These design guidelines are intended to help designers maximize the versatility of Ryton® PPS engineering thermoplastics in their products. Our suggestions regarding these common part and mold design issues are based on the results of direct experimentation and our Technical Service staff's experiences in helping customers.
For more complete and detailed information, contact our technical specialists.
Wall Thickness and Radius
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 advise using a maximum wall thickness of not more than 0.375 inch (9.52 mm).
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 should be used. 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.
Gate Location and Size
A variety of different gating methods are suitable for molding Ryton® PPS compounds. Common sprue, pin, and tab gates as well as fan, flash, spoke, diaphragm, and submarine gates have all been used for molding Ryton® PPS compounds and Ryton® PPS Alloy compounds.
The placement of the gate requires careful consideration since it has substantial ramifications on the strength and dimensional stability of the part. The designer should strive to maintain uniform flow throughout the part filling process and minimize internal stresses and weld lines. Since Ryton® PPS compounds are reinforced with glass fiber, they shrink anisotropically, about half as much in the flow direction as perpendicular to the direction of flow. This differential shrinkage can incur internal stresses that may cause part distortion or contribute to part failure.
Differences in part cross section and abrupt changes in flow patterns during mold filling can generate additional internal stresses and/or voids. Gate location should promote steady material flow starting in thicker wall sections and proceeding to thinner wall sections, must avoid back filling, and should ensure that the last places to fill can be vented, preferably at the mold parting line. Filling patterns should be planned to promote glass fiber orientation along axes that experience more stress in service. If the part cannot be molded without weld lines, the weld lines should be placed in the thickest section of the part or in areas where there is minimal stress.
Sufficient gate size is essential to ensure easy filling and adequate packing of the part, but it is also important that the gate not be so large that the time required for it to freeze off adversely affects cycle time. It is generally best to plan for the gate to be of minimal size, and then enlarge it later, if necessary. Although the wide variety of possible part geometries makes it impractical to establish criteria for any circumstance, we can provide the following general guidelines for gate sizing.
We recommend an absolute minimum gate diameter or thickness of 0.9 to 1.0 mm (0.035 to 0.040 inch) for any part, even very small parts. This is to ensure adequate pack and hold pressure for the part and to prevent excessive shear that may cause overheating of the material and/or degradation of glass fiber length. A good guideline for larger parts is for the gate diameter or thickness to be about 60% to 75% of the maximum wall thickness of the part, but the graph shown below depicts the need for increasing the gate diameter or thickness as overall part volume increases. The “Minimum Gate Diameter” for round gates in the chart shown below may also be considered the minimum gate thickness for typical rectangular gates having a width of two times the gate thickness.
Glass Fiber Orientation
The orientation of the glass fiber reinforcement affects strength, thermal expansion, and mold shrinkage. There tends to be more alignment of glass fibers parallel to the direction of flow during mold filling, and the mechanical strength of Ryton® PPS compounds and Ryton® PPS Alloy compounds can be up to 50% greater in the flow direction as compared to the transverse direction. Part and mold design should exploit the enhanced strength from glass fiber orientation along axes that will experience more stress in service, while minimizing potential stress transverse to glass fiber alignment. In some cases, it may be necessary to increase wall thickness to compensate for lower strength transverse to glass fiber alignment.
Bear in mind that the mechanical strength values reported on technical data sheets are measured in the flow direction. It is also important to note that thermal expansion will typically be at least twice as much along axes transverse to glass fiber alignment as compared to along axes parallel to glass fiber alignment. See Mold Shrinkage and Typical Molding Tolerances for information on how glass fiber alignment affects mold shrinkage.
Weld Line Strength
Weld lines are formed during the molding process when the melt flow front divides and then flows back together. Typically, the weld line interface is resin rich because the glass fibers tend not to cross the interface. The lack of glass fiber reinforcement across the interface results in lower mechanical strength along the weld line. Gate location and fill patterns should be planned so that weld lines will be eliminated or located in areas of minimal stress whenever possible.
If weld lines must bear stress, the part design should compensate for the typical weld line strengths indicated below. Weld line strength is highly dependent on molding conditions, so the part and tool design should allow for rapid injection, a hot flow front, and thorough packing. Gas entrapment is very detrimental to weld line strength, so molds must be designed to avoid back filling and should be adequately vented in areas where weld lines form.
|Nominal Weld Line Tensile Strength of Ryton® PPS Compounds|
|BR111 and BR111BL||45 MPa||6.5 kpsi|
|BR42B||55 MPa||8.0 kpsi|
|R-4 and R-4-02||40 MPa||6.0 kpsi|
|R-4-200NA and R-4-200BL||60 MPa||8.5 kpsi|
|R-4-220NA and R-4-220BL||55 MPa||8.0 kpsi|
|R-4-230NA and R-4-230BL||40 MPa||6.0 kpsi|
|R-4-240NA and R-4-240BL||80 MPa||11.5 kpsi|
|R-4XT and R-4-02XT||55 MPa||8.0 kpsi|
|R-7-120NA and R-7-120BL||45 MPa||6.5 kpsi|
|R-7-121NA and R-7-121BL||40 MPa||6.0 kpsi|
|R-7-220BL||45 MPa||6.5 kpsi|
|XE4050BL||45 MPa||6.5 kpsi|
|XE5030BL||50 MPa||7.5 kpsi|
|XE5515BL||65 MPa||9.5 kpsi|
|XK2340||60 MPa||8.5 kpsi|
Test Method: ISO 527, double end gated specimens
|Test Specimen Molding Conditions: Melt Temperature 315-343°C (600-650°F); Mold Temperature 135°C (275°F)|
*The nominal properties reported herein are typical of the products but do not reflect normal testing variances and therefore should not be used for specification purposes.
Tool Steels, Coatings, and Surface Treatments
Because of the abrasive nature of the glass and mineral fillers used in Ryton® PPS compounds and Ryton® PPS Alloy compounds, hard tool steels are required. For long run production molds, we advise using A-2, D-2, or D-7 tool steels hardened to Rockwell C-60 or higher. For low volume runs, P20, S7, and H13 are acceptable softer steels. Slow deposition dense chrome and electroless nickel coatings provide good mold release characteristics and fairly long tool life. Surface treatments including Borofuse®, LSR-1®, and Nitride® may also be used to reduce tool wear. Steels with a surface finish of 0.0001 mm (4 microinch) or better typically experience extended service life. Because especially high rates of wear are typically encountered at gates, removable (replaceable) gate blocks are often used. For more information, refer to the Abrasion Resistant Steels and Surface Treatments for Extending Tool Life technical bulletin.
Mold Temperature Control
Molds for processing Ryton® PPS compounds and Ryton® PPS Alloy compounds must be designed to provide a mold surface temperature of at least 135°C (275°F). Various methods such as hot oil, electric cartridges, or high pressure water may be used to accomplish this. We generally advise using hot oil because it allows for addition or removal of heat for better temperature control under relatively low pressure.
Sprues and Runners
Molds for processing Ryton® PPS compounds and Ryton® PPS Alloy compounds generally use standard sprue designs with nominal 2° of draft and reverse taper or Z-cut sprue puller systems. The shaft should be as short and highly polished as possible to ease part removal from the stationary half of the mold. Runners of many types can be used, however, full round and trapezoidal runners are preferred and runner length should be kept to a minimum.
Multi-cavity molds should have balanced runner systems designed so that the mold cavities fill uniformly and in a balanced fashion. On multi-cavity molds with primary and secondary runners, the primary runner should carry on beyond the intersection with the secondary runner in order to provide a cold slug well for the runner flow front. As with engineering plastics in general, it is good practice not to use family mold layouts because difficulty in controlling filling patterns may produce parts with varying physical and mechanical properties.
Proper venting is essential for processing Ryton® PPS compounds and Ryton® PPS Alloy compounds because inadequate venting results in hard to fill parts as well as burning of the part and accelerated mold wear in the areas where gas is trapped. Venting can be accomplished with 0.008 to 0.013 mm (0.0003 to 0.0005 inch) deep by 6.35 mm (0.250 inch) wide channels cut on the parting line. Venting can also be accomplished by flattening ejector pins 0.018 mm (0.0007 inch) on one to four sides. Polishing vents to an A-1 finish in the direction of flow will help prevent buildup of residues during long part runs.
Stationary vent pins are typically not used because they can become clogged over long part runs. Vacuum venting has been used successfully in areas where a blind pocket exists. The vacuum is turned on after the mold closes and prior to the start of the injection cycle. Runners should be vented with 0.025 mm (0.001 inch) deep channels cut on the parting line.
Mold Shrinkage and Typical Molding Tolerances
Mold shrinkage of Ryton® PPS compounds and Ryton® PPS Alloy compounds is affected by many factors including the type of compound, part weight and thickness, glass fiber orientation, and coring.
Glass fiber reinforced Ryton® PPS compounds and Ryton® PPS Alloy compounds will shrink anisotropically due to glass fiber alignment. For most parts having a wall thickness of 0.20 inch (5.1 mm) or less, mold shrinkage in the flow direction will typically be about 0.003 in/in for 40% glass filled PPS compounds and about 0.002 in/in for 65% glass and mineral filled PPS compounds. But, mold shrinkage will typically be about twice as much transverse to the direction of flow. Consult individual technical data sheets for mold shrinkage measurements determined on 4 inch by 4 inch by 0.125 inch (102 mm X 102 mm X 3.2 mm) plaques.
The shrinkage factors provided in our technical data sheets should provide a good indication of the shrinkage behavior for most parts having a wall thickness of 0.20 inch (5.1 mm) or less. But larger, thicker walled parts will tend to shrink more, and differential shrinkage may vary depending on part geometry and coring. For any given part, less mold shrinkage is observed with increasing part weight (less shrinkage with better part packing). Thicker wall sections generally exhibit higher shrinkage (up to an additional 0.001 in/in) than thinner ones since they hold heat longer. Lengthy thin wall sections tend to generate more glass fiber alignment resulting in greater differential shrinkage. Parts restrained during molding by coring will exhibit lower shrinkage than unrestrained parts. Mold shrinkage also increases somewhat with increasing mold temperature due to increased crystallization.
Cold molded parts will exhibit slightly less shrinkage than hot molded parts due to the lower degree of crystallinity. When heated above 190°F (88°C) after molding, cold molded parts will undergo thermal induced crystallization and additional shrinkage will occur. Conversely, hot molded parts that already have a high degree of crystallinity will undergo very little additional shrinkage when exposed to temperatures above 400°F (204°C).
Due to the low mold shrinkage of Ryton® PPS compounds, part-to-part dimensional molding tolerances are very reproducible, once shrinkage behavior under typical processing conditions has been established. Molding tolerances as tight as 0.0001 in/in may be achieved in optimally gated small parts, but molding tolerances are typically about 0.001 in/in, and may be as high as 0.002 in/in in some large parts. New molds should be cut “steel safe” to allow for later adjustments for differential shrinkage.