Mold Design Fundamentals P3 pps

31 C C OR E OR E T T ECH ECH S S YSTE M YSTE M Mold.ppt Air Air - - Trap and Gate Location Trap and Gate Location Air Bag Housing thinner section (0.5-0.8mm) thicker section (>10mm) Racetrack Effect Air-trap here PLAY When the plastic melt fills the mold, it displaces the air. The displaced air must be removed quickly, or it may cause burn spot (due to the fast compression of trapped air pocket by the low-thermal-conductivity polymer melt), or it may restrict the flow of the melt into the mold cavity, resulting in incomplete filling (short-shot problem). Consider the injection-molding of a air bag housing. Notice that the part consists of a thin central region and a thick rim around it. A single gate is adopted in the original design. Most of the melt flow along the part side since the section is thicker and the flow resistance is lower than that in the central thinner region. That is, the melt races away along the thick rim while the central region is filling at a slow rate. The filling along the rim is dominant and finally the melt backfills the central region and cause an entrapment of air there. In this case an air-trap problem is caused by the racetrack effect of melt flow. To avoid the buring or incomplete filling associated with the entrapment of air, proper venting is required. Venting is provided by the clearance between knockout/vent pins and their holes, parting lines, as well as additional venting slots (in general, 0.01 to 0.02mm deep and 3mm to 6mm wide). Gate location is directly related to the consideration of venting location. In general, the vent is located opposite the gate, area near the end of filling, or in the air-trap position. 32 C C OR E OR E T T ECH ECH S S YSTE M YSTE M Mold.ppt Viscous Heating and Gate Size Viscous Heating and Gate Size pinpoint gate (dia.=2mm) Temperature ( o C) Gapwise Scale inlet melt temperature temperature peak caused by viscous heating effect )Melt viscosity is reduced and flowability is improved by raising the melt temperature via viscous heating effect )Temperature raised <15 o C (lower value for thermal sensitive material) As the melt flows through the restricted gate, the flow velocity is sufficient high and the melt is highly sheared in the narrow passage. This frictional (viscous) heating would cause a raising in melt temperature. The temperature change is related to the melt viscosity and the local shear rate. The nomial wall shear rate in the gate is greater than 1000 sec -1 and can reach as high as 10 5 sec -1 . At this high shear rate the viscosity may be reduced due to the shear-thinning rheological character of polymer melt. The melt viscosity is further reduced by the viscous heating in the gate region. The viscosity reduction as the melt flows through the gate is important in improving the flowability of the material. A gate should be properly sized so that it could provide sufficient shearing and viscous heating in order to achieve the greatest flow length possible. If the gate is too large, it may freeze permaturely due to the insufficient viscous heating and the dominant mold cooling effect. On the other hand, if the gate is too small, filling process is highly restricted, leads to the overheating and thermal degradation of part. In general, the temperature change across the gate should be controlled within the range of 15 o C; if the material processed is thermal-sensitive, the range should be smaller. 33 C C ORE ORE T T ECH ECH S S YSTE M YSTE M Mold.ppt Gate Design vs. Part Shrinkage Gate Design vs. Part Shrinkage Gate Size Part Shrinkage Higher Packing Lower Packing Demolding Less Shrinkage Larger Shrinkage Differential Shrinkage °Back Gate design is important not only in controlling the filling pattern of the mold cavity, but also in the dimensional quality of molded part. Smaller gates freeze off sooner. Once the gates frozen, there is no melt added during the holding pressure stage, and the molded part will therefore shrink more. On the contrary, larger gates remain open longer. They freeze slowly and melt continues to feed under holding pressure through the open gate, adding more plastic as the melt shrinks in the cavity. Longer effective holding time and higher holding pressure level of larger gates lead to smaller part shrinkage values. In the mold cavity, the areas closer to the gating position are better packed than the more remote areas, which may already have cooled down enough to prevent additional melt to make up for volume contraction through shrinkage. The result is that the areas near the gate shrink less than the areas farther away. Besides, during the mold filling stage the polymer molecules undergo a stretching that results in molecular orientation and anisotropic shrinkage behavior: plastic materials tend to shrink more along the direction of flow (in- flow shrinkage) compared to the direction perpendicular to flow (cross-flow shrinkage), while the shrinkage behavior of reinforced material is restricted along the fiber-orientation direction. This differential shrinkage is the primary cause of part warpage. 34 C C ORE ORE T T ECH ECH S S YSTE M YSTE M Mold.ppt Cooling System Design Cooling System Design molding cooling channels •Layout •Size •Distance to Molding •Coolant Flow Rate •Coolant Temperature •Type of Coolant •Mold Material The mold of thermoplastics receives the hot, molten plastic in its cavity and cools it to solidify to the point of ejection. The mold is equiped with cooling channels or cooling lines that remove heat released from the part via flowing coolant. The mold temperature is controlled by regulating the temperature of coolant and its flow rate through the cooling channels. Productivity (cycle time) and quality (dimensional accuracy) of molded part depend heavily on the design and efficiency of the cooling system. High efficiency cooling system may cool down the part uniformly and quickly, hence the cycle time can be shortened, this leads to an improvement of the molding productivity. The cooling channels should be spaced evenly to prevent uneven temper-ature on the mold surface, they should be as close to the part surface as possible, taking into account the strength of the mold material. The cooling channels are connected to permit a uniform flow of the coolant, and they are thermostatically controlled to maintain a given coolant temperature. Even mold temperature distribution is important to ensure the dimensional accuracy of molded part. Uneven mold temperature leads to unbalanced cooling of part surface. The thermal stresses associated with the temperature profile across the part thickness result in part warpage or distortion. Design parameters involved in cooling system involves the type of coolant and mold material, coolant flow rate, coolant temperature, distance and size of cooling channels, and their layout. 3 5 C C ORE ORE T T ECH ECH S S YSTE M YSTE M Mold.ppt Cooling Channel Layout vs. Cooling Channel Layout vs. Part Warpage Part Warpage Lower Cooling Rate Higher Cooling Rate Unbalanced Cooling Demolding Colder surface Smaller Shrinkage Hoter surface Larger Shrinkage Warpage of Injection-Molded Part Uniform cooling throughout the part is critical to the dimensional accuracy of molded part. Consider the cooling of an injection-molded plate part by a poor-designed cooling system. The top face of the part is cooled by three cooling channels, the part surface temperature in higher due to the insufficient cooling; on the other hand, the bottom face of the part is cooler since it is cooled by four cooling channels (assume that all cooling channel has the same cooling efficiency). The hotter top surface of the part will continue to shrink more than than the cooler bottom surface after the gate frozen off and part ejection. This differential shrinkage through the part thickness is caused by the differential cooling ( difference in the cooling rate between the cavity and the core) and would cause the part to warp due to the unbalanced internal thermal stresses and their associated bending moments as the part is ejected from the mold. Non-uniform cooling plays a key role in the warpage behavior of molded part, especially in the cases of flat moldings, such as disks (records, trays, etc). The differential cooling problem can be minimized with proper mold cooling system design. 3 6 C C OR E OR E T T ECH ECH S S YSTE M YSTE M Mold.ppt Wall Thickness and Part Design Wall Thickness and Part Design q Flow Length/Wall Thickness Ratio (L/t Ratio) ) a measure of moldability of the part () L / t Ratio Maximum Flow Length From Gate to Rim Average Wall Thickness ≡ ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ L t L/t Ratio 0 100 200 300 Heavy-walled parts easy to mold Most parts relatively easy to mold Thin-walled part Difficult to mold, needs special considerations Very-difficult-to- mold part needs special equipment An important measure of the moldability of a part design is its flow length/wall thickness ratio (L/t ratio). The L/t ratio of a part is defined as its maximum flow length from gate (the pressure source) to the farthest point (end point of filling), to its average wall thickness. A smaller values of the L/t ratio indicate a shorter flow length or thicker part section, represent a smaller flow resistance and pressure loss, hence the parts are easy to mold. On the other hand, thin-walled parts or parts with longer flow length have larger L/t ratios and the molding is more difficult to carry out. The L/t ratio of a given part can assist the part designer in determining the gate locations, especially for parts of constant wall thickness. It’s rather difficult to evaluate this value for a complicated part with variable wall thicknes, this situation is further complicated by the fact that runner systems can consume a significant portion of the mold’s pressure drop. Many factors influence the L/t ratio of a given design, such as plastic materials processed, melt temperature, mold temperature, maximum injection pressure, and injection velocity, etc. 3 7 C C OR E OR E T T ECH ECH S S YSTE M YSTE M Mold.ppt Maximum Flow Length of Maximum Flow Length of Plastic Materials Plastic Materials PC,PVC Acetal Nylon Acrylic ABS PS HDPE PP LDPE 25 36 38 33-38 45 51-63 57-63 63-70 70-76 0 1020304050607080 PC,PVC Acetal Nylon Acrylic ABS PS HDPE PP LDPE (cm) Maximum Flow Length in a 2.54mm(0.1in.) thick part The flow of the plastic melt in the mold depends on various factors, such as the plastic used, melt temperature, mold temperature, length and diameter of sprue and runners, gate type, etc. In determining the minimum wall thickness of the part, all these factors have to be considered. The L/t ratio achieveable depends heavily on the type of plastic to be processed. A high viscosity (low melt index) plastic such as polycarbonate (PC), polysulfone (PSU), acrylic,etc., has a higher resistance to flow because of its microstructure (cross linking, high molecular weight) and thus has a shorter maximum flow length. It requires higher injection pressure to fill the mold cavity with sufficient filling speed. For example, in a testing mold with a thickness of 2.54mm (0.1in.), the maximum flow length of PC is 25cm. On the other hand, for easy-flow, low-viscosity plastics such as poly-propylene (PP), polyethylene (PE), the maximum flow length is longer and the minimum wall thickness that can be filled is smaller than for stiff-flowing materials. Typical maximum flow length of general purpose grades of thermoplastics, based on a cavity thickness of 2.54mm (0.1 in.) and conventional molding techniques, are provided here to illustrate their processing properties. These data are obtained from the spiral flow length experment and can be used as a reference of moldability of various resin grades. The actual maximum flow length of a plastic material depends on part design, mold design, as well as the process variables. 38 C C OR E OR E T T ECH ECH S S YSTE M YSTE M Mold.ppt Maximum Flow Length of Maximum Flow Length of Plastic Materials Plastic Materials Maximum Flow Length Part Thickness increasing injection pressure @constant injection speed, mold/melt temperature Maximum Flow Length Part Thickness increasing melt/mold temperature @constant injection pressure injection speed The maximum flow length achieveable for a particular plastic grade depends on molding conditions of the experiments. For instance, under a constant injection speed/mold temperature/melt temperature condition, the flow length increases as the applied injection pressure is increased because of the increasing driving force for mold filling. Thus easy- to-flow materials require a lower injection pressure to fill the mold cavity with sufficient filling speed. Under a constant injection speed/injection pressure condition, the maximum flow length of a given material increases as the mold temperature and/or the melt temperature is raised. A plastic material has a longer flow length at higher temperature because of its thermal-reduced melt viscosity. These flow length data of plastic materials provide valuable information about their flow behavior and processing properties. They are available from material suppliers. 39 C C OR E OR E T T ECH ECH S S YSTE M YSTE M Mold.ppt Wall Thickness of a Part Wall Thickness of a Part P l as t i c s M i n. Wa l l T h i c k n e ss ( mm ) M a x .Wa l l T h i c k n e ss ( mm ) S u g g e s t e dWa l l T h i c k n e ss ( mm ) P O M 0.4 3.0 1.6 A B S 0.75 3.0 2.3 A c r y l i c / P M M A 0.6 6.4 2.4 C e l l u l os e 0.6 4.7 1.9 T e f l on 0.25 12.7 0.9 N y l on 0.4 3.0 1.6 P C 1.0 9.5 2.4 Po l y e s t e r 0.6 12.7 1.6 L D PE 0.5 6.0 1.6 H D PE 0.9 6.0 1.6 EV A 0.5 3.0 1.6 PP 0.6 7.6 2.0 P S U 1.0 9.5 2.5 PP O 0.75 9.5 2.0 PP S 0.75 3.8 2.3 P S 0.75 6.4 1.6 S A N 0.75 6.4 1.6 PV C - R i g i d 1.0 9.5 2.4 P U 0.6 38.0 12.7 S u r l y n 0.6 19.0 1.6 The nominal minimum, maximum, and suggested wall thickness for various plastic materials is listed here. The essential issue in determining the wall thickness of a part is the flowability of polymer melt. The wall of a part should allows plastic melt to flow properly under appropriate injection pressure. The wall should permits effective transmission of packing/holding pressure during the holding stage. Finally, the wall should withstand the internal/external loading after the part is ejected from the mold cavity. The allowable minimum wall thickness is smaller for easy-flow, low-viscosity plastics such as polyethylene (PE) and polypropylene (PP). This value is larger for polycarbonate (PC) and polysulfone (PSU) that are more viscous and stiff- flow. Typically, a thin-walled part can be arbitrarily defined as a part with a L/t ratio greater than 200 or with wall thickness less than 1mm (t<1mm). In a thin-walled part mold cooling effect is dominant and the part is rapidly cooled. Cycle time is usually short (less than 10 sec). The injection pressure needed is higher for proper filling and short-shot (incomplete filling) can be a problem. A heavy-walled part can be defined as a part with a L/t ratio smaller than 100 or with wall thickness more than 2mm (t>2mm). Filling is not a problem in a heavy wall and the injection pressure needed is lower than that of the thin wall. Cycle time is long, often longer than 20 sec. Determining the proper part thickness is important to facilitate the processing and ensure product strength. 40 C C OR E OR E T T ECH ECH S S YSTE M YSTE M Mold.ppt Wall Thickness of a Part Wall Thickness of a Part q Empirical Equation ) t,L in mm for easy -flow plastics: t = 0.6 L 100 0.5 for fair -flow plastics: t = 0.7 L 100 0.8 for stiff - flow plastics: t = 0.9 L 100 1.2 + ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ + ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ + ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ e.g.,PP,PE,Nylon e.g.,POM,PMMA e.g.,PC,PSU An empirical equation is presented here to give an rough estimate of wall thickness for a plastic part. For example, if polypropylene (PP) is used as the molding compound, the wall thickness of a 50-cm long part will be: wile for the stiff-flow polycarbonate (PC) the required wall thickness is: the cooling time is about four times that of PP. tmm=+ ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ =06 500 100 05 33 tmm=+ ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ =09 500 100 12 56 [...].. .Mold. ppt Part Wall Transition flow Sharp/Stepped Transition: poor design t flow Gradual Transition: better design thick-to-thin gating 3t flow 3t Gradual Transition: thin-to-thick gating (not recommended) flow 3t Smooth/Tapered Transition: best design CORETECH SYSTEM For a variable wall thickness part, the wall transition should be gradual to ensure proper mold filling and part... thin rib is a good strategy to improve the design In practice, rib wall thicknesses are typically 40%-80% as great as the wall from which they extended, with a base radius values from 25%-40% of the wall thickness The specific rib designs are material dependent, and are influenced primarily by the shrinkage behavior of the plastic material Alternative better design is to core out the heavy section, uniform... wall thickness can be obtained in this case This results in cycle time reduction along with an overall quality improvement 42 Mold. ppt Wall Thickness and Shrinkage original design part warpage sink mark sink mark thin section thick section sink mark thick section rib better design sink mark voids stress concentration CORETECH SYSTEM Thick walls in a part will fill easily, with less pressure, but will... and sink mark/warpage problems may be caused 41 Mold. ppt Wall Thickness and Shrinkage sink mark Shrinkage voids Use two short thick ribs:good local heavy section:poor Core out the heavy section:better Use a long thin ribs:better CORETECH SYSTEM Thin wall parts with heavy boss, ribs, rims, and/or other local heavy cross sections usually is difficult to molding Usually the poorly cooled heavy sections... chages its filling velocity suddenly in the wall thickness transition region and a pressure loss is caused by the flow contraction effect The filling pattern in this design may result in air entrapment and stress concentration problems A better design is to modify the stepped transition into a gradual transition (usually tapered a transition length equal to three times the difference in thickness) The melt... lower than that of the stepped transition High stress concentration around the transition region can be avoided The best design is to vary the wall thickness as smooth as possible, usually a tapered transition is adopted Pressure loss and stress concentration can be minimized in this design Note that the melt flow should be directed in the direction from “thick-to-thin” whenever posible The thicker section... heavy cross sections usually is difficult to molding Problems such as sink marks, warpage, and shrinkage voids may be caused if the part wall is not properly desinged When parts have both thick and thin sections, gating into the thick section is preferred because it enables packing/holding of the heavy section, even if the thinner sections have frozen off The design can be further improved by coring out... only shrinks less but also takes a shorter cooling time A properly design part, with even wall thickness and adequate ribbing, is usually stronger and stiffer than a part with thicker and/or uneven walls Saving of material, reduction in part weight and cycle time, improvement in part quality , etc., are the advantages obtained if we design the part carefully 43 ... heavy sections This can be often seen by the sink marks on the surface behind these local heavy sections Also, the differential cooling and shrinkage of the thin and thick sections lead to warpage of the molded part When the cooled outer surface of the part is strong to resist sinking and the inner hot melt cools and shrinks, shrinkage holes/voids will be created within the plastic wall Thick ribs provide . S S YSTE M YSTE M Mold. ppt Cooling System Design Cooling System Design molding cooling channels •Layout •Size •Distance to Molding •Coolant Flow Rate •Coolant Temperature •Type of Coolant Mold Material The mold. be minimized with proper mold cooling system design. 3 6 C C OR E OR E T T ECH ECH S S YSTE M YSTE M Mold. ppt Wall Thickness and Part Design Wall Thickness and Part Design q Flow Length/Wall. easy to mold Thin-walled part Difficult to mold, needs special considerations Very-difficult-to- mold part needs special equipment An important measure of the moldability of a part design is