A brief introduction to principles of mold flow analysis and how to read mold flow analysis reports

 

Pre-processing in Moldflow

Current mainstream mold flow analysis software is Moldflow, which only accepts triangular units and tetrahedral units.
High-quality finite element mesh is guarantee of finite element analysis accuracy.
For injection molded parts, there are three main mesh division methods in Moldflow: neutral surface, dual-face, and 3D entity.

Part	Neutral surface	Dual-face	3D entity
Division method	Extract neutral surface of part, then divide mesh (triangular unit) on neutral surface	Extract surface of part as core and cavity surface of mold, then divide mesh (triangular unit)	Directly divide finite element mesh on 3D digital model.
Advantages	Fewer meshes, fast analysis speed, and high calculation efficiency	No need to extract neutral surface, and post-processing is more realistic	High calculation accuracy
Disadvantages	Difficult to extract the neutral surface and low analysis accuracy	Meshes on upper and lower surfaces of part require a certain correspondence, and mesh division requirements are high	Large number of units, low calculation efficiency

Assumptions in Midplane and Dual Domain Meshes
Applicable to thin-walled parts
Flow width is at least four times thickness
Use Hele-Shaw model for generalized Newtonian fluids
Follow Newtonian fluid mechanics
Ignore effects of inertia and gravity
Ignore heat conduction in the same plane
Ignore heat conduction through thickness
Ignore heat losses from edges

How to calculate change of flow front in neutral plane and dual plane
Starting from pouring point
Flow front grows to adjacent nodes
When one node is filled, another node is added
Material temperature enters model uniformly
Cooling of plastic depends on transferring heat to mold

Assumptions in 3D mesh
For thick-walled and “short and fat” products
Aspect ratio less than 4:1
Use a true 3D solid model
Calculated at each node
Pressure
Temperature
Velocity in X, Y, and Z directions
Following items can be considered for heat conduction in all directions
·Inertia
·Gravity

Gate placement design guide
Place gates to make flow
balanced
unidirectional
place gates
in thick wall areas
away from thin wall areas
towards mold wall to prevent ejection
prevent seam lines
in weaker areas of the product
appearance
prevent poor venting
add gates to
reduce filling pressure
prevent over-holding
gate placement depends on
mold type – two-plate mold or three-plate mold
runner – hot runner or cold runner
gate type – edge-in or submerged-in
mold design or functional requirements
for materials with long glass fibers, fiber orientation should also be considered
plastic material, flow capacity, filling capacity, product size, structural complexity, wall thickness and flow length ratio are comprehensively considered to determine type and approximate number of gates

Classification of thermoplastics in terms of morphology
Amorphous polymers
√Analysis of same structure during molding process
Crystalline polymers
√Compact structure when cooled, amorphous structure when heated

Crystallinity		Non-crystalline
Differences in physical properties	Opaque or translucent	Transparent
	Chemical resistance	Good dimensional stability
	Large mold shrinkage	Low mold shrinkage
	Good solvent resistance	Good impact strength
	Easy to break under stress	Easy to break under stress
Common types of plastics	PP	PS
	HDPE	PMA
	LDPE	PC
	PA(Nylon)	MPPO
	POM	EVA
	PTFE	PSF
	PEO	PES
	PET	PEI
	PBT	PAI
	HIPS	PAR

Filling area of each gate can be visually seen, and weld line can be roughly estimated.

Animation can intuitively show final filling position. End of filling is prone to defects such as weld lines and air pockets. Therefore, exhaust should be strengthened at the end of filling.

For products without thickness trend, use 3D grid analysis. Results of 3D grid analysis are more intuitive and accurate.

At the beginning of filling, it is displayed in dark blue, and last filled area is red. If product is short-shot, unfilled part has no color.
For a good filling of product, its flow pattern is balanced. A balanced filling result: all processes end at the same time, and material flow front reaches end of model at the same time.

This product is a PC lens with a size of 120mmX80mmX0.5mm. Since glue is too thin, it is not fully filled.

Contour lines are evenly spaced, and intervals between contour lines indicate flow rate of polymer. Wide contour lines indicate fast flow, while narrow contour lines indicate slow filling and stagnation. Areas with severe stagnation are prone to short shots.
Contour lines in red box in above figure are dense, resulting in slight stagnation and slow flow. Because glue thickness is too thin, main wall thickness is 2mm and wall thickness in the middle area is 1mm.

Note:
Injection pressure>=80%*holding pressure.
Holding time should be greater than time for gate to cool 100%.
Influence:
Too low holding pressure may make part unable to be filled. Normally, holding pressure does not exceed 80Mpa. Too much pressure may cause batching. For thin-walled or complex products, holding pressure will be increased to maintain pressure smoothly, which may be greater than or equal to injection pressure.
If holding time is less than 100% cooling time of gate, part may be under-pressurized and surface defects may occur.
If holding pressure is still maintained after gate is 100% cooled. Holding pressure at this time will not have any effect on part, but will only waste labor time and increase costs.

Note:
Maximum clamping force should not exceed maximum clamping force of injection molding machine used to produce part.
Clamping force varies with injection pressure and holding pressure.
Effect:
If maximum clamping force exceeds maximum clamping force of injection molding machine used to produce part, flash may be generated on part.

Above figure shows change of pressure over time during the whole process of product. Change trend of pressure in each area of product over time should be similar. If change trend is too different, you can use an XY graph with several points defined to view change trend of pressure at each point. Change trend should be consistent, otherwise it will cause uneven pressure distribution on product, uneven volume shrinkage, internal stress, and more prone to warping.

Note:
Switch between injection and holding pressure should be between 95% and 99% of injection completion.
Pressure distribution should be as balanced as possible when switching to holding pressure
Pressure drop of gating system can be visually seen, which is helpful for optimizing gating system.
If injection volume is less than 95% of part volume, insufficient holding pressure may result. That is, some areas may have defects such as shrinkage and lack of material due to insufficient filling.
Unbalanced pressure distribution may cause inconsistent shrinkage of the part material, higher residual stress, and even insufficient or over-holding pressure in a certain area.

Results show that pressure distribution is balanced when switching to pressure holding

Figure above shows temperature of polymer when flow front reaches a specified point in the center of plastic cross section. If flow front temperature is very low in thin area of product, stagnation or short shots may occur, weld lines and flow marks are obvious. If flow front temperature in a certain area is very high, material degradation and surface defects may occur. Make sure that flow front temperature is always within recommended range for polymer. Check model cooling rate to see if there are hot spots or cold spots in model.
Change in temperature of melt front should be less than 20 degrees. Excessive temperature changes can cause residual stress inside part, and presence of residual stress can cause part to warp
As shown in figure, front temperature can be overlapped with weld line results to evaluate quality of weld line.

Figure above shows weighted average temperature in thickness direction over time. Temperature trends of various regions on product should be similar over time. If trends are too different, uneven temperature distribution on product will occur, internal stress and surface defects will also occur. You can check model cooling rate to see if there are hot spots or cold spots in model.

Figure above shows thickness of frozen layer displayed as a factor of part thickness. This result shows thickness of frozen layer factor. The higher value, the thicker frozen layer and the thinner polymer melt layer. Generally, the thinnest part solidifies first and last part solidifies last.
You can check gate freezing time. If holding time is less than gate 100% cooling time, it may cause insufficient holding of part and surface defects. If holding is continued after gate is 100% cooled. Holding at this time will not have any effect on part, but will only waste work time and increase costs.
You can check whether holding channel is smooth. The thicker area in circle in left figure above is frozen first, and effective holding is not obtained, which is more likely to shrink.
When product is 100% solidified and cold runner system is solidified by more than 50%, product can be demolded, thereby determining molding cycle of product.

Note:
Shear rate is an intermediate data result.
Maximum shear rate cannot exceed allowable value of material.
Impact:
If shear rate during injection exceeds maximum shear rate of material, material may degrade during injection process, resulting in many unexpected defects such as black spots, white spots, impact marks, etc.

Note:
Shear stress on wall is an intermediate data result.
Maximum shear stress on wall cannot exceed allowable value of material.
Impact:
If maximum shear stress inside part exceeds allowable value of material, product is prone to cracking and surface quality is poor.

Note:
Keep welds as short and few as possible.
Keep welds away from exterior and load-bearing structure.
Horizontal welds are better than vertical welds.
Evaluate welds together with weld temperature.
Venting grooves should be provided near weld line.
Weld lines can be eliminated or reduced by optimizing flow port position, product structure and wall thickness. Position of weld line can also be changed.
Impact:
Weld lines can cause surface defects.
Weld lines can reduce strength of part.

Note:
Air pockets should be distributed on boundary of part.
Venting grooves should be added to mold where air pockets exist.
Air pockets should be avoided on the surface of part.
Effects:
Air pockets may cause part to be underfilled, with bubbles on the surface and pores in part.
Air pockets may cause burning and cause scorching on part.

Note:
Target value of shrinkage rate of part is about 3 times shrinkage rate of mold. Generally, the thicker wall is and the farther it is from gate, the greater volume shrinkage rate. Shrinkage difference is the most common cause of deformation.
Volume shrinkage rate of each area cannot differ by 2%. If difference is too large, large warping deformation will occur. Further optimization of pressure holding is required to make volume shrinkage rate evenly distributed. If volume shrinkage rate of adjacent areas differs by 2%, surface area of product is prone to shrinkage. Volume shrinkage can be reduced by optimizing product wall thickness, placing gate in wall thickness area, and increasing pressure holding.

Generally, if dent value is >0.03mm, surface shrinkage is obvious. Dent depth can be reduced by increasing basic wall thickness, reducing wall thickness of reinforcing ribs and bolt columns, and increasing holding pressure.

Above figure shows cooling time required for each area of product.

This figure shows approximate orientation of fibers on part. Changing gate can change fiber orientation.

Displays temperature of coolant in cooling circuit. Temperature rise from inlet to outlet should not exceed 2-3℃. If exceeded, cooling should be strengthened and water circuit should be optimized.

Generally, at the end of cooling, temperature difference on product surface is within 10℃, indicating that cooling effect is good. If temperature difference is too large, especially temperature difference between front and rear molds, product is prone to warping. In areas with high local temperatures, surface temperature of product can be ensured to be uniform by adding cooling water channels, baffles, springs, beryllium copper inserts, etc.

Above figure shows the total deformation of each node of product magnified 5 times. Among them, transparent part is product before deformation, and colored part is product after deformation. Above offset values relative to original model include normal shrinkage value of plastic and cannot be equated with product deformation value.
Note: Main factors affecting warpage of product should be caused by uneven shrinkage of material and glass fiber orientation, rather than insufficient cooling, molecular orientation and other factors.

Figure above shows the total deformation of product in X-axis direction at each node. Transparent part is product before deformation, and colored part is product after deformation. Above offset values relative to original model include normal shrinkage value of plastic and cannot be equated with product deformation value.

Above figure shows the total deformation of product in Y-axis direction at each node. Among them, transparent part is product before deformation, and colored part is product after deformation. Above offset values relative to original model include normal shrinkage value of plastic and cannot be equated with product deformation value.

Figure above shows the total deformation of product in Z-axis direction at each node. Transparent part is product before deformation, and colored part is product after deformation. Above offset value relative to original model includes normal shrinkage value of plastic and cannot be equated with product deformation value.

Main reason for product deformation is fiber orientation, followed by uneven shrinkage. Deformation can be reduced by optimizing gate position, product structure and optimizing holding pressure.

Only one gate in the middle is needed, and deformation is greatly improved compared with two gates.

Only one gate in the middle can significantly improve deformation compared with two gates. This is mainly due to significant improvement in deformation caused by orientation factors.
For materials with long glass fibers, fiber orientation has a great influence on warpage deformation of product, which can be improved by optimizing gate position.