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General Design Principles for DuPont Engineering Polymers - Module

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Figure 11.54 Tongue-in-groove<br />

0.5<br />

B<br />

0.5<br />

B<br />

A<br />

B<br />

A<br />

0.6 B<br />

0.4 B 0.6 B<br />

Dimension A 0.4 mm <strong>for</strong> B dimensions from 1.5 to 3 mm<br />

and proportionally larger or smaller <strong>for</strong><br />

other wall thicknesses<br />

Dimension B <strong>General</strong> wall thickness<br />

Dimension C Optional recess to ensure good contact<br />

with welding horn<br />

Figure 11.55 Butt joint—variations<br />

Strong joints can be achieved with amorphous resins,<br />

however, it may be difficult to obtain hermetic seals in<br />

complex parts.<br />

The main feature of the butt joints is a “V” shaped<br />

bead or “energy director” on one of the two mating<br />

surfaces which concentrates the energy and limits<br />

initial contact to a very small area <strong>for</strong> rapid heating<br />

and melting. Once the narrow area begins to soften<br />

and melt, impedance drops and further melting occurs<br />

at a faster rate. The plastic in the energy director melts<br />

first and flows across the surfaces to be joined.<br />

Amorphous plastics have a wide, poorly defined<br />

softening temperature range rather than a sharp<br />

melting point. When the plastic flows, sufficient heat<br />

is retained in the melt to produce good fusion over the<br />

entire width of the joint.<br />

Delrin ® , Zytel ® , Minlon ® and Rynite ® PET are<br />

crystalline resins with no softening be<strong>for</strong>e melting and<br />

a sharp melting point and behave different than<br />

amorphous resins. When the energy director melts and<br />

flows across the surfaces, the melt being exposed to<br />

C<br />

B<br />

1.5<br />

B<br />

10°<br />

10°<br />

60°<br />

102<br />

air can crystallize be<strong>for</strong>e sufficient heat is generated to<br />

weld the full width of the joint. It is necessary, there<strong>for</strong>e,<br />

to melt the entire joint surface be<strong>for</strong>e significant<br />

strength can be obtained. (In the case of Zytel ® ,<br />

exposure of the high temperature melt to air can cause<br />

oxidative degradation which results in brittle welds).<br />

This phase of the weld cycle is very long as can be<br />

seen in Figure 11.56 and 11.57, which are graphs<br />

showing typical weld sequences <strong>for</strong> parts of Delrin ®<br />

and Zytel ® using the basic butt joint.<br />

The dotted lines indicate the weld time at which an<br />

objectionable amount of flash occurs. This flash is a<br />

limiting factor in most applications. Beyond this time,<br />

results are extremely variable, especially with Zytel ® .<br />

Figure 11.56 Butt joint—typical per<strong>for</strong>mance, burst<br />

pressure vs. weld time<br />

Burst pressure, MPa<br />

15<br />

12.5<br />

10<br />

7.5<br />

5<br />

2.5<br />

Delrin ® 500 and 900 F<br />

Delrin ® 100<br />

0<br />

0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6<br />

Weld time, sec<br />

Figure 11.57 Butt joint—typical per<strong>for</strong>mance, burst<br />

pressure vs. weld time<br />

Burst pressure, MPa<br />

12.5<br />

10<br />

7.5<br />

5<br />

2.5<br />

Zytel ® 101 (DAM)<br />

0<br />

0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6<br />

Weld time, sec

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