The injection molding is the technique most used for processing polymers. Nowadays, due to
economic demands the ejection of the parts occur as soon as possible to lead to a short cycle times. The ejection
stage is the most critical phase during the injection molding processing. During this stage, especially in
deep core molding, it is possible the development high friction forces making difficult the ejection of the
moldings without marks or other defects. The aim of this work is to assess the coefficient of friction between
the part and the mould walls in conditions similar that occurs during the ejection in the injection molding process.
The assessment of the coefficient of friction was carried out with the help of one apparatus that possibility
this type of measurement. This equipment provides that measure under controlled conditions as test temperature,
replication temperature, material of the mould, and superficial roughness. The obtained results with
several polymeric materials (semicrystalline and amorphous) show that the tribological conditions have great
influence on the coefficient of friction in static conditions

Pontes, A. J.

Rodrigues, Angela Raquel Morgado

Universidade do Minho: RepositoriUM

 243  1 INTRODUCTION The technique most used to process parts with pol-ymers is the injection molding. In these days, due to economic demands the ejection phase occurs as soon as possible to lead to a short cycle times. This stage is the most critical during the injection molding (Pouzada et al. 2006; Correia et al. 2011; Ferreira et al. 2002) especially in molds with deep cores (Pon-tes et al. 2005; Pouzada et al. 2006). In order to understand what happens during the ejection of the parts in the injection molding is nec-essary to understand the tribological behavior of the materials. Tribology study the contact between two solid surfaces in movement. That movement produc-es effects such as friction, wear rate, adhesion, among others (Sinha & Briscoe 2009; Blau 2009). Friction is known as the opposition to the motion be-tween two bodies in contact. For the determination of the coefficient of friction is necessary measure the normal force. This force is proportional to the fric-tion force (Sinha & Briscoe 2009; Blau 2009). The coefficient of friction (µ) allow the characterization the movement resistance between two contact sur-faces. This variable neither is a material property nor a physical constant (Blau 2009). It depends on some variables, namely sliding velocity, temperature, con-tact pressure, properties of the materials in contact, superficial finishing, among others (Blau 2009). An-other important variable that have influence in the coefficient of friction is the draft angle in the case of deep parts (Pouzada et al. 2006; Ferreira et al. 2002; Correia et al. 2011). Important variables that have influence in the co-efficient of friction and are covered in this paper are the type of mold, the part material, the roughness and the coatings of the molding surfaces. Correia (Correia et al. 2011) carried out a study to understand the contributions and the mechanisms involved in the friction during the ejection in the in-jection molding process. It was studied the relevance of temperature, contact pressure and roughness in the coefficient of friction. During the work was con-cluded that friction force increases with the surface roughness and the contact pressure and reduces with the increasing temperature. Pontes (Pontes et al. 2004) have studied the effect of the superficial roughness and the temperature on the static coefficient of friction of the pair ABS/steel. It was concluded that two variables are important in the control of the friction between two surfaces. There is an optimal surface roughness that corresponds to the minimum of the static coefficient of friction, when the ejection force is lower. It was Evaluation of coefficient of friction in similar conditions to ejection of molding parts A. R. Rodrigues, A. J. Pontes Institute for Polymers and Composites/I3N,University of Minho, Guimarães, Portugal ABSTRACT: The injection molding is the technique most used for processing polymers. Nowadays, due to economic demands the ejection of the parts occur as soon as possible to lead to a short cycle times. The ejec-tion stage is the most critical phase during the injection molding processing. During this stage, especially in deep core molding, it is possible the development high friction forces making difficult the ejection of the moldings without marks or other defects. The aim of this work is to assess the coefficient of friction between the part and the mould walls in conditions similar that occurs during the ejection in the injection molding pro-cess. The assessment of the coefficient of friction was carried out with the help of one apparatus that possibil-ity this type of measurement. This equipment provides that measure under controlled conditions as test tem-perature, replication temperature, material of the mould, and superficial roughness. The obtained results with several polymeric materials (semicrystalline and amorphous) show that the tribological conditions have great influence on the coefficient of friction in static conditions. 244  also concluded that the optimal surface roughness varies with the testing temperature. Boey (Boey et al. 2005) studied the coating of the molds surfaces with DLC coatings and the use of techniques that implement ions in the nitrided steel. He concluded that the last one has much better prop-erties then the simple nitrided steel. However he also concluded that DLC coatings showed the best per-formance and have a great potential in the industry. 2 EXPERIMENTAL 2.1 Materials Five materials were studied in this work. The poly-meric materials were PP ( Domolen 1100 L by Do-mo Chemicals), reinforced PP with 30 % of glass fi-bers (Domolen P1-01-V10-N by Domo Chemicals), PC (Lexan 123 R By Sabic), PS (Polystyrene 145 D by BASF), PMMA (Plexiglas 8N by Evonik Röhm Gmbh).  The tested parts were produced in the Ferromatik-Milacron K85 injection molding machine with 85 ton of clamping force. 2.2 Equipment The study consisted in measuring the coefficient of friction to evaluate the influence of various types of steel and coatings in the mold surface in the injec-tion molding process. For this propose it was used a prototype equipment (Figure 1) described by Pouzada et al. (2006). This prototype was developed at Minho University to measure the coefficient of friction in ejection conditions and works assemble in a universal testing machine, Instron 4505, which records the data from the force to determine the co-efficient of friction.  Figure 1. Testing apparatus for coefficient of friction measure. The friction tests were performed at room tem-perature. The test time is about 15-20 min and in-cludes the heating of the mould surface up to the replication test, stabilization of the temperature, ap-plication of contact pressure, cooling down to the testing temperature and the realization of the friction test. 2.3 Stamps and coatings The dimension of the stamps (Figure 2) used for this propose are 55 × 19 × 8 mm, with two types of dif-ferent 1.2083 steel and a stamp processed with 1.2711 steel.  Figure 2. Technical design of the stamp. To manufacture the stamps they were used EDM machining and EBM finishing in the test surface. The machined stamps were used to measure the co-efficient of friction, then were coated with near fric-tionless coatings, PTFE and WS2+C. The coating of the stamps was realized to understand the influence of the coating in the coefficient of friction. 3 RESULTS AND DISCUSSION To promote a better way to organize the obtained da-ta the stamps have the following nomenclature (Ta-ble I). Table I. Stamps nomenclature Nomenclature Stamp A 1.2083 coated with PTFE B 1.2083 after re-processing C 1.2083 Supra D 1.2083 coated with WS2+C E 1.2083 coated with PTEF gradi-ent F 1.2711 3.1 Reproducibility of test For all stamps and polymeric materials they were performed five tests under the same test conditions. In Figure 3 it is presented a typical graph obtained by the universal testing machine. This graph repre-245  sents the measured force, frictional force, afterwards used to calculate the coefficient of friction between the stamps and the polymeric material.  Figure 3. Graph from the coefficient of friction tests. The maximum value obtained in the graphs (simi-lar to Figure 3) is used for the determination of the coefficient of friction, since this is the most critical moment of the ejection stage in the injection mold-ing process. This force represents the friction force. 3.2 Type of steel In the Figure 4 it is shown the influence of the steel on the coefficient of friction.  Figure 4. The effect of Steel of the stamp on the coefficient of friction. In the Figure 4 it is possible to verify that the tested materials have different coefficients of fric-tion. It was possible to observe that PP, reinforced PP had a behaviour similar to each other and that PS led to the higher values. The coefficient of friction of PMMA is almost constant with the stamp B and the stamp C. Among the stamps C and F it was veri-fied an increase of about 0.10 on the coefficient of friction. In the case of PC the coefficient of friction decreased when changing from stamp B to stamp C and increased in the case of the stamps C and F. 3.3 Coating effect The results of the tests carried out with the stamps manufactured with different types of steel are pre-sented in Figure 5 and Figure 6. In the case of the 1.2083 steel coated with PTFE (A) and PTFE gradient (E) the results are shown in the Figure 5. In Figure 5 it is possible to observe that the stamp manufactured with the 1.2083 supra steel without coating is the one that promotes smaller co-efficients of friction for PP and reinforced PP.   Figure 5. Influence of PTFE coating in the 1.2083 steel. For the 1.2083 steel (Figure 6) it was observed that for all materials the stamp without the coating promotes less coefficient of friction than the stamp coated with WS2+C.  Figure 6. Influence of the WS2+C coating in the 1.2083 supra steel. 3.4 Polymer material effect The type of polymer materials also has influence on the coefficient of friction. Figure 7 shows the effect of the type material, PP (semicrystalline polymer) 246  and PS (amorphous polymer), with the two stamps that led to a smaller coefficients of friction.  Figure 7. Effect of the type of polymer material.  It is possible to observe that the two polymer ma-terials behave differently. For the two stamps PP has a coefficient of friction lower than the PS. 3.5 Stamp coatings vs different polymer materials Different polymer materials had different behaviors during the molding process with different types of coatings in the mold. As it was discussed before it is possible to observe in Figure 8 that the coating of the stamp led to a larger coefficient of friction. In any case the coefficient of friction on demoulding PP is lower than in the case of PS.  Figure 8. Type of polymer and stamp with and without coating of WS2+C. The effect of the different types of coatings for PP and PC is shown in Figure 9. The WS2+C coating promotes higher coefficient of friction than the stamps coated with the gradient of PTFE.  Figure 9. Effect of different type of coatings.  CONCLUSIONS Analyzing the tests performed to assess the coeffi-cient of friction it is possible to observe that the dif-ferent types of steel and polymer materials had dif-ferent types of responses. The steel 1.2083 leads to lower coefficient of fric-tion than the 1.2083 supra steel in the case of PP and PS. Stamps with test surfaces without coating pro-mote less coefficient of friction when compared with the same steel covered with the studied coatings. Semicrystalline polymers, such as PP, in contact with the steel during the ejection stage had less coef-ficient of friction than amorphous polymers, as PS. With respect to the coatings, the coefficient of friction is lower with gradient of PTFE coatings than with WS2+C coatings. ACKNOWLEDGEMENTS The authors gratefully acknowledge the financial support given by PEst-C/CTM/LA0025/2013 (Stra-tegic Project - LA 25 - 2013-2014). REFERENCES Blau, Peter J. 2009. Friction Science and Technology: From Concepts to Applications. London: Taylor & Francis Group, LLC. ISBN: 978-1-4200-5404-0. Boey, P., Ho, W. and Bull, S. J. 2005. The Effect of Tempera-ture on the Abrasive Wear of Coatings and Hubrid Surface Treatments for Injection-Moulding Machines. Wear 258 (1-4): 149-156. Correia, M. S., Miranda, A. S., Oliveira, M. C., Capela, C. A., Pouzada, A. S. 2011. Analysis Of Friction in the Ejection of Thermoplastic Mouldings. The International Journal of Ad-vanced Manufacturing Technology 59 (9-12):977-986. Ferrira, E. C., Neves, N. M,. Muschalle, R and Pouzada, A. S. 2002. Friction Properties of Thermoplastics in Injection Molding. Innovation and Technology from Portugal. 247  Pontes, A. J., Ferreira, E. C. and Pouzada, A. S. 2004. Tribological Aspects during Ejection in Injection Moulds. RDP: 3-5. Pontes, A. J., Pouzada, A. S., Pantini, R., and Titomanlio, G. 2005. Ejection Force of Tubular Injection Moldings. Part II: A Prediction Model. Polymer Engineering & Science 45(3): 325-332. Pouzada, A. S., Ferreira, E. C., and Pontes, A. J. 2006. Friction Properties of Moulding Thermoplastics. Polymer Testing 25(8): 1017-1023. Sinha, S. and Briscoe, B. J. 2009. Polymer Tribology. UK. Im-perial College Press. ISBN: 978-1-84816-202-0. 

Evaluation of coefficient of friction in similar conditions to ejection of molding parts

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Universidade do Minho: RepositoriUM