Before formally introducing mold flow analysis of plastic parts, let's review aspects that should be considered for an excellent plastic injection molded part:
Uniform wall thickness: Ensure that shrinkage stress is relatively consistent when plastic shrinks from high temperature to low temperature
Insert edge chamfering: Insert edge chamfering can reduce stress at edge during high and low temperature impact
Location hole design: Location holes are also crack stop grooves, which can help release stress during injection molding process
Location hole arrangement direction design
Material selection
Pay attention to plastic strength and CTE
Through optimization of above aspects, ensure that part design meets requirements, which is also the first step to ensure quality of parts.
After part design is completed, it is delivered to part supplier, who will perform simulation analysis. Usually, multiple rounds of mold flow simulation are performed to find the best mold design and process parameters, and finally manufacture parts that meet requirements. Following is an introduction to evaluation points of mold flow simulation.
Runner and gate types mainly include: main runner, branch runner, gate, cold well, etc.
Main runner: Main runner is also called main runner, sprue or vertical runner. It is the first part that molten plastic flows through after entering mold. It starts from part where injection machine nozzle contacts main runner bushing of mold and ends at branch runner.
Branch runner: Branch runner is also called branch runner or secondary runner. It is transition area between main runner and gate, which can smoothly change flow direction of molten plastic. For multi-cavity molds, branch runner also has function of evenly distributing plastic to each cavity.
Gate: Gate is also called feed port. It is a narrow passage between branch runner and cavity. Its function is to use constricted flow surface to accelerate plastic. High shear rate can make plastic flow well. There are various types of gates, including side gates, point gates, latent gates, etc.
Cold well: Purpose is to store colder plastic wave front in initial stage of filling to prevent cold material from directly entering cavity and affecting filling quality or blocking gate. It is usually set at the end of main channel. When length of branch channel is long, a cold well should also be opened at the end.
Different types of gates and their characteristics are as follows:
Side gate: simple shape, easy processing, by changing gate size can effectively adjust shear rate and gate condensation time during mold filling, but it will leave gate marks on outer surface of plastic part.
Point gate: cross-sectional size is very small, position restriction is small, and residual trace after removing gate is small, which does not affect appearance of plastic part, but pressure loss is large and mold structure is complex.
Latent gate: evolved from point gate, feed gate is generally on inner surface or side of plastic part. It does not affect appearance of plastic part. Gate is automatically cut off when mold is opened.
Direct gate: plastic melt flows directly into cavity, pressure loss is small, and feeding speed is fast, but it is difficult to remove gate and traces are obvious.
These different types of runners and gates need to be selected and designed according to specific plastic type, part size and mold requirements when designing.
Material information
For different material selections, material parameters and process parameters are inconsistent, so it is necessary to input correct selected material information during simulation: process parameters, viscosity curve, PVT curve.

 

Process parameters
Process parameters mainly include injection time, holding time, holding pressure, cooling temperature, cooling time and holding parameter curve. This part of information must be confirmed with plastic particle supplier.

Filling process
Filling process is a dynamic diagram that characterizes injection molding process of sample. When evaluating mold flow filling process and filling quality, following points should be paid attention to:
Filling is completed at the same time as much as possible
Melt flows smoothly in mold
Is there a situation where filling in a certain area suddenly becomes faster or slower
If so, is there a sufficient reason (such as due to difference in part thickness, part structure, etc.)
No short shot position (gray area)

 

Shrinkage index
Shrinkage index refers to shrinkage of surface raw material at wall thickness due to volume shrinkage of product during cooling, resulting in shrinkage on the surface of part. Phenomenon of a dent is called shrinkage. Following points can be used to evaluate whether shrinkage is reasonable:
Is there a position where shrinkage factor is ≥ S
Is point where shrinkage factor is greater than S located on A side of part
Is position where shrinkage factor is greater than S and located on A side acceptable to a certain extent
If there is sufficient reason (such as strength requirements for ribs) for shrinkage caused by rib thickness ratio not meeting requirements, it can be accepted at discretion

 

Weld line
Line formed by intersection of two melts is weld line. Because weld lines have appearance and strength reasons, obvious weld lines should be avoided as much as possible. Rules for evaluating weld lines are as follows:
Melt Is melt intersection angle greater than 135°?
If melt intersection angle is ≤135°, is it located on A surface? Is temperature difference between two melts ≤10℃?
If it is located on A surface, is it limitable?
Generally speaking, weld marks cannot be avoided around holes with more complex structures, and can be considered as appropriate.

 

Gas entrapment
Gas entrapment refers to confluence of at least two melt flow fronts, or at the end of melt flow path, which may cause gas retention and cannot be discharged during molding. Gas entrapment may cause defects such as burnt parts, lack of meat or surface scars. Rules for evaluating gas entrapment are as follows:
Is there a phenomenon of encapsulated gas entrapment on A surface of part?
If trapped air on A side is near parting surface, it can be solved by exhausting
If trapped air on A side is not on parting surface, it is necessary to consider exhausting by inserts, etc.
If there is trapped air in speaker mesh, it is necessary to consider making inserts for exhaust
Is there any trapped air on B side of part? Based on results of CAE analysis, is it planned to add exhaust to mold?
For deep ribs (≥25mm), it is recommended to make inserts for exhaust

 

Pressure at the end of filling
Generally, gradient changes can only be controlled within two color spectrum changes

 

At the beginning of filling, pressure is the highest; as filling progresses, pressure decreases
Pressure at the end of filling is 0
Pressure distribution diagram at the end of filling is an effective tool standard for evaluating whether pressure distribution of product is balanced. Rules are as follows
Confirm whether filling pressure gradient is same as filling time distribution trend
Confirm whether change in pressure gradient is reasonable
Pressure at VP switching
When cavity is about to be filled, pressure when screw movement is converted from flow rate control to pressure control is called V/P conversion pressure. Evaluation criteria are as follows:
Pressure at V/P conversion generally does not exceed 80% of injection pressure of molding machine
Injection pressure is not tonnage of mold
When reaching V/P conversion pressure, flow channel loss pressure shall not exceed 50% of V/P conversion pressure
Filling amount at V/P conversion is required to be ≥95%

 

Pressure at injection position
According to injection pressure XY diagram, it is clear when (X time point) Ymax is reached
Ymax is generally pressure at V/P conversion. If not, it means that there is an unbalanced injection molding process, and internal stress that cannot be released may be generated inside part, causing defects such as shiny and deformation

 

Change of pressure can be seen through XY diagram of injection position pressure. Evaluation criteria are as follows:
Maximum pressure should be ≤80% of maximum pressure of injection molding equipment
Flow front temperature
Flow front temperature describes temperature at a certain time point during filling process. Flow front temperature is temperature during flow process, and is distribution of wavefront temperature during flow process. It represents temperature at the center of cross section. Evaluation criteria are as follows:
Whether flow front temperature is always within temperature range recommended for material
Confirm whether there is obvious low temperature (close to lower limit of recommended material temperature) in the area where B rib is thinner (≤0.7mm). If so, countermeasures are required
Flow front temperature difference should be ≤20℃
If there is a situation of >20℃, it is necessary to prove whether there is material retention or under-injection at low temperature; whether high temperature exceeds recommended material temperature
Where temperature difference between two melts is ≤10℃ at location where weld mark occurs

 

Cooling water temperature
Cooling water temperature shows temperature distribution of cooling water in cooling circuit. Evaluation criteria are as follows:
According to CAE analysis, inlet and outlet temperature difference of coolant is ≤3℃
If there is a deep well or parallel waterway, please provide coolant Reynolds index data to ensure that Reynolds index is ≥10000 (ideal Reynolds number is 10000)
Standard ② is only used for deep well structures (well depth ≥50mm) or parallel waterways

 

Deformation
Deformation diagram feedbacks the overall deformation of product and deformation in three directions of X/Y/Z
Refer to requirements of different products: uneven cooling, uneven shrinkage, uneven fiber orientation, corner effect
Uneven cooling: Design of cooling waterway is unreasonable, so that product cannot obtain uniform cooling
Uneven shrinkage: Shrinkage of product is inconsistent
Fiber extraction Uneven orientation: Uneven fiber orientation causes large warping deformation of product
Corner effect: Heat is concentrated at the corners of deep box-shaped products, shrinking more, causing bending deformation
Solution: Optimize cooling water channels
Solution: Change materials, product structure, number and position of gates, and pressure holding curve
Solution: Number and position of gates, product structure
Solution: Strengthen cooling at corners and reduce wall thickness at corners

 

If shrinkage compensation is done in CAE analysis, deformation should meet standard one CAE shrinkage compensation, which is not applicable to 3D networks or multi-cavity molds
In order to facilitate your learning, I will organize key information into following content, hoping to help you.