The effect of melt compressibility on the mold filling of thin-walled parts

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1 The effect of melt compressibility on the mold filling of thin-walled parts Andy Rose, Lucien Douven, DSM Ahead BV Autodesk Moldfow, Benelux user meeting Eindhoven May 2 nd, 2017

2 Agenda Introduction Background Molding trials Mathematical model Dimensionless model & Gamma Simulations vs practice Model setup Results Conclusions 1

3 Things you may not know about DSM Engineering Plastics

materials* Melt temperature ( C) 280 240 200 PA410 Akulon PA6, PA66 Stanyl PA46 * Enabling High Tg Optimum

4 DSM has the most extensive portfolio in high temperature polyamides DSM launched the first halogen free materials in 2008 and has a range of thermally and electrically conductive materials Innovative materials* Melt temperature ( C) PA410 Akulon PA6, PA66 Stanyl PA46 * Enabling High Tg Optimum chemical resistance High peak temperatures High tensile modulus Glass transition temperature (ºC)

5 You can find DSM materials in 87% of all new cars

6 You can find DSM materials in EVERY mobile device

connectors Trends in connectors: Smaller

7 Introduction DSM Engineering Plastics supplies materials (PA46, PA4T, PA6, PPS, PBT, PET) for electrics & electronics applications, e.g. connectors Trends in connectors: Smaller height Smaller pitch More pins longer flow length Thinner parts: thin-wall molding 6

8 Introduction What flow length can be reached in thin-walled parts? depends on competition between filling rate and solidification rate Conceptually no difference between conventional and thin-wall molding 7

9 Flow behavior: Filling pressure vs filling speed filling pressure viscosity increase: cooling crystallization For specific material and mold thickness Newtonian, isothermal shear-thinning, viscous heating filling speed (= actual flow speed in cavity) 8

10 How to achieve longer flow lengths via processing? filling pressure pressure dominated by cooling & crystallization: thin parts pressure dominated by filling speed: thick parts max. injection pressure of machine increase v fill decrease v fill filling speed 9

11 Flow rates in injection molding pressure sensors h b filling rate Q fill injection rate Q screw flow rate (if melt is incompressible!): 1 4 pd 2 Q v screw screw = = Q fill n b h v fill Q in cc/s v in mm/s n: number of cavities 10

12 Molding of 0.2 mm thin flow bar expected result L = 50 mm h = 0.2 mm b = 5 mm measured result: mold filling rate <2% of imposed injection rate! 11

13 Molding of commercial connector in 32-cavity mold measured result: mold filling rate 15% of imposed injection rate! 12

14 pressure (bar) Explanation: melt compression effect Start of filling: melt at mold entrance mold melt volume screw 13

15 Explanation: melt compression effect During of filling pressure (bar) pressure increase screw displacement compresses melt v inj screw displacement to fill mold 14

16 Explanation: melt compression effect End of filling pressure (bar) cushion v inj melt compression part >> mold filling part 15

17 Melt compression vs. flow length Recap: Mold filling = competition filling rate solidification rate Filling rate can be slowed down significantly by melt compression Melt compression limits flow length Questions: What determines magnitude of melt compression effect? How to reduce compression effect? How to obtain longer flow lengths for thin-walled parts? 16

18 Melt compression model Melt flow balance (Chan et al, 2000; Basov and Kazankov, 1970): dp k V melt + Q mold = dt melt compression rate mold filling rate screw Assumptions: Leakage (over check ring) neglected V melt >> V cavity (Þ V melt constant) Pressure drop Dp = 0 from screw tip to gate Isothermal flow Newtonian liquid Compressibility k : approx. 10% volume reduction at 2000 bar Q screw injection rate 17

19 Melt compression governed by one dimensionless parameter Substitute Q mold and p in flow balance using equations for slit flow 12h V 2 h melt d æ dl ö k çl + dt è dt ø melt compression rate dl hb dt mold filling rate = Q screw screw injection rate Make dimensionless using scaling parameters : with boundary conditions l(t=0)=0 and p(t=0)=p init G d dt æ ç è ö ø * * * dl dl çl + = * * * dt dt 1 L = cavity length t = hbl Q screw = incompressible filling time 18

20 Interpretation of parameter G Dimensionless flow balance G Parameter G governs effect of melt compression on mold filling G = d dt æ ç è * * * çl = * * * 12hk V b dl dt melt screw» 2 4 h ö + ø Q dl dt material machine process mold Note: melt compression rate ~ 1 melt compression rate mold filling rate dp dt depends on mold filling rate 19

21 Significant compression effect in thin-wall molding 4 mm tensile bar Incompressible filling 1 mm flow spiral 0.5 mm flow bar 2-cavity 0.2 mm flow bar 0.2 mm flow bar 20

22 Good agreement between experiment and model (material: PA46) calculated filling rate very sensitive to errors: dp Qmold = Q screw - k Vmelt dt 1 cc/s = 40 cc/s 39 cc/s 21

23 How to decrease compression effect? G = 12hkV melt 2 4 b h Q screw Increase part width b and thickness h Minimize injection rate Q screw Decrease compressibility k Decrease viscosity h = the Holy Grail of material development Decrease melt volume V melt (cylinder head, nozzle, sprue) 22

24 Comparison of practice with flow simulation software results Similar cases analysed in Autodesk Moldflow 3 different approaches were considered: Standard Part & runner modeled with injection onto end of the sprue Melt pot - Vmelt simulated via 1D hot runner elements Full process - Molding machine defined in analysis 23

25 Moldflow model Full 3d mesh including feed system Finite element solver No of elements elements over thickness Section over thickness of flow bar 24

26 Moldflow model Process definition Choice to enter injection time or flow rate. Constant flow rate specified at inlet condition, 40 cm³/s 25

27 Moldflow model Process definition for IM machine Representative IM machine selected from the standard database Constant flow rate specified at inlet condition, 40 cm³/s 26

Moldflow results Standard Melt Pot IM machine Qscrew cm³/s 40 40

28 Moldflow results Standard Melt Pot IM machine Qscrew cm³/s Vmelt cm³ Qmold cm³/s See next slide

29 Moldflow results 28

30 Comparison: simulated vs actual results Where are the main differences? Melt compressibility factor If pvt coefficients are adjusted to give the same melt compressibility as the experimental data then the results are as follows: 29

31 Results with modified pvt data Where are the main differences? Melt compressibility factor If pvt coefficients are adjusted to give the same melt compressibility as the mathematical model then the results are as follows: Moldflow Melt Pot IM machine Qscrew cm³/s Vmelt cm³ Qmold cm³/s

32 Conclusions Effect of melt compression on mold filling is large in thin-wall molding. Melt compression effect can be captured by the analytical model and there is only one governing dimensionless parameter. Automatic stroke volume of 120% is too small for small volumes to capture the true compressibility of the material especially where there is a high pressure increase in a very short time period. Relatively small differences in the melt compressibility between the pvt data and that seen at the machine can give a large error in the assumed flow rate in the mold. 31

Moldflow Report Beaumont Logo. Performed by: Beaumont Technologies Requested by: Customer

Moldflow Report Beaumont Logo. Performed by: Beaumont Technologies Requested by: Customer Moldflow Report Beaumont Logo Performed by: Beaumont Technologies Requested by: Customer Objective To analyze the BTI Logo part in order to determine an optimal gate location. To thoroughly evaluate the