Rubber & Tyre Machinery World

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A Beginner’s Guide to Autoclave

You use an Autoclave or Vulcanizer (Vulcanizing autoclave) to convert natural rubber into a cured and chemically cross-linked rubber product. A vulcanized rubber is less sticky and gives you superior mechanical properties.

Rubber Vulcanization (a term coined by Thomas Hancock) “process” was discovered accidentally by Charles Goodyear in 1839.

If you are curious about history, this is how it goes – Charles Goodyear left a piece of natural rubber mixed with sulphur on a hotplate one evening. Overnight, it turned into elastic rubber and that caught his attention! Since sulphur and heat were associated with the ancient Roman god Vulcan, the process was named vulcanization (or vulcanisation).

Your friends in rubber industry use autoclaves to cure tires, hoses, and many other products but not limited to vulcanization and forming of extrudates (like car radiator hoses), rubber mats, sleeves, joints, gaskets and boots. Typically these are products that you cannot achieve on a standard press-based compression molding process.

Depending on the type of your rubber, the vulcanizing process can occur from room temperature (as in Silicone) up to 350°F (170°C) or more like in case of Tire.

Autoclave 2

The Machinery:

Autoclaves are predominantly cylindrical pressure vessels with lids or doors to process your rubber parts that require exposure to elevated pressure and temperature. They are available in a wide range of sizes and design pressures in horizontal (or sometimes vertical) configuration.

The key component of your autoclave is the door. For a manufacturer, this is also the critical component in cost of autoclave construction. On one hand, your operator must be able to open and close the door quickly and easily; on the other, the door must satisfy stringent safety requirements of manufacturing and usage.

Let me explain.

Rubber vulcanizing is a batch process. Your autoclave door must be of full diameter to allow easy access to the chamber inside. So you need a fast-acting door to reduce batch change time and increase productivity. Depending on the size of your autoclave and manufacturer’s offered features, there are several types of fast-opening doors.

An autoclave design is driven by various safety standards, primary among them being the ASME Pressure Vessel Code. Of all safety-related concerns, the most critical are those which relate to the door’s operation. The manufacturer of your autoclave should ensure that the door seals tightly against rated pressure at the highest shell temperature; operate readily and quickly and meets all safety guidelines of a pressure vessel. Regular testing is a must along with foolproof interlock mechanisms to prevent door opening under pressure.

Autoclave 1

Leading manufacturers deploy Solid-Modelling and Finite Element Analysis (FEA) to design and validate autoclave pressure vessels and quick-opening doors.

CE Standard in Europe applies to vessels as well as to electrical controls.

The process medium in autoclaves may be steam, a combination of steam and air or inert gas, “Dry Heat” produced by electrical heaters, steam or hot oil autoclave jackets, or circulating hot gases. Despite electric heat being 100 per cent efficient, most precise to control, and lowest maintenance; the cost per kilowatt-hour makes them expensive for your use. And because rubber vulcanizing cannot be done in an air atmosphere, you mostly use steam.

Two types of autoclaves are in common use – Non-jacketed and Jacketed.

In the non-jacketed type, steam is introduced directly into the autoclave chamber. The steam condenses on the walls of your autoclave and on the surface of your rubber products under vulcanization. However, this leaves behind a mark on the surface of your product. And when you adopt preventive measures, you end up getting a dull finish on your rubber products. For some products like Radiator hoses for cars, you can cure in open steam.

Autoclave 4

The jacketed autoclaves has a double wall. Steam is circulated in the jacket to provide heating. There is  no direct contact of steam with your rubber products. An inert gas (like Nitrogen) is then introduced in the autoclave to eliminate oxidation. (Oxidation aids polymer degradation and hence you need to eliminate them). Gases are normally poor conductors of heat and thus increases the curing time. You may also come across this process referred to as “gas curing” by technologists.

Additionally, if you have brightly colored articles, the jacketed autoclave lends a good surface finish on your rubber product.

The vulcanizing process in an autoclave is a function of temperature and time. Traditionally, in most applications there is no indication of when your cure is complete. For this reason, to ensure that all components in the autoclave have been universally cured, your process cycle times are usually longer than is necessary.

Modern autoclaves come with PLC controlled systems and offer extended features including variable temperatures, pressures, cycle times, cycle and system alarms, multiple cycle recipe storage and selection.  Operator Consoles with HMI provide an operator friendly interface with touch screens, for cycle parameter entry, cycle/system/alarm status monitoring, operating and diagnostic messages.

pc smart autoclave

When you invite offers to purchase an autoclave, you should not be surprised if price variations confuses you or you are unable to comprehend immediately – be it prices among manufacturers and between features.

Price is sensitive to some factor and insensitive to others. For example, doubling the design pressure might increase the cost of the autoclave by over fifty percent. And doubling the diameter might more than double your purchase price. On the other hand, you may find that increases in length are inexpensive (relatively).

So, your purchase price varies as a function of what has been designed and built into your proposed autoclave. Options of features offered by your suppliers has to be carefully decided so as to optimize your overall investment.

Operation Cycle

Your typical cycle of autoclave curing has the following steps.

The autoclave is loaded with your rubber component, and the connections are made to the autoclave. The door is closed and locked. Pressure is applied until the required level is attained. The circulation fan starts. Heating begins and is maintained at a specified ramp up rate. Once the required temperature is reached, a timed soak at that temperature begins and runs for the necessary duration. At the end of the soak period, the cooling function brings the temperature down to a set value at a specified ramp rate. The vessel is depressurized and the circulation stopped.

Steam curing requires a great deal of manual work from one processing stage to the other, and this also contributes to the high cost of your autoclave cured products. Hence, some of your new generation peers advocate the more advanced microwave process (more on that later…).

Summarizing autoclaves are pressure vessels for rubber product vulcanization, available in a wide range of sizes.  A discontinuous curing method, the curing of your rubber component takes place in a vessel (autoclave) where pressurized steam is the medium of heating.

Let me have your comments to add; or contact me if you seek more information on this machinery and their manufacturers.

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