Investigations on glass-to-mold sticking in the hot forming process

Investigations on glass-to-mold sticking in the hot forming process

Daniel Rieser 1,Gerd Spie?,Peter Manns *

Fraunhofer-Institut fu ¨r Werksto?mechanik IWM,Wo ¨hlerstra?e 11,D-79108Freiburg i.Br.,Germany

Available online 11December 2007

Abstract

The sticking behavior of various mold materials and coatings for hot glass melt forming processes,like,e.g.glass container manu-facturing,was investigated using a new testing procedure.The mold material specimens under test were subjected to frequent contact

with hot viscous glass gobs in a pressing process with presetting well de?ned non-isothermal pressing parameters to simulate industrial working conditions.Three di?erent glass compositions were used in this investigation,soda-lime silicate glass,lead crystal glass,and borosilicate glass.The sticking characteristics of the tested mold materials and coatings were described by two quantities,a ‘lower’and an ‘upper’sticking temperature,which are speci?c for each mold material and type of glass in the non-isothermal pressing process.The ‘lower’sticking temperatures of uncoated mold materials were found to depend monotonically on the thermal e?usivity (heat pen-etration coe?cient)of the bulk mold materials.All of the coating materials applied to various substrate mold materials were found to reduce the ‘lower’sticking temperature as compared to the uncoated materials.Most of the coating materials were found to reduce also the ‘upper’sticking temperature.

ó2007Elsevier B.V.All rights reserved.

PACS:89.20.Bb;83.80.Ab;81.05.Kf;68.35.Np;46.80.+j

Keywords:Measurement techniques;Mechanical properties;Modeling and simulation;Oxide glasses;Surfaces and interfaces;Thermal properties;Viscosity and relaxation

1.Introduction

The phenomenon of glass-to-mold sticking is a major problem for industrial glass forming processes,like,e.g.container glass fabrication,as well as for precision mold-ing of optical components from inorganic glasses.The conditions,which lead to glass-to-mold sticking are not completely understood.Numerous investigations on stick-ing behavior of various mold materials with glass melts of di?erent compositions have been conducted during the past decades using di?erent experimental approaches [1–9].The results of several of those investigations are con-tradictory,and are even in contradiction with practical

experience in glass industry.Some of those discrepancies may be due to the fact that until now neither a precise de?nition of ‘the sticking temperature’nor a standardized or commonly accepted test procedure have been developed.

For systematic materials research and development of mold materials and coatings with regard to higher ‘sticking temperature’and improved wear behavior,a reliable test method is required,by which the service behavior of mold materials can be characterized with high relevance for industrial practice.In a ?rst step to overcome these di?cul-ties and to come to clearer results with respect to sticking behavior,a new testing method and a respective laboratory scale testing device were developed.By this method the sticking behavior of mold materials and coatings can be characterized,and in addition the corrosion and wear behavior of mold materials in hot forming of glass melts can be quanti?ed.

0022-3093/$-see front matter ó2007Elsevier B.V.All rights reserved.doi:10.1016/j.jnoncrysol.2007.02.095

*

Corresponding author.Tel.:+497615142135;fax:+497615142402.E-mail address:peter.manns@iwm.fraunhofer.de (P.Manns).1

Present address:RENA Sondermaschinen GmbH,D-78148Gu ¨ten-bach,Germany.

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Available online at https://www.360docs.net/doc/e516261093.html,

Journal of Non-Crystalline Solids 354(2008)

1393–1397

2.Experimental setup

The testing method consists of a non-isothermal paral-lel-plate pressing process,a simpli?ed model of common industrial glass molding processes.The mold material spec-imens are subjected to pressing of series of small portions of hot glass melt at precisely de?ned process conditions.Sticking behavior as well as wear of the mold surfaces and surface quality of the pressed glass blanks are recorded as a function of the process parameters,i.e.glass tempera-ture,mold temperature,pressing pressure,pressing dura-tion,service time of the molds.

The experimental setup consists of a melting crucible,a servo-electrical parallel shear,a transfer channel,a pressing unit with heated inserts of the mold material,a cooling device and a pushing mechanism for separating the pressed glass blanks from the mold,and equipment for controlling and recording the process parameters.A schematic draw-ing of the testing apparatus is shown in Fig.1.Technical details of the experimental setup and the testing method are described elsewhere [10,11].

The pressing unit itself consists of a symmetrical setup with a lower and upper die,and is operated by a pneumatic actuator.The mold specimens are clamped onto heating units which are controlled to achieve a preset steady state temperature of the molds.The temperature of each mold specimen is monitored continuously with a miniature ther-mocouple embedded in the center of the pressing area,2.5mm underneath the mold surface.The absolute overall accuracy of the temperature measurements was better than ±3K.The glass melt streaming from the melting crucible is portioned into small gobs of constant mass by an appropri-ately controlled shear.The temperature of the glass melt is monitored by a thermocouple in the spout of the melting crucible.The glass gobs are transferred into the pressing unit and are pressed semi-automatically.In order to insure that the pressed glass blanks would stick preferably to the

upper mold,the temperature of the lower mold was adjusted 20K lower than the temperature of the upper mold.The term ‘mold temperature’used in this paper denotes the steady-state temperature inside the upper mold, 2.5mm underneath the mold surface,before the glass gob is pressed against the mold surface.

The occurrence of sticking of a pressed glass blank to the surface of the upper mold was detected by a mechanical pushing mechanism,as sketched in Fig.1.The intensity of sticking was quanti?ed by recording the time interval nec-essary to detach a pressed glass blank from the mold sur-face while applying to the glass blank a de?ned cooling procedure with a jet of cold air combined with a mechani-cal separation procedure which avoids damaging of the mold surfaces.The time interval between the end of the pressing duration,i.e.opening of the pressing unit and lift-ing of the upper die,and the instant when the pressed glass blank is released from the mold surface is denoted by the term ‘sticking time’as used in Fig.2.3.Results and discussion

The intensity of sticking of glass blanks to the surface of the pressing molds was found to depend on numerous parameters:type of glass,glass temperature,size of the glass gob,size of shear mark,type of mold material,mold temperature,molding pressure,duration of pressing,mold surface roughness,etc.From the process parameters which may be set for each pressing cycle,the mold temperature appeared to be the most important and dominating param-eter.Initial surface roughness of the mold specimen (mirror polished),mass of the glass gob (2.5g),molding pressure (2.5MPa)and duration of pressing (5s)were kept constant throughout these investigations.While keeping all the other parameters constant,the intensity of sticking was found to increase monotonically with mold temperature.Sticking of the glass occurred only above some critical mold temperature,and the intensity of sticking

increased

Fig. 1.Schematic drawing of the testing device,which consists of a melting crucible,servo-electrical parallel shear,transfer channel,pressing unit with heated inserts,cooling and pushing mechanism for separating the pressed glass blanks from the mold,devices for controlling and recording the process

parameters.

Fig.2.Sticking curve for pressing soda-lime silicate glass melt (type B270)with mold material Stellite 12ò.The values of ‘lower’and ‘upper’sticking temperature are marked with arrows.

1394 D.Rieser et al./Journal of Non-Crystalline Solids 354(2008)1393–1397

steeply with increasing mold temperature within a rather small interval of mold temperature until the glass blank adhered very strongly,and in many cases even permanently to the mold surface.Fig.2shows a typical experimental sticking curve,recorded from pressing soda-lime silicate glass with commercial mold material Stelliteò,which is widely used for plungers in industrial glass container man-ufacturing.All the mold materials and coatings tested in this study were found to exhibit essentially very similar dependence of sticking intensity on mold temperature, but with distinct di?erences in the temperature of onset of sticking as well as in the slope of the steep part of the curves.Sticking curves for numerous mold materials tested with hot forming of three di?erent glass compositions, soda-lime silicate glass(optical glass B270),lead crystal glass containing24%PbO,and Pyrexò-type borosilicate glass,have been reported in[10].

Summarizing the observations from the pressing exper-iments conducted throughout this investigation,from the sticking curves of the tested mold materials and coatings in each case two quantities,‘lower’and‘upper’sticking temperature were extracted.The position of these two characteristic temperatures in a typical pressing experi-ment are indicated in the sticking curve of Fig.2.The ‘lower sticking temperature’was de?ned as the lowest mold temperature at which pressed glass blanks would stick to that mold material.Below this characteristic tem-perature no sticking of glass blanks to the mold was observed,above the‘lower sticking temperature’all of the pressed glass blanks did always stick to the mold. The‘upper sticking temperature’was de?ned as the lowest mold temperature at which glass blanks would stick very ?rmly to the mold surface with sticking time of more than 60s.The values of‘lower’and‘upper’sticking tempera-tures were determined to an accuracy of approximately ±5K by conducting multiple pressing experiments in the respective temperature ranges.Statistical analysis on the observation of occurrence and duration of sticking in the region of the‘lower’sticking temperature is re?ected in[10–12].

The determined values of the‘lower’sticking tempera-tures for a variety of mold materials employed for pressing of three di?erent types of silicate glasses are plotted versus the thermal e?usivity(heat penetration coe?cient)of the mold materials in Fig.3.The expression de?ning the ther-mal e?usivity

b?

????????????????kác páq p

;

comprises heat conduction coe?cient k,speci?c heat c p, and density q of the material[13].The numerical values of thermal e?usivity of metallic mold materials were taken from the literature[14]and data sheets provided by the var-ious producers of the respective materials,and were extrap-olated to the temperature region of these pressing experiments,450–750°C.As Fig.3shows,the‘lower’sticking temperature of mold materials appears to increase monotonically with thermal e?usivity of the mold materials.

For the interpretation of this dependency and the appar-ent di?erences in‘lower’sticking temperature of di?erent mold materials for pressing of di?erent types of glass a mathematical model for the heat exchange process in the glass-mold interface was applied assuming‘ideal contact’of glass melt and mold surface.For such pressing condi-tions which lead to sticking,it is assumed that the viscous glass melt essentially comes into very close contact with the mold surface without any gap in between,in the very?rst instant of the pressing process.This assumption is sup-ported by the experimental observation that an air-?lled gap of some300nm width can be detected only after the sticking glass blanks were at least partially released from the molds.

For the mathematical model it is also assumed that the temperatures inside the glass blank and the mold body are initially homogeneous,and the two bodies are brought into contact instantly.Starting with di?erent temperatures of glass and mold,the resulting temperature distribution inside the glass and mold as function of time and distance from the interface can be derived[13].This model yields the mathematical parameter‘contact temperature’in the inter-face,which is independent of time,and must not be misun-derstood as the actual surface https://www.360docs.net/doc/e516261093.html,ing the experimentally determined values of the‘lower’sticking temperatures to calculate‘contact temperatures’at the onset of sticking,the model yields equal values of glass vis-cosity for all mold materials and types of glass tested.The experimental and mathematical results of this investigation lead to the conclusion that sticking will occur,if the glass viscosity which corresponds to the‘contact

temperature’Fig.3.Lower sticking temperatures of various mold materials for three di?erent glass types(borosilicate glass,soda-lime glass,lead crystal glass) plotted as a function of the thermal e?usivity of the mold materials.The numbers at the data points denote the respective mold materials:(1) Stellite12ò,(2)Cast iron GGG40,(3)Steel1.7335with JetCode,(4)Steel 1.7335boronized,(5)Bronze NiBz15,(6)AlN sintered ceramic,(7)CrNi-Steel1.4057,(8)Steel1.4057coated with TiAlN,(9)ODS-Nickel based ST015,(10)Nickel based Alloy30ò,and(11)ODS-Iron based ST020.The dashed lines are drawn as a guide to the eye.

D.Rieser et al./Journal of Non-Crystalline Solids354(2008)1393–13971395

in the glass-mold interface is less than the unique value of g c %108.8Pa s [15].The experimental results of this investi-gation on bulk mold materials are in very good qualitative agreement with the empirical experiences in industrial glass manufacturing,thus providing evidence for the suitability of this testing method.The applied model proves capable to explain the experimentally determined values of ‘lower’sticking temperature for bulk materials and di?erent types of glass.

The testing method was then also used for testing of coated mold specimens.Coatings of thin ?lms of various metallic and ceramic coating materials (Cr,TiAlN,AlN)were applied by di?erent techniques onto various substrate mold materials.From pressing experiments with these coated molds it was found that all tested coatings lead to a signi?cant lowering of the ‘lower’sticking temperature as compared to the uncoated mold materials,see Fig.4.The e?ect of lowered sticking temperature appeared very pronounced for the specimen of sintered ceramic material AlN with thin layers (0.5l m)of ceramic PVD-AlN coat-ings on the substrate mold material (see data points 9–11in Fig.4).The observed reduction of ‘lower’sticking tem-perature leads to the conclusion that the heat penetration from the glass body into the substrate mold material was substantially reduced by the coatings applied to the sub-strate material surface,thus shifting the contact tempera-ture towards higher values and reducing the contact viscosity in the glass-mold interface below the critical value for onset of sticking,g c %108.8Pa s [16].The presently used approximation in the mathematical model as well as the data base of thermal properties of thin ?lm composites will

have to be extended in order to predict the in?uence of mold coatings and surface layers on sticking correctly.The experimentally determined ‘upper’sticking temper-atures of several mold materials,uncoated and coated with various metallic and ceramic coating materials,are plotted versus thermal e?usivity of the respective substrate mold materials in Fig.5.Most of the coating materials also lead to a signi?cant lowering of the ‘upper’sticking tempera-ture.However,one type of coating material was identi?ed by this testing method,which apparently raises the ‘upper’sticking temperature to a large extent of approximately 50K,see marked data points 3–4in Fig.5.This type of thin ?lm ceramic coating containing compositions of chro-mium deposited on a nickel based alloy substrate has also proved to exhibit advantageous properties in isothermal precision molding of glasses for optical components [17,18].

4.Summary and outlook

Using a newly developed testing procedure,the sticking behavior of various mold materials (metals,ceramics,and coatings)was investigated for non-isothermal pressing of glass melts.The sticking characteristics of the tested mold materials and coatings were described by two quantities,the ‘lower’and the ‘upper’sticking temperatures.The ‘lower’sticking temperatures were found to depend mono-tonically on thermal e?usivity of the mold materials,but not on chemical composition of the glass or mold material.This tendency is explained in terms of a critical viscosity g c %108.8Pa s of the glass melt in the glass-mold interface from a mathematical model assuming ‘ideal contact’.The ‘upper’sticking temperatures depend on additional param-eters,which have not been quanti?ed completely

yet.

Fig. 4.‘Lower’sticking temperatures from pressing soda-lime silicate glass using various mold materials with bare surface (open symbols)and with surface coating (?lled symbols)plotted versus thermal e?usivity of the substrate materials.The arrows indicate that all of the tested coatings lead to a lowering of the ‘lower’sticking temperature.The numbers at the data points denote the respective mold materials:(1)Stellite12ò,(2)TiAlN on Stellite,(3)Nickel-base alloy,(4)Coating containing chromium on Nickel-base alloy,(5)Cast iron GGG40,(6)TiAlN/ZrN (superlattice)on Cast iron,(7)Stainless steel 1.4057,(8)TiAlN on Stainless steel,(9)AlN ceramic,(10)AlN coating on AlN ceramic,and (11)Two layers of AlN coating on AlN

ceramic.

Fig. 5.‘Upper’sticking temperatures from pressing soda-lime silicate glass using various mold materials with bare and coated surfaces.Most of the thin ?lm coatings lead to a lowering of the ‘upper’sticking temperature.Only one type of coating containing compositions of chromium (marked with solid arrow)raises the ‘upper’sticking temper-ature signi?cantly (>50K).The numbers at the data points denote the respective mold materials,same as in Fig.4.

1396 D.Rieser et al./Journal of Non-Crystalline Solids 354(2008)1393–1397

Coating materials(metallic and ceramic thin?lms) deposited onto various substrate mold materials did not lead to an improvement of sticking behavior with respect to raising sticking temperatures for non-isothermal https://www.360docs.net/doc/e516261093.html,pared to the uncoated mold materials,all types of coatings reduced the‘lower’sticking temperatures of the molds.And almost all of the coating materials also reduced the‘upper’sticking temperature.With some coat-ing compositions,however,improvements towards higher values of the‘upper’sticking temperature were observed. Therefore future investigations will focus on the develop-ment of such coating materials which would raise the ‘upper’sticking temperature,and on the development of an advanced quantitative testing method for such coatings. Acknowledgments

These investigations were conducted with the kind sup-port of the Arbeitsgemeinschaft industrieller Forschungs-vereinigungen(AiF),Ko¨ln,(AiF-No.13508N)under the auspices of the Hu¨ttentechnische Vereinigung der Deut-schen Glasindustrie(HVG),O?enbach/Main,utilizing re-sources provided by the Bundesministerium fu¨r Wirtschaft und Arbeit(BMWA),Berlin.Thanks are due to all these institutions.

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