Injection Moulding Flashcards

Question

What is the cycle time in injection moulding?

Answer 1

 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.

Answer 2

 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

Answer 3

 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.

Answer 4

 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.

Answer 5

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.

Answer 6

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.

Answer 7

 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.

Answer 8

 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.

Answer 9

 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.

Answer 10

 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.

Answer 11

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.

Answer 12

1. Preventing Flash: Decompression helps to relieve pressure that could cause excess material to escape from the mould cavity, which is known as flash. 2. 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. 3. Enhancing Material Flow: It allows the molten material to flow more easily, ensuring that it fills the mould completely and uniformly lower the pressure 4. Controlling Shrinkage: Decompression can help manage the cooling and solidification process, reducing the likelihood of warping or uneven shrinkage in the final product. 5. Preventing Sticking: It reduces the adhesion of the moulded part to the mould, making it easier to eject the part without damage.

Answer 13

 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.

Answer 14

Primary Settings: Temperature – melt Temperature – mould tool Injection speed (cm/s) Packing pressure Packing time  Secondary Variables : Stroke, Speed, Time  Response Variables: ΔP (injection pressure) ie machine parameters (pressure, temp, speed, stroke, time) Mould design (feed system layout and size, gate number and position) CAN'T CONTROL:  Controllable / off-line: mould tool / mould system design  Uncontrollable: variability in material quality (MFR), additives, humidity, water temperature, hydraulic oil …….

Answer 15

- Mould filling (reduce ΔP) - Effect on properties (nucleation etc)- Surface finish / tool replication oil/water would be circulated through internal channels

Answer 16

 Orientation  Residual Stress  Shrinkage & Distortion  Microstructure  Weld-line Formation Large interaction between processing and properties for injection moulding: Flow and cooling  Predictions using theory and simulation

Answer 17

1.Mould Design Parameters:  Factors such as cooling system design, runner and gate design, and the number of cavities in the mould can impact the cycle time.  Efficient cooling channels, well-designed runners and gates, and optimal cavity arrangements can contribute to reducing the overall cycle time. 2. Product Design Parameters:  Factors such as wall thickness and part geometry influence the cooling time and overall cycle time.  Designing parts with uniform wall thickness and minimising complex geometries can help optimise the cycle time. 3.Injection Moulding Process Parameters  Injection speed and pressure, melt temperature, mould temperature, and holding time and pressure all play a role in determining the cycle time.  Optimising these parameters can result in shorter cycle times and increased production efficiency 4.Material Selection  Different materials have varying melt temperatures and cooling rates, which affect the overall cycle time.  Selecting materials with optimal flow characteristics and cooling properties can contribute to reducing the cycle time.

Answer 18

 Utilise high-speed injection to fill the mould quickly.  Set the injection pressure at the minimum required for proper part filling.  Optimise the gate design to ensure smooth material flow and minimise pressure drop.

Answer 19

Affects the quality of the moulded part = reduced cycle times and improved production efficiency.  Design efficient cooling channels in the mould to ensure uniform cooling. Or use metal 3d printed conformal cooling channels to help get the cooled water to the exact areas that the tool requires.  Use advanced cooling systems, such as chilled water, to enhance cooling efficiency.  Optimise the mould temperature control to achieve optimal cooling rates for the specific material.

Answer 20

hold time allows the material to fully solidify in the mould and reduces the risk of warping or distortion. Reducing hold time without compromising part quality can contribute to cycle time reduction.  Optimise the holding time and pressure to the minimum required for proper part packing.  Utilise advanced process control systems to ensure accurate and efficient packing.  Employ mould flow simulation software to analyse and optimise the hold time.

Answer 21

=time taken to open and close the mould between cycles  Invest in injection moulding machines with fast clamping systems.  Optimise the mould design to minimise the complexity and size of the mould.  Regularly inspect and maintain the mould to ensure smooth and efficient mould movement.

Answer 22

 Use fast ejection systems to reduce the time taken for part ejection.  Ensure sufficient ejection force to avoid part sticking or damage during ejection.  Regularly maintain and lubricate mould components to ensure smooth and efficient mould movement

Answer 23

the force must exceed the pressure(consider filling and packing phases) X projected area(normal to mould face, total area is relevant so mould design is important), but if it is less this causes mould flashing [ref equation pg 75-80]

Answer 24

(A) Injection phase:  Screw moves forward at a set speed (velocity, v), hence controlled displacement of material  Therefore: this is a ‘velocity-imposed’ flow: “V” Volume flow rate (Q) – is set / fixed; Pressure (ΔP) – is a dependent variable(B) Packing phase:  Screw moves forward according to the set pressure in the hydraulic cylinder  Therefore: this is a ‘pressure-imposed’ flow: “P” V/P switchover: when the injection phase ends, packing phase begins

Answer 25

When plotting Pressure (MPa) Specific volume (cm3 g-1) Temperature (oC), it Allows estimation of shrinkage following processing, taking account of both process temperature (T) and pressure (P). Typical pressures for injection moulding can be as high as 100 MPa.

Answer 26

From the injection unit it heats and therefore softens the viscous fluid into the mould for shape stabilisation. Thermoplastics cool Thermosets and elastomers- cross link/cure in the mould

Answer 27

The tool is machined with a series of channels (the feed system) through which the polymer flows on injection: from the machine to the ends of the cavities.

Answer 28

Sprue – this is a tapered channel machined through the sprue brush, fed by and in-line with the machine nozzle. Allows easy separation from the mould. Runner – these are the basis of the feed system design that takes the polymer melt to the mould cavities. Runners are positioned on or perpendicular to the parting surface of the mould and are designed to deliver the melt to the cavities with minimal pressure drop. Runners should always be balanced and be symmetrical (wherever possible) for multi cavity tools. Gate(s) – these are the entrances to the mould cavities, fed by runners. Many variations in gate design exist. Once filled, their function is to allow the polymer to freeze rapidly (preventing back flow) and to provide easy de-gating at ejection. Cavities – the cavity (or cavities) are machined to high-precision. Depending on the product complexity, inserts and/or cores may be necessary to simplify mould manufacture.

Answer 29

 Unbalanced Feed Systems (multi-cavity moulds): - cavities which fill first are being ‘packed’ whilst others are still filling- this leads to non-uniform dimensions, weight, quality, part distortion etc. Unbalanced Feed System (complex, single cavity mould): contains a flow obstruction, therefore weld lines form BALANCED FEED SYSTEM:  more complex layout  greater amount of material in the runners weld lines positioned appropriately more uniform filling  Equalising the pressure requirement (ΔP) to fill each cavity;  Attempt to achieve symmetrical flow wherever possible.

Answer 30

1) Unbalanced – varying part quality (2) Naturally Balanced – limited applications (3) Artificially Balanced – requires analysis – secondary runners have smaller diameter (middle components) MUST CHOSE WHICH IS BETTER FOR USAGE

Answer 31

Lateral dimensions of the runners (e.g. diameter, for a circular section) are different Objectives:  to deliver plastic melt to all cavities at the same pressure  all the cavities fill at the same time – switch to packing phase

Answer 32

1. An edge gate at the end of the component would require a very high pressure = more orientation. The gate is in a better position in regard to avoiding mechanical weaknesses in the middle. Better for MULTI CAVITY mould/ SMALL components. 2. A gate at the centre of the panel centre gate gives the possibility of a direct feed from the machine, simplifying the mould tool and reducing the pressure requirement; BETTER FOR a SINGLE, LARGE component.

Answer 33

It is a non -isothermal flow in mould filling (temperature not the same throughout). There is a net heat flow away from the flowing melt (thermoplastics), as the cavity fills.  The opposite is true for rubbers / thermosets: heat flow to the flowing material). As Tmelt >> Tmould theres non-isothermal flow

Answer 34

1. By circulation of a heat transfer medium through ‘cooling channels’ drilled into the mould plates. Water (chilled, or frozen from a mains supply) is used for sub-ambient temperatures and heated oil is usually used (from a separate, independently controlled mould heater) for temperatures higher than ambient. 2. Electrical resistance ‘cartridge’ heaters are used for heating specific areas of the mould, for example, in heated runner systems.

Answer 35

cooling the mould is typically essential when moulding thermoplastics, in order to stabilise the new shape that has been formed in an economically acceptable cooling time. 1. Moulding feasibility – effect on non-isothermal flow and pressure requirement (ΔP) for mould filling. 2. Structure development - mould temperature influences microstructural features such as orientation (flow-induced), residual cooling stress (thermally–induced), shrinkage and distortion, and strength of weld lines. 3. Promotion of crystallisation – thermoplastics crystallise at maximum rate around halfway between Tg and Tm. Elevated mould temperatures will enhance the rate of crystallinity development in specific engineering polymers (nylons, polyesters) for applications where rigidity / high modulus is required. 4. Product quality – other attributes of the product are dependent upon mould temperature, e.g. replication of the mould and surface quality.

Answer 36

Increased cooling time, therefore: * Increased cycle time, in production

Answer 37

crystallinity, residual stress, microstructure

Answer 38

1. Isothermal - actually are non-isothermal as Tc

Answer 39

The maximum pressure on the machined must be > that the sum of pressures from the sprue, runner, gate and cavity. These pressures are a function of melt temp, mould temperature, injection speed Q and therfore shear rate. ref eqs pg 112-115

Answer 40

Newtonian - Linear Pseudoplastic fluids - non-linear

Answer 41

1. The molten plastic is injected down to a relatively cold flow channel: heat transfer from the melt is extremely rapid at the mould wall, such that local temperature drops dramatically and the polymer solidifies; 2. As moving away from the mould surface, the colling rate diminishes, due to the thermal insulating character of the developing solid layer of polymer; 3. More shear heat is being generated as fresh melt flow through the channel, which itself decreases in thickness as the frozen layer grows; 4. Eventually, a pseudo thermal equilibrium is set up in the frozen layer, where the heat losses to the mould are counterbalanced by prolonged contact with the flowing melt; further growth of the frozen layer does not occur; 5. Since these events take place relatively quickly, the thickness of the frozen layer is assumed to be constant as a function of distance along the cavity (thickness H). ref eqs pg 117-124

Answer 42

 Of great significance to manufacturing feasibility is the cooling time for a moulded component.  An approach equivalent to frozen-layer development can also be used to estimate cooling time, if the correct demould (ejection) temperature (TEJ ) for the centre-plane of the thickest part-section, can be specified.  These represent the maximum temperatures at which mouldings can be ejected and remain form-stable.  Ejection temperatures are related to the thermal transition temperatures and can be summarised:  Amorphous Tg ± 20ºC  Semi-crystalline Tm – (20 to 40)ºC ref eq pg 126-127

Answer 43

Melt Transportation; breaking secondary bonds

Answer 44

Reactions; breaking primary bonds. This can include degradation or cross linking of primary chains.

Answer 45

Melting- Polymer-additive interactions:  MIXING - dispersive - distributive  DEVOLATILISATION - loss of volatiles / gases - venting Shaping Phase (Flow & Cooling)-  MOLECULAR ORIENTATION - induced by stress (during flow) and rapid cooling  RESIDUAL STRESS: - induced by non-uniform cooling and thermal constraint  SHRINKAGE / DISTORTION: - anisotropic effects of orientation  MICROSTRUCTURALDIFFERENCES: - crystallinity - crystalline structure / texture - weld-lines - voids

Answer 46

Melting- A) Decreases in molecular weight - mechanisms of degradation / decomposition:  Thermal  Oxidation (O2)  Hydrolysis (H2O) Accelerated by:  Mechanical stress  High residence time Shaping Phase (Flow and cooling)- B) Increases in molecular weight:  Crosslinking (either deliberate, or a by product of degradation)  Reactive processing(Note that thermosets and rubbers are deliberately crosslinked (‘cured’) in the final stages of processing)

Answer 47

Preferred alignment of molecular chains in a fabricated product. It is similar to the concept of molecular relaxation so that in a random coil this is the lowest free energy state. When a HIGH stress is applied to this it can either slow cool(molecular relaxation) or fast cool(orientation).

Answer 48

Either through direction which is uniaxial or biaxial or through the magnitude of orientation - degree of alignment.

Answer 49

In through thickness profiles of orientation in thermoplastics: 1. As molten polymer enters the mold, it flows faster in the center of the channel and slower near the cooler mold walls. There is highest shear stress at the walls and lowest at the center. At the flow front, polymer at the center moves outward and toward the mold walls, "fountaining" out, while previously arrived material near the walls gets pushed sideways and backward. Therefore chains in the center are less oriented due to lower shear rates but are highly stretched and oriented near the walls from the high shear. Final levels of orientation depend on cooling rate and shear stress, each altered by other variables.

Answer 50

(1) Complex pattern close to the gate - due to effects of packing phase (2) Non-linear shear stress / pressure profile, due to: - pseudoplastic melt; - non-isothermal flow. Orientation increases at a smaller flow length. There is maximum shar stress and pressure at the gate but it is a minimum at the end of fill (furthest away). Ref eqs pg 141 for prediction.

Answer 51

Mould temp- increases then the degree of orientation decreases linearly - bc rapid cooling melt temp- increases then the degree of orientation decreases exponentially - bc high viscosity and shear stress Cavity thickness- increases then the degree of orientation decreases linearly - bc high shear stress and cooling rate Hold on time- increases then the degree of orientation increases exponentially - bc hold pressure - effect of packing Hold pressure-increases then the degree of orientation increases linearly

Answer 52

If: mould temp is low , melt temp is low , cavity thickness is thin( high shear stress and cooling rate), hold on time is high, hold pressure is high (bc high shear stress)

Answer 53

the modulus and the refractive index of the material are a function of direction which leads to birefringence. To analyse light wave components must be vibrating in parallel or perpendicular to the orientation direction.

Answer 54

A state of internal stress in fabricated products, as a result of inhomogeneous (nonuniform) cooling and constrained thermal contraction. AKA thermal, cooling or quenching stress. caused by NON UNIFORM COOLING not flow.

Answer 55

Typical residual stress profiles in injection moulded parts show tensile stress in the core of moulded parts, with compressive stress close to the surfaces. Theoretically (plate glass forming) the material cools from compression to being in tension back to compression (bell curve). Tensile stresses develop in the slower-cooling core. Compressive stresses form in the fast-cooling outer layers causing balanced residual stress profile (different in real life). This can cause non-uniform shrinkage as the outer layer solidify and shrink earlier than the inside and a temp gradient is created.

Injection Moulding Flashcards

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