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Vibration Welding of Thermoplastics. Part IV: Strengths of Poly(Buty1ene Terephthalate), Polyetherimide, and
Modified Polyphenylene Oxide Butt Welds
V. K. STOKES
Corporate Research and Development General Electric Company
Schenectady, New York 12301
In vibration welding of thermoplastics, frictional work done by vibrating two parts under pressure, along their common interface, is used to generate heat to effect a weld. The main process parameters are the weld frequency, the amplitude of the vibratory motion, the weld pressure, and the weld time or weld penetration. The effects of these parameters on weld quality were systematically studied by first butt welding thermoplastic specimens under con- trolled conditions, over a wide range of process parame- ters, and by then determining the strengths and ductilities of these welds by tensile tests. The three thermoplastics investigated are poly(buty1ene terephthalate), polyether- imide, and modified polyphenylene oxide. Changes in the weld pressure are shown to have opposite effects on the strengths of polyetherimide and modified polyphenylene oxide welds: Also, the weld frequency is shown to have a significant effect on the weldability of polyetherimide. The weldability data for these three thermoplastics are com- pared with data for polycarbonate. Under the right condi- tions, the strengths of butt welds in these materials are shown to equal the strength of the virgin polymer.
INTRODUCTION n vibration welding of thermoplastics, fric- I tional work done by vibrating two parts under
pressure, along their common interface, is used to generate heat to effect a weld. The phenom- enology and analysis of the weld process have been described in a series of papers (1-3). The effect of various weld process parameters, such as the weld pressure, the weld amplitude, the weld frequency, and the weld penetration, on the strength and ductility of vibration-welded polycarbonate (LEXAN@ 9030) was considered in a companion paper (4).
This paper is concerned with the strengths of vibration-welded butt joints in three polymers: the semicrystalline polymer poly(buty1ene ter- ephthalate) (VALOX@ 3 lo), the high-tempera- ture amorphous polymer polyetherimide (UL- TEM@ lOOO), and a modified polyphenylene ox- ide (NORYL@ 190). These materials will be re- ferred to as PBT [poly(butylene terephthalate)], PEI (polyetherimide), and M-PPO (modified polyphenylene oxide). The work reported in this
paper is part of a weldability study (5) that included polycarbonate (PC).
All the weld data in this paper were obtained from welds performed on a research vibration welding machine (1, 3) on which accurately aligned butt welds can be made. The weld pa- rameters can be controlled independently on this machine. The interfacial vibratory dis- placement during a weld is z = a sin 27r nt, where a is the weld amplitude and n the weld frequency. The geometry of the test specimens and the procedures for determining strength are described in detail in a companion paper (4, 5).
STRENGTH OF PBT WELDS All the data for PBT were obtained by tests on
6.35-mm (0.25-inch) thick specimens cut from 6.35-mm thick by 153 mm by 203 mm (6 inch by 8 inch) (edge-gated) injection-molded plaques. Figure 1 shows the weld strengths of PBT as a function of the weld penetration, at a frequency of 120 Hz and a weld pressure of 0.9 MPa (0.13 ksi) for two amplitudes, a = 0.95 mm
998 POLYMER ENGINEERING AND SCIENCE, MID-AUGUST 1988, Vol. 28, No. 15
Vibration Welding of Thermoplastics
0 -
v -
h
a I
F :' 0 5 50- F - v)
(0.0375 inch) and a = 1.59 mm (0.0625 inch), at two different strain rates of i = 0.01 s-' and i = 1 s-'. The strengths of the virgin material at these two strain rates are indicated by points on the zero-penetration line. At the lower strain rate, the strengths (maximum stress] of four different virgin specimens were found to be 65.2 (9.461, 60.1 (8.72), 61.8 (8.971, and 58.7 MPa (8.51 ksi). The value shown by the open circle on the zero-penetration line corresponds to the highest value 65.2 MPa (9.46 ksi). In these four tests, the percent strains measured by the extensometer were 6.25, > 18.6, 19.25, and 7.5, respectively. The strength for the higher strain rate based on one test, shown by the solid circle, was 76.6 MPa (1 1.1 1 ksi). The corresponding strain was 7.5%.
In Fig. 1, unfilled and filled circles represent weld-strength data for strain rates of lo-' s-' and 1 s-', respectively, for a weld amplitude of 0.95 mm (0.0375 inch). Unfilled and filled tri- angles represent the strengths at these two strain rates for a weld amplitude of u = 1.59 mm (0.0625 inch). As in the case of PC (4). the weld strength is not affected by the amplitude of the vibratory motion: the weld strengths are quite high as long as the penetration is greater than about 0.25 mm (0.01 inch); the weld strength is not affected by strain rate, in the sense that the weld strengths at the higher strain rate are close to the strength of the virgin material: and weld strength decreases quite dra- matically for weld penetrations smaller than about 0.25 mm (0.01 inch).
The effect of weld pressure on PBT welds at a frequency of 120 Hz and a weld amplitude of a = 1.59 mm (0.0625 inch) is shown by the strength-penetration plot in Fig. 2, in which the parameter, po, varies from 0.9 MPa (0.13 ksi) to 13.8 MPa (2.0 ksi). For pressures in the range of 0.9 MPa to 3.45 MPa, the weld strength is not very sensitive to the weld pressure as long as the penetration is greater than 0.25 mm
- 8=0.01 s-1 c
- F A 0 - 0
- F X A v)
- 01 Y
-10 - X z - w A
0 5
WELD PENETRATION (1 O-' in)
0 10 20 30 40 50
n= 120 Hz po=0.9 MPa (0.13 ksi)
Y 10 -
0 A F
a I V
Z
? 5 v )
3 0
0 0.5 1 .o 1.5
WELD PENETRATION (mm)
Fig. 1. Strength of 120-Hz PBT welds us. the penetration with weld amplitude as parameter.
(0.01 inch). At a pressure of po = 6.9 MPa (1.0 ksi), the weld strength remains low even at a penetration of 0.5 mm (0.02 inch]. Finally, at still higher penetrations, the weld strength does approach the strength at lower pressures. This behavior is similar to the behavior of PC at po = 13.8 MPa (see Fig. 4 in Ref. 4). However, when the weld pressure is raised to 13.8 MPa (2.0 ksi), the weld strength appears to increase again, as indicated by the two data points represented by squares.
The effect of pressure also can be inferred from the strength versus pressure data shown in Fig. 3, in which the final penetration vF is the parameter. The strength is low or erratic for low penetrations on the order of 0.152 mm (0.006 inch]. Except for weld pressures around 7 MPa, high weld strengths are attained at all pressures as long as the weld penetration is greater than 0.25 mm (0.01 inch). The corre- sponding data for PC are shown in Fig. 5 in Ref. 4.
Strength and ductility data for 120-Hz welds
WELD PRESSURE (ksi)
0 c 5 a
10 - I
a=1.59 mm (0.0625 in)
Y V
F 4
0 s Y li
5 1 E z W w
0 0.152 (0.006) X 0.305 (0.012) A 0.533 (0.021) 0 1.270 (0.050)
0 1 I 1 I I I I I ; I , I I I 0 0 5 10 15
WELD PRESSURE (MPa)
Fig. 3. Strength of 120-Hz PBT welds us. the pressure with the weld penetration as parameter.
POLYMER ENGINEERING AND SCIENCE, MID-AUGUST 7988, Yo/. 28, No. 75 999
V. K. Stokes
of PBT at two different amplitudes, a = 0.95 mm and 1.59 mm, and a strain rate of i = 0.01 s-’ are given in Table 1. uW is the actual weld strength based on the original cross-sectional area. The relative weld strength uW/uu compares the strength of the weld, uw, to that of the base resin, uu. At i = 0.01 s-’, four different virgin specimens of PBT gave base strengths of 65.2, 60.1, 61.8, and 58.7 MPa, indicating a strength variation of about 10% in the base material. To obtain conservative relative strength estimates, uU = 65.2 MPa has been used for the ultimate strength of PBT. Thus, although the highest relative strength indicated in Table 1 is uW/uu = 0.963, corresponding to uW = 62.8 MPa, any value of uW above 58.7 MPa has the potential of having close to 100% strength. Strains at fail- ure were as large as 7.22%. Just as in the discussion on the ductility of PC (4), it must be noted that the actual strains at failure are, in all probability, larger than the values indicated in the table. With these interpretations, it is clear that PBT welds well over a wide range of weld parameters.
Table 2 lists weldability data for 120 Hz-PBT welds for a pressure of po = 0.9 MPa at a higher
strain rate of i = 1 s-’. For a = 1.59 mm, the relative strengths and ductilities are about the same as for i = 0.01 s-’. However, the relative strengths are significantly lower for a = 0.95 mm.
The effect of weld pressure on weld strength at a frequency of n = 25 Hz is shown in the strength-pressure plot in Fig. 4 , in which the weld amplitude is a parameter. These data are from welds that were made at a nominal weld penetration of 0.58 mm (0.023 inch). The weld strength appears to increase with the weld am- plitude. Corresponding strength and ductility data are given in Table 3.
Weld-strength data for 250-Hz PBT welds, as a function of the weld penetration with the weld pressure as parameter, are shown in Fig. 5. High strengths can be achieved for penetrations greater than 0.25 mm (0.01 inch). No systematic effect of the weld pressure can be inferred from this data. The actual data in Table 4 show that very high strengths, with nominal strains-to- failure of about 2.5%, can be achieved.
Weld-strength data for PBT at a frequency of 400 Hz, at a weld amplitude of a = 0.318 mm (0.0125 inch), and for three weld pressures are
Table 1. Strength and Ductility Data for 120-Hz Welds of PET at a Strain Rate of i = 0.01 s-’.
n a Hz mm (lo-” in)
120 0.95 (37.5) 120 0.95 (37.5) 120 0.95 (37.5) 120 1.59 (62.5) 120 1.59 (62.5) 120 1.59 (62.5) 120 1.59 (62.5) 120 1.59 (62.5) 120 1.59 (62.5) 120 1.59 (62.5) 120 1.59 (62.5) 120 1.59 (62.5) 120 1.59 (62.5) 120 1.59 (62.5) 120 1.59 (62.5) 120 1.59 (62.5) 120 1.59 (62.5) 120 1.59 (62.5) 120 1.59 (62.5) 120 1.59 (62.5) 120 1.59 (62.5) 120 1.59 (62.5) 120 1.59 (62.5) 120 1.59 (62.5)
Po MPa (ksi)
o w V F to o w - mm in) S MPa (ksi) ‘p. 100 €0
0.9 (0.1 3)
0.9 (0.1 3) 0.9 (0.1 3)
0.9 (0.13) 0.9 (0.13)
0.9 (0.13) 0.9 (0.13) 0.9 (0.1 3) 0.9 (0.13) 0.9 (0.1 3) 3.45 (0.5) 3.45 (0.5) 3.45 (0.5) 3.45 (0.5) 3.45 (0.5) 3.45 (0.5) 6.9 (1 .O) 6.9 (1 .O) 6.9 (1 .O) 6.9 (1 .O) 6.9 (1 .O) 6.9 (1 .O) 13.8 (2.0) 13.8 (2.0)
0.1 8 (7) 0.33 (1 3) 0.61 (24) 0.1 5 (6) 0.20 (8) 0.3 (12) 0.35 (14) 0.53 (21) 0.58 (23) 1.32 (52) 0.15 (6) 0.28 (11) 0.33 (1 3) 0.53 (21) 0.58 (23) 1.30 (51) 0.15 (6) 0.28 (1 1) 0.33 (13) 0.53 (21) 0.58 (23) 1.27 (50) 0.33 (13) 0.58 (23)
3.41 4.06 5.33 1.48 1.58 1.71 2.0 2.29 2.1 1 3.38 0.75 0.94 1 .o 1.41 1.28 2.23 0.53 0.82 0.66 1.07 1 .o 2.0 0.6 0.83
45.2 (6.56) 62.4 (9.05) 62.0 (8.99) 57.8 (8.39) 32.5 (4.71) 61 .O (8.84) 61.9 (8.98) 60.0 (8.70) 61.2 (8.87) 60.6 (8.79) 40.9 (5.93) 53.9 (7.82) 57.4 (8.32) 57.2 (8.30) 56.3 (8.17) 50.1 (7.26) 56.5 (8.20) 39.6 (5.74) 61.7 (8.95) 45.3 (6.57) 55.0 (7.97) 57.9 (8.40) 62.8 (9.1 1) 57.8 (8.38)
0.693 0.957 0.950 0.887 0.498 0.934 0.949 0.920 0.938 0.929 0.341 0.827 0.879 0.877 0.864 0.767 0.867 0.607 0.946 0.695 0.842 0.888 0.963 0.886
1.77 3.75 3.75 3.48 1.22 5.48 3.22 7.22
7.05 1.82 2.8 2.65 3.26 2.60 2.51 3.23 1.7 3.31 2.0 2.39 3.93 3.44 2.6
-
Table 2. Strength and Ductility Data for 120-Hz Welds of PET at a Strain Rate of i = 1 s-‘. ~
O W n a Po VF to O W - Hz mm in) MPa (ksi) mm in) S MPa (ksi) 0” 100 co
120 0.95 (37.5) 0.9 (0.13) 0.2 (8) 4.12 0.48 (0.07) 0.007 - 120 0.95 (37.5) 0.9 (0.13) 0.35 (14) 4.98 25.2 (3.65) 0.386 - 120 0.95 (37.5) 0.9 (0.13) 0.61 (24) 4.88 62.3 (9.03) 0.955 - 120 1.59 (62.5) 0.9 (0.13) 0.15 (6) 1.45 13.6 (1.97) 0.177 - 120 1.59 (62.5) 0.9 (0.13) 0.33 (13) 2.07 71.8 (10.41) 0.937 3.8 120 1.59 (62.5) 0.9 (0.13) 0.61 (24) 2.24 72.8 (1 0.56) 0.951 4.6 120 1.59 (62.5) 0.9 (0.13) 1.35 (53) 3.19 72.9 (10.58) 0.952 4.6
1000 POLYMER ENGINEERING AND SCIENCE, MID-AUGUST 1988, VOI. 28, No. 15
Vibration Welding of Thermoplastics
100 - - 0 - a
I v -
shown in Fig. 6. For penetrations greater than 0.25 mm (0.10 inch), the strength is highest at the lowest pressure (0.9 MPa) and decreases systematically with a n increase in the weld pressure. This trend is confirmed by the actual data in Table 5, which show that in addition to high strengths, the corresponding strains-to- failure are high for po = 0.9 MPa. As in earlier discussions on PBT, the actual value of (TJCT,
- c?
F l - - n=iOOkr I I I I I
a=0.318 rnrn (0.0125 in)
I I
6=0.01 s-1 - Y
WELD PRESSURE (ksi)
n=25 Hz 'lF=O.58 rnrn (0.023 in)
c Y
0 d=O.Ol s-' a
10 - I W
A A F 5 0 1 X 8 0
v) X
X
5.08 (0.20) 2.54 (0.10)
I A 1.27 (0.05) 1 0 1 I I I I I I I I I I 0 0 5 10
WELD PRESSURE (MPa) Fig. 4 . Strength of 2 5 - H z P B T welds us. thepressure with weld amplitude as parameter.
WELD PENETRAllON (10-'in)
0 10 20 30 40 50
- 100 n=;50IHz I I I I I I I I -
- a=0.44 rnm (0.0175 in) -
9 Y 0.90 (0.13)
3.45 (0.50) A 6.90 (1.00)
0 0 0.5 1 .o 1.5
WELD PENETRATION (mrn) Fig. 5. Strength of 2 5 0 - H z P B T welds us. the penetration with weld pressure as parameter.
0 0.5 1.0 1.5 WELD PENETRATION (mm)
Fig. 6. Strength of 4 0 0 - H z P B T welds us. the penetration with weld pressure as parameter.
could be much higher because a high value of vu has been used in computing this ratio.
In general, the weldability of semicrystalline PBT is qualitatively similar to that of amor- phous PC (4), although higher strains-to-failure are achieved in PC welds. PBT appears to have a sharper transition to higher weld strength at a penetration threshold of 0.25 mm (0.01 inch) than does PC.
The static strength of PBT welds can be as high as that of the virgin polymer. This can be seen from Fig. 7, in which Fig. 7a shows the geometry of a weld specimen before a tensile test. On the same scale, Fig. 7b shows a (differ- ent) specimen in which necking initiated just above the weld. This weld was made at a fre- quency of 120 Hz, at a weld amplitude of 1.59 mm (0.0625 inch), a weld pressure of 0.9 MPa (0.13 ksi), and a weld penetration of 0.58 mm (0.023 inch). Figure 7c shows another speci- men in which stable necking occurred above the weld, and the specimen failed by rupture in the necked material while the weld remained intact. This was a 250-Hz weld, made at a weld amplitude of 0.44 mm (0.0175 inch), a weld pressure of 0.9 MPa (0.13 ksi), and a weld pen- etration of 0.61 mm (0.024 inch).
To achieve the best weld results for PBT in the frequency range of 120 Hz to 400 Hz, it is recommended (1) that the penetration be main- tained at values above 0.25 mm (0.01 inch), and
Table 3. Strength and Ductility Data for 25-Hz Welds of PBT at a Strain Rate of := 0.01 s-'.
n a Po IJF to U W - HZ mm in) MPa (ksi) mm in) S MPa (ksi) 0" 100 t o
ow
25 1.27 (50) 25 1.27 (50) 25 1.27 (50) 25 2.54 (1 00) 25 2.54 (100) 25 2.54 (100) 25 5.1 (200) 25 5.1 (200) 25 5.1 (200)
0.9 (0.1 3) 3.45 (0.5) 6.9 (1 .O) 0.9 (0.13) 3.45 (0.5) 6.9 (1 .O) 0.9 (0.13) 3.45 (0.5) 6.9 (1 .O)
0.61 (24) 0.61 (24) 0.64 (25) 0.61 (24) 0.58 (23) 0.58 (23) 0.53 (21) 0.51 (20) 0.46 (1 8)
63.3 21.4 11.3 14.4 4.08 2.76 4.20 1.72 0.88
28.6 (4.1 5) 0.439 51.5 (7.47) 0.790 57.5 (8.34) 0.882 21.2 (3.08) 0.326 39.3 (5.70) 0.603 36.2 (5.25) 0.555 22.3 (3.23) 0.342 50.6 (7.34) 0.776 46.6 (6.76) 0.715
1 .o 2.17 2.66 0.78 1.53 1.37 0.81 2.1 3 1.89
POLYMER ENGINEERING AND SCIENCE, MID-AUGUST 1988, Vol. 28, NO. 15 1001
V. K. Stokes
00 - 0 l l l l ~ l l l l ~ l l l l
Table 4. Strength and Ductility Data for a = 0.44 mm (0.0175 inch), 250-Hr Welds of PBT at a Strain Rate of = 0.01 s-'.
- - - 0
ow n a Po VF t o ow - Hz mm in) MPa (ksi) mm (lo-' in) S MPa (ksi) 0" 100 t o
250 0.44 250 0.44 250 0.44 250 0.44 250 0.44 250 0.44 250 0.44 250 0.44 250 0.44
(17.5) (17.5) (1 7.5) (17.5) (17.5) (17.5) (17.5) (17.5) (17.5)
0.9 (0.13) 0.9 (0.1 3) 0.9 (0.13) 3.45 (0.5) 3.45 (0.5) 3.45 (0.5) 6.9 (1 .O) 6.9 (1 .O) 6.9 (1 .O)
0.20 (8) 0.33 (13) 0.61 (24) 0.20 (8) 0.33 (13) 0.58 (23) 0.20 (8) 0.33 (1 3) 0.58 (23)
3.36 22.1 (3.20) 0.338 3.97 49.0 (7.1 0) 0.751 4.60 61.6 (8.94) 0.945 1.62 52.7 (7.65) 0.809 1.80 59.0 (8.55) 0.904 2.42 52.3 (7.58) 0.801 0.91 49.3 (7.15) 0.756 1.38 52.1 (7.55) 0.798 2.31 55.9 (8.1 1) 0.857
0.78 2.0 3.7 (e) 2.4 2.85 2.17 2.05 2.2 2.44
(e) Specimen fractured niter necking at a point approximately 25 mm ( -1 inch) above yield.
(2) that weld pressures lower than 6.9 MPa (1 .O ksi) be used. At 400 Hz the highest strengths are achieved at weld pressures on the order of 0.9 MPa (0.13 ksi). The penetration threshold of 0.25 mm (0.01 inch) is valid for nominally 6.35-mm (0.25-inch) thick weld specimens. This threshold may have different values for other specimen thicknesses.
STRENGTH OF PEI WELDS All the data for PEI were obtained by tests on
6.35-mm (0.25-inch) thick specimens cut from 6.35-mm thick by 153 mm by 203 mm (6 inch by 8 inch) (edge-gated) injection-molded plaques. Figure 8 shows weld strength data as a function of weld penetration for a frequency of R = 120 Hz, a = 1.59 mm (0.0625 inch), and i = 0.01 s-', with the weld pressure as param- eter. Weld strength is quite low for pressure in the range of 0.9 MPa (1 30 psi) to 1.72 MPa (250
(a) (b) (C)
Fig. 7 . StabZe necking in welded PBT specimens.
psi). The weld strength increases steadily with the weld pressure until po = 13.8 MPa (2.0 ksi), but appears to fall off at po = 20.7 MPA (3.0 ksi). However, even at a weld pressure of 13.8 MPa (2.0 ksi) the weld strength, which is on the order of 90 MPa (1 3.0 ksi), is still substantially lower than 119 MPa (17.3 ksi), the strength of the virgin resin.
Table 6 lists actual strength and ductility data for 120-Hz welds of PEI for a weld ampli- tude of a = 1.59 mm (0.0625 inch) and for a strain rate of i = 0.01 s-' . A maximum relative strength of u,/u, = 76.4% and a strain-to-fail- ure of G, = 3.3% were attained at a pressure of 13.8 MPa (2.0 ksi).
Higher weld strengths can be achieved at a frequency of 250 Hz, as shown by the data in Fig. 9. HeKe again, the strength increases stead- ily with the weld pressure until p o = 13.8 MPa (2.0 ksi), but falls off at po = 20.7 MPa (3.0 ksi). For the same weld pressure, the strengths of the 250-Hz welds are higher than those of the 120-Hz welds. Even more interestingly, the weld strength at po = 6.9 MPa (1 .O ksi) is as high as that of the virgin material. Furthermore, the weld strengths fall off for low penetrations. At lower pressures, higher penetrations are re- quired for achieving the optimal weld strength.
1002 POLYMER ENGINEERING AND SCIENCE, MID-AUGUST 1988, Vol. 28, No. 15
Vibration Welding of Thermoplastics
Table 5. Strength and Ductility Data for a = 0.32 mm (0.0125 inch), 400-Hz Welds of PET at a Strain Rate of = 0.01 s-'.
- 0
O W n a Po 1)F t o U W - HZ mm in) MPa (ksi) mm (lob3 in) S MPa (ksi) 0" 100 (0
400 0.32 (12.5) 0.9 (0.13) 0.1 5 (6) 3.31 17.4 (2.53) 0.267 0.94 400 0.32 (1 2.5) 0.9 (0.13) 0.35 (1 4) 4.06 61.1 (8.86) 0.937 3.46 ( f ) 400 0.32 (12.5) 0.9 (0.13) 0.61 (24) 5.73 62.0 (8.99) 0.950 3.7 400 0.32 (12.5) 0.9 (0.1 3) 1.37 (54) 7.26 62.3 (9.03) 0.955 3.7 (s) 400 0.32 (12.5) 3.45 (0.5) 0.20 (8) 1.73 53.2 (7.72) 0.816 2.22
400 0.32 (12.5) 3.45 (0.5) 1.37 (54) 5.28 54.9 (7.96) 0.841 3.35
400 0.32 (12.5) 3.45 (0.5) 0.35 (1 4) 2.24 56.1 (8.14) 0.860 2.48 ( f ) 400 0.32 (12.5) 3.45 (0.5) 0.58 (23) 2.94 56.9 (8.25) 0.872 2.45
400 0.32 (12.5) 6.9 (1 .O) 0.20 (8) 1.22 55.9 (8.1 1) 0.857 2.4 400 0.32 (12.5) 6.9 (1.0) 0.33 (1 3) 1.8 51.8 (7.51) 0.794 2.1 6 400 0.32 (12.5) 6.9 (1 .O) 0.61 (24) 2.1 2 54.5 (7.90) 0.835 2.28 400 0.32 (12.5) 6.9 (1 .O) 1.37 (54) 4.20 45.6 (6.62) 0.700 1.83
(I) Specimen fractured above the weld because of internal defect. (g) Specimen fractured at the weld after neck formation in the weld area.
I 13.8 (2.00) I I 20.7 (3.00) -
l l l l ~ l l l l ~ l l l ~
Table 7 lists actual weldability data for 250- Hz welds of PEI for a weld amplitude of a = 0.44 mm (0.0175 inch) and a strain rate of i = 0.01 s-' . These data confirm that very high strengths and ductilites can be achieved at this frequency. A maximum relative strength of gUt/gu = 99.7% and a strain-to-failure of c0 = 7.1% were achieved at a weld pressure of 6.9 MPa (1 .O ksi).
Finally, the weld strength data for n = 400 Hz is shown in Fig. 10. Here again, the weld strength equals the virgin polymer strength at po = 6.9 MPa (1 .O ksi). Also, the weld strengths. at po= 3.45 MPa are higher for the 400-Hz welds than for the 250-Hz welds. The weldability data in Table 8 show that very high weld strengths and ductilities can be attained at this fre- quency.
Thus, weld frequency appears to have a sig- nificant effect on the weld strength of PEI, and virgin-material strengths can only be achieved at high frequencies. There does not appear to be any straightforward explanation for this ef- fect of frequency.
As in the case of PC (4), there is a weld pene- tration threshold of about 0.25 mm (0.01 inch), above which weld strength is independent of
WELD PENETRATION (10-3in) 0 10 20 30 40 50
150' I ' I I I I I I ' A
A
X
0 X
0
a * X
0
n=250 Hz a=0.44 mm (0.0175 in) &=O.Ol s-1
3.45 (0.50) 6.90 (1.00)
0 0.5 1.00 1.5
WELD PENETRATION (rnrn) Fig. 9. Strength of 250-Hz PEI welds us. the penetration with weld pressure as parameter.
WELD PENETRATION (lO-'in) 0 10 20 30 40 50
r, lPEll Y v,
=loo$ € *
50 D
0 0 0
v
n=400 Hz a=0.318 mm (0.0125 in) 8=0.01 s-'
0.90 (0.13) 3.45 (0.50)
0 0.5 1 .oo 1.5 WELD PENETRATION (mm)
Fig. 10. Strength of 400-Hz PEI welds us. the penetration with weld pressure as parameter.
the penetration, while the strength drops off dramatically for penetrations below this thresh- old.
In summary, high weld strengths in PEI can be achieved under the right conditions. For the weld amplitude range of a < 1.59 mm (0.0625 inch), the highest relative strength of a 120-Hz weld was 91.1 % with a corresponding strain- to-failure of 3.3%. To achieve the best results, it is recommended that welds be made in the frequency range of 250 to 400 Hz; that a weld pressure in the range of 6.9 (1.0) to 13.8 MPa (2.0 ksi) be used; and that the penetration be maintained at values above 0.25 mm (0.01 inch]. This penetration threshold may be differ- ent for specimens having nominal thicknesses other than 6.35 mm (0.25 inch).
STRENGTH OF M-PPO WELDS
All the data for M-PPO were obtained by tests on 5.08-mm (0.2-inch) thick specimens cut from 5.08-mm thick by 153 mm by 203 mm (6 inch by 8 inch) (edge-gated) injection-molded plaques. Weld-strength data for M-PPO at a fre- quency of n = 120 Hz and an amplitude of a = 1.59 mm (0.0625 inch) for different weld pres-
POLYMER ENGINEERING AND SCIENCE, MID-AUGUST 1988, VOl. 28, NO. 15 1003
V. K. Stokes
loo -
3 -
F -
a I - - c3 6 50- F !' v)
Table 6. Strength and Ductility Data for a = 1.59 mm (0.0625 inch), 120-Hz Welds of PEI at a Strain Rate of = 0.01 s-'.
n a Po U W ?F to B W - HZ mm ( in) MPa (ksi) mm in) S MPa (ksi) a" 100 t o
120 1.59 (62.5) 0.9 (0.13) 0.13 (5) 1.46 18.6 (2.70) 0.156 0.53 120. 1.59 (62.5) 0.9 (0.1 3) 0.1 8 (7) 2.1 1 19.8 (2.87) 0.166 0.61 120 1.59 (62.5) 0.9 (0.13) 0.33 (13) 2.19 44.6 (6.47) 0.374 1.30 120 1.59 (62.5) 0.9 (0.13) 0.33 (13) 2.19 43.4 (6.29) 0.363 1.23
1.59 (62.5) 0.9 (0.13) 0.36 (1 4) 2.4 43.1 (6.25) 0.361 1.36 120 120 1.59 (62.5) 0.9 (0.13) 0.53 (21) 2.76 48.2 (6.99) 0.404 120 1.59 (62.5) 0.9 (0.13) 0.58 (23) 2.68 43.2 (6.27) 0.362 1.23 120 1.59 (62.5) 0.9 (0.1 3) 0.58 (23) 2.57 57.0 (8.27) 0.478 1.70 120 1.59 (62.5) 0.9 (0.1 3) 0.97 (38) 0.32 37.0 (5.36) 0.310 1.14
-
120 1.59 (62.5) 1.72 (0.25) 0.53 (21) 1.64 52.4 (7.60) 0.439 1.57 120 1.59 (62.5) 3.45 (0.5) 0.1 8 (7) 0.83 63.8 (9.26) 0.535 2.0 120 1.59 (62.5) 3.45 (0.5) 0.30 (12) 0.99 62.7 (9.10) 0.526 2.0 120 1.59 (62.5) 3.45 (0.5) 0.51 (20) 1.36 74.0 (10.74) 0.620 2.37 120 1.59 (62.5) 3.45 (0.5) 0.56 (22) 1.33 72.2 (1 0.47) 0.605 2.3 120 1.59 (62.5) 6.9 (1 .O) 0.1 8 (7) 0.53 73.8 (10.7) 0.618 2.37 120 1.59 (62.5) 6.9 (1 .O) 0.30 (12) 0.76 79.8 (1 1.57) 0.668 2.77 120 1.59 (62.5) 6.9 (1 .O) 0.51 (20) 0.81 76.9 (1 1.15) 0.644 2.62 120 1.59 (62.5) 6.9 (1 .O) 0.56 (22) 0.91 83.8 (12.16) 0.702 2.80 120 1.59 (62.5) 6.9 (1 .O) 0.79 (31) 1.13 69.5 (10.08) 0.582 2.23 120 1.59 (62.5) 10.3 (1 5) 0.56 (22) 0.71 66.7 (9.68) 0.559 1.67
120 1.59 (62.5) 13.8 (2.0) 0.30 (1 2) 0.34 86.0 (12.48) 0.721 2.97 120 1.59 (62.5) 13.8 (2.0) 0.51 (20) 0.58 82.9 (12.02) 0.694 2.9 120 1.59 (62.5) 13.8 (2.0) 0.56 (22) 0.53 89.1 (12.93) 0.747 3.2 120 1.59 (62.5) 20.7 (3.0) 0.56 (22) 0.38 78.7 (1 1.41) 0.659 2.61
120 1.59 (62.5) 13.8 (2.0) 0.18 (7) 0.23 91.1 (13.22) 0.764 3.3
- c= Y
n=;50:z I I I
a=0.44 rnm (0.0125 in)
I ' - & = O . O l s-1 -
-10 - - F - a z I ?
0 v) 0 0
- 5
sures are given in Fig. 11. Weld strength ap- pears to be independent of the penetration, but is quite sensitive to the weld pressure. Best strengths appear to be achieved at pressures below 0.52 MPa (75 psi). For the data shown in this figure, the highest strength is achieved at po = 0.34 MPa (50 psi). Thus, M-PPO welds extremely well at low weld pressures.
The actual weld strength and ductility data for 120-Hz M-PPO welds, given in Table 9, con- firm that the best welds are obtained at weld pressures below 0.52 MPa (75 psi). During a tensile test, M-PPO undergoes a load drop-off (while the deformations are still homogeneous. The relative strengths, uw/uur in Table 9 are based on a maximum stress of u, = 45.5 MPa (6.60 ksi).
100 -
h - [M-PPOI - n = : 2 0 L z 1 I I I I I ' I - a=1.59 mm (0.0625 in)
B=O.Ol s-1 -
X O
0.52 (0.075) 0.83 io.12oj
A 1.72 (0.250) * I 0 3.58 (0.520)
X
0
0 1 5
* 6.79 (0.985)l 0
0 0.5 1 .o 1.5
WELD PENETRATION (rnrn)
Fig. 1 1 . Strength of 120-Hz M-PPO welds us. the penetra- tion with the weld pressure as parameter.
Weld-strength data for M-PPO at a frequency of 250 Hz and at a n amplitude of a = 0.44 mm (0.0175 inch) for different weld pressures are shown in Fig. 12. Weld strength again appears to be independent of the penetration, but it is quite sensitive to the pressure, with higher strengths at lower pressures. For the same weld pressures and penetrations, the weld strengths of the 250-Hz welds are lower than for the 120- Hz welds. Weld strength and ductility data for this frequency are given in Table 10.
Weldability data for M-PPO at a frequency of n = 400 Hz for a weld amplitude of a = 0.318 mm (0.0125 inch) are shown in Fig. 13. Here again, higher strengths are obtained at low weld pressures. For the same weld pressures and penetrations, the weld strengths of the 400-Hz
1004 POLYMER ENGINEERING AND SCIENCE, MID-AUGUST 1988, Vol. 28, No. 15
Vibration Welding of Thermoplastics
Table 7. Strength and Ductility Data for a = 0.44 mm (0.0175 inch), 250-Hr Welds of PEI at a Strain Rate of i = 0.01 s-'.
n a Hz mm ( in)
P O MPa (ksi)
1)F
mm (lo+ in) t o S
250 250 250 250 250 250 250 250 250 250 250
0.44 (17.5) 0.44 (17.5) 0.44 (17.5) 0.44 (17.5) 0.44 (17.5) 0.44 (1 7.5) 0.44 (17.5) 0.44 (17.5) 0.44 (17.5) 0.44 (17.5) 0.44 (17.5)
0.9 (0.13) 0.9 (0.13) 0.9 (0.13) 3.45 (0.5) 3.45 (0.5) 3.45 (0.5) 6.9 (1 .O) 6.9 (1 .O) 6.9 (1 .O) 13.8 (2.0) 20.7 (3.0)
0.20 (8) 0.33 (13) 0.58 (23) 0.18 (7) 0.33 (1 3) 0.58 (23) 0.1 8 (7) 0.30 (12) 0.56 (22) 0.56 (22) 0.53 (21)
4.90 6.32 7.94 2.15 2.65 3.34 1.27 1.70 2.31 1.33 0.94
nw MPa (ksi)
49.4 (7.17) 69.1 (10.03) 80.1 (1 1.62) 78.5 (1 1.38) 88.7 (12.86) 93.0 (1 3.49) 104.7 (15.18) 119.0 (17.26) 117.8 (17.09) 113.5 (16.47) 96.9 (14.05)
0.41 4 1.47 0.579 2.16 0.671 2.66 0.657 2.65 0.743 3.1 0.779 3.4 0.877 4.4 0.997 7.1 0.987 6.45 0.951 4.85 0.81 2 3.49
WELD PENETRATION (10-3in)
a=0.318 mrn (0.0125 in)
a I
$ I X x A
i' 0.90 (0.13) 3.45 (0.50) 1 A 6.89 (1.00) It
0 0.5 1.0 1.5 WELD PENETRATION (mm)
Fig. 13. Strength of 400-Hz M-PPO welds us. the penetra- tion with the weld pressure as parameter.
01 I I I , ~ l l l l I I I ' I I 0
welds appear to be slightly lower than for the 250-Hz welds. Actual weld strength and ductil- ity data for this frequency are given in Table 10.
Thus, very high strength and ductilities can be attained in M-PPO welds. The penetration threshold for M-PPO appears to be a little lower than for the other polymers considered in this paper. For best results, it is recommended (1) that welds be made at a frequency of 120 Hz, (2) that weld pressures lower than 0.69 MPa ( 100 psi) be used, and (3) that weld penetration be maintained at values above 0.25 mm (0.01
inch). The value of the penetration threshold may depend on the thickness of the specimen. High strength also can be achieved at n = 250 Hz.
CONCLUDING REMARKS It has been shown that under the right con-
ditions, very high weld strengths and ductili- ties-equal to those of the virgin resin-can be achieved in thermoplastic welds made by the vibration welding process. High weld strengths and ductilities have been demonstrated in PC, PBT, PEI, and M-PPO over a frequency range of 25 to 400 Hz, a weld pressure range of 0.34 (0.05) to 20.7 MPa (3.0 ksi), and a range of weld amplitudes from 0.32 (0.0125) to 1.59 mm (0.0625 inch). However, the optimum weld pa- rameters are different for each thermoplastic.
Of the many weld parameters, namely the amplitude, a, the frequency, R , the penetration, V F , and the weld time, to, the parameter having the largest effect on weld strength is the pene- tration. It has been shown that for a large range of amplitudes, frequencies, and pressures, the weld strengths of nominally 6.35-mm (0.25- inch) thick specimens are not very sensitive to the weld process parameters as long as the penetration, vF, is greater than a threshold value of about 0.25 mm (0.01 inch). For pene- trations below this threshold, weld strength ap- pears to fall off and become unpredictable, while predictable and relatively constant strengths are achieved for penetrations above
Table 8. Strength and Ductility Data for a = 0.32 mm (0.0125 inch), 400-Hz Welds of PEI at a Strain Rate of k = 0.01 s-'.
0, n a Po 1)F to o w - H r mm in) MPa (ksi) mm (1 in) S MPa (ksi) 0" 100 €0
400 400 400 400 400 400 400 400 400 400
0.32 (12.5) 0.32 (12.5) 0.32 (12.5) 0.32 (1 2.5) 0.32 (12.5) 0.32 (1 2.5) 0.32 (12.5) 0.32 (12.5) 0.32 (12.5) 0.32 (1 2.5)
0.9 (0.13) 0.9 (0.13) 0.9 (0.13) 3.45 (0.5) 3.45 (0.5) 3.45 (0.5) 6.9 (1 .O) 6.9 (1 .O) 6.9 (1 .O) 20.7 (3.0)
0.20 (8) 0.33 (13) 0.58 (23) 0.20 (8) 0.33 (13) 0.56 (22) 0.1 8 (7) 0.30 (1 2) 0.56 (22) 0.53 (21)
8.12 11.7 13.28 3.54 4.43 5.89 2.02 2.95 3.78 1.3
79.2 (1 1.49) 74.6 (1 0.82) 72.7 (10.54) 103.9 (1 5.07) 110.0 (15.96) 1 1 1.3 (16.15) 116.8 (16.94) 114.7 (16.64) 115.3 (16.73) 91.6 (13.29)
0.664 0.625 0.609 0.871 0.922 0.933 0.979 0.961 0.966 0.768
2.71 2.51 2.43 4.3 4.72 2.25 6.21 5.8 6.57 3.23
POLYMER ENGINEERING AND SCIENCE, MID-AUGUST 1988, Vol. 28, NO. 15 1005
V. K. Stokes
this threshold. In fact, the penetration appears to be the single most important factor affecting weld strength. I t would seem that the weld time, by itself, is not a good indicator of weld quality and, therefore, should not be used as a weld machine control parameter. However, the pen- etration is not the only variable affecting strength. Also, the penetration threshold may be a function of the weld-specimen thickness.
A perusal of the penetration-time curves for PBT, PEI, and M-PPO (Figs. 12 through 18 in Ref. 1 ) shows that a penetration of 0.25 mm (0.01 inch) corresponds to a penetration in the steady-state regime of melting and flow. Just as for PC (4). it would appear that the attainment of near steady-state flow conditions is neces- sary for a good weld.
The next important parameter appears to be the weld pressure po, which affects each poly- mer differently. The weld strength and ductility of PC (4) are relatively insensitive to the pres- sure p o in the range of 0.9 (0.13) to 6.9 MPa (1 .O ksi), but fall off at pressures on the order of 13.8 MPa (2.0 ksi). In a similar manner, the strength of PBT welds are also not very sensitive to po, although the highest strengths of 400-Hz welds appear to correspond to the lowest pres- sures tried, 0.9 MPa (0.13 ksi). In contrast to PC and PBT, the strengths of PEI welds increase systematically with the weld pressure. On the other hand, M-PPO welds appear to attain the highest strengths at the lowest pressures.
The analysis in Ref. 2 shows that for a con- stant-temperature molten film of constant vis- cosity, the steady-state film thickness varies as ho = C ( ~ , / p ~ ) l / ~ . Thus, increasing weld pres- sures should result in thinner steady-state mol- ten film thicknesses. Why PEI and M-PPO are affected differently by an increase in po is not clear from such an analysis. The explanation
for these contrary effects probably has to do with different material orientation effects in these two materials.
In three of the four polymers studied, namely PC, PBT, and M-PPO, the weld strengths are relatively insensitive to the frequency as long as an adequate weld amplitude is used. This insensitivity would appear to be in agreement with the modeling assumption that the ampli- tude, a, and the frequency, R = 27r/w, only affect the welding process through the maximum ve- locity, w, = ao. However, the frequency appears to have a large effect on PEL Weld strengths corresponding to those of the virgin PEI mate- rial can be achieved only at high weld frequen- cies-250 Hz and 400 Hz in these tests-for which relative weld strengths in excess of 99.7% and 97.9%. respectively, have been achieved. In contrast to these high values, the highest relative strength in the 120-Hz welds was 9 1.1 %. There is no straightforward expla- nation for this effect of frequency on the strength of PEI welds.
Weld amplitude appears to have the least ef- fect of all on the resulting weld strength and ductility. The inertia forces during welding are proportional to u w 2 = w,o, where ws is the max- imum velocity of the vibratory motion. Analyses in Ref. 2 have shown that for a constant po, the effect of the weld frequency appears in combi- nation with the amplitude, a, through w,. Now, to a first approximation, a constant ws = aw implies the same weld conditions although a and w may vary. Thus, for “fixed” weld condi- tions, the inertia forces will be proportional to W. By using higher weld pressures, for the “same” weld conditions, the inertia forces ac- tually could be reduced in proportion to u2. From the perspective of designing vibration welding machines, lower weld frequencies are more de-
Table 9. Strength and Ductility Data for a = 1.59 mm (0.0625 inch), 120-Hr Welds of M-PPO at a Strain Rate of = 0.01 s-’. ~~~ ~ ~
n a Hr mm in)
120 1.59 (62.5) 120 1.59 (62.5) 120 1.59 (62.5) 120 1.59 (62.5) 120 1.59 (62.5) 120 1.59 (62.5) 120 1.59 (62.5) 120 1.59 (62.5) 120 1.59 (62.5) 120 1.59 (62.5) 120 1.59 (62.5) 120 1.59 (62.5) 120 1.59 (62.5) 120 1.59 (62.5) 120 1.59 (62.5) 120 1.59 (62.5) 120 1.59 (62.5) 120 1.59 (62.5) 120 1.59 (62.5) 120 1.59 (62.5) 120 1.59 (62.5)
O W Po IIF t o 0 W - MPa (ksi) mm in) S MPa (ksi) 0” 100 €0
~~
0.34 (0.05) 0.52 (0.075) 0.52 (0.075) 0.52 (0.075) 0.9 (0.13) 0.9 (0.13) 0.9 (0.13) 0.9 (0.13) 0.9 (0.13) 1.72 (0.25) 1.72 (0.25) 1.72 (0.25) 1.72 (0.25) 3.45 (0.5) 3.45 (0.5) 3.45 (0.5) 3.45 (0.5) 6.9 (1 .O) 6.9 (1 .O) 6.9 (1 .O) 6.9 (1 .O)
0.61 (24) 0.18 (7) 0.33 (13) 0.58 (23) 0.13 (5) 0.28 (1 1) 0.53 (21) 0.79 (31) 1.30 (51) 0.15 (6) 0.13 (5) 0.28 (1 1) 0.53 (21) 0.13 (5) 0.28 (11) 0.53 (21) 1.30 (51) 0.10 (4) 0.28 (1 1) 0.53 (21) 1.32 (52)
4.28 1.85 2.39 3.1 0 1.28 1.57 1 .88 2.58 3.27 0.82 0.96 1.13 1.51 0.53 0.73 1.06 1.99 0.28 0.55 0.8 1.69
45.6 (6.61) 44.1 (6.40) 45.2 (6.55) 45.0 (6.53) 41.7 (6.05) 42.8 (6.21) 43.2 (6.27) 42.5 (6.17) 43.4 (6.29) 39.2 (5.69) 38.5 (5.59) 38.4 (5.57) 39.7 (5.76) 33.3 (4.83) 32.4 (4.70) 32.1 (4.65) 31 .O (4.50) 24.7 (3.58) 25.4 (3.69) 23.3 (3.38) 22.9 (3.32)
1 .oo 0.969 0.991 0.988 0.91 6 0.940 0.949 0.934 0.952 0.861 0.846 0.843 0.872 0.731 0.71 1 0.704 0.681 0.542 0.558 0.512 0.502
2.4 2.45 2.4 2.4 (h) 1.92 2.12 2.63 2.85 3.09 1.73 1.68 1.63 1.74 1.13 1.34 1.29 1.25 0.88 0.99 0.91 0.88
( h ) Specimen undement homogeneous deionnation and did not fail.
1006 POLYMER ENGINEERING A N D SCIENCE, MID-AUGUST 1988, Vol . 28, NO. 15
Vibration Welding of Thermoplastics
Table 10. Strength and Ductility of 250- and 400-Hz Welds of M-PPO at a Strain Rate of = 0.01 s-'.
u w to C W - n a P O 1)F HZ mm in) MPa (ksi) mm (w3 in) S MPa (ksi) 0" 100 t o
250 0.44 (17.5) 0.9 (0.13) 0.20 (8) 6.04 40.5 (5.88) 0.890 1.88 250 0.44 (17.5) 0.9 (0.1 3) 0.41 (16) 7.56 40.4 (5.86) 0.887 1.84 250 0.44 (17.5) 0.9 (0.13) 0.69 (27) 9.63 39.4 (5.72) 0.866 1.82 250 0.44 (17.5) 3.45 (0.5) 0.18 (7) 2.20 30.1 (4.36) 0.660 1.18 250 0.44 (17.5) 3.45 (0.5) 0.33 (13) 2.85 28.4 (4.12) 0.624 1.14
250 0.44 (17.5) 6.9 (1 .O) 0.20 (8) 1.11 24.9 (3.61) 0.546 0.96
250 0.44 (17.5) 6.9 (1 .O) 0.58 (23) 2.26 23.9 (3.47) 0.525 0.92
400 0.32 (12.5) 0.9 (0.13) 0.38 (15) 6.41 39.0 (5.65) 0.855 1.83
250 0.44 (17.5) 3.45 (0.5) 0.61 (24) 3.71 28.0 (4.06) 0.61 4 1.09
250 0.44 (17.5) 6.9 (1 .O) 0.33 (13) 1.55 23.5 (3.41) 0.51 6 0.90
400 0.32 (12.5) 0.9 (0.13) 0.23 (9) 5.1 8 40.5 (5.87) 0.888 2.00
400 0.32 (12.5) 0.9 (0.13) 0.66 (26) 8.43 39.8 (5.78) 0.875 1.97 400 0.32 (12.5) 3.45 (0.5) 0.20 (8) 1.99 29.0 (4.21) 0.637 1.19 400 0.32 (12.5) 3.45 (0.5) 0.36 (1 4) 2.44 26.9 (3.90) 0.590 1.09 400 0.32 (12.5) 3.45 (0.5) 0.61 (24) 3.14 26.8 (3.89) 0.589 1.10 400 0.32 (12.5) 6.9 (1 .O) 0.58 (23) 1.92 23.0 (3.34) 0.506 0.92
sirable. Since good welds have been demon- strated at low frequencies, machines that op- erate at frequencies below the industry stand-. ard of 120 Hz would seem to be desirable. Low- frequency machines would be especially useful for reducing the inertial loads during the weld- ing of large parts.
The demonstration of high-quality high-fre- quency welds, at weld amplitudes as low as 0.32 mm (0.0125 inch), is important. As pointed out in Rej. 1 , low amplitudes are useful in applica- tions where a part has to be welded into a recessed cavity, wherein the clearances may not be sufficient for large-amplitude vibration. Of course, a high-frequency capability is desir- able also because some polymers, such as PEI, may only weld well at high frequencies.
The vibration-welding process, which is ideally suited to the welding of thermoplastics along relatively flat seams, has been shown to be capable of producing high-strength, high- ductility welds over a wide range of process parameters. Now that an understanding has been developed for the basic process mechan- ics, a better control of the process should be possible. A penetration-based control is recom- mended. This process has many advantages: I t requires very little surface preparation. The vi- bratory shearing motion promotes material mixing. No foreign material is introduced in the weld. In fact, it is a self-cleaning process, in the sense that molten material flows outward from the midthickness of the weld, thereby flushing out any impurities and trapped air. With the right penetration, the weld consists of clean freshly molten material. The cycle time is low; the actual weld time can be as low as a fraction of a second. Furthermore, the heat-affected zone is very small (2). thereby resulting in very localized residual stresses.
A comparison of the data in this paper with other data is not feasible because these data appear to be the only data on vibration welding. The closest process is friction, or spin welding
(6), for which, again, no weld-strength are avail- able.
Once the vibratory motion is stopped, the sub- sequent flow and solidification in the molten film is "similar" to that in the hot-plate welding process. However, in contrast to hot-plate weld- ing, the molten film is hottest at the center. Barber and Atkinson (7) demonstrated that very high weld strengths and ductilities can be achieved in polyethylene, polybutene- 1, and polypropylene by the butt-fusion welding proc- ess. In their tests, the weld pressure was main- tained constant at 0.1 MPa, except for one ma- terial for which it was 0.15 MPa. The weld time, the time for which the two parts were held together under pressure after heating, was 180 s. Thus, in these tests there was no control of penetration. Rather, the weld was allowed to cool under a low pressure of 0.1 MPa. In con- trast to this, in the hot-plate welding study by Bucknall, et al. (8), the weld pressure was not specified. Instead, the penetration was con- trolled by means of stops. Their study looked at the factors affecting the fusion welding of poly(methy1 and methacrylate) and polypropyl- ene. In a study of the fusion welding of poly(viny1idene fluoride), Andrews and Bevis (9) varied the weld pressure in the range of 0.01 to 0.12 MPa and achieved high strengths at appro- priate weld temperatures. Because the inital temperature distribution in the molten film and the solid material in fusion welds is very differ- ent from that in vibration welding and because the materials are different, the results in the papers cited in this paragraph cannot be com- pared with those in this paper.
ACKNOWLEDGMENTS
Partial support provided by General Electric's Plastics Business Group is gratefully acknowl- edged. All the tests were carried out by L. P. Inzinna. The enthusiasm with which he did the work and his contributions are greatly appre-
POLYMER ENGlNEERfNG AND SCIENCE, MID-AUGUST 1988, Vol. 28, NO. 15 1007
V. K . Stokes
ciated. Special thanks are due to Julia A. Kin- loch for her help and patience during the prep- aration of this paper.
5. V. K. Stokes, ”Vibration Welding of Thermoplastics. Part 11: Strength of Butt Welds,” General Electric Cor- porate Research and Development Report No. 87CRD061, Schenectadv, NY (19871.
REFERENCES V. K. Stokes, Polym. Eng. Sci. , 28, 718 (1988). V. K. Stokes, Polym. Eng. Sci., 28, 728 (1988). V. K. Stokes, “Vibration Welding of Thermoplastics. Part I: Phenomenology and Analysis of the Welding Process,” General Electric Corporate Research and De- velopment Report No. 86CRD223. Schenectady, NY (1986). V. K. Stokes, “Vibration Welding of Thermoplastics. Part 111” (this issue).
6. R. J. Crawford and Y. ?am, b. Mater. Sci., 16, 3275 (1981).
7. P. Barber and J. R. Atkinson, J. Mater. Sc i . , 9, 1456 (1 974).
8. C. B. Bucknall, I. C. Drinkwater, and G. R. Smith, Polym. Eng. Sci., 20, 432 (1980).
9. J. R. F. Andrews and M. Bevis, J. Mater. Sc i . . 19, 645 (1984).
Received 2 April 1987
1008 POLYMER ENGINEERING AND SCIENCE, MID-AUGUST 1988, YO/. 28, NO. 15