a starters guide on compression moulding and autoclave

Machinery World

Rubber & Tyre Knowledge On-The-Go Special Supplement

http://rubbermachineryworld.com

A Starter’s

GUIDE

Vulcanizing Autoclave

Compression Moulding Press

Inside:

How To Select?

This edition features two key machinery used in the

rubber industry viz. Autoclave and Compression

Moulding.

Autoclave is a vulcanizing equipment, and the other

is the equipment used in your most common rubber

moulding technique - compression moulding. These

contents first appeared on our portal and witnessed

high traffic.

This high interest level, prompted us to publish the

details in this Knowledge On-The-Go Special

Supplement.

Our previous issue on Mixing Mills was well-received.

Thanks again to each of you - esteemed readers and

supportive advertisers.

While, the last edition was tracking the advancements

of a very popular rubber machinery, this edition is

aimed at fulfilling the curiosity of the new generation

of business leaders - who are either taking over from

their parents or those young entrepreneurs setting up

their own venture.

Hence, the theme - ‘A Starter’s Guide’.

I hope you find this special supplement different and

informative.

As always, I welcome your feedback on

[email protected].

BestRegards

Prasanth Warrier

| Editor’s Note |

A Starter’s Guide

Autoclave is a

vulcanizing

equipment, and the

other is used in your

common rubber

moulding technique

viz. compression

moulding.

3

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| COMPRESSION MOULDING |


Image: Bharaj Machineries Make Automatic Compression Moulding Press

5

COMPRESSION

MOULDING

MACHINERY

Compression moulding is the

most common moulding

technique you will notice in

the rubber industry.

The other two are transfer

moulding and injection

moulding.

Compression moulding is a

closed mould process and you

apply high pressure on the job.

It is best used when you have

low to medium production

volumes for larger parts

where tightest tolerances and

flawless finishes are not

required.

Examples may include

diaphragms, valves and

gaskets, Dampers, O-rings,

seals, rubber wear parts, etc

depending upon their areas of

application.

Prasanth Warrier



Schematic From JW Elastomer Engineering Guide

As shown in the schematic here, in

compression moulding, you place an

uncured rubber profile in a heated

mould. Then you soften the uncured

rubber by the heat and press them

forcing the rubber compound to fill

the mould cavity. The rubber is thus

made to conform to the shape of the

mould and then you cure it to

produce your finished article.

6



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Compression Moulding Press:

The machinery you will use is

called Compression Moulding

Press. These presses are mostly

hydraulically driven, where a

hydraulic pump powers a

hydraulic cylinder (or cylinders)

that drive the ram or slide.

(Caution: there are pneumatically

powered presses too).

Hence, the terms that apply to a

standard hydraulic press are

relevant here.

Hydraulic Press Terms

Stroke: The amount of possible ram travel.

Stroke is the total distance that the ram can

travel, from full extension to full retraction.

Shut Height: The distance between the bed

bolster and the ram bolster when the ram is

fully extended. This is also commonly

known as the Closed Height. As standard,

this dimension is usually within “0.25” due

to assembly processes.

Daylight: The distance between the bed

bolster and the ram bolster when the ram is

fully retracted. This is also commonly

known as the Open Height.

Bolster: The removable plate that serves as

the working surface for the bed and ram.

8

Terms & Image Taken from Beckwood Hydraulic Presses and Machinery



The plate is usually bolted to the bed and

rams substructures.

Bed Height: Bed Height is the distance from

the bottom of the hydraulic press structure

to the working height or top of the bed bolster.

Remote Power Skid: Some press applications

may require that the hydraulics be located

remotely from the press itself. Others may

preclude the power system from being able

to be installed at the top around the crown

structure of the press. In these cases, the

power system and even the controls may be

designed into a separate unit capable of being

placed adjacent to the press or away from the

press.

Heated Platens: These are plates that have

heating capabilities.

Image: A 60" Mixing Mill With Advanced Features,

Controls, and Automation from Bharaj Machineries.



Hydraulic presses are

preferred by your peers in the

industry because they give

Performance, Reliability, and

Versatility. This means you can

get full pressing force (or

tonnage) anywhere in the

range of stroke using hydraulic

press unlike a mechanical

press.

By varying the ram speed and

other operating parameters,

you can optimize cycle time for

each of your job. Further, you

could customize your bed size;

say lower tonnage on a larger

work area or higher tonnage

on a smaller area of work.

You will see that compression

moulding presses offered by

most reputed manufacturers

are vertical and the moulds

can be heated using electric

rods, steam, oil, water or other

medium.

Depending on requirement,

your moulds may be fixed to

both the platens; or part fixed

to a platen, or totally free to be

drawn out. When you have

free loose moulds, your

operators need to load and

unload them on a press table

manually. You call it charging

(loading) and stripping

(unloading) by hand.

On the other hand, the

presence of ejector pins

or strippers on fixed

mould presses will speed

up your operation.

One of the platens is

fixed in your

compression moulding

press. When the lower platen

is raised or lowered in the

vertical plane, you call it

upstroking; while in down-

stroking, the upper platen is

the driven plate (moving up

and down).

Hydraulic presses are preferred by your

peers in the industry because they give

Performance, Reliability, and Versatility.

Bharaj Machineries Make Automatic Vacuum Type

Compression Moulding Press 10



How will you decide (select)?

Apart from safety and ease of

operations, your final choice

of a compression moulding

press depends on these six

important considerations:

1. Calculation of the proper

amount of rubber for your

product.

2. Calculating the minimum

amount of energy required

to heat your rubber.

3. Calculating the minimum

time required to heat your

rubber.

4. Identifying the appropriate

heating technique.

5. Predicting the required

force, to ensure that stroke

attains the proper shape for

your rubber.

6. Designing the mould for

rapid cooling after the

rubber has been compressed

into the mould.

You try to optimize the Time,

Temperature and Pressure

– the three critical

parameters for your

moulding process.

Manufacturers compete to

offer your robust hydraulic

presses with superior drives

and controls, advanced levels

of hydraulics and

instrumentation features.

A Reconditioned Press

By Pelmar Engineering

Your final choice of a compression

moulding press depends on these six

important considerations.

12

Santec Group Make Compression Moulding Press



Advantages and Disadvantages

of Compression Moulding

Process

The advantages of compression

rubber moulding are lower cost

of your moulds, the large sizes of

mouldings possible, and the

relatively quick changeover

between different moulds

helping you expedite shorter

production runs.

Additionally, there are no tooling

features like gates, spruces or

runners that could increase the

time for your mould to enter the

cavity or potentially affect the

aesthetics of your product.

The main disadvantage is your

output when loaded and

unloaded manually. This

increases your cost on skilled

labour.

There is product wastage too

as compression moulding is

not precise like injection

rubber moulding. Since

thermal conductivity of

rubber is poor and you

often place your profile

into the cavity ‘cold’,

your cure times are

longer.

Additional difficulties

you face will be from

positioning the blank

properly in the cavity

and the difficulty to

control the ‘flash’

that results from the

additional material

placed in the cavity.

Unsuitable for complex

moulding, compared to a

transfer moulding or

injection moulding, your

product contamination is also

higher in compression

moulding.

Summarizing, compression

moulding is the most

common moulding technique

that you will find in rubber

industry and the common

machinery for this process is

the a hydraulic compression

moulding press.

Presses Inside A Plant of Elastomer Engineering Ltd UK

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14

| AUTOCLAVE |


VULCANIZING 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

moulding 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.

Image: Bondtech Autoclave

Prasanth Warrier

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| AUTOCLAVE |


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.

Autoclave Image From Web

Tyre Vulcanizing in Autoclaves (Web)

Rubber vulcanizing is a batch

process. Your autoclave door

must be of full diameter to

allow easy access to the

chamber inside.

16

| AUTOCLAVE |


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 fool

proof interlock mechanisms to prevent

door opening under pressure.

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.

Inside View of an Autoclave

Your manufacturer 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.

18

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20

| AUTOCLAVE |


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.

Separate Images of Controls and HMI Screen (Inset) from different sources

on web (Only for representation purpose).

| 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 eg:, 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

Price is sensitive to some factor

and insensitive to others.

21

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| AUTOCLAVE |


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.

RMW

Hence, some of your new generation

peers advocate the more advanced

microwave process.

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.

22

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