it is very important to select the correct cost-effective and productive method to dry the components.

It is usually a prerequisite of any requirement of the vibratory mass finishing technique that after the process the components should be dry, clean, stain-free and protected against any possibility of corrosion. There are a number of ways to achieve this target; the most common is the vibratory maize drier, also known as the corncob drier.

The principle of the vibratory maize drier has been around for many years and is typically used for the high-speed drying of large volumes of small- to medium-sized components. Modern machines have cost-effective heating systems and incorporate many features that facilitate automation and improve efficiency.

The drying media is normally maize (corncob): the crushed maize is granular in shape and calibrated into different sizes. The appropriate size for each application is selected depending upon the size and shape of the particular components in question. Maize is an incredibly absorbent material and is extremely effective at absorbing moisture, other drying materials can be used such as crushed walnut shells, glass balls, nylon and plastic granules but none of these materials are as absorbent as maize or have the same recovery properties.

Typically, the vibratory maize dryer is linked directly to the (wet) process machine; the transfer of components is normally direct and automated so there is no requirement for operator intervention.

In "continuous" mode, components enter the vibratory dryer at the base of the spiral chamber; the components are cushioned by the soft/dry maize so part-on-part contact is minimal and component damage avoided, then both components and maize are transported around the spiral inclined chamber until they reach the maize/component separation screening deck. Here, the granular maize passes through a (normally mesh) separation screen and the components are gently transported out of the machine into collection trays or directly onto the next operation. Components that require additional drying cycle times can be dried in "batch" mode: here, a batch of components are loaded into the dryer; the separation door that bridges the gap to the entrance point of the separation screen is left open for the desired cycle time; when the components are completely dry, the separation door is activated (normally automatically) and the components are discharged from the machine. Batch mode is often used when the components have internal holes or complicated shapes that require longer drying times or some additional polishing as the maize will enhance the aesthetic appearance.

Modern vibratory dryers are available with many different features, depending upon the application of the heating method possibly via coiled elements positioned under the drying chamber or external hot air blowers. The latter is normally applicable when the drying chamber is lined with polyurethane or rubber to protect very delicate components from scratching, or the chamber can be enclosed with a simple dust cover or a hinged cover that also incorporates dust extraction and noise reduction, so all the operating cycles can be very easily automated.

While the vibratory maize dryer is probably the most cost-effective method to rapidly dry large volumes of components it cannot always be applied - What if?

The components are large and or heavy?
The components have holes or apertures where the maize or other media will stick?
The components are extremely fragile?

Fortunately, other types of machine are available.

The hot-air drying oven is normally used when the components in question are large, heavy, sensitive to damage, or may have many different holes where a drying media will lodge.
The hot air is thermostatically controlled and recycled in the drying chamber. A stainless steel woven wire mesh belt transports the components through the drying chamber. Drying times are variable and machines are available in lengths and dimensions to suit each application.

The drum dryer uses a drying media (normally Maize/Corncob) the same as the vibratory dryer but is especially applicable when the components are cup-shaped or when extremely high through-put and additional screening/separation times are required. When the components enter the drying section of the continuous scroll, they are completely covered in the heated maize, and the gentle turning of the components ensures complete removal of any moisture, while a relatively long separation section with gently induced tumbling action ensures complete media removal.

The centrifugal dryer is perfect for drying very small components such as rivets, screws, nails etc., however this technique is not continuous, meaning the components must be dried in batches. For component batches up to 25 kg, the machine is normally loaded and unloaded by the operator manually, while larger batches utilise a power-lifting device to aid handling.

Good Vibrations
by Paul Rawlinson
Contributing Editor MFN & Business Development Manager, Aerospace International for Rösler

Tel. +49.9533.924 647
Fax +49.9533.924 601
E-mail: P.Rawlinson@rosler.com