The Role of Back Pressure in Injection Moulding

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THE ROLE OF BACKPRESSURE IN INJECTION

MOLDING
The concept of backpressure is interesting. This in part because

it is an elegant demonstration of thermodynamics. One of many
such in the injection moulding process. It is one that is often
overlooked. You’ll often find descriptions of injection moulding
with no mention of backpressure. In the fundamental sense, it
relates the pressure exerted on the system to the screw motion.
But beyond this, it also affects several other aspects of the
process. Injection moulding is an interplay of different events
culminating in a formed product. Several parameters need
control and are monitored in the process. Many of these
parameters relate to each other. One of such is backpressure.
Backpressure is essential in injection moulding. For an efficient
injection moulding run, back pressure needs controlling. Another
name for backpressure is plasticizing pressure. When thought of
as the latter it has a more positive ring to it. This article helps to
understand the concept of backpressure. It discusses the role
back pressure plays in injection moulding and how it gets
managed. We look at the components of the injection moulding
machine that handle backpressure. We also look at how
backpressure affects other aspects of injection moulding
systems.
Back Pressure in a Nutshell

The pressure is an important physical concept in injection
moulding. Pressure control gets used at different points of the
injection moulding machine. There’s the sheer pressure on the
melt as it forces its way between the screw and barrel. There’s
the holding pressure that the moulded part gets held at while it
cools. There’s also the pressure applied on the screw which in
turn exerts pressure on the melt. It is with this pressure the melt
fills the runners and the mould. Somewhere around all this lies
backpressure.
To understand back pressure, one needs to have a good idea of

how injection moulding works. An injection moulding machine
mixes and melts plastics and moulds them into shapes. The
process begins when plastic pellets get fed into the hopper. The
pellets get moved along a barrel which is in the typical case
horizontal. As they pass through the heated barrel they get
sheared by a plasticizing screw. The shearing occurs by the
rotating action of the screw. As the screw rotates, it moves
backward. This backward motion of the screw pushes the melted
plastics to the front of the screw. There now exists this volume of
fluid in front of the screw. This fluid exerts pressure on the screw
causing its backward motion. The backward motion of the screw
increases the volume in the melt chamber. This achieves the
desired shot size. But this volume must get controlled to exert
enough compression pressure on the melt. This is necessary for
even well-compacted melt. So, the pressure applied at the other
end of the screw counters the pressure exerted by the melt in
front of it. This pressure is backpressure. This pressure was
generated by the plasticizing of the plastics. Hence its alternative
name, plasticizing pressure.
How Back Pressure Affects Injection Moulding

Backpressure has a lot to do with how the injection process
progresses. The screw does much of the work in the process.
So, the way it moves is crucial. The injection moulding process
centres around the back and forth motion of the screw. There are
several parameters at play here. The rotation speed of the
screw, measured in rotation per minute (rpm). The screw rotates
anywhere between 20 and 60 rpm in most machines. The
rotation speed for any given process depends also on the
property of the plastics. Another parameter to consider is the
size of the shot. The shot size gets determined by the size of the
injection chamber. Since this is set by the limit switch which
determines how far back the screw goes.
This pressure sends back the screw towards the hopper end of
the machine. The speed at which the screw moves back
depends on a balance of different factors. These are; the size of
the screw, friction, and the net pressure. Once the screw hits the
limit switch it gets forced forward by hydraulic pressure. This
forward action injects melt into the mould. In injection moulding
timing is important. How does the back pressure affect timings?
Here’s how. As the screw moves back the volume of the melt
chamber increases. Meanwhile, the volume on the other end of
the screw decreases. This results in increased pressure.
The rate at which the screw moves back gets controlled by the
gradual increase in volume. So how much melt the screw
pushes forward per unit time affects how fast it moves backward.
The net pressure moving the screw back. It is a balance between
the pressure exerted by the melt and the backpressure. The
acceleration of the screw is proportional to the net pressure
exerted. So, to control how fast the screw moves back, control
the backpressure. The illustration in the image provided below
further explains this.
Diagram illustrating backpressure. P2 indicates the pressure
exerted by the melt on the screw. P1 is the backpressure
applied.
The type of mechanism used for controlling pressure varies. It

depends on the design of the injection moulding machine. In
particular how it gets powered. This determines how the
pressure gets applied. The injection moulding machine can be
hydraulic, mechanical, electrical, or a hybrid. In the hydraulic
system, it occurs by controlling the flow of oil in and out of the
cylinder. This gets done using valves controlling oil flow. In the
electric type systems, this occurs through AC servo valves.
Backpressure starts from around 50 psi. This then rises by 10 psi
until a peak of around 300 psi. If the back pressure is too low,
the screw’s backward motion is too easy. If the backpressure is
too high the melt experiences too much shear.
Thus far, we have based the discussion on backpressure on

thermoplastics. But bear in mind that the same principle applies
to thermosets. The difference is that the plastics get fed in as
liquid resins. This is unlike thermoplastics which get fed in as
solid pellets or granules.
6 Things to Know About Back Pressure

Knowing how backpressure occurs in the injection moulding
process is important. It gives a better understanding of how the
process gets controlled. The injection moulding process is a
combination of well-orchestrated events. Every event has an
impact on the quality of the final process. Here are 6 roles of
back pressure in injection moulding.
Compacting
To get uniform smooth materials pressure must get exerted on
the melt. A higher pressure brings the molecules closer together.
This improves density and makes for a better product. If the melt
is not compact enough it could result in insufficient flow down the
line. So, the back pressure does more than control the screw
motion. It also maintains the compacting pressure needed. The
image below is an illustration of compaction. As the pressure
gets increased the molecules move closer together.
Image illustrating compaction under increased pressure (say for
example three times the pressure)
Venting
Even where the product gets dried it is difficult to avoid air getting
into the melt. This can be from the condensation of volatiles in
the plastic. It could also be from condensed air or liquid from the
environment. When the pressure gets exerted, trapped air
escapes. This gets driven by a pressure gradient which moves
the trapped air from the melt chamber. This trapped air gets
moved to the lower pressure region and out of the system.
Trapped air can result in loss of design precision or surface
imperfection. Even when not visible on the product it can result in
loss of functionality. Often the injection moulding machine gets
fitted with a venting and decompression zone. The back
pressure aids the effectiveness of such features.
Process timing
Timing gets managed to even fractions of a second in injection
moulding. A one-second difference in one cycle can have an
impact on profits. Accurate control of backpressure reflects the
inaccuracy of screw speed. While the screw moves back,
another process is taking place. The melt in the mould from the
previous cycle is cooling. The backward motion of the screw thus
needs to be in sync with the ejection of the product. Such that by
the time the screw stops moving back that product has left and
mould gets closed. This going wrong by even a second could
halt the whole process.
Shrinkage
Exerting pressure on the melt helps address shrinkage. It
encourages isotropic shrinkage. It also reduces the impact of
shrinkage. Where the melt is under pressure a reduction in total
volume will result in fewer gaps. Compared to where there is less
pressure exerted. So back pressure allows for better dimensional
stability. This is not to say that shrinkage will not occur, it will. But
its effect will be less pronounced.
Mould filling
The pressure at which the melt gets held while filling the mould is
important. This is more so in multi-cavity systems. The pressure
must remain as the melt travels through the runners. The
backpressure contributes to the speed and pressure of mold
filling.
Melt temperature
The pressure relates to temperature. As the pressure of a
system increases so does temperature. As melt gets pushed
forward into the melt chamber, the volume of melt there
increases. But the volume of the space available to the melt gets
reduced by the backpressure. By physical laws, the system
responds by increasing temperature. The chaos generated by
fitting more melt into a smaller space gets converted to heat.
This is good for managing the cost of energy. The increased
temperature gets achieved in this zone by applying
backpressure. For many plastics, this increase in temperature
decreases viscosity. This is good for more effective mold filling
and product formation. If backpressure is too high the
temperature rise, as a result, can lead to degradation of the
plastic. Bear in mind that this is only for plastics that are
temperature sensitive. Or when working with quite a high
backpressure. Temperature increase from back pressure is not
as high as that from screw rotation.
Filled Plastics/ Composites

In materials filled with fibres, care is necessary not to have a
detrimental effect. High backpressure can reduce fibre length.
Long fibres in particular are liable to damage. The compaction
and increased shearing from back pressure can result in
breakage. Plastics get filled with long fibres to improve their
properties. The length and arrangement of the fibres within the
plastic affect these properties. Many plastic products for injection
moulding have recommendations from the manufacturer. For
such plastics, take note of the pressure the material can
withstand.
Conclusion
Backpressure, amongst other things it does, controls the screw
backward motion. Besides this, it contributes to other important
factors crucial to product quality. Good control over
backpressure has a significant impact on product formation.
Properties like melt uniformity and dimensional stability get
improved through backpressure.

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• info@cavitymold.

THE ROLE OF BACKPRESSURE IN INJECTION

The concept of backpressure is interesting. This in part because

Back Pressure in a Nutshell

To understand back pressure, one needs to have a good idea of

How Back Pressure Affects Injection Moulding

The type of mechanism used for controlling pressure varies. It

Thus far, we have based the discussion on backpressure on

6 Things to Know About Back Pressure

Filled Plastics/ Composites

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