Injection Moulding Flashcards
What are some advantages of injection moulding?
Complete design freedom; ability to mould complex shapes with ribs, bosses etc. in a single process.
High levels of versatility, in terms of size / shape of products and the types of materials that can be moulded successfully.
Close tolerances can be achieved, with high levels of repetitive accuracy across relatively short machine cycle times.
Suitable for high levels of process automation in a mass production, high volume manufacturing sector.
Large-area, thin-section parts can be shaped easily and quickly, but often require high pressure to fill the mould cavity.
Multi-impression and family moulds can be utilised to optimise the number of units produced per cycle. PET preforms for stretch blow moulding are made in large, multi-impression moulding tools.
Computer-aided design (CAD) and process simulation is well advanced and well-used in the injection moulding sector. Examples include Moldflow ® MPA software
What are some disadvantages of injection moulding?
Injection moulding is a capital-intensive process, with large financial outlay required for machines, moulding tools and ancillary equipment.
Long production runs are usually required to ensure financial viability.
Changes in process conditions can have a profound influence on
microstructural features (crystallinity, orientation, fibre alignment in composites) and formation of defects (weld lines, air traps). These will then compromise the quality of the finished parts**. Injection moulding grades of polymers are often limited to relatively low molecular weight grades.
Waste material is produced unless hot runner feed systems are used
KEy way to answer questions
structure (microstructure)
processing
properties
Give a light processing overview of injection moulding
PROCESS DESCRIPTION
Machine & Process Sequence
Machine Size & Specification
MANUFACTURING CYCLE
Time & pressure cycles
Process variables and machine control
MOULD CLAMP FORCE
Mould filling
Mould packing/dwelling/holding
What are the 3 units of an injection moulding machine?
a clamping unit
a mould (unit) and
an injection unit.
What is the function of the clamping unit? and what are the 2 types of clamping methods?
The functions of the clamping unit are opening and closing a die (mould),and the ejection of products.
There are 2 types of clamping
methods, namely:
the toggle type and
the straight-hydraulic type in
which a mould is directly opened
and closed with a hydraulic
cylinder.
What is the mould ?
A mould is a hollow metal block into which molten plastic is
injected to from a certain fixed shape. There are many holes drilled in the block for temperature control by means of hot water, oil or heaters.
Molten plastic flows into a mould through a sprue and fills cavities
by way of runners and gates.
Then, the mould is opened after cooling process and the ejector
rod of the injection moulding machine pushes the ejector plate of
the mould to further eject mouldings.
How is the moulding area of an injection moulder be designed for efficiency?
ref diagram pg 13
Since obtaining only one product by one shot is very inefficient, a
mould is usually designed to have multiple cavities connected with a runner so that many products can be made by one shot. If the length of the runner to each cavity is different in this case, the
cavities may not be filled simultaneously, so that dimensions,
appearances or properties of the mouldings are often different cavity by cavity.
Therefore the runner is usually designed so as to have the same
length from the sprue to each cavity.
What is the mould function?
Shaping by injection; shear flow at high pressure.
Displacement of air from the closed cavity.
Heating/cooling environment.
Ejection mechanism
minimise cycle time
What is the function of the injection moulding machine?
The functions of the injection unit
are to melt plastic by heat and then
to inject molten plastic into a mould.
What are the 3 main components of the injection unit?
Screw / barrel assembly - rotation and axial, backwards motion - similar to extrusion, injection/screw forward - screw acts as a piston assembly to inject into the mould no rotation.
Nozzle (to feed the mould)
Hydraulic motor & cylinder
What should the injection unit do?
The screw is rotated to melt plastic introduced from the hopper and to accumulate molten plastic in front of the screw (metering zone) . After the required amount of molten plastic is accumulated, injection process is started.
While molten plastic is flowing in a mould, the machine controls the
moving speed of the screw, or injection speed. On the other hand, it controls dwell (holding) pressure after molten plastic fills out cavities. The position of change from speed control to pressure control (The ‘V-P Switchover’) is set at the point where either screw position or injection pressure reaches a certain fixed value.
What are the sizes and specifications of the injection unit?
- Injection capacity (max. shot weight or volume) (g or cm3)
- Screw details (diameter, L / D, stroke, max. speed or drive power)
- Hopper capacity (filling mechanism and drying power / temperature)
- Heating capacity (kg / hr)
- Pressures: maximum injection pressure capability
- Injection phase: screw forward speed, injection time
- Additional: Special alloy steels?
Special features (e.g. swivel carriage, machine control etc.)
What are the machine size and specifications of the clamp unit?
- Clamp type, maximum clamp force (next session)
- Mould opening stroke, speeds, maximum ‘daylight’ between platens3. Platen size (area, m2)
- Tie-bar (clearance, diameter, constructional features)
- Mould (thickness range, opening force, height adjustment)
- Ejection system
What is the machine size and specification for the injection unit/clamping frame ?
1 Maximum Shot Weight / Volume
Maximum volume (weight of PS)
Range: ‘grams’ – x 10 (kg’s)
2 Maximum Clamp Force
Oppose melt pressure in the mould cavity – prevents the mould from opening Range few tonnes – (several x 1000) tonnes
What is the clamp force required for the moulding machine specification?
ref pg 21
F = ΔP x A (projected area of parts)
ΔP = pressure developed during mould filling /packing
Machine size / specification:
Injection capacity - maximum shot-weight (g) or shot volume (cm3).
Clamp force (in kN, or tonnes-force).
Screw / barrel diameter (mm) – determines output.
Power – motor main drive, total power requirements
Overall dimensions and weight
Hydraulics – oil tank capacity, pressure capabilities
Machine control system / data storage and processing facilities
Interface with mainframe computer system, production planning
Additional features:
I. Mould temperature / pressure monitoring
II. Hours and / or cycle counter
III. Core-pulling systems
IV. Product withdrawal, robotics, material conveying systemsV. Software capability; statistical process control (SPC) and data
processing facilities.
What is the main 5 steps of the injection moulding process?
Injection Moulding is the process of pushing or injecting molten plastic into a mould cavity.
Clamping
Injection
Dwelling / Packing / Holding
Cooling Mould
Opening / Ejection
What is melting in injection moulding?
Material granules from the hopper feed into the heated barrel
& rotating screw.
Material melted by heat, friction & shear force is forced
through a check valve to the front by the rotating screw
What is step 1 - clamping for injection moulding?
The high strength clamp operates by holding the two halves of
the injection mould together during the injection and cooling.
The clamping is accomplished through hydraulic or electric
pressure
What is step 2 - injection for injection moulding?
During the injection phase plastic pellet material flows into a
hopper on top of the injection unit.
The pellets feed into a cylinder where they are heated until they
turn molten.
A motorised screw, or ram, within the heating cylinder then mixes the molten resin and force the polymer to the end of the cylinder.
Once enough material has accumulated in front of the screw, the injection process begins.
The molten plastic is inserted into the mould through a sprue, while the pressure and speed are controlled by the screw.
What is step 3 - dwelling/packing/holding for injection moulding?
The dwelling phase provides increased pressure building within the injection process.
Once the molten plastic has been injected into the mould, pressure is applied to make sure all the mould cavities are filled.
This portion of the injection moulding process helps to build the
moulded parts overall wall thickness.
What is Stage 4 - Cooling for injection moulding?
The cooling stage of the injection moulding process allows the
moulded part to properly cool.
Cooling times vary depending on thickness of the part wall. This
is often the longest part of the injection moulding process.
What is Stage 5 – Opening / Ejection for injection moulding?
The mould separates as the clamping pressure releases. Once this occurs the mould is separated into two halves.
The ejection of the mould is accomplished by using an ejection rod and plate to eject the newly generated finished part.
The tool is closed and the injection moulding process starts again at stage 1.
The unused sprues and runners can be recycled for use again in
future production runs.
ref equations for shot volume and clamp force..
What is the difference between extrusions and injection moulding?
EXTRUSION – continuous rotation / steady state
INJECTION MOULDING – intermittent rotation / cyclic process
What is the cycle time in injection moulding?
The cycle time in injection moulding refers to the total time required to complete one full cycle of the injection moulding process.
It encompasses various stages, including
injection time,
cooling time,
dwelling (packing/holding) time,
ejection time, and
mould opening/closing time.
Each stage plays a crucial role in determining the overall cycle
time and the efficiency of the production process.
How does the cycle time vary for each of the 5 stages of injection moulding?
Injection Time
The injection time is the duration required to fill the mould cavity with
molten plastic.
It depends on factors such as the material’s flow characteristics,
injection speed, and part geometry.
Optimising the injection time can significantly contribute to reducing the overall cycle time.
Dwelling (Packing/Holding) Time
This is the phase after the Injection time during which the material remains in the mould but is held under pressure to finally fill the cavity and prevent sink and distortion while it solidifies fully.
Reducing the Holding time without compromising the quality of the part can help optimise the cycle time.
Cooling Time
Once the mould cavity is filled with molten plastic, the material needs time to cool and solidify.
The cooling time is a critical part of the cycle as it affects the part’s
dimensional stability and quality.
Factors such as the type of material used, the thickness of the part, and the efficiency of the mould cooling system influence the cooling time.
Ejection Time
Once the cooling and dwelling stages are complete, the finished part is ejected from the mould using ejector pins or other mechanisms.
The ejection time is the duration required to remove the part from the mould.
Efficient ejection mechanisms and proper ejection force can minimise
the ejection time.
Mould Opening/Closing Time
The time taken to open and close the mould between cycles is also
part of the overall cycle time.
The complexity and size of the mould, as well as the capabilities of the moulding machine, influence the mould opening/closing time.
Streamlining this stage can contribute to reducing the cycle time.
Calculating Cycle Time
Calculating the cycle time in injection moulding involves considering the duration of each stage in the moulding process.
By summing up the time spent in each stage, manufacturers can
optimise the cycle time and improve production efficiency. REF eq pg 42/43
what is the 1st and 2nd stage of mould filling in injection moulding?
The first stage - Injection for getting most of the plastic into the
part, normally 90% to 99.9% full by volume;
The second stage –packing/holding, to pack the part to replicate
the steel cavity texture and shape;
The second stage normally moves relatively little plastics in the
cavities, but critically important for the finishes, cosmetics and part dimensions.
How is packing pressure created and why?
Packing pressure is created by the compressibility of the melt and the continued forward motion of the screw after all the cavities are
completely filled.
This could be considered as overfilling in which a certain % of cavity volume is overfilled with melt, creating a rapid rise in pressure.
Packing is the last stage during injection, which is the only pressure controlled stage so its pressure has to be set carefully to avoid flashing.
After the packing stage, the process switches to holding pressure. The position of the screw at the transition is called the transition point.
What is the purpose of holding pressure?
to maintain a pressure at the
screw tip to continue filling material into the cavities as the melt in
the mould cools and contracts.
The space created by the contraction is taken up by more melt to reduce or avoid sink marks in the solidified parts.
The holding pressure setting should not exceed the packing stage pressure setting, otherwise, flashing could be created at the holding pressure stage.
The purpose of holding pressure is to compensate for the defects
caused by the cooling mould shrinkage rate of the melt and the
drop in melt temperature, while also ensuring the longitudinal and
transverse consistency of the product.
The process of holding pressure is to maintain the pressure of the
injection moulding machine for a period of time immediately after
injection moulding.
This allows the plastic material to be partially melted through the
interaction of pressure and temperature to ensure the quality and performance of injection moulding.
What are the main roles of pressure holding?
1.Solve the problem of shrinkage holes and burrs - when the pressure is released after injection moulding, the melt will shrink leaving shrinkage holes/burrs at the corners so by maintaining pressure the machine can be maintained in a good pressure state and prevent product defects.
2.Improve the firmness of the product - pressure holding ensures that the plastic material is well filled into every corner and blind hole of the injection mould. When the melt is compacted the molecules of the plastic are arranged more closely which makes the product stronger and more durable and increases the density of the product.
3.Improve the moulding accuracy of the product - During the pressure maintaining process of the injection
moulding machine, the plasticity of the melt is increased.
This allows the plastic material to fill the injection mould more
completely, thus effectively improving the moulding accuracy of the product.
What is the screwback and why do it come about in injection moulding?
Since the sprue (the entry channel for melt flow into the mould) is
already occupied by rapidly-cooling melt, the material being pumped forward by rotary shear action of the screw generates a positive pressure (back pressure) in front of the screw tip.
Once it exceeds the back pressure pre-set, the pressure forces the screw back down to the barrel (whilst still rotating) to the desired
set-point (pre-set limit) which determines the swept-volume
available for the next shot, the stroke.
The screw rotation phase is usually termed screwback.
When this position is reached, the screw will stop rotating.
What are the several factors that create back pressure?
Viscosity of the Material: Thicker or more viscous materials
require higher pressure to flow through the injection system.
Mould Design: Complex geometries, narrow gates, or long flow paths can increase resistance and thus back pressure.
Injection Speed: Rapid injection can lead to increased back
pressure if the material cannot flow quickly enough.
Temperature: Higher temperatures generally reduce viscosity, which can lower back pressure, while lower temperatures can increase it.
What is the importance of backpressure?
Material Homogeneity: Proper back pressure helps ensure that the material is well-mixed, which is crucial for achieving consistent properties in the final product.
Control of Injection: Managing back pressure allows for better
control over the injection process, leading to improved part
quality and reduced defects.
Cycle Time Optimisation: Balancing back pressure can help
optimise cycle times by ensuring that the material fills the mould efficiently without causing defects.
How can backpressure be managed and adjusted in injection moulding?
Managing Back Pressure, Back pressure can be adjusted by:
Modifying the injection speed.
Adjusting the temperature of the barrel and nozzle.
Changing the design of the injection system or mould.
Understanding and controlling back pressure is essential for
achieving high-quality injection-moulded parts and efficient
manufacturing processes.
As backpressure is increased, the time for the screw to recover to
set-point is increased. Screw recovery speeds should be increased as well to maintain cycle time.
What is decompression in injection moulding?
Decompression in injection moulding refers to the process of reducing pressure within the injection unit or mould after the injection of molten
plastic into the mould cavity.
Why is decompression important?
- Preventing Flash: Decompression helps to relieve pressure that
could cause excess material to escape from the mould cavity, which is known as flash. - Improving Surface Finish: By reducing pressure, decompression can help improve the surface finish of the moulded part, as it minimizes the risk of surface defects caused by excess material flow.
- Enhancing Material Flow: It allows the molten material to flow more easily, ensuring that it fills the mould completely and uniformly lower the pressure
- Controlling Shrinkage: Decompression can help manage the cooling
and solidification process, reducing the likelihood of warping or
uneven shrinkage in the final product. - Preventing Sticking: It reduces the adhesion of the moulded part to the mould, making it easier to eject the part without damage.
How is the decompression process typically achieved?
This is often achieved by retracting the screw slightly or using a vacuum system to lower the pressure in the cavity.
Properly managing decompression is essential for producing high-quality
moulded parts with consistent characteristics.
Too much decompression can result in excessive air being pulled into the melt stream, resulting in bubbles or splay in the moulded part.
