实践与实验室测试塑料注射成型.pdf

.Surface and Coatings Technology 142?144 2001 143?145 Practice vs. laboratory tests for plastic injection moulding M. Van Stappen?, K. Vandierendonck, C. Mol, E. Beeckman, E. De Clercq WTCM?CRIF, Scientifi c and Technical Centre for the Metalworking Industry, Uni?ersitaire Campus, 3590 Diepenbeek, Belgium Abstract Different types of anti-sticking coatings have been applied industrially on injection moulds for various types of plastics. Very often these tests are being done on a trial-and-error basis and results obtained are diffi cult to interpret. WTCM?CRIF has developed laboratory equipment where the injection moulding process can be simulated and demoulding forces and friction coeffi cients can be measured. These measurements were compared with surface energy calculations of the coated surfaces and of the plastic materials in order to fi nd a correlation. Using this approach it must be possible to make an easy and cheap selection of promising coatings towards plastic injection moulding. Another important advantage is that the understanding and modelling of the mould?plastic interface becomes possible. This new way of coating selection for plastic injection moulding has been demonstrated for various PVD coatings and verifi ed for different industrial injection moulding applications. Keywords: Injection moulding; PVD coating; Modeling; Surface energy 1. Introduction PVD coatings have found their way into industry for several applications like metal cutting and deep draw- ing. Their use in plastic injection moulds has given both ?positive and negative results 1?3 . The unreproducible character of the results hinders further implementation in industry. To valorise the intrinsically good coating properties like chemical in- ertness vs. plastics to enhance demoulding, more in- sight is needed into the mechanism of interaction between the mould surface and the plastic material during injection moulding. To our knowledge, a systematic study of the infl u- ence of mould surface roughness, mould coating, properties of the polymer like Youngs modulus, sur- face energy, polarity, structures, etc. on possible bind- ing mechanisms between the mould surface and the plastic material has never been carried out. This makes it practically impossible to understand demoulding ?Corresponding author. Tel.: ?32-11-26-88-26; fax: ?32-11-26- 88-99. mechanisms and, as a consequence of this, to select a proper coating for the injection mould. The purpose of this work was to try to simulate the injection moulding process in the laboratory and to correlate the results with surface energy measurements of the coated mould and of the plastic material. This could result in an approach to select the proper coating for a certain kind of plastic to be injected. 2. Experimental details Laboratory equipment has been built to measure demoulding forces and friction coeffi cients. The mould itself is made out of tool steel 1.2083 and has a diame- .ter of 64 mm and a height of 30 mmFig. 1 . The thickness of the moulded part is 2 mm. A pressure sensor measures the demoulding forces. The tempera- ture inside the mould is measured by thermocouples as presented in Fig. 1. All moulds were hardened to a hardness of 56 HRC. After a running-in period of 40 injections, the de- moulding force was measured 10 times for each coat- ing?plastic material combination. ()M. Van Stappen et al.?Surface and Coatings Technology 142?144 2001 143?145144 Fig. 1. A cylindrical plastic part injection moulded around a mould. Surface energy was measured on the surface of the coating and on the surface of the plastic material using the model of Owens and Wendt. A Digidrop GBX apparatus has been used based on water and di- iodomethane as testing liquids. To measure the total surface energy, the dispersive surface energy and the polar surface energy are measured. Injection moulding was carried out as follows. In the fi rst application, a polyurethane plastic material with tradename DESMOPAN 385 S was injection moulded using uncoated moulds and moulds coated with, respec- tively, a TiN and a CrN coating. In the second applica- tion, three types of polymers were tested on a TiN coated mould and an uncoated mould. Two elastomers trade name HYTREL G 3548 W, which is a block- copolyester, and SANTOPRENE 101-73, which is a .blend of polypropylene and EPDM , and EVOPRENE, which consists of polystyrene and butadiene. 3. Results and discussion The demoulding forces measured for the fi rst appli- cation are given in Table 1. The demoulding forces for the second application are given in Fig. 2. This demoulding behaviour has also been observed in industrial practice, so the demoulding laboratory apparatus is a good simulation of reality. To explain these results, an attempt was made to fi nd a correlation with the surface energy measurements. Both total surface energy as well as polar surface Table 1 .Demoulding forces N for DESMOPAN Uncoated mould7757 N TiN coated mould?2810 N CrN coated mould?415 N .Fig. 2. Demoulding forcesin Nfor three materials: HYTREL, EVOPRENE, SANTOPRENE. energy in mJ?m2were compared for both coated sur- .faces and plastic materials Fig. 3 . In order to explain the demoulding behaviour, an attempt was made to make a correlation between de- moulding forces measured and the surface energy val- ues. It should be expected that when the surface energy of the coated surface is lower than the surface energy of the plastic material, an easy demoulding behaviour could result as a consequence of low material affi nity between coating and plastic material. Because the ratio of polar vs. dispersive surface energy varies for the different plastic materials, both surface energy values are taken into account. For the demoulding forces measured in the fi rst case .Table 1 , it could be seen that a CrN coating, espe- cially, could offer good demoulding behaviour. When .we compareFig. 3the surface energy values of DESMOPAN with the values for the mould surfaces .? STAVAX ?uncoated , CrN and TiN ? then it can be seen, for both total surface energy as polar surface energy, that the measured values for DESMO- 2. Fig. 3. Total surface energies mJ?mof the different coatings and plastic materials. ()M. Van Stappen et al.?Surface and Coatings Technology 142?144 2001 143?145145 2. Fig. 4. Polar surface energies mJ?mof the different coatings and plastic materials. PAN are lower compared to the mould surface values. This means that there is no correlation between the demoulding forces measured and the surface energy values. It seems, however, that a CrN surface has the lowest surface energy compared to a TiN coated sur- face and an uncoated surface. When one looks to the total surface energy values .Fig. 3 , one can see that SANTOPRENE has the lowest value and HYTREL the highest. If our hypothesis was correct from the beginning, we shouldconcludethatthedemouldingforcefor HYTREL should be small and should be large for SANTOPRENE. One can see from Fig. 2 that this is not the case. When one looks at the polar surface energy values .Fig. 4 , the three plastic materials have a lower value than the mould surface and SANTOPRENE and EVOPRENE have a lower value than HYTREL. Even when other surface energy criteria are used, e.g. the lower the energy of the mould surface the .lower the demoulding force 3 , even then no correla- tion can be found. It can be seen that a TiN coating always increases the surface energy and, on the other hand, good de- moulding is sometimes seen, e.g. for HYTREL and DESMOPAN, and sometimes bad demoulding results, e.g. for EVOPRENE. Hence, we can conclude that, based on the surface energy values measured, no correlation could be found withinthedemouldingforces.Obviously,other parameters, such as roughness and injection tempera- ture, also play an important role in explaining the demoulding beha