11.4 Moisture Absorption Curves

Figure 11.28 Nylon moisture content as a function of time for three different thicknesses of molded nylon Zytel® parts while immersed in water and at 50% relative humidity.[9] Courtesy o

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Figure 11.28 Nylon moisture content as a function of time

for three different thicknesses of molded nylon (Zytel®) parts

while immersed in water and at 50% relative humidity.[9]

(Courtesy of DuPont.)

Figure 11.29 Water absorption of a variety of materials

when immersed in water for 24 hours.[40] (Courtesy of Hoechst Celanese.) This figure (see Fig 7.6) is reproduced

here for the reader’s convenience

Notes:

Delrin® (Fig 11.26) absorbs relatively little water[17] when compared with some other resins such as nylon (Fig 11.28).[9] Nylon swells with the absorption of water Moisture absorption can cause a nylon part to become larger than the mold from which the part was made Figure 11.27 shows nylon water absorption as high as 9% by weight.[40]

Delrin, on the other hand, absorbs less than 1% water by weight.

11.4 Moisture Absorption Curves

Figure 11.26 Rate of water absorption at various conditions

of humidity for Delrin®.[17] (Courtesy of DuPont.)

Figure 11.27 Change in dimensions with moisture content

for Zytel® 101 in the stress-free (annealed) condition.[35]

(Courtesy of DuPont.) This figure (Fig 7.12) is reproduced

here for the reader’s convenience

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11.5 Pressure Volume Temperature (PVT) Curves

Subject to the conditions discussed in Ch 4, PVT curves can give a close approximation of the volumetric shrinkage of a plastic, molded part These curves give no indication of actual linear shrinkage because they do not account for molecular or fiber orientation, nor do they account in any way for physical restraints such as ribs, walls,

or cores that may restrict shrinkage while the part is still in the mold The point at which the gate freezes and the holding pressure becomes ineffective is difficult to determine with exactitude Nevertheless, a PVT curve gives a great deal of insight into the shrinkage behavior of the plastic.

Most of the curves shown herein are presented in a 2D format This format is generally easier to use The 3D curves presented give a graphic picture of the effects of pressure, volume, and temperature on a given plastic, espe- cially semicrystalline plastics, but are more difficult to use in predicting plastic shrinkage.

The PVT curves shown here are given as a representation of a huge database that is available from various plastic suppliers GE has PVT curves for over 500 different plastic materials This type of data must be requested from the supplier for the particular material you wish to mold.

Tait equation variables are given for each material.

Figure 11.30 A 3D PVT curve for the GE Cycolac® T grade unfilled ABS amorphous plastic (same material as shown in Fig

11.31) (Courtesy of GE Plastics.)

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Figure 11.38 A 3D PVT curve for unfilled Nylon 6/6 (Zytel® 101L) See 2D curves in Fig 11.39 (Courtesy of GE Plastics.)

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Figure 11.40 A 3D PVT curve for unfilled PBT (GE Valox® 327) See 2D curves in Fig 11.41 (Courtesy of GE Plastics.)

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11.6 Shrinkage and Warpage of Molded Disks

The following shrinkage and warpage data was obtained by molding a circular disk with a single edge-gate The change in size from the gate to the opposite side of the disk was measured to determine the flow-direction shrink rate The cross-flow shrinkage was measured perpendicular to the flow-direction shrinkage The warpage is the offset of the edge of the disk opposite the gate over the diameter of the disk when the gate side is held tightly against the measurement surface See Fig 11.44.[6]

Shrinkage Rate (in/in) Flow Cross Flow Warpage

Figure 11.44 Flow, cross flow, and warpage (Cup/Diameter) (A/D in Tables 11.2–11.5).[6] (Courtesy of Hanser-Gardner.) This

Table 11.2 Flow and Cross Flow Shrinkage and A/D Warpage

*A/D is Cup/Diameter, see Fig 11.44

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with Increasing Glass-Fiber Loading[47]

Glass Fiber

Content (%)

Flow Shrinkage (in/in)

Transverse Shrinkage (in/in)

Differential Shrinkage (in/in × 10 -3 )

Warpage (A/D*)

4 inch diameter × 1/16 inch thick disks

Table 11.4 Comparison of the Warpage of Polycarbonate and SAN at Various Filler-Loading Levels[47]

Base Resin Modifier Type Loading Level

(%)

Plaque Warpage (in) a

Disk Warpage (A/D*) b

Measurements must be taken at least forty-eight hours after molding Hygroscopic materials must be kept dry for this period.

Many process variables affect warpage data before annealing If parts are annealed, process variables have little effect on measured warpage.

Table 11.4 shows warpage results when molding polycarbonate and SAN.[4]

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Table 11.5 Shrinkage and Warpage Data for Injection-Molded Neat and Filled Thermoplastic Polymers[4]

Base Polymer Modifier Type Loading Level

4 in diameter × 1/16 thick disk

3ASTM D955 test bar

11.7 Angular Warpage

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Notes:

Figures 11.46 and 11.47 indicate the effects of fiber reinforcement and gussets on bow angles of the walls of the plaque in Fig 11.45.[46] The angles are measured as deviations from the perpendicular The bowing is caused by the delayed cooling of the inside corner of the mold where the wall meets the main part of the plaque The gusset resists the bending stress caused by the slower-cooling inside corner, thus reducing the bow angle.

Notice in Figure 11.47 that the gusset reduces the bow angle to less than half the un-gusseted angle.

Figure 11.46 Bow angle of side wall without gusset vs

thickness for unfilled and filled polycarbonate and

nylon 6/6.[46] (Courtesy of SPE.)

Figure 11.47 Bow angle of front wall with gusset vs

thickness for unfilled and filled polycarbonate andnylon 6/6.[46] (Courtesy of SPE.)

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Notes:

Hoechst Celanese ran tests[40] to determine warpage using 40% glass-filled and 65% mineral/glass-filled PPS using the sample part shown in Fig 11.48.[40] Unfortunately, gate location was not specified Figure 11.49 shows the dimensions and points at which measurements were taken Figures 11.50–53 show the test results.[40]

As one might expect, the warpage of the 65% mineral/filled material was less than that of the 40% fiber-filled material The mineral/glass-filled material has less glass fiber in it than the 40% glass-fiber-filled material The improved warpage characteristics therefore result from two sources First, the aspect ratio of the mineral fill

glass-is less than the glass fiber, therefore the anglass-isotropic shrinkage glass-is less Second, the higher fill ratio results in less overall shrinkage These tests give some indication of the variations one might expect when molding a complicated part from PPS.

Once a mold is built and proven, the molder may expect good consistency from the mold provided he exercises good control over the molding conditions.

Figure 11.49 Measurement points of the Hoechst Celanese test plaque molded of PPS.[40] (Courtesy of Hoechst Celanese.)

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Figure 11.50 Warpage with respect to flatness in the

Hoechst Celanese test plaque molded of PPS.[40] (Courtesy

of Hoechst Celanese.)

Figure 11.51 Warpage with respect to roundness of a

cylinder in the Hoechst Celanese test plaque molded ofPPS.[40] (Courtesy of Hoechst Celanese.)

Figure 11.52 Warpage with respect to roundness of a hole

in the Hoechst Celanese test plaque molded of PPS.[40]

(Courtesy of Hoechst Celanese.)

Figure 11.53 Warpage with respect to bowing angle in the

Hoechst Celanese test plaque molded of PPS.[40] (Courtesy

of Hoechst Celanese.)

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ó × 100

(%)

3 x

ó × 100

(%) for 3 days

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Note:

While these data indicate that increasing thickness causes increased shrinkage, parts of greater thickness may not shrink significantly more than indicated for 6-mm thickness because thicker parts often develop voids instead of more shrinkage Gate/runner size and flow direction also influence the above data.

Usually the shrinkage in the thickness of the part is not of significant interest because the thickness is normally about 1/8 in (3 mm) One study (Fig 11.2) measured the in-mold thickness shrinkage of polypropylene, polyethyl- ene, and polystyrene in an 1/8-in thick tensile test bar The measurements are in microns, each of which is about 40/

Average Rate* per ASTM D955 Material Reinforcement 0.125 in

(3.18 mm)

0.250 in (6.35 mm)

*Rates in in/in (Courtesy ICI-LNP)

11.8 General Shrinkage Characteristics for Various Plastics

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Shrinkage Material

Flow mil/in

Transverse mil/in

mil (3.2mm)

5-8Cycoloy PC/ABS GPM6300 125

mil (3.2mm)

Flow mil/in

Cycoloy C1110HF 125 mil(3.2mm)

5-7Cycoloy C1200 125 mil (3.2mm) 5-7Cycoloy C1200HF 125 mil

(3.2mm)

5-7

Delrin 100 NC010 125 mil(3.2mm)

Delrin 100P NC010 125 mil(3.2mm)

Delrin 1700P NC010 125 mil(3.2mm)

Delrin 500 NC010 125 mil(3.2mm) test bar

Delrin 500 NC010 125 mil(3.2mm) plaque

Delrin 570 NC010 125 mil(3.2mm) 110°C

13Delrin 570 NC010 125 mil

(3.2mm) 124°C

Delrin 500 AF (20%PTFE) 125mil (3.2mm)

Enduran PBT 7065 125 mil(3.2mm)

Table 11.8 Comparative Mold Shrinkage Values for Flow and Cross Flow (Transverse) Directions

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Flow mil/in

Noryl HS1000X 125 mil(3.2mm)

5-7Noryl N190HX 125 mil (3.2mm) 5-7

Rynite 520 NC010 20% GF 125mil (3.2mm)

Table 11.8 (Cont’d.)

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Flow mil/in

Valox 195,307,310,311 (PBT)30-90 mil

Valox 195,307,310,311 (PBT)90-180 mil

Xenoy 1200 (PC/PBT) 125 mil(3.2mm)

16-18

Xenoy 1402B (PC/PBT) 125 mil(3.2mm)

5-8

Flow mil/in

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