Water-assisted injection molding of thermoplastic materials: Effects of processing parameters

Water- Ass is t ed Injection M o I d i n g of The r m o p I a st i c Mate r i a Is: Effects of Processing Parameters

SHIH-JUNG LIU* and YJ3N-SHOW CHEN

Polymer Rheology and Processing Lab Department of Mechanical Engineering, Chang Gung University

Tuo-Yuan 333. Tuiwan

The objective of this study was to experimentally investigate the effects of various processing parameters on the water-assisted injection molding of thermoplastic materials. Experiments were carried out on a lab-developed water-assisted injection molding system, which included a water pump, a water injection pin, a water tank equipped with a temperature regulator, and a control circuit. Two types of water injection pins designs were proposed to mold the parts. After molding, the lengths of water penetration in molded parts were measured. The effects of different processing parameters on the lengths of water penetration were determined. It was found that the shrinkage rate and the viscosity of the polymeric materials, and the void shapes of the hollowed cores mainly determined the water-penetration lengths in molded products. In addition, a comparison has been made between the parts molded by water-assisted injection molding and gas-assisted injection molding. I t was found that water-assisted injection molded parts exhibit less uniform void sizes along the water channel. The cycle time for water-assisted injection molded parts was shorter than that of conventional injection molded parts and gas-assisted injection molded parts.

INTRODUCTION injection molding is shown in F&. 1 a. One of the limi-

newly developed water-assisted injection molding A technology (1) has received extensive attention in recent years, owing to its light weight, relatively lower resin cost per part, faster cycle time, and its flexibility in the design and manufacture of plastic parts. Water- assisted injection molding can produce parts incorpo- rating both thick and thin sections with less shrinkage and warpage and a better surface finish, but with a shorter cycle time. Typical applications include: 1.) tube and rod-like parts, 2.) large sheet-like structural parts with a built-in water channel network, and 3.) complex parts consisting of both thick and thin sections. The water-assisted injection molding process can enable greater freedom of design, material savings, weight production, and cost savings in tooling and press capacity requirements (2 - 5).

Water-assisted injection molded parts can be either short-shot molded or full-shot molded. In a short-shot water-assisted injection molding, the mold cavity is partially filled with the polymer melt followed by the injection of water into the core of the polymer melt. A schematic diagram of the short-shot water-assisted

T o whom correspondence should be addressed

1806

tations of short-shot water-assisted injection molded parts is the switchover mark on the surface of molded products. Full-shot water-assisted injection molding has the advantage of eliminating the switchover mark on the surface of molded parts. In a full-shot molding, the mold cavity is first completely filled with the polymer melt followed by the high-pressure water into the core of the polymer melt because of the shrinkage of the polymer. Figure l b schematically shows the full- shot water-assisted injection molding process.

Water-assisted injection molding has advantages over its better known competitor, gas-assisted injection molding (6- 10). because it incorporates a shorter cycle time to successfully mold a part, due to the higher cooling capacity of the water during the molding process. The incompressibility, low cost, and ease of recycling the water make it an ideal medium for the process. Since water does not dissolve and diffuse into the polymer melts during the molding process, the internal foaming phenomenon (11) that usually occurs in gas- assisted injection molded parts can be eliminated. In addition, water-assisted injection molding is more ca- pable of molding larger parts with small residual wall thicknesses.

Despite the advantages associated with the process, the molding window and process control are more

POLYMER ENGINEERING AND SCIENCE, NOVEMBER 2003, Vol. 43, No. 71

Water-Assisted Injection Molding

Fig. 1. Schemtic Of short-skot, and (b) full-shot water-assisted i n jection-molding process.

HOLLOW COW WATER PENETRATION

critical and difficult since additional processing paran- eters are involved. These new water-related processing parameters include the amount of melt injection, water pressure, water temperature, and water injection delay time (12). The water-assisted injection molding process involves the dynamic interaction of two totally different materials (polymer melt and filling water), and the product, tooling and mold process design are quite complicated. The previous experience with the conventional injection molding process is no longer s ~ % - cient to deal with this process.

This report is devoted to studying the effect of various processing parameters on the short-shot water- assisted injection molding of thermoplastic materials. The first part of this report describes a water-assisted injection molding system. A water injection unit, which

thermoplastic materials, including polystyrene, polyethylene, polypropyleine, and acrylonitrile-butadiene- styrene. A comparison has been made between the parts molded by water-assisted injection molding and gas-assisted injection molding. The third part of this report studies the effects of different processing parameters on the molding of water-assisted injection molded products. The fmal goal of this research is to gain a better understanding of various processing parameters on the moldability of water-assisted injection molded parts, so that steps can be taken to optimize the process. This prclvides significant advantages in improving part quality.

EXPERIMENTAL Materials

includes a water pump, a water injection pin, a water tank equipped with a temperature regulator, and a control circuit, has been developed in this study. Two types of water injection pin design were also proposed to mold the parts. The second part of this report tests the developed system for its moldability of various

The resins used in this study included general- purpose polystyrene (GPPS), high impact polystyrene (HIPS), polypropylene (PP), polyethylene (PE) , and acrylonitrile-butadiene-styrene (ABS). Table 1 lists the materials used and their melt flow indexes. The melt flow indexes of these materials were measured on a

POLYMER ENGINEERING AND SCIENCE, NOVEMBER 2003, Vol. 43, No. 11 1807

Shih-Jung Liu and Yen-Show C h e n

Table 1. Various Thermoplastic Materials Used in the Experiments.

Material Melt flow index Producer ~

1. General purpose polystyrene (GPPS) 2. High impact polystyrene (HIPS) 3. Polypropylene (PP) 4. Polyethylene (PE) 5. Acrylonitrile-butadiene-styrene (ABS)

7.0 2.3 3.0

22.0 1.8

Formosa Chemical 5250 Formosa Chemical 9450 Taiwan Polypropylene PC-366-3 US1 Far East NA-208 Formosa Chemical AG15EO

Kayeness melt flow indexer, according to the ASTM D1238 standard.

Water-Assisted Injection Unit

A lab scale water injection unit, which included a water pump, a water tank equipped with a temperature regulator, and a control circuit, was developed in our lab as shown schematically in FQ. 2. During experiments, the control circuit of the water injection unit received a signal from the molding machine and controlled the time and pressure of the injected water

Water Injection Pin Two types of water pin design were developed in our

lab: the ring type pin and the orifice type. Figure 3 shows the dimensions of the water injection pins used in this study. The ring type pin has four shallow openings (0.1 x 2.0 mm in cross section) between the pin body and the cap aronnd the water injection (Fig. 3a). The orifice type water injection pin has a small orifice at the side of the pin body. Three different orifice sizes (0.5, 1.0 and 1.5 mm in diameter) were used (Fig. 3b).

(Fig. 2). Before injection into the mold cavity, the water was stored in a tank with a temperature regula- Injection Molding Machine and Mold - tor for 30 minutes to sustain an isothermal water temperature (1 2).

Water-assisted injection molding experiments were conducted on an 80-ton conventional injection molding

Liquid Tank

Injection Molding Machine 3 I Signal Feedback

Valve

Rg. 2. Schematically, the setup for water-assisted injection molding.

k!! T- t I

I (x, cu

t

iii M 1 04

(b) Rg. 3. Dimensions of (a) ring type, and (b) on@e type water injection pins (unit: mm).

1 808 POLYMER ENGINEERING AND SCIENCE, NOVEMBER 2003, Vol. 43, No. 11


270 - 1

Fig. 4. Layout and dimensions of mold cauity (unit: mml.

machine, with a highest injection rate of 109 cm3/sec. A spiral cavity with a square cross section of 15 mm X 15 mm was used for all experiments. Figure 4 shows the layout and dimensions of the mold. The temperature of the mold was regulated by a water-circulating mold temperature control unit. After molding, the length of water penetration in molded parts was measured. The variation of the hollowed core ratio of molded parts was also measured and calculated according to the following (10):

R - S R

Hollowed core ratio = ~

Where R is the average radius of the water channel, and S is the skin thickness of molded parts.

Various processing variables were studied in terms of their influence on the length of water penetration in molded parts: melt temperature, mold temperature, melt filling speed and pressure, water temperature and pressure, water injection delay time and hold time, and short-shot size of the polymer melt. Table 2 lists these processing variables as well as the values used in the experiments.

RESULTS AND DISCUSSION Water-assisted injection molding experiments were

conducted with an 80-ton reciprocating injection

molding machine equipped with a lab-developed water injection unit. Five specimens were performed for each test trial.

Moldability of Various Materials To test the water-assisted injection molding system

developed in this study, various materials were used to mold the parts, including GPPS, PP, HIPS, PE and ABS. The molding conditions used for all materials were a melt temperature of 235"C, a mold temperature of 65"C, a melt injection speed of 76.3 cm3/sec, a melt filling pressure of 115 Mpa, a water temperature of 60T, a water pressure of 7.5 Mpa, a water injection delay time of 1 second, a water hold time of 30 seconds, and a short-shot size of 72%. The experimental results suggested that all materials could be successfully molded by the water-assisted injection molding system developed in this study. Figure 5 shows the cross sections and the void shapes of the molded spiral parts along the water channel (from top to bottom in the figure). Two distinct void sizes can be ob- served: large and small. The large voids are formed by flow-induced water peinetration, so-called primary penetration, which occurs during water-assisted filling. Near the far end of the tube (bottom of Fig. 5), all voids are comparatively small and round in shape. They are formed during postfilling, namely water-assisted packing and cooling. During postfilling, the solidifyrng polymer undergoes volum&-ic shrinkage, allowing water to penetrate further into the parts for purposes of compensation. The small and round voids are formed by shrinkage-induced water penetration, or secondary water penetration.

The hollowed core ratios of molded parts were also measured and calculated according to Eq 1 . Figure 6 shows the hollowed core ratios as a function of water penetration length in molded parts. All materials exhibited quite uniform hollowed core ratios along the water channel, except the polyethylene material, which has a smaller hollowed core ratio near the beginning of the water channel. Molded semicrystalline parts were found to have larger water-penetration lengths than molded amorphous parts.

A Comparison of Water and Gas To compare the gas-assisted injection molding and

water-assisted injection molding methods, an amorphous polystyrene and a semicrystalline polypropylene

Table 2. The Processing Variables as Well as the Values Used in the Experiments.

A B C D E F G H I

Melt Mold Filling Fill Water Water Delay Hold Short Processing temp temp speed pressure temp pressure time time shot parameter ("C) ("C) ("/.I (MP4 ("C) (MPd (set) (see) size (%)

1 225 40 50 60 (70) 25 50 0 10 70

3 235 50 70 a5 (loo) 45 70 2 20 80 4 240 55 80 95 (115) 55 80 3 25 85

2 230 45 60 70 (85) 35 60 1 15 75

'The values in parentheses are the settings used for polypropylene.


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Shih-Jung Liu and Yen-Show Chen

PP

Fig. 5. Void shapes of water-assisted injection molded thermoplastic materials.

POLYMER ENGINEERING AND SCIENCE, NOVEMBER 2003, Vol. 43, No. 11


I I I

-a- ABS -*- PS -A- HIPS 70 - - --~-ppP -O-pE

h 60- - s W

material were used for molding. Gas-assisted injection molding experiments were carried out on the same injection molding machine, but equipped with a high- pressure nitrogen gas injection unit (Gas Injection model PPC- 1000, U.K.). Details of the gas injection setup can be found in the references (6-8). After molding, the gas as well as the water-penetration lengths of molded parts and their hollowed core ratios was measured. The experimental result suggested that gas-assisted injection molded thermoplastic parts exhibited a more uniform hollowed core ratio along the gas channel

than those molded by water-assisted injection molding, as shown in FLg. 7. This might be because water has a higher cooling ciapacity and cools the parts faster than the gas. During the molding process, as long as the water enters into the cavity, it begins to cool the polymer melt and increases the viscosity of the melt significantly. It then becomes more difficult for the water to penetrate into the core of the parts than for the gas. Water-assisted injection molded parts may thus exhibit nonuniform hollowed core ratios along the water channel.

80 , I I 1

h

W s 0 .- .u E

-.-- PS (WAIM) -0- PS (GAlM) PP(WA1M) 0 PP(GA1M)

60

0 10 20 30 40 50

Length of water penetration (cm) Fig. 7. Hollowed core ratios as afunction of penetration length in gas and water-asskted injection molded parts.

POLYMER ENGlNEERlNG AND SCIENCE, NOVEMBER 2003, Vol. 43, No. 11 1811


Parameter

Melt filling Water pressure pressure

(MP4 WPa)

100 5 100 7 100 9 140 5 140 7 140 9 180 5 180 7 180 9

250 I I I I I

Type of water pin

Ring Orifice Orifice Orifice

X 0 0 0 X 0 0 0 0 0 0 0 X 0 0 0 X 0 0 0 0 0 0 0 X X X 0 X X 0 0 X 0 0 0

(0.1 mm x 2.0 mm) (+ 0.5 mm) (9 1 .O mm) (9 1.5 mm)

I --.- IM -0- GAIM -A- WAIM

0 I I I I I

0 10 20 30 40 50 60

Elapsed time (sec)

m. 8. Temperature variations of injection molding (IM), gas-assisted injection molding [GAlM), and water-assisted injection molding WAIM) .

The temperatures inside the mold cavity of polypro- Design of Water Injection Pin pylene parts molded by both gas-assisted injection molding and water-assisted injection molding were measured and compared. Unsheathed 0.5-mm-diameter thermocouples were used for these measurements. The beads of the thermocouple wires were placed 5 ~ ~ l l ~ l deep into the cavity from the mold/part interface. These thermocouples were connected to a personal computer through a €7,9232 interface. The temperature profiles were recorded by LabVIEW (National Instru- ment, U.S.A.) data acquisition software. The measured results in Fig. 8 suggested that the cooling time for water-assisted injection molding is much shorter than that of conventional injection molding and gas-assisted injection molding. For the part geometry in this study, water-assisted injection molding needs only about 1 / 3 the cycle time for conventional injection molding, and needs only 1 /2 the cycle time for gas-assisted injection molding.

Two types of water pin design were proposed and used in this study: the ring type pin and the orifice type. The structure of the water pins has been ex- plained in Fig. 3. All different water injection pins were tested against polypropylene materials for their moldability. The results in Table 3 show that the ring type water pin (Fig. 3a) had a higher probability of molding parts with an incomplete filling. On the other hand, Table 4 showed that the orifice type water pin (Fig. 3b) had a higher chance to get stuck by the polymer melt.

During the water-assisted injection molding process, the mold cavity is first partially filled with the polymer melt followed by the injection of water into the core of the polymer melt. As the polymer melt enters the cavity, it may flow back into the small openings of water injection pins and stick to the openings. To prevent the polymer melt to flow back into the water injection

Table 3. Moldability of Water Injection Pin Designs Against Polypropylene Material.

1812 POLYMER ENGINEERING AND SCIENCE, NOVEMBER 2003, Vol. 43, No. 11


Melt filling Water pressure pressure

(MP4 (MP4

100 5 100 7 I00 9 140 5 140 7 140 9 180 5 180 7 180 9

Table 4. Moldability of Water Injection Pin Designs Against PolvProPvlene Material.

Ring Orifice Orifice Orifice

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 * 0 0 0 itt 0 0 0 * 0 0 * 3% 0 0 % * 0 0 * itt

(0.1 mm x 2.0 mm) ($I 0.5 mm) (4) 1 .O mm) ($I 1.5 mm)

Parameter I Type of water pin

* 0 = pin unstuck; 40 = pin stuck

pins, the clearance of the pin opening needs to be small. The advantage of the ring type water pin is that it has quite a smaller opening (0.1 mm) and thus has less chance to be stuck by the polymer melt. Never- theless, because of a smaller clearance, the volumetric flow rate of water of a ring type pin is limited. This might then lead to an incomplete filling of the parts during the molding process. On the other hand, the orifice type pins have larger holes (from 0.5 to 1.5 mm in diameter) as well as higher volumetric flow rates of injection water. Therefore they can be used to mold larger parts. But there is also a higher probability of polymer melt flowing back as well as the opening being stuck by the melt.

I t should be noted that whether or not the pin becomes stuck depends strongly on the viscosity of the polymer melt. The viscosity of a polymeric material depends on several factors such as the polymer’s mo- lecular weight and structure, melt temperature and mold temperature. The optimum sizes of the openings for water injection pins may vary depending on the materials and the molding conditions. Nevertheless, the experimental results here showed that both water pin designs proposed in this study could be successfully used to mold thermoplastic parts, as long as the size of the pin was properly selected.

Effects of Processing Parameters

Various processing variables were studied in terms of their influence on the lengths of water penetration in molded parts. Table 2 lists these processing var- ables as well as the values used in the experiments. To mold the parts, one a r b i t r q processing condition was chosen as a reference (Table 2). After molding, the length of water penetration into the parts was measured. By changmg one of the parameters in each test, we were able to better understand the effect of every parameter on the water-penetration lengths in water- assisted injection molded parts. The measured results were as shown in Figs. 9-1 5. In addition to the parm- eters shown in these figures, the lengths of water penetration in molded products decreased somewhat with melt filling speed and filling pressure (121.

There are at least three factors that might affect the length of water penetration in molded parts (10, 12). The first is the shrinkage of the polymeric materials. In water-assisted injection molding, the cavity is first partially filled by polymer melt followed by the injection of high-pressure water for packing the materials. During post-filling, the solidifymg polymer undergoes volumetric shrinkage, allowing water to penetrate into the parts for the purpose of compensation. The more the polymer shrinks, the longer the water will penetrate, The second factor that affects the water-penetration length is the viscosity of the polymer melt. During cooling, the viscosity of polymer melts increases significantly as the material cools down. It then becomes more difficult for the water to push and penetrate into the core of the materials. Since the water has a larger heat capacity and can take away more heat than gas can, this factor becomes more significant when compared to that of gas-assisted injection molding. Another factor that affects the water- penetration length is ithe void shape of the parts, The larger the void area at the beginning of the water channel, based on a fured volumetric shrinkage as- sumption of the same material, the water will penetrate less, as shown schematically in Rg. 16.

The experimental results in this study suggest that water penetrates further in semicrystalline parts (PE and PP) than in amorphous parts (PS and ABS). This is because the semicrystalline materials have a higher level of crystallinity and have larger volumetric shrinkage. Therefore they mold parts with a longer water- penetration length.

For the processing parameters used in the experiments, increasing the melt temperature increases the water penetration in polystyrene materials, but decreases the water penetration in polypropylene parts. This might be because increasing the melt temperature decreases viscosity of the polymer melt. The material at the core of the parts takes a longer time to cool. It helps the water to overcome the high viscosity of the polymer melt ([especially for polystyrene) and penetrate into the core of the material. The length of water penetration may thus increase as well. However,

POLYMER ENGINEERING AND SCIENCE, NOVEMBER 2003, Vol. 43, NO. 11 1813


-.- pp -*- ps

45 -

40 -

35 -

45 -

40 -

. .. -...-..-___..;I ....... __ .---a - _, -

- --:!!! __ .

I -t---__--_,,I 7c -

i-- 1 :

T

i ...I ..*.. 3- _._̂ _ .. .. ....._.__.._

... .. i

35 - -

30 I I I I

30 225 230 235 240

Melt ternperamre (“C)

Q, 9. Effect of melt temperatwe on the water penetration length of water-assisted injection molded parts.

c 0 .I ... B 3 35- u

a

- - i

-

[ --I-PP--...-PS

Q. 10. Effect of mold temperature on the water penetration length of water-assisted injection molded parts.

50 I I I I I I ’

4 5 ~ 40 i



i

45 -

40 -

35 -

50 I I I 'I

45 -

40 -

35 -

40 -

35 -

- T

30 I I 1 I T I

30 40 50 60 70 ao Water pressure (MPa)

Rg. 12. Effect of waterpressure on the waterpenetration length of water-assisted injection molded parts.

50

I

I --.-PP-.---PS I

301 . I . . , I . I , I . 4 -0.5 0.0 0.5 1 .o 1.5 2.0 2.5 3.0 3.5

Water injection delay time (sec)

Fig. 3 3. Effect of water injection delay time on the water penetration length of water-assisted injection molded parts.

I -x f3 I I W

30 I I I I I 10 15 20 25

Water hold time (sec)

Fig. 14. Effect of water hold time on the water penetration length of water-assisted injection molded parts.



Short shot size (mm) Q. 15. Effect of short shot size on the water penetration length of water-assisted injection molded parts.

a lower viscosity of the materials also helps the water to pack the water channel and increase its void area, instead of penetrating further into the parts. The hollow core ratio at the beginning of the water channel increases and the length of water penetration for polypropylene may thus decrease, as shown in Fig. 9.

Increasing the mold temperature decreases the cooling rate as well as the viscosity of the materials. The water then packs the channel and increases its void area near the beginning of the water channel, instead of penetrating further into the parts. Molded parts thus have a shorter water-penetration length (Fig. 10).

Filling the mold cavity as rapidly as possible should keep the polymer melt at high temperatures for a longer time. It helps the water to push the materials against the mold wall instead of penetrating further into the core of the parts. Molded parts thus exhibited shorter water-penetration lengths. Increasing the melt filling pressure increases the resistance for water penetration. It becomes more difficult for the water to penetrate into the parts. The lengths of water penetration decrease accordingly.

Increasing the water temperature decreases the cooling rate of the materials and keeps the polymer melt hot for a longer time. The viscosity of the materials decreases accordingly. This will help the water penetrate further into the core of the parts, especially for amorphous polystyrene materials. On the other hand, a lower viscosity of the polypropylene materials helps the water push the channel and increases its void area near the beginning of the water channel, instead of penetrating further into the parts, as shown schematically in Fig. 16. The water penetration length in molded polypropylene parts thus decreases (Rg. 1 I ) .

Increasing the water pressure helps the water penetrate into the materials (polystyrene material in Fig. 12). However, if the pressure is too high, the water may push against the mold wall and decrease the penetration length in molded parts (polypropylene).

The experimental result suggests that increasing the water-injection delay time increases the length of water penetration. This is because increasing the water-injection time increases the cooling time. The temperature distribution in molded parts is lower and the

I I

MOLTEN HOLLOW CORE LAYER

\

HOLLOW CORE

Fg. 16. Schematic showing that for a part having a larger void area at the beginning of the water channel the water penetrates less.


Water-Assisted injection Molding

solidifylng layer of the polymer melts increases accordingly. The molded part has a smaller void area (hollowed core ratio) at the beginning of the water channel. The water can thus penetrate further (FQ. 13).

Finally, increasing the water-hold time increases the water penetration length. In water-assisted injection molding, the water pressure acts as a holding pressure just as in conventional injection molding. Increasing the water-hold time keeps the water at a high pressure for a longer time. This helps the water penetrate further into the core of the materials. The length of water penetration thus increases (m. 14) (12).

CONCLUSIONS

The following conclusions can be drawn based on

1. A water-assisted injection-molding system, which includes a water pump, a water injection pin, a water tank equipped with a temperature con- troller, and a control circuit, has been successfully developed to mold various thermoplastic materials.

2. Two types of water pin design have been proposed and proved to be successful in molding thermoplastic parts. The ring type pin has the advantage of being less stuck by the hot polymer melt during filling. The orifice type pin has a higher volumetric flow rate and can be used to mold larger parts.

3. Water-assisted injection molded parts exhibit less uniform void sizes along the water channel.

4. The cycle time for water-assisted injection molded parts is shorter than that of conventional injection molded parts and gas-assisted injection molded

the current study.

parts.

This study has examined the effects of various processing parameters on t h e moldability of water-assisted injection molded parts: this being done, steps can thus be taken to optimize the process. This optimization pro- vides significant advantages in improving part quality. Future studies will work on the morphology and structural development in water-assisted injection molded parts due to a faster cooling of the water.

ACKNOWLEDGMENT The National Scieince Council of Taiwan, R.O.C.,

under the grant NSC91-2216-E-182-006, has sup- ported this work financially.

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Water-assisted injection molding of thermoplastic materials: Effects of processing parameters