Recycling plastics by two-shot molding

Recycling Plastics by Two-Shot Molding

R. C. DONOVAN, K. S. RABE, W. K. MAMMEL, and H. A. LORD

Engineering Research Center Western Electric Company

Princeton, New Jersey

In response to increasing ecological and economic pressures, a two-shot molding process has been developed to recycle molding plastics. This process buries scrap lastic under a skin

that has the appearance of and similar mechanical properties to the skin material only. Two injection units and a speciaI nozzle design are used to achieve the desired lamination. The- oretical and experimental studies have been conducted to de- termine the effects of parameters on the amount of scrap material that can be buried and its effect on impact strength. With conventional production molding dies, scrap plastic com-

of the total shot has been

of virgin plastic in the molded part, resu P ting in a laminate

parts having stringent ap- prising approximat$ 40 percent molded beneath virgin plastic in pearance requirements.

INTRODUCTION lastics have served a valuable role in a variety of p products, and like most other materials, their im-

pact on our environment is causing increasing con- cern. Reuse or recycling of plastics reduces the amount and cost of refuse requiring disposal, con- serves the natural resources used in the formulation of plastic compounds, provides a supply of plastics for the manufacture of products, and has potential cost savings.

The plastics industry has actively practiced reproc- essing of noncontaminated plastic compounds of a single type for many years by recycling sprue systems and process dropouts in injection molding. Even slight contamination, however, markedly de- creases the value of processed plastic in the current technology and causes large quantities of processed plastics to become unacceptable for further use. This is particularly true for molded parts with stringent appearance and strength requirements such as telephone handles and housings. A considerable amount of processed plastic is either being sold or junked as scrap plastic, and recycling of this plastic would be beneficial from both the economic and ecological viewpoints. Some of this plastic results from the manufacturing process and some originates from molded product retired from customer service.

This study describes a two-shot injection molding process developed for recycling molding plastic. The process characteristics and the equipment design have been structured so that the process is a viable manufacturing operation, The scrap plastic is processed in an injection molding machine such that it is buried beneath virgin plastic in the molded part.

Since the scrap plastic does not appear on the surface of the part, stringent appearance requirements can be maintained; and since the integrity of the outer skin in the molded part is maintained with the virgin plastic, the molded part can retain good mechanical properties. This process also shows promise for other applications where a lamination of two plastics provides flame retardance, mechanical strength and/or appearance qualities at less cost than a single plastic.

The principle of producing a laminated part construction by sequentially injecting two materials has been utilized in previously reported work ( 1-6). Combinations of solid materials in a laminated struc- ture have been illustrated but little detail or data is given and the emphasis has been on creating foamed cores in relatively thick and simple configurations. The work described here involves thin walls, complex shapes and surface requirements that will tolerate no contamination of the surface material or marking that may result from mechanical valves.

TWO-SHOT MOLDING CONCEPT

Single-Shot Injection Molding A cross-section of a typical reciprocating-screw in-

jection molding machine is shown in Fig. 1 . At the start of a cycle the molded part has just been ejected from the open mold and the empty mold closes. A shot of melted plastic at the head of the screw is injected into the empty cavity by hydraulic pressure. After injection, the screw begins to rotate, pumping melted plastic into the shot at the head of the screw

714 POLYMER €NG/NEER/NG AND SCIENCE, NOV€MBER, 7975, Vol. 15, No. 7 1

Recycling Plostics by Two-Shot Molding

and causing the screw to translate back. Meanwhile, the plastic in the mold has been cooling, and when it is sufficiently cool, the mold o ens and the plastic part is ejected. The molding cyc P e then begins again,

Two-Shot Injection Molding In studies on mold filling (e.g., Ref. ( 7 ) ) , it is

noted that the molten plastic fills the mold cavity in the manner shown in Fig. 2. As the molten plastic en- ters the cold cavity the layer of flowing plastic near the cooled metallic walls cools by heat conduction, increases in viscosity and decelerates to a zero velocity as it solidifies. Thus, when the cavity is partially filled, a non-moving layer of solidified plastic exists at the wall and a flowing region of molten plastic exists in the center of the flow channel (Fig. 2). As additional plastic is injected, the molten plastic flows within the flow channel formed by the solidified plastic toward the flow front where the flow lines turn as the plastic flows toward the cavity wall. Thus, as shown in Fig. 3, if at the time the cavity is half- filled with one plastic, a second molten plastic is injected into the mold, it will force the molten first plastic already in the cavity toward the flow front while flowing beneath the solid layer formed by the first plastic, This continues until the mold is filled and a two-shot part is produced with the first plastic at the surface of the product and at the end of the mold cavity, and the second plastic completely buried within the product. This two-shot concept permits recycled plastic to be buried beneath virgin plastic.

Ftg. 2. Mold filling process.

Fig. 3. Two-shot mold f l h g process.

The formation of the solid layer is a key factor in the two-shot process. Suppose, for example, that plastic A, which is one-ha1 of the shot, is to be injected first, followed by plastic B, which is the other half of the shot, and they are to be injected into a single-cavity die ( Fig. 4). If plastic A is processed in such a way that its solid layer is very thin, plastic B will be hidden beneath a thin layer of plastic A in the upstream part of the die and a considerable amount of plastic A will exist beneath the surface of the part at the downstream end of the cavity. In this situation, a higher percentage of the shot could have been plastic B, e.g., 70 percent plastic B, 30 percent plastic A, without plastic B surfacing (Fig. 4). How- ever, if plastic A is processed in such a way that its solid layer is quite thick, 50 percent of the shot cannot be hidden beneath the surface of the part and the amount of plastic B must be decreased to 40 percent of the shot to keep it beneath the surface.

This example illustrates ( 1) molding conditions which provide a thin solid layer permit a higher percentage of the second plastic to be molded beneath the surface and ( 2 ) the location at which the

TWO-SHOT INJECTION MOLDING

THIN SOLID LAYER, 50x1 II S O U I I THICK SOLID LAYER. 6 O U A II S O U 1

THIN SOLID L A Y i X S O X A II 7 O X I I THICN SOLID LAYER. 5 O X A II 4 0 % 1

Fig. 4. Two-shot injection molding.

POLYMER ENGINEERING AND SCIENCE, NOVEMBER, 1975, Vol. 15, No. 1 1 775

R. C . Donovan, K . S . Rabe, W. K . Mammel, and H . A. Lord

second plastic will surface is at the downstream end of the cavity. It is essential that the equipment design and process characteristics be structured to permit a high percentage of the second plastic to be successfully buried under the first plastic in order for the process to be a viable manufacturing operation.

Two-Shot Molding Equipment To utilize the above molding concepts in the re-

cycling of plastic, a two-shot molding process has been developed. This is accomplished by processing one plastic, the virgin plastic in one plasticating unit, a second plastic, the recycled lastic in a second plasticating unit, and injecting 0th lastics into a

necessary to permit the injection of plastic from two injection units into the same molding die. The design requirements chosen for the nozzle were: (1) it should have no moving parts so that wear and leakage problems are minimized when used in production and so that both sequenced and overlapping injection of the two plastics can be accomplished, and (2 ) the nozzle tip should be free of mixing or contamination of the two plastics when the sprue is removed during each cycle. This second require- ment is important for successful two-shot molding and it is essential for molding appearance parts.

In the two-shot nozzle, one plastic flows axially through a cylindrical passage at the center of the nozzle while the other plastic flows in an annulus surrounding the first plastic ( F i g . 5). A second injection unit was mounted perpendicular to the ex- isting injection unit and connected to the annular channel in the new nozzle, as illustrated in Fig . 5. The equipment could then be used in one of several ways, including injection of the in-line unit first followed by the injection of the perpendicular unit, injection of the perpendicular unit first followed by the injection of the in-line unit, or overlapping injection of the two units. In the illustration in Fig. 5, the perpendicular unit was injected first with approximately half of the shot followed by injection of the in-line unit with the last half of the shot. The product in the molding die contains the plastic from the perpendicular unit at the surface and the plastic from the in-line unit hidden beneath the surface.

common molding die. A special g Y nozz e design was

The molding machine (Lombard Model 300L- 24s) used in this study has a 300-ton clamping force and a 2.5 in. diameter, 20/1 L/D in-line reciprocating screw extruder. The second injection unit (Egan reciprocating series B ) also has a 2.5 in. diameter 20/1 L/D reciprocating screw extruder. Both units have conventional hydraulic systems and a conventional check ring at the screw tip.

Experiments were conducted with a variety of molds but data will be reported on five molding dies: 1. Plaque Mold (two-cavity, 2 X 7 X l/S and y4 in. thick, end gated), 2. Disc Mold (single-cavity, 7 in. diameter x 1/16 in. thick, center gated), 3. Housing Mold I (two-cavity, tele hone housing, wall thickness approximately 0.120 in5, 4. Housing Mold I1 ( two-cavity, telephone housing, wall thickness approximately 0,090 in.), and 5. Handle Mold I ( single-cavity telephone handle).

The sprue system and part configurations for the housings and handle are illustrated in Fig . 6. A wide variety of acrylonitrile-butadiene-styrene ( ABS ) molding com ounds were processed, including vari-

grades of scrap ABS compounds. ous colors o f! virgin ABS compounds and various

TWO-SHOT MOLD FILLING THEORY A mathematical model of the two-shot injection

molding process has been developed in order to provide a thorough understanding of the effects of molding parameters on the skin thickness and the amount of scrap material that can be buried.

A complete description of a computer model which computes the temperature, pressure, and velocity fields in a cavity during conventional (sin le shot) mold fiIIing was provided in reference f 7 ) . The model of two-shot mold filling is very similar but includes some additional features which permit the two-shot process to be described more completely and the effects of some additional variables to be studied. The mathematics of the model will not be described in detail in this report due to space limita- tions. In the model, it is assumed that at the time the flow front reaches a selected point, the material entering the cavity is changed from material No. 1

Fig. 5. Two-shot nozzle cross-section.

776

Fig. 6. Sprue systems and part confguraticms for telephone housing and handle molding dies.

POLYMER ENGINEERING AND SCIENCE, NOVEMBER, 1975, Vol. 15, No. I 1

Recycling Plustics by Two-Shot Molding

to material No. 2. At this time, the interface between the materials is assumed to be a straight line (or, in three dimensions, a plane) across the cavity entrance. As the cavity continues to fill, the material toward the center of the channel moves more rapidly than that closer to the walls, and the interface between the materials consequently distorts, forming the pat- tern shown in Fig. 4. The prediction of the amount of second material that can be buried for a given set of conditions is made by finding the time in the filling process at which the second material may be introduced so that the interface catches the flow front at exactly the time that the cavity fills. This method results in a slight overestimate of the amount of material that could actually be buried because the interface between the two materials at the time the second material just reaches the cavity entrance is not a straight line (or plane), but is qualitatively similar in shape to the interface shape in the cavity.

Typical results of the model computations are shown in Fig. 7 for the plaque mold cavity at the conditions listed in T a b l e 1. The lines represent the location of imaginary interfaces between material that entered the cavity in each increment of 10 percent of the total cavity volume. Thus, for example,

4 0 % \ I =r G 4L,30% \ I I

I

7

20% \ \ \ I

DISTANCE FROM CAVITY CENTERLINE (64th~ ins.)

Fig. 7. Compted interface projiles in cavity cross-section.

Table 1. Molding Parameters-Standard Case

Material ,No. 1-ABS (Cycolac T2098) Material No. 2-ABS (Cycolac T2098) Shot Temperature No. 1450°F Shot Temperature No. 2 4 5 0 ° F Initial Injection Rate No. 1-1.5 in.a/sec. Initial Injection Rate No. 2-1.5 in.3/sec. Injection Pressure Setting No. 1-co Injection Pressure Setting No. 2-co Coolant Temperature-125°F Cavity Material-Steel Cavity Thickness-3/32 in. Cavity Width-2 in, Cavity Length-7 in.

the area between the lines labeled 20 percent and 30 percent entered the cavity when between 20 percent and 30 percent of the cavity volume remained unfilled. The region between the wall and the line labeled 90 percent represents both the first 10 percent of the material to enter the cavity and material which entered later but reached the flow front. No attempt is made to describe the path followed by material after it reaches the flaw front, since it is enough to know that if second shot material reaches the front, some of it will surface and the part will not be acceptable. In the case illustrated in Fig. 7, the amount of second material that could be buried is somewhat greater than 60 percent of the shot volume.

To provide a quantitative evaluation of the model, experiments were performed with simple molds to compare to detailed predictions of interface shapes and to predictions of the effect of a wide range of parameters on the amount of second material that could be buried.

Detailed measurements of the shapes of the interfaces between white and black Cycolac T ABS molding compounds were obtained for center- gated discs and end-gated rectangular plaques. The measurements were obtained by sectioning the parts in several places and measuring the thicknesses of the white and black layers at a number of positions on the section with an optical comparator attached to a microscope. The fraction of buried material in the parts used to compute the predicted interface sha e was calculated from the measured interface

Figure 8 includes measurements of five different sections of a single disc. The measurements on the different sections are denoted by different symbols. The calculated inner material fractions based on these five profiles varied from .52 to .62; the data points should thus fall on or between the lines on the figure. Generally, they fall close to the predictions. Figure 9 is a similar plot for a disc molded at a lower rate, showing excellent agreement. Note the thinner surface layers in Fig. 8 as compared to Fig. 9, due to the higher injection rate.

For the plaque molds, there is a tendency for the interface to be slightly less flat and therefore more advanced at the centerline than predicted, but generally the agreement between measured and pre-

pro Fi les.

POLYMER ENGlNEERlNG AND SCIENCE, NOVEMBER, 1975, Vol. 15, No. 1 1 777

R. C . Donovan, K . S . Rabe, W. K . Mammel, and H . A. Lord

k! 4 -

x : 2 -

8

8

DISC RADIUS ( in

44% LINE 0

0

0

Fig. 8. Predicted (lines) and m e w r e d (points) interface profiles, 1/16 in. thick disc mold.

1 5

1 i 8

MOLD TEMPERATURE @ l 3 . F k MEASURED INNER MKERIAL FRACTIONS: a 0 DENOTES .We 0 DENOTES .40

g e

!i w Y 4

i.2t I I I I ) I ] 3.0 3.5 B O 1.5 20 2.5 I .o 0.25 0.5

DISC RADIUS (in.)

Fig. 9. Predicted (lines) and measured (points) interface profiles, 1/16 in. thick disc mold.

dicted results is reasonable. Comparisons are illustrated in Fig. 10 for a ?h in. thick plaque and in Fig. 11 for a Y4 in. thick plaque.

One important observation from both the theoreti- cal and experimental evaluations is that up to 60 percent inner material can be buried in simple molds by the two-shot process. The minimum skin thiclc- ness at these conditions is typically 10-15 percent of the part thickness.

$1.0 I I I I I - I I

I I I I I I 2 3 4 5 6 7

Y DISTANCE FROM GATE (In

Fig. 10. Predicted (line) and measured (pointsJ interface profiles, 1/8 in. thick plaque mold.

C K O W T2098 ABS @ 450.F OUTSIDE CYCOLX GT45W ABS @ 45O.F INSIDE F W INJECTION RATE g Y l in3/*ic SECOND INJECTION RATE Q 4 B mm3/n.c MLAV TIME BETWEEN NJECTICUS. OTBUE

MOLD TEMPERATURE Q 6O.F

-

- MEASURED INNER MTERIAL FRACTWNS, 47

z 0 G 06 I I 2 I I I I I 0

3 4 5 6 e DISTANCE FROM GATEIln I

Fig. 11. Predicted (line) and measured (points) interface profiles, 1 / 4 in. thick plaque mold.

Process Results for Different Molds Plaque Mold. Inner material comprising nearly

50 percent of the part volume was molded in this two-cavity plaque mold with the cavity thickness set at .125 in.

Disc Mold. The data in Figs. 8 and 9 show that inner material comprising approximately 60 percent of the part volume was successfully buried in this single-cavity disc mold with the cavity thickness set at 1/16 in.

Housing Mold I. By injecting black virgin ABS (Cycolac GT 4502) through the inner cylinder of the nozzle followed by re rocessed white ABS, (Cy-

nozzle, white ABS, equal to approximately 40 percent of the shot, was molded beneath the surface of the black plastic ( F i g . 1 2 ) . The total cycle time for the two-shot molding was comparable to that used in single-shot molding, and the molded housings had appearance qualities (color, gloss, absence of surface defects) equivalent to single-shot housings, The inner material was only observable at the gate after the runner is detached from the housing but this cannot be seen in normal part usage. A short shot containing all of the black plastic but none of the white showed imbalance in the cavity filling. When the proportion of the second material was increased past 45 percent, the inner material would surface at the front weld line in cavity #3 while the inner material remained corn letely hidden and only reached the midpoint oPcavity #4. This is illustrated by the molding of white ABS under clear (Cycolac CIT 31336) in Fig. 13. Thus, balancing of the cavity filling would permit an increase in the percentage of inner material.

colac T 2098) through t ph e annular portion of the

Fig. 12. Two-shot thickwall telephone housings.

778 POLYMER ENGINEERING AND SCIENCE, NOVEMBER, 1975, Vol. IS, No. 1 1

Recycling Plastics by Two-Shot Molding

Fig. 16. Two-shot telephone hande.

Fig. 13. Two-shot thickwall telephone housings with trans- parent outer material to illustrate imbalance.

Housing Mold II. Using scrap ABS as the second material, parts were molded with more than 40 percent inner material in the shot ( F i g . 14) on a machine cycle equivalent to single-shot molding. This high percentage in a thinwall, complicated molding geometry suggests that the two-shot concept holds high promise for a wide variety of production tools. Again, cavity imbalance was noted, although not as severe as in Housing Mold I. To eliminate any cavity imbalance, one cavity was blocked off, and single- cavity housings were molded with the percentage of inner material increased to approximately 50 percent of the shot ( F i g . 15).

Handle Mold L Using a single-cavity handle molding die, handles were molded using approximately 45 percent inner material in the shot (F ig . 16) on a machine cycle equivalent to the single-shot cycle.

PROPERTIES OF TWO-SHOT PRODUCTS Mechanical Properties of Test Specimens

One advantage of the laminated construction of two-shot parts is that inexpensive materials having

Fig. 15. Single cavity two-shot thinwall telephone housing.

properties that would not be adequate for the entire cross-section may be used as the core material without altering the part a pearance or other part properties, such as surface Lar dness, which depend entirely on the layer of material close to the. surface. Some part properties, however, may be influenced by the material properties of both the surface and the core materials, and it becomes necessary to know what these relationships are in order to take full economic advantage of the two-shot molding process without unacceptably altering the properties of the finished product.

One of the important properties of many molded parts is the ability to withstand impact loading without fracture. In order to gain a quantitative understanding of the relationship between the individual impact strengths of the two materials and their impact strength in the laminated two-shot product, impact tests were conducted on plaques molded from combinations of two ABS materials. Izod im act values obtained from specimens cut from tensile E ars for the two materials were 3.73 and 1.85 ft-lbs/in., respectively. The plaques were molded from each material separately, from blends of the two materials, and by the two-shot molding process over a range of material fractions with each of the materials on the outside. The test laques were sub-

from various heights (GarLer Impact Tester); the energy of impact at which cracking typically occurred was taken to be a measure of part strength. The falling weight impact test was considered most appropriate for mul- tilayer parts, since it determines the level of impact that causes the initiation of fracture (presumably usually in the outer skin) rather than the energy absorbed when fracture occurs across the entire cross-section of the part.

The test plaques were molded on a 75 ton, 4 02, two-color injection molding machine. The materials used were a virgin white ABS and a virgin black ABS. The plaques, molded in a test mold, measured 1% x 6 x .090 in. Single shot plaques were molded from each material, and from blends which con- sisted of 20, 40, 60, and 80 percent of each material by weight, Two-shot plaques were molded with each of the two materials on the outer surface of the laques. A range of conditions was run, with the

gaction of inner material in the part varying from the maximum that could be achieved (over 50 percent) down to essentially zero inner material, by varying the stroke length of the unit that injected first.

The impact tests were performed on a Gardner Heavy-Duty Variable Impact Tester. Forty to sixty

jected to im act by weights faling P

POLYMER ENGINEERING AND SCIENCE, NOVEMBER, 1975, Val. 15, No. 11 179

R. C. Donovan, K . S. Rabe, W. K . Mammel, and H . A. Lord

test plaques at each condition were impacted at three locations along the centerline of each plaque, at 3/4, 3, and 5Y4 in. from the gate. The location 3 in. from the gate was centered over a ?h in. diameter knockout pin that produced a sharp edged cir- cular rotrusion about .005 in. high on the parts on the si B e opposite to that on which they were struck. This resulted in a sizable stress concentration and much lower impact values at this location than at either end, A failure was arbitrarily defined as a crack or a number of cracks exceeding a total crack length of 1 mm. The tests were conducted at 77 2 2°F.

For the present study an integral of the fraction of parts that did not fail over all impact loads was taken to be an appropriate measure of impact strength, since it was expected that the integration process would tend to minimize the random scatter in the data that resulted from relatively small Sam le sizes, The value of this integral is similar to, gut more readily computed than, the value of impact energy at which half of the specimens would be expected to fail.

Results for the sum of the integrals at the gate end, the middle, and far end are plotted vs percentage of white material for single shot, blends, and two-shot parts in Fig. 17. The lines on the figures represent least-square fits to the data.

Referring to the figure, blending of the two materials appeared to result in an approximately linear variation of the impact energy integral between the values for the individual materials. In the two- shot specimens, the buried material appeared to exert only a minor influence on the impact strength; the impact energy integral of two-shot parts was in all cases close to that of specimens made wholly from the surface material. Thus the two-shot process appears to be a good way to use low-strength materials with minimum loss of part strength.

Mechanical Properties of Product Telephone housings processed by conventional

molding were compared against two-shot housings with respect to functional impact strength by means of a specified drop test. The test consists of mounting each housing on a weighted dummy base and then dropping the assembly from a height of 25 in. onto a steel surface using a prescribed sequence of ten drops. The housings are dropped specifically on each of the four corners and on the top for two cycles, A failure is recorded when either a major break occurs or when a mounting insert has completely pulled out. Comparison of failure rates gives a measure of a housings’ relative impact strength. The results of these tests compared well

I I I I

I I I I

20 40 60 80 AVERAGE % WHITE MATERIAL

GATE END, MIDDLE, AND FAR END

Fig. 17. Impact energy integral sum us percentage of white material: A two-shot, white over black; Q single shot, blends; 0 two-shot, black O V ~ T white.

with the Gardner impact test results previously men- tioned, both showing that the im act strength of a

strength of only the outer material. Additional preliminary tests on two-shot parts

with recycled plastic have been completed, The handles and housings have been subjected to product drop tests and finishing operations such as buffing and solvent polishing. Delamination has not been observed in any test as yet. The product has performed satisfactorily in each of these preliminary tests, and more detailed product testing is continu- ing.

ACKNOWLEDGMENT Messrs. J. J. Taylor and D. E. Thomas contributed

to the design of the two-shot nozzle and to the design specifications of the EPCO plasticating unit. Mr. A. Karp constructed the apparatus which permits either sequential or o v e r l a p g injection of the two units. Each of these e orts is gratefully acknowledged.

REFERENCES 1. I. E. Escoles, Kunstofe, 60, 847 (1970). 2. I. G. Grausch and 0. Schleith, Kunstofe, 63,656 (1973). 3. J. H. Sandiford and D. F. Oxley, SPE J. , 27, 38 (1971). 4. I. R. Schrewe, Kunstofe, 61, 623 (1971). 5. Europlastic Monthly, . 8, (Nov. 1973). 6. Plastic Machinery ancfEpuipment, p. 11 (Feb. 1973). 7. H. A. Lord, and G. Williams, SPE Antec Papers, 21

two-shot housing is largely depen B ent on the impact

( 1975).

780 POLYMER ENGINEERING AND SCIENCE, NOVEMBER, 1975, Vol. 15, No. 11

Documents

Recycling plastics by two-shot molding