An Experimental Study of Sandwich Injection Molding of

Two Polymer Melts Using Simultaneous Injection JAMES L. WHITE

and BIING-LIN LEE"

Department of Chemical and Metallurgical Engineering

Th.e University of Tennessee Knoxville, Tennessee

An experimental study of two phase flow of polymer melts relating to injection molding has been carried out. The ex- periments specifically concern rheological phenomena which may occur when two melts are simultaneously injected through a runner system leading to a mold cavity. Injection in the form of annulus-core configurations and simple stratded flow are investigated. A new injection molding method is proposed which is based upon the latter technique. Encapsulation of one melt by the other occurs in the runner system and formation of a sandwich structure in the cavity results. Annulus-core in- jected configurations were only found to be stable when the lower-viscosity melt is in the annular region.

INTRODUCTION O n e of the more interesting developmats in plas-

tics fabrication during the past decade is the sandwich molding process in which two different molten polymers are sequentially injected into a mold cavity to form parts with a skin of the first polymer injected and a core of the second polymer (1-3). Studies of sandwich molded parts and flow visualization investigations of the process show it to be sensitive to differences in the rheological prop- erties of the two melts (4, 5 ) . It appears that if skin- core structured parts are to be produced, the first melt injected must possess a lower viscosity than the second melt.

It would also seem possible to produce sandwich parts by simultaneous injection of two melts. The simplest idea would be to have a runner possessing a cross-section of a cylindrical hole (for the core melt) surrounded by an annular slit (for the skin melt) or at least an annular die of this type to inject into a runner. Such a design has indeed been developed ( 6 ) . Menges and his coworkers ( 7 ) have considered the similar problem in which a melt injected into a mold is separated into two streams of this type and a foaming agent is injected into the annular stream. We suspect, however, that this process will also be rheologically sensitive and in this paper will present an experimental investigation of the behavior of an- nular flow of polymer melts in a geometry simulating mold filling.

0 Current address: B. F. Goodrich Company, Research Center, Brecks- ville, Ohio.

POLYMER ENGINEERING AND SCIENCE, JULY, 1975, Vol. 15, No. 7

We will describe in this paper a new injection molding process for making skin-core structured parts which is also based upon simultaneous injec- tion of the two melts. In this process the difference in the rheological properties, specifically the viscos- ities, of the two melts is used as a mechanism to secure the skin-core configuration. In recent years various researchers have studied the fluid mechanics of stratified two phase fiow of polymer melts through dies (8-15) for the general purpose of understanding flow phenomena occurring during the melt spinning of bicomponent fibers (16-18) and of co-extrusion of sheet and multilayer film (19, 20). Nagano ( 8 ) and Southern and Ballman (10) have shown that in stratified flow through cylindrical dies low viscosity melts gradually encapsulate a higher viscosity melt. This has been verified by later researchers (11, 12, 14) and the hydrodynamic mechanism investigated (12, 15). We propose and demonstrate in this paper that if two stratified melts with different viscosity levels are injected through a long runner system and gate into a mold that the lower viscosity melt will encapsulate the higher viscosity melt and a molded part with a skin-core configuration will result.

EXPERIMENTAL

Materials Six polymers were used in this study. These were

( 1) low-density polyethylene rod (LDPE and LDPE-R), ( 2 ) polystyrene rods (PS and PS-R), ( 3 ) low-density polyethylene tubing (LDPE-T), and (4 ) polymethyl methacrylate tubing ( PMMA-T)

481

James L. White and Biing-Lin Lee

purchased from Cadillac Plastics, Louisville, Ken- tucky. The LDPE-R and PS-R rods were slightly larger than in. in diameter and the tubing had an inside diameter of y4 in, and an outside diameter slightly larger than 3/s in, The LDPE and PS rods with diameters slightly larger than 3/s in. are the same polymers as the rod studied by White and Dee (4) and are similar in character to those used in earlier studies of melt extrusion and injection mold- ing by our group (9, 14, 21). The shear viscosity function p of these six melts was determined over a wide range of shear rates using a Weissenberg rheogoniometer and an Instron capillary rheometer. The viscosity of the LDPE and PS were determined at 160, 180, and 2OO0C, and the LDPE-R, PS-R, LDPE-T and PMMA-T at 160°C. The procedures used were standard and equivalent to those of our earlier papers (4, 9, 14, 21) and we will not repeat them here. The results are plotted in Figs. I and 2. The viscosities are seen to order according to

PS-R N PMMA-T > LDPE-T > LDPE-R (1)

PS > LDPE

I I I

0 0 P S - R 0 LDPE-T + 0 PMMA-T

A A LDPE - P

>

, - 1 0-3 10-2 lo-' 1 0 l o

Shear Rate (sec-1)

Fig. 1 . Viscosity of LDPE-R, PS-R, LDPE-T and P M M A - T as a function of shear rate at 160°C.

A a - LDPE

Annular Flow Experiments Two phase annular flow was investigated using

the Instron capillary rheometer. The PMMA-T tub- ing was used in conjunction with the LDPE-R rod and the LDPE-T with the PS-R rod. The outer diam- eter of the polymer tubing was trimmed down to the size of the barrel of the Instron capillary rheometer, while the polymer rods were machined to the level of the inside diameter of the tubing. Polymer rods were then inserted into the tubing to form a full composite cylinder which was placed inside of the Instron rheometer barrel (see Fig. 3) . The length of the composite rod was ten ins. It is believed that ex- trusion of finite lengths of melts with a composite in- terface in the direction of flow contacting air simu- lates mold filling. A 34s in. diameter flat ended steel bar eight in. long was inserted into the barrel to en- sure the initial configuration. The system was then heated to 1W"C and the steel bar removed. The cross-head was then allowed to descend at a rate of 0.2 in./min. (No capillary die was used.) After four in. of flow the heater was turned off and the system was cooled down. The solid composite rod was then pushed out of the barrel and the samples cross-sec- tioned.

Injection Molding of Stratified Melts Cylindrical rods of PS and LDPE were sliced in

half to form semi-cylinders which were inserted into the barrel of Instron capillary rheometer below which was attached the nozzle-mold system designed by Dee (4) (see Fig. 4 ) . The nozzle is made of steel and has a l/s in. bore 1% in. long. This acts as a runner. This is screwed flush with the bottom of the Instron barrel, The mold is constructed from alumi- num in three parts and is held together by four sym- metrically placed bolts. It also includes a stainless steel nozzle adaptor. The mold was primarily de- signed for flow visualization and the outer clamping blocks are provided with Pyrex plate glass windows 1 , x 2 x 4 in. These glass plates are fitted into a hollow carved out portion of the aluminum blocks and cushioned by teflon gaskets. The mold cavity is formed from 5/32 in. thick aluminum plate. The plate contains a 13/16 in. long continuation of the runner from the nozzle followed by a hollowed out rectangular cavity 3 x 1 x 5/32 in. This is mold ( a ) described by White and Dee (4).

The two polymeric half cylinder rods were heated to 200°C and brought into the molten state. The stratified two phase melt system was then injected into the runner system by movement of the Instron cross-head. The cross-head velocity was 5 in./min. The mold was unheated. After it had cooled, the mold was opened and the now solidified two phase plastic injection molded part removed and inspected.

RESULTS AND DISCUSSION

Annular Flow The final flow configurations as exhibited in the

solid cylinders extruded from the Instron barrel are shown in Fig. 3. In the PS-R/LDPE-T set, the initial

POLYMER ENGINEERING AND SCIENCE, JULY, 1975, Vol. 15, No. 7 482

An Experimental Study of Sandwich Injection Molding of Two Polymer Melts Using Simultaneous Injection

a u B - P B - - ar 1 annulus

annular configuration was found to persist. This was not the case for the LDPE-R/PMMA-T combination. Indeed a phase inversion was found.

The pressure development of the two systems was recorded by the Instron force recorder. With the low viscous component initially in the skin region ( PS-R/LDPE-T), the force readings increase mono-

ADPE-T

(2)

core

-R I I I I

Top View I I I ’ I Side View

( a ) Initial Configuration

Top View

Side View

( b ) After Four Inches of Flow PMMA-T

LDPE- R

Side View ( a 1 Initial Configuration

LDPE - R PMMA-T

Side View

(b )A f te r Four Inches of Flow

Fig. 3. Initial and final configurations of annular flow of two melts in the lnstron rheometer barrel. A. LDPE-T/PS-R; B. PMMA-T/LDPE-R.

Fig. 4. Schemutic diagram for iniecting two melts simul- taneously into a mold in stratified flow.

POLYMER ENGINEERING AND SCIENCE, JULY, 1975, Vol. 15, No. 7 483

James L. White and Biing-Lin Lee

Tadmor (24) and White (25). In essence the lower viscosity fluid core would flow past the higher viscos- ity annulus and then flow radially outward to the wall. This would separate the original annular con- figuration into a forward (lower) region of low vis- cosity melt and a back (upper) region of high vis- cosity melt. Continued application of pressure would then result, as observed in the work of White and Dee (4) for sequential injection, in the higher vis- cosity melt moving as a piercing finger into the cen- tral core of the lower viscosity melt. The observed phase inversion would then be accomplished. With low viscosity melt now near the wall, a fixed extru- sion rate will result in reduced shear stresses and force readings.

Interestingly, Han (%), in an unpublished manu- script received while this paper was being written, has found that in annular extrusion of two polymer melts, that if the core is slightly eccentrically placed, the configuration is stable for a higher viscosity core but is unstable for a lower viscosity core. In the lat- ter case the core melt pushes to the wall establishing a near simple stratified configuration which would be expected to evolve in a long enough tube to en- capsulation by the lower viscosity melt.

From the above experiments and interpretations, it was concluded that injecting a two phase slug of polymer melt consisting of an annulus of a low vis- cosity melt about a high viscosity melt is feasible, but injecting a slug in which the lower viscosity melt is in the core gives rise to an unstable system. More work is required to quantify the influence of the magnitude of the viscosity difference and ratio.

Injection Molding of Stratified Melts Both the solidified stratified two phase melt in the

runner and the distribution of the two phases in the molded parts were determined. These are sum- marized in Fig. 5. It is clear that as the two phase melt system flowed through the runner, the LDPE encapsulated the PS. When the melts entered the mold there was already an annulus of LDPE around the PS. The distribution of the phases in the molded part show an LDPE skin and a PS core. This clearly arises from the annular distribution of the phases at the gate.

It would seem clear from the above result that the flow encapsulation phenomena which takes place when two stratified polymer melts of differing vis- cosity flow side by side through a tube may be used in injection molding to obtain parts with skin-core configurations. More work is needed to establish ( a ) criteria for required runner lengths as a function of viscosity ratio to accomplish encapsulation, ( b ) uni- form skins especially near the gate and positions where the advancing front meets the mold cavity wall, and ( c ) influence of mold temperature and the dependence of melt viscosity upon temperature. Clearly a better understanding of the hydrody- namics of the encapsulation process is required to answer these questions. However, a good idea of the criteria for complete encapsulation in the runner

A . s e c t i o n a a '

LDPE1

C . lonqitudinal R . s e c t i o n b b '

Fig. 5 . Observed configuration of phases in injection molded parts of various cross-sections. A. section aa'; B . section bb'; C. longitudinal.

might be expected to be obtained by combining the work of Nagano (S), Southern and Ballman ( lo) , and Lee and White (14) . The work of the latter authors includes the two polymers PS and LDPE considered here. Nagano gives the criterion for en- capsulation

The studies of Southern and Ballman and especially Lee and White would suggest that this is perhaps not in general sufficient to obtain complete encapsu- lation. The smaller the viscosity difference, the greater the error. However, inlet conditions, extru- sion rates and materials differ between Nagano and other researchers. More work is clearly needed.

CONCLUSIONS The problem of simultaneous injection molding of

two polymer melts to produce parts with skin-core configurations has been investigated:

0 If two melts are injected into a mold in the form of an annular ring and a core, the configuration would seem stable only if the annulus contains the lower viscosity melt. If the annulus contains a higher viscosity melt, phase inversion can occur.

0 A sandwich injection molding process based upon simultaneous injection of two stratified layers each with the initial shape of a half circle is pro- posed. As the two phase system flows through the runner, the lower viscosity melt will encapsulate the

484 POLYMER ENGINEERING AND SCIENCE, JULY, 1975, Vol. 15, No. 7

An Experimental Study of Sandwich Injection Molding of Two Polymer Melts Using Simultaneous Injection

;her viscosity melt. Criteria for runner designs are cussed. iome further general comments are useful here. injection molding is a non-isothermal process in- ving temperature gradients in space and time and : viscosity levels of the two melts vary with tem- .ature in different manners, heat transfer effects y well play an important role in these processes. r instance, a melt at a wall will tend to have a ver temperature than a melt in an interior region. is will tend to make the phase in the neighborhood the wall more viscous than one would expect from thermal data. This is well known in injection ilding ( 2 5 ) . The implications of this behavior to cosity sensitive two phase injection molding is not 3wn and needs further investigation.

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