MOULDING TERMINOLOGY
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Terminology Language for Plastics Injection-Moulding
Plastics Injection-moulding is just about as hard to
learn as another language. Learning these terms will not make you a
moulding expert, but it helps if you know what people in this industry are
saying.
Here are a few explanations which might help...
Average Velocity
The overall speed of the material as it is flowing through a part of
the mould, which has a constant cross section. It is usually described as
(inches) millimetres per second of material travel.
Barrel Capacity
The amount of material that a machine will hold with the screw fully
retracted. The machine is prepared to deliver that amount of material when
it is at its largest shot size setting. Barrel capacity can be described
as either the weight or the volume of material that is in front of the
screw when it is at its maximum shot size setting.
Cavity Pressure
The force that is operating directly on the moulding material at a
specific mould location and time during the moulding cycle.
Clamping Force
The force or pressure applied by the machine's clamping unit to the
mould during the filling, packing and holding phases of the moulding
cycle. Clamping force is described in terms of pressure acting on the
mould's projected surface. The units are usually kN / m^2 or tons / in^2.
Cushion
The small amount of material that is left in the barrel when the screw
has stopped moving forward at the end of the holding pressure or packing
phase. The cushion prevents the screw from reaching the end of its stroke
during injection forward movement. Without a cushion, differences in
material, temperature, pressure, speed and mechanical resistance will
cause short shorts or over packing.
Cycle Time
The length of time that it takes to make a part and complete
preparations to produce the next one. It is the time required to complete
one trip through the moulding cycle.
Density & specific gravity
Terms that refer to the fact that some materials are relatively
heavier than others. For example, polypropylene is not as dense as ABS.
This can be shown by weighing equal volumes of both materials.
Feed System
The main connecting link between the machine's nozzle and the part
cavities of the mould.
Fill/Pack Transfer
A series of events that happen at the transition between the filling
phase and the packing phase (holding pressure start) of the moulding
cycle. These events involve changes to the state of the machine, the
material and the mould.
Fill Pattern
A visual history of how a mould fills under a specific set of moulding
conditions. The lines on the part show the progress of the melt front at
particular points in time after filling has begun.
Fill Time
The length of time in seconds that it takes to just fill the mould
with melted plastic material during the filling phase of the moulding
cycle.
Flow Balancing
The process of choosing a mould design strategy which promotes even
filling of the mould. First, a gating strategy is selected which will
promote ease of filling. Second, a runner layout is developed to feed
material to the gates. Finally, runner dimensions are identified which
will cause all of the flow paths to fill at the same time.
Flow Length
The total distance that plastic material must travel from the machine
barrel along a particular flow path in order to completely fill a section
of the mould.
Flow Path
The route which is travelled by a melt front as material fills a
section of a mould.
Flow Rate
A way of describing how much material goes through or past a specific
point in a fixed period of time. If the nozzle tip is used as a reference,
flow rate can describe how much material is flowing out of the machine per
second during injection. Flow rate by volume is usually measured in units
of cm^3 / sec or in^3 / sec.
Fountain Effect
This is how the material at the melt front behaves during the filling
of the mould. The leading material swells into the shape of a bubble and
behaves in much the same way as water flowing from a fountain.
Frozen Layer
A skin of solid material that forms next to the mould walls during the
filling phase of the moulding cycle.
Gating Strategy
The approach you use to choose the number and location of gates in a
mould.
Hydraulic Pressure
The liquid force that is acting on the machine screw during moulding.
Hydraulic pressure is developed by pumping oil into the chamber behind the
injection ram.
Injection Velocity
A machine control you set that determines how fast the screw moves
forward during the filling phase of the moulding cycle. It describes how
far the screw travels in a fixed period of time and is usually reported in
inches per second of screw travel.
Knit Lines
Are locations in the moulded part where two or more melt fronts meet.
Knit lines are often described by breaking them down into two
subcategories: meld lines or weld lines.
Melt Expansion
Refers to the fact that all plastics expand when they are melted--the
same weight of a given material takes up more space. The exact degree to
which the volume expands is unique to each plastic material. Said another
way, a plastic's solid density is greater that its melt density.
Melt Front
The location of the leading edge of flow as material is filling a
mould. The location of the melt front is constantly changing with time.
Melt Temperature
Is the actual temperature of the material at a given mould location
during processing. The melt temperature is constantly changing. It varies
with time and will not be the same at different locations in the mould.
Mould Cooling
The process by which the melt temperature of the material is reduced
to the point where parts can be removed from the mould.
Mould Packing
The process of delivering an additional amount of material to the
mould which has already been filled with melted material. The packing
portion of the shot compensates for the amount of volumetric shrinkage
that will take place when the filling portion of the shot cools and
solidifies.
Mould Temperature
The surface temperature of the mould as measured by a contact
pyrometer. While it is considered to be a single number for control
purposes, mould temperature usually varies with location over the surface
of the mould.
Moulding Cycle
The series of steps that result in the machine producing a part. The
cycle is usually described by breaking it down into four separate phases.
The amount of time that it takes to complete one trip through the cycle is
called the cycle time.
No Flow Temperature
The temperature at which the viscosity of a material is so high that
it effectively cannot be made to flow.
Orientation
The change in shape that plastic molecules can undergo when they are
made to flow.
Optimum Conditions
The machine settings you use that will allow the mould to be filled
with the lowest pressure drop and produce parts with the least amount of
residual stress.
Polymer Structure
The composition and makeup of a given thermoplastic material from
fundamental elements.
Pressure
This is how the net effect of force is distributed when it is applied
over a specific area. Pressure is defined as the force per unit area. For
injection moulding purposes, it is normally reported in units of psi or
MPa.
Pressure Drop
The loss in pressure that occurs when the material is pushed into a
section of the mould during the filling phase. Pressure drop is reported
in the same units as pressure, normally psi or MPa.
Residence Time
The length of time that the material is held at melt range
temperatures. For any given machine and mould combination, the residence
time is determined by two factors: the length of the overall cycle time
and the shot size setting.
Residual Stress
A general term that describes the level and pattern of stress which is
left in the part after it is removed from the mould.
Runner Layout
The approach used to get material from the machine nozzle to the
gates.
Shear Heating
A term that describes how friction caused by flow can increase the
melt temperature of the material as it is flowing during the filling
phase.
Shear Rate
A way to describe how quickly the velocity of the material changes
from the mould wall to the centre of flow for a given cross section. The
size of the shear rate gives an indication of the shape of the velocity
profile for a given flow situation.
Shear Stress
The result of the force that is generated in a plastic material to
overcome its resistance to a particular flow situation. Shear stress is
the product of the viscosity of a material and the shear rate. It is
reported in units of stress or pressure (psi or Pa).
Shear Thinning
A way of describing how the physical effects of orientation influence
the flow behaviour of the material. Shear thinning causes the material's
viscosity to drop when it is made to flow within a certain shear rate
range.
Shot Composition
This describes the relationship between machine barrel settings and
the events which occur during the moulding cycle. The shot size setting
and the transfer point give the machine information about how material is
to be injected into the mould. The distances for these settings correspond
to volumes of material that play a role in different portions of the
moulding cycle.
Shot Size Setting
A machine control setting which limits how far back the screw will
travel as it rotates in the cooling/plastication phase of the moulding
cycle. The shot size setting is measured as a distance from the front of
the barrel. You can set the shot size to any distance along the length of
the barrel up to the stroke of the machine.
Split Points
Locations in a mould cavity where a melt front will split up and
advance in more than one direction at the same time. Split points are also
referred to as branch points, flow junctions or nodes.
Thermal Properties
Description of how a given plastic material will behave during the
exchange of heat. They are an important factor in determining how
susceptible a material is to shear heating; they also characterize how
quickly a given material will cool down.
Transfer Point
A position on the length of the barrel which signals the machine to
switch from filling at constant flow rate to packing (holding pressure
start) at constant pressure. When set properly, it corresponds to the fill
time for the mould.
Velocity Profile
Refers to the fact that plastic material moves at different speeds
relative to the flow cross section as it flows into the mould during the
filling phase. The velocity of the material depends upon its location
relative to the mould wall. Material at the centre of flow away from the
mould walls flows the fastest.
Viscosity
A way to describe how much resistance a plastic material exhibits when
an attempt is made to make it flow. High viscosity means the material is
thick and resistant to flow. Low viscosity means the material is thin and
will flow easily.
Viscosity Response
A way of describing how the viscosity of a particular material
responds to changes in temperature and shear rate.
Last Updated:
Monday, 09 August 2010 12:54 AM
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