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Since material cost and cycle time are directly related to wall thickness, optimum part design balances minimum wall thickness versus sufficient strength. However, wall sections must be thick enough for the material to fill the mold under typical processing conditions. The maximum material flow that may be expected at various wall thicknesses under typical processing conditions is tabulated below. Many Ryton® PPS parts have walls as thin as 0.015 to 0.020 inch (0.38 to 0.51 mm), however no more than 2 inches (5 cm) of flow can be expected when filling such thin sections. To avoid the formation of sinks, internal voids, and internal stress cracking in Ryton® PPS parts, we generally recommend a maximum wall thickness of not more than 0.375 inch (9.52 mm).
|
|
Maximum Flow Length in 275°F (135°C) Mold |
| Wall Thickness |
Ryton®
R-4 Series |
Ryton® R-7, R-10,
and BR111 Series |
Xtel®
XK Series |
| in |
mm |
in |
cm |
|
in |
cm |
|
in |
cm |
| 0.020 |
0.51 |
2 |
5 |
2 |
5 |
4 |
10 |
| 0.040 |
1.02 |
5 |
13 |
5 |
13 |
11 |
28 |
| 0.060 |
1.52 |
13 |
33 |
12 |
30 |
17 |
43 |
| 0.080 |
2.03 |
16 |
41 |
14 |
36 |
24 |
61 |
| 0.100 |
2.54 |
19 |
48 |
16 |
41 |
30 |
76 |
| 0.120 |
3.05 |
22 |
56 |
19 |
48 |
36 |
91 |
Uniform wall thickness throughout the entire part is the single most important design feature that can be incorporated into a part, so thick part sections should be cored to achieve a uniform wall thickness. Uniform wall thickness provides for a more uniform pressure drop, even temperature distribution, constant shear stress, and balanced cooling. As a result, molded parts will exhibit more uniform shrinkage, which decreases warpage and residual stress. When uniform wall thickness is not possible, the thinnest wall thickness should be no less than 40% of the thickest wall and intersections should have a radius of at least 60% of the thinner wall thickness. Parts should be gated to fill from thicker sections to thinner sections.
Being a semi-crystalline thermoplastic, Ryton® PPS is notch sensitive, so sharp corners should be avoided in part designs. Abrupt changes in part geometry, like sharp inside corners, cause amplification of stress in the area of the abrupt change, as well as additional problems such as impeding flow, development of molded-in stress, and formation of voids. To avoid these problems, corners in Ryton® PPS parts should have a radius of 60% of the wall thickness.
Ribs and bosses should have a minimum wall thickness of 60% of the thickness of the intersecting wall and the height should be no more than three times the thickness of the intersecting wall. At the base, there should be a radius of 60% of the thickness of the intersecting wall, and 0.5° to 2° of draft is recommended. The outer diameter of a hollow boss should be 2.5 to 3.0 times the inner diameter, and the depth of the core should extend to 60% of the thickness of the intersecting wall. The cross section of a typical boss design is shown below. |
