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Moldflow_Design_Guide_A_Resource_for_Plastics_Engineers.pdf

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MOLDFLOW_DESIGN_GUIDE_A_RESOURCE_FOR_PLASTICS_ENGINEERS
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10 Shrinkage and Warpage ? Injection molding and shrinkage ? Basic causes of shrinkage and warpage ? Designing accurate parts considering warpage 10.1 Injection Molding and Shrinkage In this section the relationship between processing and shrinkage is considered. In particular, the effect of packing pressure on shrinkage is described. 10.1.1 What Are Shrinkage and Warpage? Part shrinkage may be thought of as a geometric reduction in the size of the part. If the shrinkage is uniform, the part does not deform and change its shape, it simply becomes smaller. Warpage results when shrinkage is not uniform. If regions of the part shrink unequally, stresses are created within the part which, depending on part stiffness, may cause the part to deform or change shape. In the long term parts can even crack. 10.1.2 Shrinkage and Machine Settings All molders know that shrinkage and consequently warpage is affected by processing conditions. Figure 10.1 shows some of the classic relationships between machine settings and shrinkage, also shown is the effect of wall thickness. These curves apply only to a particular mold and material combination. It is clear from Figure 10.1 that the final shrinkage of a component is a complex function of machine settings. Nevertheless, a major factor is the pressure and time history of the material as it fills, packs, and cools in the mold.174 Shrinkage and Warpage Figure 10.1 Effect of machine settings on shrinkage 10.1.3 Mold Filling and Packing Plastic melts are very compressible at the pressures used in injection molding. As the ram moves forward, the material in the barrel is compressed so that the flow rate in the cavity is less than indicated by the ram movement. As the ram slows down, the plastic expands under pressure. Melt compressibility causes a smooth transition from mold filling to packing. The molding process is frequently divided into two phases. Injection molders will commonly talk about the filling and packing stages because this corresponds to machine settings. Experiments on an instrumented mold show this concept is far from the truth. Figure 10.2 illustrates a simple mold with pressure transducers PT1, PT2, and PT3 positioned as shown. The lines labeled PT1, PT2, and PT3 show the pressures recorded by these transducers during filling of the mold. Because of the compressibility of plastic, there is a time delay between ram displacement and plastic movement. This actual switch from filling to packing on the machine usually occurs before the cavity is filled (see Figure 10.2) and the final stages of filling occur by expansion of the pressurized material. Shrinkage Shrinkage Shrinkage Shrinkage Shrinkage Shrinkage Melt Temperature Mold Temperature Injection Rate Packing Pressure Packing Time Part ThicknessInjection Molding and Shrinkage 175 Figure 10.2 Pressure traces for a simple molding 10.1.4 How Pressure and Time Affect Shrinkage The magnitude of pressure and the time for which pressure is applied greatly affect the shrinkage of material in the cavity. The actual pressure to which the material is subjected is determined not only by machine settings, but also by the viscosity of the material and the geometry of the cavity. Although a complicated matter, it is possible to restrict attention to two important regions: close to the gate and at the end of flow. 10.1.4.1 Shrinkage near the Gate Areas near the gate are easier to pressurize (and depressurize) than areas at the end of flow and generally the relationship between pressure, time and shrinkage is simple. High packing pressure gives lower shrinkages as long as the pressure is kept on until the gate has frozen. In this case the shrinkage around the gate will generally be lower than that at the end of flow. If the packing pressure is not held on until the gate or runner system has frozen, then the pressure in the cavity will cause plastic to reverse flow back into the runner system. This can result in a higher shrinkage around the gate area than in the rest of the cavity. 20 10 0 PT1 PT2 PT3 Pressure (MPa) Ram position PT1 press trace PT2 press trace PT3 press trace Time at which the ram stops Time at which the flow front reaches PT3176 Shrinkage and Warpage 10.1.4.2 Shrinkage at the End of Flow Pressure has to be transmitted through the plastic to reach the extremities of the cavity. Cavity geometry, viscosity, and the time the melt channel in both the feed system and cavity remain open determine how well pressure is transmitted. A high packing pressure results in a high initial flow as the pressure is quickly distributed throughout the cavity. Once the cavity is pressurized, the flow into the cavity will result from the contraction of the material and may be very slow in comparison with the initial flow. In other words there will be a high initial flow followed by a very slow flow. A low packing pressure may give the opposite effect. Initially the flow rate will be much smaller than with the high pressure so the frozen layer will grow quickly. However as the material cools the volumetric change (from high to low temperature) is much greater at low pressures so the flow rate due to compensation will be greater than for the higher pressure. High packing pressures do not automatically mean that there will be less shrinkage at the end of flow. This is because the plastic will freeze off in the upstream section earlier in the cycle, thus preventing the pressure packing out the area at the end of flow. 10.1.5 Thermally Unstable Flow Plastic flow is self-reinforcing, that is, flow will carry heat into an area thereby maintaining flow. This was illustrated in Chapter 1. A disk with a thick outer rim was packed out to give a high compensating flow to the thick outer rim. The plastic does not flow as a thin disk but forms a series of flow channels that are self-reinforcing, maintaining plastic temperature and heating the mold, while other areas with low flows freeze off early in the packing phase. The flow channels will be filled with highly orientated material that cools off at a later time than the remainder of the part. They act as tension members that will cause warping. Two important applications of this effect occur opposite the sprue and at corners. Plastics are not simply viscous materials but have certain mechanical strength. As the plastic melt changes direction at the sprue, some force is required to physically deform the material as the direction of flow changes. This force comes from the face opposite the sprue and results in a highly asymmetric flow pattern. A similar effect occurs at corners where a slight temperature difference or elastic effects will initiate asymmetric flow. Very small mold temperature variations that have virtually no effect in the filling stage will have a major effect in the packing stage. The position of cooling lines can dramatically affect packing stage flow. Once established, these flow patterns will not just be maintained but will continue to self-reinforce in the later stages of packing.Basic Causes of Shrinkage and Warpage 177 10.2 Basic Causes of Shrinkage and Warpage This section describes the main causes of shrinkage and warpage. Instead of relating shrinkage to processing parameters, we consider some fundamental factors that affect shrinkage. These factors are volumetric shrinkage, crystalline content, stress relaxation and orientation. Describing shrinkage and warpage in terms of these variables is preferable to using machine parameters, as the relationships of the latter to shrinkage are too complex to be used as design criteria. 10.2.1 Causes of Shrinkage Shrinkage of plastic components is driven by the volumetric change of the material as it cools from the melt state to solid. Despite the apparent simplicity of this statement, it is important to note that the relationship between the volumetric shrinkage and the linear shrinkage of the component is affected by mold restraint, crystallinity and orientation. Warpage is caused by variations in shrinkage. 10.2.1.1 Volumetric Shrinkage To understand shrinkage it is first necessary to appreciate just how large the volumetric shrinkage of plastics is. All plastic materials have high volumetric shrinkages as they cool from the melt state to the solid. Without pressure, this is typically about 25%. Plastic parts cannot be made without, in some way, offsetting this large volumetric shrinkage. In injection molding, the application of high pressure can reduce this volumetric shrinkage, but by no means eliminate it. Pressure: The relationship between pressure, volume, and temperature for a plastic material can be conveniently represented with a PVT diagram. Such a diagram relates specific volume (the inverse of density) to temperature and pressure. Figure 10.3 is an example of a PVT diagram. The specific volume is given by the surface over the plane defined by the pressure and temperature axes. Figure 10.3 3D PVT diagram V P T178 Shrinkage and Warpage PVT data for polymers usually is displayed as a projection onto the plane formed by the specific volume and temperature axes. Figure10.4 shows this type of display for an amorphous and a semicrystalline material. This diagram shows that normal injection molding pressures will only reduce volumetric shrinkage by around half. To see this, consider the points A, B, and C on Figure 10.4. Point A indicates the specific volume at room temperature and pressure, point B indicates the specific volume at a typical molding temperature, and Point C indicates the specific volume at a typical molding and packing pressure. The line going through point D is an extrapolated pressure line showing the pressure required to give zero shrinkage from the melt to the solid phase. Such a pressure would be well in excess of that available on an injection-molding machine and clearly shows the impracticality of trying to eliminate shrinkage by the simple application of pressure alone. Figure 10.4 PVT diagrams for polymers Crystallinity: PVT plots are usually measured at a constant temperature or very slow cooling rates. Under these conditions, the crystalline content will have reached equilibrium value. Volumetric shrinkage derived from PVT is therefore called equilibrium volumetric shrinkage. Both cooling rate and orientation level will affect crystalline content. It is very difficult to obtain PVT data under conditions of fast cooling. In view of this, actual or net volumetric shrinkage is usually found by modifying equilibrium volumetric shrinkage with a mathematical model of crystallization kinetics. 10.2.1.2 Relationship between Linear and Volumetric Shrinkages Linear shrinkage is driven by volumetric shrinkage, but there is not a one-to-one relationship. If the plastic were free to shrink in all directions isotropically, the linear shrinkage S lwould be approximately one third of the volumetric shrinkage S v . In fact the exact relationship is(10.1) 0 50 100 150 200 250 300 0 50 100 150 200 250 300 350 1.10 1.05 1.00 0.95 0.90 0.85 0.80 Temperature ( o C) Specific Volume [cm^3/g] Amorphous material Semicrystalline material A B C D A B C D P=0[MPa] P=50[MPa] P=100[MPa] P=150[MPa] P=200[MPa] S l 11 S v – () 13 ? – =Basic Causes of Shrinkage and Warpage 179 Volumetric shrinkage for a given pressure, temperature and level of crystallinity will always be the same. However, the way volumetric shrinkage is divided into the three linear shrinkage components (thickness, parallel to flow, and perpendicular to flow) will vary. The relationship between volumetric and linear shrinkages depends on stress relaxation and orientation. Stress Relaxation: In practice, the two linear shrinkage components in the plane of the molding will have values much less than one third the volumetric shrinkage value. This is because the material is constrained in its own plane while within the cavity. It is however free to shrink in the thickness direction as shown in Figure 10.5. Figure 10.5 Effect of mold restraint As the material tries to shrink in its own plane, stresses are created due to mold restraint. These stresses relax at a rate that depends on the relaxation characteristics of the material and the temperature-time history the part is subject to while constrained in the mold. These stresses will relax while the part is cooling, leading to permanent deformation of the part. This is analogous to a stress relaxation experiment in which the material is stressed by applying a constant strain. Some of the stress will relax and result in permanent deformation while the residual stress will result in elastic deformation. While in the mold, for each drop in temperature the material will receive a new stress input. At high temperatures most of this stress will simply relax while at lower temperatures a higher percentage of the stress will be retained elastically. If a cold part is simply left in the cavity longer, the effect on shrinkage will be quite small. The cooling rate has a significant effect on the degree of relaxation. Raising mold temperature will reduce linear shrinkage relative to the volumetric shrinkage by allowing the material to relax (here, we ignore the additional crystalline content that may be produced by reducing the cooling rate). However, this may extend the cooling time. Materials that relax slowly (materials with high resistance to creep) will be highly stressed in the cavity and so will spring off the mold and exhibit high linear shrinkage. Materials that relax quickly will tend to conform to the dimensions of the cavity and therefore have lower linear shrinkages. For a given volumetric shrinkage, materials that relax slowly will exhibit higher linear shrinkage than those with rapid relaxation characteristics. Free Free Fixed Fixed180 Shrinkage and Warpage Orientation: Orientation will cause the plastic to shrink by different amounts parallel and perpendicular to flow. Orientation of long stringy molecules is an easy concept to understand. Molecular orientation is initially generated by shear. At high temperatures, molecular mobility allows orientation to relax, so if shear stresses are removed the material will rapidly return to an unoriented state. Orientation is locked in by the combination of freezing while shearing. Two factors influence the relationship between orientation and linear shrinkage. Usually oriented material will tend to relax, giving a higher shrinkage in the direction of flow than across the flow. For materials that crystallize, closer packing can occur perpendicular to flow, increasing shrinkage across the flow relative to shrinkage in the direction of flow. This effect is noticeable in materials prone to shear-induced crystallization. It is important to note that for fiber-reinforced materials, orientation of the fibers has more effect than the molecular orientation. Also, the fiber orientation direction need not be in the direction of flow. For these materials,
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