How to read a mold flow analysis report?

Time:2019-04-22 08:24:17 / Popularity: / Source:

Product design is to ensure that designed parts are mold-openable. Now finite element simulation of injection moulding, holding, cooling and warping of plastic parts can be carried out with CAE software (Moldflow, C-Mold, Z-Mold, etc.). When mold is reviewed, plastic mold factory generally provides a mold flow analysis report. As a plastic mold factory, how do we interpret a mold flow analysis report?
First understand definition of results and know how to use results. Definitions of commonly used results and how to use them are listed below.
Process parameter setting: Process setting includes all relevant equipments such as injection molding mold, injection moulding machines and their cooling, holding pressure, opening and closing modes, other parameters throughout injection moulding cycle. Therefore, setting of process parameters is actually a process of abstracting actual manufacturing process and production equipment. Setting of process parameters will directly affect analysis results of product injection moulding.
  • Filling analysis
(1) Fill Time
Filling time shows expansion of melt flow front. Default drawing method is shadow map, which is easier to interpret using moiré pattern. Moiré shows results in isoline, contour spacing is relatively uniform, sparse contours indicate a gentle flow rate, and dense equivalent red indicates a rapid flow rate. Any position on product can show time melt reached that position. For most analyses, filling time is a very important key result.
More balanced filling process is mainly reflected in: melt reaches remote of cavity at the same time.
Using filling time results, you can find following problems in injection moulding process:
1) Short Shot and hesitation, short shot is shown in gray, very obvious, and there is a case where hysteresis often occurs when contour is densely populated in a small area, resulting in short shot.
2) Overpacking, if melt first fills cavity in flow path in a certain direction, there may be a case where pressure is maintained, and overpressure may cause uneven distribution of product, thereby product is out of design weight, wasting material, and more serious is causing warpage.
(2) Weld lines
When flow fronts of two melts are brought together, or a flow front is separated and brought together, a weld line is created, such as melt flowing along a hole. When there is a significant difference in flow rate, a weld line is formed, such as a fast flow at a thick wall, a slow flow at a thin wall, and a weld line may be formed at a thick junction. Weld line can be displayed along with fill time and can also be displayed with temperature and pressure maps. Reducing number of nozzles can eliminate some weld lines, changing nozzle position or changing wall thickness of product can change position of weld line.
(3) Air traps
When material flows from all directions to same node, air traps is formed. Air traps is displayed in its actual position. When air traps is on parting surface, gas can be discharged. In position where air traps is present, exhaust groove should be placed on injection molding mold. Air traps on product should be eliminated, changing wall thickness, nozzle position and injection time of product will help eliminate air traps.
(4) Temperature at flow front
Flow front temperature is intermediate flow temperature at which polymer melt fills a node. Because it represents temperature at the center of section, it does not change much. Flow front temperature map can be used in conjunction with weld line diagram. Temperature of melt is high when weld line is formed, quality of weld line is good (the first place where weld line is formed in one section is center of section, therefore, if temperature of flow front is high, weld line strength is usually high). Change in melt tip temperature should be less than 30F. Excessive temperature changes can cause residual stresses inside part, and presence of residual stress can cause part to warp.
(5) Pressure end of filling
Pressure end of filling belongs to a single set of data, which is an effective tool for observing whether pressure distribution of product is balanced. Since pressure at the end of filling is very sensitive to balance, if pressure map at this time is balanced, product is well balanced. Unbalanced pressure distribution may cause inconsistent shrinkage of product material, higher residual stress, and even lead to insufficient holding pressure or over-pressure in some areas.
Maximum pressure used should be below pressure limit of injection machine, and many injection machines have a pressure limit of 140 MPa (20,000 psi). Design pressure limit of injection molding mold is preferably around 100 MPa (14,500 psi). If pressure limit of injection moulding machine used is higher than 140 MPa, design limit can be increased accordingly. Design pressure limit of injection molding mold should be approximately 70% of limit of injection machine. If analysis does not include a gating system, design pressure limit should be 50% of limit of injection machine.
  • Pressure analysis
(1) Frozen layer fraction
Percentage of solidified layer simulates whole process of product from cooling to solidification. Red area indicates the first solidified area. Generally, the thinnest part is first solidified, and the thickest part is finally solidified. If solidified layer percentage value is 1, then Indicates that section has completely solidified. Percentage of solidified layer is result of intermediate data. It is useful to observe time at which product and nozzle are solidified. If some areas of product close to nozzle are solidified early, area away from nozzle will have a high shrinkage rate; if thicker area is solidified first and pressure-holding path is cut, thicker area will not be effectively held. Typically, an XY plot is created at a critical location (such as a nozzle) to observe changes in percentage of solidified layer. In general, percentage of solidified layer at the end of filling is required to be more than 80% to open injection molding mold.
(2) Pressure at injection location: XY Plot
Nozzle position pressure XY diagram is change of pressure of product nozzle position during whole process of injection, pressure keeping and cooling. Nozzle node is a common node for observing 2-dimensional XY maps. Change in pressure can be easily seen by XY plot of nozzle position pressure. When melt is injected into cavity, pressure continues to increase. If there is a spike in pressure (usually at the end of filling), it indicates that product does not reach equilibrium filling well, or velocity of flow front is increased due to a significant reduction in volume of flow front material.
(3) Volume shrinkage
Volume shrinkage is a percentage reduction in volume of part due to holding pressure. Volume shrinkage is an intermediate data result that shows change in shrinkage of product during holding and cooling. This result is usually not used because shrinkage at ejection is final volume shrinkage of part. Volume shrinkage at ejection is result of a single set of data. Shrinkage of entire cavity should be uniform, but it is usually difficult to achieve, and shrinkage rate can be made uniform by adjusting pressure holding curve.
(4) Sink Index (Sink Index)
Sink index gives relative likelihood of a sink mark on product, the higher value, the greater likelihood that a sink or crater will appear. Volume shrinkage and product wall thickness values are used together when calculating sink index. Indentation index map is a very useful relative tool when comparing different scenarios.
(5) Clamp force: XY Plot
Clamping force XY diagram indicates case where clamping force changes with time. Clamping force is very sensitive to filling of mold, holding pressure and volume/pressure control switching time. A slight adjustment to these parameters will cause a large change in clamping force. Maximum clamping force must not exceed maximum clamping force of injection moulding machine used to produce part.
  • Warpage analysis (Deflection)
Results of warpage analysis are divided into four categories by plastic mold factory: overall deformation, deformation caused by cooling factors, shrinkage factors, and molecular orientation. Each type of deformation is further divided into total deformation and components in three directions of X, Y, and Z. Main factor affecting product warpage is shrinkage of material rather than insufficient cooling, molecular orientation and other factors.
Overall deformation
Material shrinkage deformation
Cooling factor deformation
Molecular alignment factor deformation
  • Temperature analysis
(1) Product surface temperature (Temperature, part)
Product master face (Top) is shown in blue, and product male face (Bottom) is shown in red. Die surface temperature should be near set value, distribution should be uniform, and variation range should not exceed 30F.
(2) Temperature difference between male and female products (Temperature difference, part)
This result describes difference between product die face temperature and die face temperature, which is difference between die face temperature minus die face temperature. Mold temperature difference should not exceed 20F, and exceeding 20F may cause warpage.

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