Posted: 10 Mar 2011 04:13 PM PST

Shape Memory Alloys (SMA's) are novel materials which have the ability to return to a predetermined shape when heated. When an SMA is cold, or below its transformation temperature, it has a very low yield strength and can be deformed quite easily into any new shape--which it will retain. However, when the material is heated above its transformation temperature it undergoes a change in crystal structure which causes it to return to its original shape. If the SMA encounters any resistance during this transformation, it can generate extremely large forces. This phenomenon provides a unique mechanism for remote actuation.

The most common shape memory material is an alloy of nickel and titanium called Nitinol. This particular alloy has very good electrical and mechanical properties, long fatigue life, and high corrosion resistance. As an actuator, it is capable of up to 5% strain recovery and 50,000 psi restoration stress with many cycles. By example, a Nitinol wire 0.020 inches in diameter can lift as much as 16 pounds. Nitinol also has the resistance properties which enable it to be actuated electrically by joule heating. When an electric current is passed directly through the wire, it can generate enough heat to cause the phase transformation. In most cases, the transition temperature of the SMA is chosen such that room temperature is well below the transformation point of the material. Only with the intentional addition of heat can the SMA exhibit actuation. In essence, Nitinol is an actuator, sensor, and heater all in one material.

Calendering: Plastic Sheets & Paper Roll Manufacturing

Posted: 10 Mar 2011 06:55 AM PST

Has anyone wondered how plastic sheets or paper rolls are manufactured? Well, the
process is called 'Calendering', heard of it? Yes, it is the manufacturing process by
which plastic or paper sheets are formed. Using this process basically any thickness of sheet can be formed. This process incorporates the use of heavy cylinders and a simple transmission mechanism. In other words the cylinders themselves guide and transfer sheets forward.

The process goes like this. The raw material to be used, present in the form of dough is made to pass through two heavy cylinders, which apply pressure on it and convert it in the form of a sheet.

Usually there are many rollers as you can see in the diagram. The more number of
cylinders the better but the thing to be taken in mind is that the process should be
economical. You should not use these cylinders unnecessarily in your enthusiasm
which can make the set-up complicated and costly.

The processed sheet is then wound on a cylinder and thus the bundle is formed.
Calendering generally takes place hot, because hot plastics often are soft and easy
to shape. As a thumb rule always remember that heated things are easy to work with.
They are easily forged. Even if less pressure is applied you will get superior quality
of sheets. The pressure which is generally applied is about 890N/Cm2.But it actually
depends as to how thin sheet you want. There is also an arrangement some times to
make the cylinders hot so that the plastic does not cool in the process.

You must have seen big rollers guiding the newspapers whenever there is a scene of
paper industry in a movie well, actually you are seeing the calendaring machine.

Still not got? Well, try visiting your local bakery next time.

There is also a type of calendaring called belt calendering applying a pressure of about 1/10 that in hot calendaring.

Some companies provide a thickness of 0.17mm to 1.27mm.The process then is to
smooth, polish and glaze to get the required surface quality.
The products are many and this is the major and possibly single process to make the sheets.

The construction is rugged and the thickness obtained varies from 0.5mm but different
manufacturers claim different ranges. The advantages include no wastage of material.
The process can be stopped at the precise moment you want. So what more do we
want?

Compression molding

Posted: 10 Mar 2011 06:52 AM PST

The words compression and tension go hand in hand for any mechanical system. But
fortunately or unfortunately we have only compression molding. Compression molding is
a process of molding plastics in to desired form. This is actually a type of forging only.

The apparatus essentially usually consists of piston and a block which applies pressure
on the material and acts as a moving component and the mould in which the polymer is
kept it acts a lower part. It is generally kept fixed.

The raw material used is generally thermosetting plastics like polyesters and epoxys the
raw material used is in the form of granules or preform. The material is used in the form
of granules and preform because it is easier to do so. If we use the plastic in the form of
a big dough it will naturally take larger time to melt.

The process consists of following three simple steps The charge is placed in the mould
cavity. Later on, the mould is pressurized so that the charge takes the shape of cavity.
Only that much pressure is applied so as to ensure uniform density of the product. The
material has to be cured and solidified before removing.Lastly; the mould is removed to
get the finished component.

The process is generally performed 'hot' with molding temperature reaching the range
of about 1100 to 2200 degree Celsius. The pressure applied varies from about 3.5MPa
for polyesters and epoxys. But general pressured exerted on thermoplastics is about 14
to 41.5MPa.

The process time required is 12-15 Sec for a small object of 3 mm thick. It may vary
from 7 to 10 min for larger objects.

The process is also done 'cold' but preheating the material reduces the process time
drastically to half. The cold molding cannot maintain high tolerances and has to be
supplemented by binders.

There can be following three divisions of compression molding

1. Positive type 2.Semi positive 3.Flash type. The flash type is the cheapest and most
accurate tolerances can be achieved.

Compression molding products consist of electrical parts, flatware, gears,
buttons, buckles, knobs, handles, electronic device cases, appliance housing,
and large container.

It is used to make moderately curved or flat parts like lift gates, spoilers,
scoops etc.

The advantages include low initial costs. Secondly, there is practically no
wastage and accurate dimensional tolerances can be maintained.

We can apply this technique to composite thermoplastics with unidirectional
tapes, woven fabrics, randomly orientated fiber mat or chopped strand.

Also one of the greatest advantages is its ability to mold large, fairly
intricate parts. Compression molding produces fewer knit lines and less
fiber-length degradation than injection molding. Excellent surface finish is of
course achieved.

However, if you have ever experienced injection molding then you will
come to know that Production by injection molding is much faster. Also this
process is mostly limited to large parts with less or no curvature at all.

Transfer molding

Posted: 10 Mar 2011 06:49 AM PST

Hello everyone ever wondered or for want of a better
word 'compared' various manufacturing process? I guess many
actually do. It makes remembering simple. You only need to
remember the minor differences which set various processes
apart.
One such process is transfer molding. This process when seen
practically is strikingly similar to Compression and injection
molding.
This process is basically a combination of injection and
compression molding.
The simple apparatus consists of a plunger or a piston which
reciprocates in a transfer pot ('Transfer pot' is nothing but the
barrel as in injection molding). The transfer pot has a sprue
attached to it. This sprue then opens in a mould which is already

Prepared according to
required dimensions.
The process consists of basic steps as in the other two molding
processes. The material which is generally a thermoset (In
some cases thermoplastics may also be used) is preheated to a
sufficient temperature to make it soft. As soon as the material
softens, this near liquid material is forced into the mold by the
plunger overhead.
The plastic takes the shape of mold and it is then cured. When
the ready product is cooled to room temperature, it is taken out.

Now let us take into account its similarity with other processes.
Firstly, the process makes use of hydraulic press as in
compression molding and a Ram which forces the material in the
mold using sprue as in injection molding.
However there is one major difference in compression and
transfer that instead of loading polymer in a mold that is open the
polymer in compression molding, in transfer molding we is force
it in a closed mold after it is melted.
The mold is generally a two piece arrangement so that the
product can be removed easily.
The advantages are also mixed i.e.; it is better than compression
molding in time consumed and production capacity than
compression but injection molding consumes less time and can
produce complicated shapes than transfer molding.
Another such advantage is lower scrap produced. Quality can of
course be ensured.
Another related process is known as Resin transfer molding. It
uses unvulcanised rubber as its raw material. The only care we
should take is not to let trapped air in the mold else uniform
material density cannot be ensured.

We can very easily produce covers for electronic components,
also protective covers for pins, studs and other hardware
components.

Crazy Engineers .CoM

Thursday, March 10, 2011

CrazyEngineers Forum - Mechanical & Civil Engineering

CrazyEngineers Forum - Mechanical & Civil Engineering

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