measurement of residual stresser in injection mold

443

Measurement of Residual Stresses in Injection Molded Polymeric Part by

Applying Layer-removal and Incremental Hole-drilling Methods

Chae Hwan Kim, Sungho Kim, Hwajin Oh, and Jae Ryoun Y oun *

Department of Materials Science and Engineering, Seoul National University, Seoul 151-742, Korea

(Received May 7, 2007; Revised June 3, 2007; Accepted June 5, 2007)

Abstract: Injection molding is a flexible production method for manufacturing polymer products, but introduces residual stresses. This study employs the incremental hole-drilling method to measure the residual stresses in injection molded parts.To compare the results, the layer-removal method is utilized for measurement of residual stresses. A commercial software,Moldflow, is used for prediction of residual stresses in molded plastic parts. Results obtained by experiments and Moldflow analysis are compared. The incremental hole-drilling method is applicable to determination of the residual stresses in com-plex geometries and can be used as an adoptable technique for measurement of the residual stress in polymeric materials.Keywords: Injection molding, Polymeric materials, Residual stress, Incremental hole-drilling method, Layer-removal method

Introduction

In Injection molding is one of the most important processes for manufacturing plastic parts. It is suitable for mass production because raw materials can be converted into the net-shape by a single molding procedure. An important advantage of the injection molding is that complex geometry parts can be produced by a molding process with automatic control [1]. In addition, the method is popular because it enables fast production of high-quality products without considerable post-processing. The processing method has been used more frequently in recent years in the production of automobile and airplane parts as the molding process becomes automatic and lighter components are preferred.However, despite such advantages, the injection molding method has a few problems, e.g., residual stresses [2-6],warpage, and shrinkage of molded parts. These problems affect the quality of products seriously. Therefore, an extensive study is required for better understanding of the problems. In particular, residual stresses cause viscoelastic deformation of the final product when the product is used for an extended period of time or has been exposed to relatively high temperature. Residual stresses are internal stresses of the molded part in the absence of external forces. They are caused mainly by the non-uniform temperature profile in the part during filling, packing, and cooling steps. Residual stresses in a molded part have a considerable influence on its deformation, stress cracking, and durability, and processing conditions should be optimized to minimize these problems.In order to optimize the molding conditions, residual stresses must be investigated. Residual stress measurement methods currently in use are destructive and non-destructive methods,and the layer-removal method is one of the most commonly used destructive methods for injection molded parts.

Layer-removal Method

Developed by Treuting and Read, the layer removal method is based on the fact that residual stresses within an unconstrained object maintain equilibrium by itself. Removing thin uniform layers from the plate surface will remove the residual stresses and disrupt the balance of stresses. Residual stresses can be determined by measuring the curvature of the deformed plate and utilizing curvature-stress relations derived from elasticity. The method is widely used to measure residual stresses, but is limited to the measurement of residual stresses in flat plates. It is impossible to apply the method to plastic parts with complex geometry [4-9].

Incremental Hole-drilling Method

The incremental hole-drilling method is a semi-destructive residual stress measurement technique. The hole-drilling method was first proposed by Mathar for residual stress measurement. Introduction of a hole into a stressed body relaxes the stresses at that location. The removal of stressed material causes localized stress relaxation and deformation around the hole. Strain distribution caused by the deformation can be measured by using a specially designed strain gage,i.e., rosette. The procedure is relatively simple and has been standardized for metallic plates as ASTM Standard E837. A special three-element strain gage, rosette, is installed on the specimen at the point where residual stresses are to be determined. A precision milling guide is attached to the specimen and accurately centered over a drilling target at the center of the rosette. One can measure the induced strain at each drilling step and consider the relationship between the strain and principal stresses to determine the level of residual stresses. The equation for elastic strain (εrr ) which is measured by the rosette at the periphery of the hole is given as

(1)

εrr A B cos2β+()σmax A B cos2β–()σmin

+=*Corresponding author: jaeryoun@snu.ac.kr

444Fibers and Polymers 2007, Vol.8, No.4 Chae Hwan Kim et al.

(2)

where σmax and σmin are principal stresses, β is an angle measured counterclockwise from the maximum principal stress direction to the axis of the strain gage, and are calibration constants, and are dimensionless constants,E is Young’s modulus, and v is Poisson’s ratio. Magnitude and direction of the two principal stresses are obtained in terms of the measured strains.

(3)(4)

where ε1, ε2, and ε3 are relaxed strains measured by the strain gage [10-15].

This method is applicable to small area of an object, or an object with complex geometry. But it is assumed that the stress is uniform through the thickness. In many cases,residual stresses are not uniform with respect to the depth of the molded part. The current study is focused on investigation of the residual stresses in injection molded parts in which the residual stress is varying throughout the thickness. Moreover,unlike metal products which show elastic behavior, the injection molded polymeric parts have viscoelastic properties and the hole-drilling method has limited applications on polymeric parts. For this reason, there have been few reports on application of the hole drilling method to polymeric parts. In this study the integral method is used to measure the residual stresses because it is suitable for determination of the residual stress

field that varys with depth. The integral method evaluates the residual stress for each incremental depth during the hole-drilling measurement [16-19].

In this study, the incremental hole-drilling method which has been used frequently for metal parts is applied to a polystyrene plate produced at different injection molding

conditions in order to determine the residual stress variation

with respect to depth. To compare the results, the layer-removal method is utilized for residual stress measurement and the commercial software, Moldflow, is used for residual stress analysis. Figure 1 shows the finite element mesh generated for numerical analysis.

Experimental

For residual stress experiments, polystyrene specimens were injection molded as a rectangular flat plate with the dimension of 120×30×2mm 3. Details of the molding conditions are listed in Table 1.

For the layer-removal method, three rectangular specimens 90×6.5×2mm 3 were prepared from the central region of the polystyrene plate. A high speed milling machine with 20,000rpm was used to prevent the occurrence of stresses during removal of layers and a profile projector of high magnifica-tion was used to measure the curvature. During layer removal,the curvature was measured after every layer removal of 0.1mm for ten times. For hole-drilling, RS-200 milling guide and rosettes were used and strains were measured by following the ASTM procedure. To drill a hole that has the same radius repeatedly, a double ended boring mill cutter and a hand drill were used. Strains were measured for five times at every drilling of 0.2 mm from the surface to the center of the specimens using the blind hole method. To test various conditions, three different specimens were prepared, i.e., an injection molded plate, a specimen that was annealed in an oven at 90o C for 60 hours, and a specimen that was annealed in 90o C and quenched in ice water at 0o C. Three measurements were conducted for each specimen.

Results and Discussion

Three measurements were carried out for each polystyrene specimen. Experimental results obtained from the layer-removal method are shown in Figure 2. The results show the tensile-compressive-tensile stress distribution from the surface to the center, with the stress ranging from 2.3 to −1.1 MPa.There is a slight difference between the measurement results for different specimens but the stress variation shows the same trend. The results of hole-drilling method showed tensile-compressive-tensile stress distribution with stresses ranging from 2.8 to −0.8 MPa as plotted in Figure 3. In the

A a 1ν+()2E -------------------

B ,b 2E

------–==A B a b σmax σmin ,ε1ε3

+4A --------------2ε2ε1–ε3–()2

ε1ε3–()2

+4B -----------------------------------------------------------------+−=β12

--tan 1–2ε2ε1–ε3–ε1ε3–---------------------------⎝⎠⎛⎞

=Figure 1. Finite element mesh for injection molding simulation (mesh type: mid-plane).

Table 1. Injection molding conditions employed for simulation and

sample preparation

Barrel temperature 200o C Mold temperature 50o C Injection pressure 5 MPa Holding pressure 0.5 MPa Cycle time

60 sec

Measurement of Residual Stresses in Injection Molded Part Fibers and Polymers 2007, Vol.8, No.4445

case of the annealed panel, the highest residual stress is less than 0.5 MPa, and the quenched plate has the highest stress of around 6 MPa.

Moldflow was used to simulate polymer melt flow in the cavity and predict residual stresses in the final part assuming isotropic elastic material. As shown in Figure 4, the numerical simulation predicts residual stress distribution of tensile-compressive-tensile mode, with the stress level ranging from 1.6 to −9 MPa. The predicted residual stress distribution is comparable with the experimental results despite some differences in stress values. The predicted residual stresses are not exactly the same as the measured values. However, it has been shown that the measured values are the same as the predict ones at the center of the part. Since Moldflow is initially developed for numerical simulation of injection molding, the residual stresses predicted by the molding simulation program are not as accurate as expected. Structural analysis programs with viscoelastic capability should be utilized for better prediction of the residual stresses. It is also believed that the difference is originated from experimental errors, slipping of polymer molecules on the mold surface,viscoelastic stress relaxation of the solid polymer during cooling, stress relaxation after ejection from the mold,viscoelastic deformation during destructive measurements,and simulation errors. In the case of layer-removal and hole-drilling methods, forces are applied to the specimen locally and additional strain can be generated due to contact between the cutter and the specimen. For more reliable numerical analysis and measurement results, slip conditions on the mold surface must be considered in the numerical simulation and viscoelastic stress relaxation must be minimized before and during measurement. A new material removal method that does not cause any addition stress should be investigated for more reliable stress measurement.

The hole-drilling method employed in this study is developed mostly for measurement of residual stresses of metal parts with elastic behavior, but rarely utilized for polymeric materials with viscoelastic properties. However, the experimental results showed that it is possible to apply the method to injection molded parts despite some differences between experimental and numerical results. The incremental hole-drilling method is useful for measuring residual stresses in industrial products with complex geometry and may have applications for complex polymeric composites. The integral method should be improved to yield more accurate stress distribution from raw experimental data.

Conclusion

Residual stresses in an injection molded part were measured by layer-removal and incremental hole-drilling methods and the experimental results were compared with predicted numerical results. Residual stresses in an injection molded polystyrene plate was numerically predicted by Moldflow and the residual stress variation was determined with respect to depth. It was confirmed that the incremental hole-drilling method can be applied to the measurement of

residual

Figure 2.

Residual stresses obtained for the as-molded specimens by using the layer-removal method.

Figure 3. Residual stresses obtained for the specimens with various treatments by using the hole-drilling method.

Figure 4. Comparison of residual stresses obtained by Moldflow and experiments for molded specimens.

stresses in injection molded plastic parts. The measured residual stress is certainly affected by additional stresses generated during layer-removal and hole-drilling. Therefore the experimental environment needs to be improved to minimize such errors. The hole-drilling is deemed as a useful method to identify residual stresses in injection molded parts. However, for the incremental hole-drilling method to be used as an effective tool, the current integral method must be improved to calculate the residual stress variation with respect to depth more accurately.

Acknowledgements

This study was supported by the Korea Science and Engineering Foundation through the Applied Rheology Center (ARC). The authors are grateful for the support.

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