模具设计外文翻译资料4

Ž.
Surface and Coatings Technology142᎐1442001143᎐145
Practice boratory tests for plastic injection moulding
M.Van Stappen U,K.Vandierendonck,C.Mol,E.Beeckman,E.De Clercq
WTCM r CRIF,Scientific and Technical Centre for the Metalworking Industry,Uni¨ersitaire Campus,3590Diepenbeek,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 difficult to interpret.WTCM r CRIF has developed laboratory equipment where the injection moulding process can be simulated and demoulding forces and friction coefficients can be measured.These measurements were compared with surface energy calculations of the coated surfaces and of the plastic materials in order tofind a ing 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 verified 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
w x
positive and negative results1᎐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 influ-ence of mould surface roughness,mould coating, properties of the polymer like Young’s 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
U Corresponding author.Tel.:q32-11-26-88-26;fax:q32-11-26-88-99.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 coefficients.The mould itself is made out of tool steel1.2083and has a diame-
Ž.
ter of64mm and a height of30mm Fig.1.The thickness of the moulded part is2mm.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 of56HRC.
After a running-in period of40injections,the de-moulding force was measured10times for each coat-ing᎐plastic material combination.
()M.Van Stappen et al.r Surface and Coatings Technology 142᎐1442001143᎐145
144Fig.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 first application,a polyurethane plastic material with tradename DESMOPAN 385S 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 Žtrade name HYTREL G 3548W,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 first 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 find a correlation with the surface energy measurements.Both total surface energy as well as polar surface
Table 1
Ž.Demoulding forces N for DESMOPAN Uncoated mould 7757N TiN coated mould -2810N CrN coated mould
<415N
Ž.Fig. 2.Demoulding forces in N for three materials:HYTREL,EVOPRENE,SANTOPRENE.
energy in mJ r m 2were compared for both coated sur-Ž.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 affinity 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 first case Ž.Table 1,it could be seen that a CrN
coating,espe-cially,could offer good demoulding behaviour.When Ž.we compare Fig.3the surface energy values of DESMOPAN with the values for the mould surfaces Ž.ᎏSTAVAX s uncoated ,CrN and TiN ᎏthen it can be seen,for both total surface energy as polar surface energy,that the measured values for DESMO-
Ž2.Fig.3.Total surface energies mJ r m of the different coatings and plastic materials.
()
M.Van Stappen et al.r Surface and Coatings Technology142᎐1442001143᎐145
145
Ž2.
Fig.4.Polar surface energies mJ r m of 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 Ž.
Fig.3,one can see that SANTOPRENE has the lowest value and HYTREL the highest.
If our hypothesis was correct from the beginning,we should conclude that the demoulding force for HYTREL should be small and should be large for SANTOPRENE.One can see from Fig.2that this is not the case.
When one looks at the polar surface energy values Ž.
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
Ž.
lower the demoulding force3,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 within the demoulding forces.Obviously,other parameters,such as roughness and injection tempera-ture,also play an important role in explaining the demoulding behaviour.
In order to continue the research work to explain the demoulding behaviour,we will focus onfive industrial demonstrations and try to incorporate all relevant parameters:coating properties,plastic material proper-ties and injection parameters.
4.Conclusions
No correlation could be found between the demould-ing behaviour of plastics vs.coated moulds and the measured surface energy values.
Other parameters must also influence this demould-ing behaviour.Further research will focus on other parameters like coating properties,plastic properties and injection parameters.
References
w x1Annonymous,Big savings made with coated injection moulding
Ž.
tool,Precision Toolmaker61998,138
w x2O.Kayser,PVD-Beschichtungen schutzen werkzeug und
¨
Ž.
schmelze,Kunststoffe7199598.
w x3M.Grischke,Hartstoffschichten mit niedriger Klebneigung,JOT Ž.
1199615.。