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1 INNOVATION AND TECHNOLOGY FROM PORTUGAL PORTUGUESE PROCEEDINGS AT SPE ANTEC CONFERENCES PORTUGAL s section copyright 22 SPE - PORTUGAL S SECTION THIS PAPER IS COPYRIGHTED BY THE SOCIETY OF PLASTICS ENGINEERS AND WAS ORIGINALLY PRESENTED AT SPE ANTEC, MAY 21 universidade do minho

2 FRICTION PROPERTIES OF THERMOPLASTICS IN INJECTION MOLDING Ferreira, E.C., Neves, N.M., Muschalle, R. and Pouzada, A.S. Departamento de Engenharia de Polímeros, Universidade do Minho, Guimarães, Portugal Abstract In the ejection stage of parts injection molded over cores the knowledge of the friction properties between the mould surface and the part are important to optimize the ejection system solution. The coefficient of friction depends strongly on the mould surface and the temperature at the moment of ejection. Prototype equipment was developed to measure the friction properties in as-molding conditions, and methods developed to perform the testing. Data will be presented for two thermoplastics (polycarbonate and polypropylene). Introduction Injection molding is one of the leading technologies for the production of engineering plastic parts. The molding surfaces of the injection molds are usually obtained from steel blocks by various processes like spark erosion or milling. Those techniques have limitations in terms of attainable finishing smoothness of the surface. In the ejection stage of injection molding the parts are mechanically forced to separate from the molding surfaces (especially from the cores). The efficiency of the ejection is related with a number of factors that are of concern to the designer. Namely, the draft angles of the core and its surface finish the properties of the plastic material at the ejection temperature and the dimensioning of actuation devices (such as hydraulic or pneumatic cylinders). Aesthetics and functionality of products may require the use of small draft angles. Small draft angles lead to an increase of the ejection forces. Good surface finishing is obtained by time consuming techniques like polishing, leading to more expensive molds. Economy in the mold making industry puts pressure to the use of not so smooth surface finish of cores (molding the inner part of moldings that are normally invisible). In addition, it is known that very good polished surfaces (mirror-like) can be difficult to separate by the local formation of vacuum. To minimize these problems it is common practice however to make the finishing in the ejection direction. Productivity in injection molding requires the minimization of the cooling time at the cost of leading to higher ejection temperatures and poorer mechanical properties of the polymer materials. These additional factors further contribute against an easy and safe ejection of the parts from the mold. Moreover, the thermal expansion coefficient of thermoplastics is considerably different ( ºC -1 ) than that of steel ( ºC -1 ). Thus, after cooling from melt temperature, the plastic tends to stick over the surface of the cores, reproducing closely its surface finish. This unusual circumstance could lead to significant variation of the coefficient of friction, since in common standard test methods this condition is never considered. That was the motivation to develop a prototype equipment to study the effective coefficient of friction under those conditions. In this work the prototype equipment is presented and some preliminary results obtained using PP and PC introduced. Solid friction Friction is normally understood as the resistance offered by bodies in contact to relative motion. In injection molding the bodies in contact are steel molding surfaces and polymer moldings. Coulomb (ca. 1781) experimentally confirmed the fundamental theory that governs the friction behavior of materials. Leonardo da Vinci and Amontons preceded his work, the latter being the first to propose the use the coefficient of friction and to establish a distinction between static and kinetic coefficients of friction. Those laws can be stated in very simple terms as: Static friction may be larger than kinetic (dynamic) friction Friction force is proportional to normal force Friction force is independent of the contact area

3 The conclusions of Coulomb s work are used in many practical situations with good results. However, there are some situations in which laws of friction are not in agreement with experimental observations [1]. Friction is caused by forces acting at the interface between the surfaces of contacting bodies. The magnitude of those forces is related to the properties of the two surfaces in contact and the properties of the two materials. These forces are usually difficult to predict because the surface properties continuously change over time by deformation, wear, segregation of components or oxidation. Moreover, the real contact area between the bodies is also different than the apparent area of bodies owing to the roughness of the contact surfaces. The friction properties of pairs of materials are usually represented by the coefficient of friction, µ. The coefficient of friction is defined (e.g. in ASTM G4 [2] test standard) as: In which, F = N µ (1) F friction force N normal contact force The same standard defines a coefficient of static friction, µ s, corresponding to the maximum force that must be overcome to initiate macroscopic motion between the two bodies. A coefficient of kinetic friction is obtained from the average friction force necessary to maintain the macroscopic relative motion between the two bodies. It is represented by µ k. In our case, because of the draft angles used in injection molds, this parameter is relatively less interesting. The coefficients of static friction range from.3 in specially lubricated bearings to in the case of dry sliding [3]. Between very smooth and clean metal surfaces in vacuum, values of the coefficient of static friction exceeding 5 were obtained. Typical values for the coefficient of friction of various polymers in contact with steel are listed in Table 1. Friction in the ejection stage of injection molding Menges et al. [4] first attempted the determination of the coefficient of friction relevant for the prediction of ejection forces in the early 8 s. In their work a mould was developed enabling the study of the effect of different molding conditions on the coefficient of friction. The surface roughness and the presence of a release agent were observed to be the most important parameters. The melt temperature and the mould temperature were identified as second order parameters concerning its influence over the coefficient of static friction. Some results of the maximum coefficient of static friction obtained for PE, PP, PS, ABS and PC were reported. However, the scatter in their results was pointed out as a problem limiting the broadness of the conclusions obtained. Vaziri and co-workers studied the dynamic friction between polymers and steel [5]. This property is less interesting for the ejection stage in injection molding owing to the draft angles used in this case. In the late 8 s Malloy et al. [6] studied the design of ejector pins to be used in ejection moulds. In this work the same difficulty of prediction of the coefficient of friction was identified as one of the key problems to design the ejection system in injection moulds. Later in the early 9 s Burke et al. [7] used the thermal expansion coefficient, the stiffness at the temperature of ejection and coefficients of friction obtained from [4] to predict ejection forces. The error in the predictions was of the order of 16 % for ABS and HDPE. Later, Malloy et al. [8] reported on a standard test procedure but with a temperature controlled chamber. Steel, nickel-plated steel and PTFE/nickel plated steel specimens were studied in contact with PS, PP, PC/polyester alloy and 1 % glass fiber reinforced PC specimens. The effect of the test temperature on the coefficient of friction was also analyzed. Plating and the use of release agents were reported as the most effective ways of decreasing the coefficient of friction polymer/steel. Recently, Dearnley [9] reported a study on the friction force developing between a ring of polymer molded over a ring of steel with different surface roughness. Good correlation was observed between the roughness of the steel and of the polymer ring. Coatings of TiN, CrN and MoS 2 were studied in terms of the friction force against acetal (POM). CrN coatings in P2 steel lead to the lowest observed friction forces in spite of the slightly higher surface roughness. This result was attributed to the chemical behavior of the coating at the interface. Prototype equipment It was designed and developed a new equipment enabling to study the effect of different parameters on the coefficient of friction relevant for the ejection of plastic parts from moulds. The concept selected is illustrated in Figure 1. The specifications for the equipment were: Range of operating temperatures (2-18ºC) Range of testing speeds (1-1 mm/min)

4 Replication of the surface roughness of the molding surface into the plastic specimen Control of the normal contact force between molding surface and specimen Control of the evolution of the friction force with time To meet the specifications various functional systems were used: temperature control, control of contact pressure for replication of surface and for testing and guiding system. A brief description of the functional systems will follow. Temperature control system The control of the temperature is important to enable good replication of surface at temperatures close to melt temperature of semi-crystalline materials or above the glass transition temperature in the case of amorphous materials under study. It is also important to ensure stability of temperature during the friction test. Cartridge heaters mounted into the structure allow to raising the temperature from room temperature up to the replication temperature within a reasonable time (typically 5 minutes). Additionally, 5 mm insulating plates were used to minimize heat losses. The control is made with a Fe/Constantan thermocouple connected to a temperature controller. The depth between the tip of the thermocouple and the steelmolding surface is 2 mm. Cooling down from replication temperature to the testing temperature is obtained by circulating water in the cooling circuit. System of control of contact pressure A pneumatic cylinder is used to actuate the contact pressure. The control of the pressure obtained with a piezo-resistive pressure sensor. System of control of friction force The use of a tensile test machine was considered the easiest and more reliable way to control and acquire the data of the friction force during the test. Thus, the prototype apparatus (Figure 2) was designed to be mounted and work with a tensile test machine. Experimental Two thermoplastic materials were used to produce.2 m thick injection-molded square testing specimens ( m 2 ): a polypropylene HIFAX BA238G3 from MONTELL and a polycarbonate LEXAN 141 R from GE Plastics. The testing specimens, half of the moldings, were cut from those moldings. The normal contact force applied by the pneumatic cylinder was set to ca. 44 N. The testing speed during friction test was kept at 1 mm/min. The material of the molding surface in contact with the polymer specimen in all cases was tool steel (DIN Ck 45). The testing routine included the following stages: 1. Heating of the molding surface up to the replication temperature 2. Stabilization of the temperature 3. Application (during 6 s) of the contact pressure (1.4 MPa) to obtain surface replication 4. Cooling down to testing temperature 5. Stabilization of temperature (6 s) 6. Friction test (at 1 mm/min cross-head speed) For each condition seven specimens were tested. The cycle time for the complete routine is of about 17 minutes for PC specimens and 15 minutes for PP. The testing parameters were adjusted to the specific properties of the materials to be tested and are listed in Table 2. Results and discussion The results presented (Table 3) show the expected pattern for the evolution of the force during the friction tests. The force rises up to a maximum, corresponding to the force used in the calculation of the static coefficient of friction. Over that maximum, the force tends to level off at lower value, enough to keep the relative motion between the two surfaces. That lower force is not constant and it may oscillate in a periodic way. The equipment developed seems to produce reproducible results as shown in Figure 3. The polycarbonate specimens show a coefficient of static friction at 8ºC, with polishing transversal to the testing direction of.47 (Figure 4). This is a relatively high value when compared with values reported in literature [3]. However, one must keep in mind that the testing conditions are not comparable. In similar testing conditions but at ºC, polypropylene specimens show (Figure 5) a much lower

5 coefficient of static friction (.19). This result is considerably lower than previously published coefficients of friction for this material [3] using standard tests. The effect of testing temperature over the coefficient of static friction is shown in Figure 6. Higher testing temperature seems to increase the value of the coefficient of static friction of polycarbonate. Conclusions New prototype equipment was developed to study the coefficients of static friction relevant for the ejection of injection-molded parts. Preliminary tests indicate that the equipment is able to replicate the surfaces of molding surfaces into plastic specimens. Tests using PC and PP seem to lead to very different behaviors. At 25ºC the coefficient of static friction of PC is consistent with previously published data. At 8ºC the coefficient of static friction is larger than at room temperature. The coefficient of static friction obtained at ºC for PP is lower than the values reported in literature. Acknowledgments The Fundação para a Ciência e a Tecnologia (FCT) for their support to the Project Mouldfriction, the Institute of Materials (U. Minho) and the EU program Socrates/Erasmus for the grant to Mr. R. Muschalle. References 1. Pontes, A. J., Pinho, A. M., Miranda, A. S. and Pouzada, A. S., O Molde, 1:34, 25-34, G 4 test standard, Annual Book of ASTM Standards, ASTM 3. ASM Handbook, 18, Friction, Lubrication and Wear Technology, ASM, Menges, G. and Bangert, H., Kunststoffe German Plastics, 71:9, , Vaziri, M., Stott, F. H. and Spurr, R. T., Wear, 122, , Malloy, R. and Majeski, P., Proceedings of Antec'89, New York, , Burke, C. and Malloy, R., Proceedings of Antec'91, Montreal, p , Balsamo, R., Hayward, D. and Malloy, R., Proceedings of Antec'93, , Dearnley, P. A., Wear, 225:229, , ASM Handbook, Appendix: Static and Kinetic Friction Coefficients for Selected Materials, ASM, 73, 1992 Table 1 Coefficients of friction between various polymers and steel [1] Polymer µ s µ k PP PC * ABS.4.27 PA * Mild steel/polycarbonate Table 2 Testing parameters Parameter PP PC Replication temp. (ºC) 1 17 Test temp. (ºC) 25 and 8 App. contact area (mm 2 ) 3 3 Roughness, Ra (µm).5.5 Table 3 Summary of coefficients of friction Material Surface replication Friction force Temperature (ºC) 25 8 Literature [1] PP PC Controlled temperature Contact pressure Figure 1 Illustration of the concept for the development of the prototype equipment

6 Figure 2 Mounting of the testing apparatus in a tensile testing machine Figure 5 Polypropylene, friction test result with surface replication, roughness Ra=.5 µm machined in the across testing direction, ºC test temperature (µ s =.19) ºC (.47) 25ºC (.32) Figure 3 Reproducibility of testing results (PC) Figure 6 The effect of testing temperature over the coefficient of friction of PC Figure 4 Polycarbonate friction test result with surface replication, roughness Ra=.5 µm machined in the across testing direction, 8ºC test temperature (µ s =.47)