WARPAGE OF PMMA INJECTION MOULDED PARTS

Transcription

1 ADVANCES IN PLASTICS TECHNOLOGY WARPAGE OF PMMA INJECTION MOULDED PARTS Odkształcenia wyprasek wtryskowych z PMMA K. Mazik, T. Jaruga, M. Modławski Czestochowa University of Technology, Faculty of Mechanical Engineering and Computer Science, Poland Paper November 2011, Katowice, Poland

2 Warpage of PMMA injection moulded parts K. Mazik, T. Jaruga, M. Modławski Częstochowa University of Technology, Faculty of Mechanical Engineering and Computer Science, Poland Summary PMMA poly(methyl methacrylate) is a transparent polymer used often to manufacture products for optical applications by injection moulding. In case of such products like lens, keeping the tolerances of size and shape is very important. Since processing parameters have significant influence on material shrinkage and, as the result, on the size and shape of the product, it is possible to control the dimensions by adjusting processing parameters. The results of warpage investigation of PMMA samples are presented in this paper. The warpage was caused by injection mould temperature difference at the opposite mould walls. The samples were rectangular bars coming from mould cavities of the following dimensions: 150 (length) 23 (width). The sample thickness (cavity depth) was changed (2.6, 3.2, 4.0 mm). It was found that higher mould temperature difference (ΔT) results in higher sample deflection. The deflection of PMMA samples did not significantly depend on the sample thickness and deflection dependence on ΔT was linear. Residual stress in the samples was investigated using photoelasticity method. It was shown that more uniform stress distribution occurs in parts of higher thickness. 1. Introduction Injection mould temperature is an important factor to obtain desired part s properties. In case of semicrystalline polymers it should be kept at higher values so that polymer could be slow cooled and crystalline phase could develop in the material. On the other hand, mould temperature can cause many problems if it is not well controlled or not equal on the entire cavity surface. This, in turn, can lead to problems with non-uniform shrinkage in different part s areas and, as the result, to part s deformation warpage. This can occur in such areas like ribs, undercuts, sharp edges etc. Moreover, it is very important to assure the same temperature values on the opposite mould walls of the cavity, because, otherwise, part will warp in the way shown in Fig. 1. The concave side of the injection moulded part is created by the cavity wall of higher temperature. Figure 1 General rule of parts warpage due to difference in mould temperature [1] The reason of parts deformation due to mould temperature difference at opposite walls is the shift of the fluid core, what is presented in Fig. 2. Since higher temperature causes slower cooling and polymer solidification, the material inside the cavity stays liquid near hightemperature mould wall for a longer time. In this region more intensive shrinkage occurs what results in bending moment causing part s deformation [2]. Even in the earlier stage of the injection moulding process the injection (cavity filling) stage, asymmetry in runner or cavity 285

3 cross-section can occur in this case it is the asymmetry of polymer flow front that is more advanced near the cavity wall of higher temperature [3,4]. Figure 2 Fluid center as the reason of more intensive shrinkage at mould cavity wall of higher temperature [2] Since there is no equal cooling in the entire part s cross-section and bending moment occurs in the part, a stress in the parts also occurs that is not released totally after part s ejection from the cavity. Therefore usually residual stress is observed in injection moulded parts after processing. Residual stress can be released further by heating up the part but often causes the change of parts shape and dimensions during exploitation. The problem of residual stress occurrence is especially important for parts made from transparent plastics because it can disturb their optical functions, for example in case of such parts like lens. Thus it is especially important to control injection mould temperature when processing transparent polymers. Non uniform cooling can be caused by improper cooling circuit design, especially when there is no efficient cooling channels concentration in the areas being intensively heated by molten polymer. Another possible reason is the difference in coolant temperature in two cooling circuits located from both sides of the part. Injection moulded parts can also warp after the manufacturing process due to water absorption if one side of the part is more exposed to the humidity or if one surface was covered with non-permeable substance layer [5]. The behaviour of plastics during flow through runners, mould gate and the way it flows through the mould cavity has an essential effect on parts properties [6-12]. However, except of mechanical properties, a high impact on functional properties of the parts exerts residual stress that occurs in parts after injection moulding. Typically residual stress depends on holding pressure, mould and injection temperature. Residual stress in parts is typically caused by differentiation of cooling speed in different areas of the part s cross-section. Quickly cooled and solidified surface layers form stiff coating what limits the shrinkage of still liquid (plasticized) core. This results in appearance of tensile stress in internal layers and compression stress in external layers [13-15]. Another reason of the residual stress is rapid rise in plastic pressure in the mould cavity during holding phase and excessive clamping pressure. It results in compression of macromolecules, what causes high tensile stresses in the solidifying outer layer of the part. There are a number of factors which might impact negatively or positively the formation of stress state in injection moulded parts. During processing the lowest possible values of residual stress should be achieved since they usually cause many adverse and sometimes even destructive phenomena in the part during the further use. These phenomena are, for example: 286

4 self-deformation, deformation as a result of temperature changes (especially concerning temperature rise, since relaxation of the residual stresses occurs), stress corrosion (occurs also as a result of environmental factors, such as UV radiation, chemicals, water vapour, salts, alkalies etc.), which can lead to the formation of microcracks and then to self-induced cracking of the part. Sometimes the value of residual stress caused by high difference in cooling rate of different part s areas is so high that it exceeds the value of cohesion forces appearing in the material, what causes the destruction of the part just after being ejected from the injection mould [15-17]. 2. Experimental The studies were carried out in order to find out how the parts made from a transparent polymer PMMA, deform when mould wall temperature is not equal at opposite sides of the part. The influence of mould temperature difference and part s thickness on deformation was evaluated. 2.1 Injection mould A double cavity injection mould was used for this investigation. The scheme of the forming insert of the mould is shown in Fig. 3. Figure 3 View of the moulding insert of the injection mould The parts of different thickness were formed by changing the inserts in the bottom of the cavity. Three part thickness values h were used for this investigation. For all the thickness one 287

5 height of the film gate was used: 2.7 mm. The injection moulded parts cavities were of the following dimensions: length: 150 mm, width: 23 mm, depth: 4.0; 3.2 and 2.6 mm. The cavity depth corresponds with parts thickness h. The cooling circuit and location of temperature sensors are presented in Fig. 4. Mould temperature was measured in the left (movable) mould part (this temperature value was marked T 1 ) and in the right (stationary) mould part (marked T 2 ). Figure 4 Cooling circuit and temperature sensors in the mould 2.2 Material The polymer tested was poly(methyl metacrylate) Optix CA-1000 I Clear produced by PLASKOLITE West. Its properties are as follows: Density: 1160 kg/m 3, Melt Flow Rate MFR (3.8 kg / 230 C): 2.8 g/10 min., Yield strength: 42.1 MPa, Young modulus E t : 2034 MPa, Izod notched impact strength: 59 J/m, Vicat softening temperature (ASTM D1525): 108 C, Heat deflection temperature (1.8 MPa): 78 C. 288

6 2.3 Processing parameters Mould temperature was changed in the range recommended for PMMA: from 40 C to 80 C, in the steps of 10 C. While the temperature of one mould part (T 1 ) was increased, the temperature of the second part (T 2 ) was decreased. The parts were marked by the sequence of numbers, for example 80/40 means that T 1 =80 C and T 2 =40 C. In case of equal mould temperature the designation was 60. Mould temperature difference was defined as: ΔT = T 1 T 2 (1) The match of mould temperature values, as they were used in investigation, is presented in Table 1. Table 1 Matched mould temperature values for 5 cases Movable mould part temperature T1 C Stationary mould part temperature T2 C The other processing parameters were as follows: Melt volume rate (screw diameter: 30 mm): 60 cm 3 /s, Melt temperature: 240 C, Injection time: 0.3 s, Holding pressure: 70 MPa, Holding time: 20 s, Cooling time: 10 s. 2.4 Tests The injected samples were tested in two ways: the deflection was measured after 2-dimensional scanning of part s side surface and residual stress was evaluated by photoelasticity investigation. The scanner used for obtaining the image of samples was Hewlett Packard, model Dekjet Ink Advant K209a-z, with maximum scanning resolution of 4800 ppi. The software used for scanning was Centrum obsługi HP, version To measure the deflection, Microsoft Visio software was used. Deflection was defined as the y coordinate of the highest point of the edge between the side- and upper (convex) surface. The coordinate system was created for each part on the basis of two points located in the lowest y coordinate (in vertical direction) Linear polariscope (strain viewer) manufactured by Strainoptics, Inc., model SV-1000 was used for the photo-elasticity investigations. The samples were placed between analyzer and polarizer. The image, in the form of colourful fringes and areas, could be observed on the analyzer. Each colour of isochromatic lines defines a particular value of the difference in the main stress (σ 1 - σ 2 ), given in MPa [6, 16]. 3. Results and discussion After scanning the parts and photo-elasticity investigation, the parts profiles and deflection were compared and the photos from strain viewer were analysed. 289

7 3.1 Deflection measurements The profiles representing the edge between upper and side surfaces are matched for different values of T and presented in Fig. 5. The coordinates of points y are of average values obtained for three scanned samples from the same cavity. Figure 5 The profiles of the parts deformed by mould temperature difference. Parts are of different thickness h: a) h=4.0 mm, b) h=3.2 mm, c) h=2.6 mm 290

8 The parts warped in different direction when the mould temperature was higher at the right mould side and at the left mould side. The rule was that the concave surface was on the side of higher mould temperature. The profile lines in plots in Fig. 5 and 6 are all drawn for positive y values, although, for example, part 40/80 was deformed in the opposite direction as a part 80/40. This approach allowed better comparison of the parts profiles for different ΔT. Gate location always corresponds to the coordinate x=0. As it can be observed in Fig. 5 part s deflection is highly dependent on the injection mould temperature difference ΔT. Moreover, when the parts manufactured with the same absolute values of ΔT are compared, higher deflection is registered for parts manufactured with higher temperature of movable mould part (T 1 ). This can be influenced by some other factors like ejection forces or different cooling at cavity side than at the core (flat plate) side. While increasing part s thickness the deflection did not change in accordance with a specified tendency. When comparing the difference in deflection for the part 80/40 and 60 it will be approximately: 0.64 mm (h=4.0 mm), 0.70 mm (h=3.2 mm) and 0.61 mm (h=2.6 mm). However, this is not a rule that the highest deflection occurs for parts of 3.2 mm thickness. The parts profiles are not exactly symmetrical to the middle-length point. The maximum y points are shifted towards the part s end they are located further from the gate. The comparison of three different profiles obtained from the samples injected with the same conditions is shown in Fig. 6. It was important to obtain small differences in the profiles of these samples because otherwise it would mean that there is no real influence of the mould temperature difference on the deflection. The average difference in deflection for the parts obtained under the same conditions (thickness, ΔT) was mm. Figure 6 Parts profile measured for 3 different samples (numbered: 1, 2, 3). Parts thickness: h=4.0 mm, mould temperature: T 1 =80 C, T 2 = 40 C In order to differentiate the direction of parts warpage, for cases when T 2 was higher than T 1, the deflection was taken as a negative value. By making this assumption it was possible to present the f (ΔT) function plot not in the shape similar to parabola but as a straight line. The plot of this function for different parts thickness h is shown in Fig

9 Figure 7 Parts deflection f in function of ΔT, for different parts thickness h The results of measurements of deflection showed that it does not significantly depend on parts thickness see Fig. 7. To confirm this statement, an analysis with the use of Statistica software was performed. The deflection data were collected and the function f (ΔT, h) was created in the form of a polynomial: f ( T, h) = A T 2 + B T + C h 2 + D h + E T h + F (2) where A, B, C, D, E, F were coefficients. Afterwards the analysis was performed to find out the suitable coefficients in equation (2). Then this equation had the following form: f ( T, h) = -0,03420 T 2 + 0,74291 T - 0,06772 h 2 + 0,05450 h - 0,00017 T h + 0,13605 (3) and correlation coefficient was in this case R 2 = The shape of surface function f ( T, h) for the full equation is presented in Fig. 8. Pareto analysis was also performed using Statistical software. It was found that the result of equation (3) (deflection f) is mostly influenced by the term B T. Then the terms of equation (3) that had little influence on the result were omitted and after that the correlation coefficient was calculated. Then the other function s forms were analysed by omitting the non-influencing terms of equation (3). After all, the form of equation containing only two terms was proposed: f ( T, h) = B T + F (4) The analysis was performed to find out the coefficients of the equation (4). Finally, the function f ( T, h) was proposed in the following form: f ( T, h) = 0,74292 T + 0,07775 (5) 292

10 Figure 8 The deflection of parts f in the function of mould temperature difference T and part thickness h, described by function: f ( T, h) = -0,03420 T 2 + 0,74291 T - 0,06772 h 2 + 0,05450 h - 0,00017 T h + 0,13605 Figure 9 The deflection of the parts f in the function of mould temperature difference T and part thickness h, described by function: f ( T, h) = 0,74292 T + 0,

11 In this case the correlation coefficient was R 2 = The surface representing the function described by equation (5) is shown in Fig Residual stress investigation The photos recorded during photo-elasticity investigation are presented in Table 2. The distribution of isochromatic lines shows the difference in main (normal) stress in the parting plane. This image does not correspond directly with the stress that caused parts deflection, because the parts were deformed in the perpendicular plane and it would be difficult to investigate the stress in this plane by this method. However, the obtained results can visualize the difference in stress along the flow direction and it can be helpful to understand for example why the deflection was not symmetrical. Table 2 Part thickness h [mm] 4.0 Photo-elasticity pictures of PMMA parts Mould temperature T 1 /T 2 [ C] 40/80 50/70 60/60 70/50 80/ /80 50/70 60/60 70/50 80/40 294

12 2.6 40/80 50/70 60/60 70/50 80/40 The parts have proper stress distribution in the cavity. The differences in main stress for all parts can be observed only in the area of the gate. This is because the injection moulding process was proceeded with sufficiently long holding phase that was controlled by pressure sensor inside the cavity. Holding phase was finished after polymer has solidificated in the gates and after that no flow occurred inside the cavity what could result in stress creation. It means also that the pressure was equal in the entire cavity. The best (more advantageous) stress distribution was obtained for parts of the higher thickness (h=4.0 mm), where the difference in main stress is almost the same in the entire cavity. More significant differences in main stress can be noticed for thinner parts, especially of thickness h=2.6 mm longer areas of yellow colour that begin from the gate were observed. In the case of these parts there was more intensive shear rate and this led to higher drop of pressure on the flow length. High concentration of stress in the gate (left side of the observed parts in Table 2) for all thickness values is caused by high shear rate in gates during polymer flow and by the forces that occur near gate when parts shrink in the mould. The differences in stress between gate area and part s end caused asymmetry in parts profile. It is important that the test results proved no influence of mould temperature difference on stress distribution. 4. Summary and conclusions It was found that the deflection of PMMA parts is highly dependent on injection mould temperature difference between two opposite walls that are parallel to the parting surface. Higher mould temperature difference affects higher deflection. The deflection does not depend on the thickness in the case of investigated polymer. Therefore the injection moulded parts from this material should be manufactured with equal mould temperature of both mould parts. Deflection of the parts was described with the simple linear equation that represents the function of mould temperature difference and parts thickness is to neglect. However, these results cannot be extrapolated to other materials and further research is needed for the part of more complicated shape because some problems can occur in areas where part s thickness changes. Residual stress investigated in the parting surface has distribution that is not dependent on injection mould temperature difference. This result was obtained by sufficiently long holding phase time so it is important to keep this time until polymer solidificates in the gate. Differences in stress distribution were noticed when comparing parts of different thickness. More uniform stress distribution was observed for parts of higher thickness. This should be 295

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