Prediction of Shrinkage in Plastic Injected Parts Due to Cooling

Transcription

1 Prediction of Shrinkage in Plastic Injected Parts Due to Cooling Members: Megan Bunch and Patrick Delp Date: 5/5/03 Spring Semester Instructor: Dr. H.U. Akay ME450 Computer-Aided Engineering Analysis

2 Introduction to the Project Background information.

3 Background Information Warping in parts is caused by three things: Incorrect or insufficient injection points Variations in part thickness Irregular shrinkage due to cooling

4 Shrinkage Due to Cooling When plastic parts are injected into a mold they are at 500 F and are rapid cooled to F. Once released from the mold, the part will deform from the mold shape due to residual stresses from the cooling process.

5 What s the problem? Plastic parts that require tight tolerances may warp out of tolerance (even if made by the most experienced of mold makers). New companies may not have the expertise to start making molds correctly for even the simplest of parts.

6 The point. Big $$ are spent on molds and included in the cost of making parts. Much of the money spent on making molds goes into reworking them. There are companies devoted to reworking plastic injection molds. Ex. Ideal Tech, Inc.

7 Basically If the mold can be made right the first time: saving large money on mold => customer saving money on end product

8 Objectives List of Objectives, basic and possible, and Work accomplished.

9 Basic Objectives Discover if, by the use of FEA, shrinkage due to the rapid cooling of plastic injected parts can be predicted. Use ANSYS to model, mesh, and analyze simple plastic components given thermal boundary conditions.

10 Possible Objectives Create a program, CAE, that will take the desired part geometry and output the appropriate mold geometry. Would Require: extensive research and testing on each type of material used professional programming experience Co-operation with ANSYS or other comparable FEA program

11 Work Accomplished Obtained results from Simple Plate showing the predictability of the deformation due to cooling. Found stress concentrations for 3-D models to demonstrate FEA abilities.

12 Theoretical Background Illustrations, Principles and relevant equations.

13 Solely Due to Cooling In order to analyze the deformation of a part solely due to cooling the following assumptions need to be made: Part is at uniform temperature at beginning of cooling, i.e. perfectly injected with no heat loss. Part is allowed to come to equilibrium with mold temperature at end of cooling.

14 Minimum Cooling Time, t c t c = h 2 απ 2 ln 4 π T T M E T T W W Where a = thermal diffusivity h = plate thickness TW = mold wall temperature TM = melt temperature TE = ejection temperature

15 Material Used Material Used: Polycarbonate Typical Injection Polymer General purpose polymer: Car interiors Traffic Lights (lens housing) Baby Bottles Cell Phone Casing

16 Illustration of Process

17 Model Details Description of model as well as boundary conditions, loads applied, and meshes used.

18 Boundary Conditions & Applied Loads For all models: Thermal Boundary Conditions are that of being in the mold. Ambient Temperature is 40 F Ambient Temperature is applied to the volume s surfaces or the area s lines Transient Thermal Load is applied to the volume. Thermal Load is given to the volume such that it cools from 500 F to 40 F within 30 seconds

19 Special Boundary Conditions For Simple Plate Analysis Thermal Load is applied as with all models. Structural analysis is performed by taking a quarter of the plate and reapplying the stresses to the part to find deformation.

20 Model List Simple Flat Plate Computer Component (Bay) Cover Typical Plastic Casing

21 Simple Flat Plate No Features to keep analysis simple Modeled as area with thickness 4 X 4 plate

22 Simple Flat Plate Mesh:

23 Computer Bay Cover Has snaps for assembly to computer Minimum wall thickness = 1/16 Maximum wall thickness = 3/16 Overall size 5 17/32 x 2 3/32 x.5 (at snaps)

24 Bay Cover Illustrations Modeled from an actual snap fit computer bay cover shown below

25 Component Cover Mesh

26 Typical Casing Casing with features typical to snap fit plastic components: Ventilation holes Snaps Thin Walls (Shell) Rounded Edges

27 Typical Casing Mesh

28 Results Plots of important results.

29 Results Obtained Results were obtained for Simple Flat Plate. Stresses were shown in more complex models.

30 Simple Plate Results Step One: Stresses obtained along the edge of the part.

31 Simple Plate Stress Propagation

32 Stresses along the part edge:

33 Simple Plate Results Step Two: Reapply the stresses from the thermal analysis to the part in a structural analysis

34 Reapply Stresses:

35 Simple Plate Deflection in part was as predicted FEA analysis can be used to predict shrinkage in parts from stress concentrations

36 3-D models More in-depth FEA abilities Used to show the stress concentrations in actual and general case parts

37 Computer Component Cover Modeled from an actual snap fit computer bay cover shown below

38 Component Cover Mesh

39 Stress Concentrations

40 Max Stress

41 Typical Casing

42 Stress Concentrations

43 Max Stress

44 Impact Statement Societal aspects of the Project.

45 This project was based on the idea of Cutting Cost: Increase company s profits Lower End-Product Price Quicken time-to-market Higher efficiency => Better Economy

46 Conclusions Conclusions and Recommendations for future work.

47 Successful Project Since FEA can be used to predict shrinkage in rapid cooled plastic injection molded parts. More testing and research needs to be done in this area.

48 Recommendations Future testing and research for different types of plastics. New program, CAE, that is capable of taking desired geometry and outputting the mold geometry.

49 Bibliography All Resources knowingly used.

50 ool.html tech/guide/display.htm?guide=ther_st.ht m 97/07/004.html

51 /ansyshelp/Hlp_V_VM33.html s_and_processes/processes/process 8.asp s_and_processes/polymers/polymer 9.asp