Autodesk Moldflow Communicator 2012 Molding processes
Revision 1, 23 March 2011.
Contents Chapter 1 Molding processes..................................... 1 Thermoplastic injection molding analysis types and technologies........... 1 Overmolding............................................. 2 Overmolding analysis.................................... 2 Thermoplastic Overmolding analysis types and analysis technologies..... 3 Gas-assisted Injection Molding................................. 3 Gas-assisted injection molding analysis types and analysis technologies.... 5 Gas entrances......................................... 5 Gas injection methods.................................... 6 Overflow wells gas injection molding......................... 8 Co-injection molding overview................................. 9 Co-injection molding analysis types and analysis technologies......... 10 Overcoming Co-injection molding problems.................... 11 Injection-compression analysis................................ 12 Injection-compression molding analysis types and analysis technologies.. 14 Injection-compression analysis process........................ 14 Reactive Injection-compression Molding analysis types and analysis technologies. 16 Reactive Molding analysis types................................ 16 iii
Reactive Molding analysis types and analysis technologies............ 17 Reactive Molding analysis................................. 17 Reactive Molding analysis process........................... 18 Reactive Molding analysis technical information.................. 19 Microchip Encapsulation analysis............................... 19 Microchip Encapsulation analysis types and analysis technologies....... 20 Microchip Encapsulation analysis types........................ 20 Microcellular Injection Molding analysis.......................... 21 Microcellular injection molding analysis types and analysis technologies.. 23 Resin Transfer Molding..................................... 23 RTM/SRIM analysis types and analysis technologies................ 24 Underfill Encapsulation analysis............................... 24 Underfill Encapsulation analysis types and analysis technologies....... 25 Shape factor calculation for underfill encapsulation................ 25 Multiple-barrel Reactive Molding analysis types and analysis technologies.... 27 iv
Molding processes 1 Once you have meshed your model, you are ready to select the molding process. It is important that you select the molding process that represents the real-case scenario that you are simulating. Once you have selected the molding process, the supported analysis sequences are updated. Thermoplastic injection molding analysis types and technologies The analysis technologies that are available to you depend on the analysis type that you select. This table lists the available analysis technologies for a given analysis type. Table 1: Thermoplastics Injection Molding process and analysis types Analysis type Analysis technology Fast Fill Fill Fill+Pack Core shift (Process Settings option) Standalone Pack Fiber (Process Settings option) Cool Cooling Quality Sink Mark Warp Birefringence (Process Settings option) Stress Shrink 1
Analysis type Analysis technology Process Optimization Design of Experiments Molding Window Gate Location Runner Balance Runner Adviser Design Adviser Venting (Process Settings option) Overmolding Overmolding is an injection molding process where two materials are molded together. Types of overmolding include two shot sequential overmolding, multi-shot injection molding or insert overmolding. Multi-shot injection molding injects multiple materials into the cavity during the same molding cycle. Insert overmolding uses a pre-molded insert placed into the mold before injecting the second material. Two shot sequential overmolding is where the molding machine injects the first material into a closed cavity, and then moves the mold or cores to create a second cavity, using the first component as an insert for the second shot using a different material. Materials are usually chosen specifically to bond together, using the heat from the injection of the second material to form that bond. This avoids the use of adhesives or assembly of the completed part. It can result in a robust multi-material part with a high quality finish. When designing an overmolded part, wall thicknesses of both the insert and the overmolded component should be as uniform as possible to ensure an even and robust bond. Avoid ribs and sharp corners to reduce flow problems. Overmolded parts take longer to cool than single shot injection molded part, and cooling systems are less effective. The insert acts as an insulator and heat is less efficiently extracted from the part. However, optimising the cooling system can help reduce the cycle time. Overmolding analysis An Overmolding analysis is used to analyse two shot sequentially overmolded parts. 2 Molding processes
Overmolding analyses consist of a two step process, where a Fill+Pack analysis is performed on the first cavity (first component stage), and then a Fill+Pack analysis or a Fill+Pack+Warp analysis is performed on the overmolding cavity (overmolding stage). The overmolding stage on the second cavity uses a different material from the first component stage. As the temperature of the insert, injected in the first component stage, is not uniform, mold and melt temperatures used in the overmolding stage are initialized by the temperatures recorded at the end of the first component stage. NOTE: It is assumed that the material injected in the the first component stage does not melt and flow when the second material is injected in the overmolding stage, even though the temperature does rise. NOTE: Warp analyses on overmolded components on 3D models which contain part inserts, take into account the influence of contact between the part inserts and the overmolded component. Thermoplastic Overmolding analysis types and analysis technologies The following table shows the available analysis technologies for a Thermoplastic Overmolding analysis type. Table 2: Thermoplastic overmolding process and analysis types Analysis Type Analysis Technology Fill Fill+Pack Fiber Fill+Pack Overmolding Warp Venting (Process Settings option) Gas-assisted Injection Molding Gas-Assisted Injection Molding is a process where an inert gas is introduced at pressure, into the polymer melt stream at the end of the polymer injection phase. The gas injection displaces the molten polymer core ahead of the gas, into the as yet unfilled sections of the mold, and compensates for the effects of Molding processes 3
volumetric shrinkage, thus completing the filling and packing phases of the cycle and producing a hollow part. Traditionally, injection molded components have been designed with a relatively constant wall thickness throughout the component. This design guideline helps to avoid major flaws or defects such as sink marks and warpage. However, apart from the simplest of parts, it is impossible to design a component where all sections are of identical thickness. These variations in wall thickness result in different sections of the part packing differently, which in turn means that there will be differentials in shrinkage throughout the molding and that subsequently distortion and sinkage can often occur in these situations. By coring out the melt center, gas injection molding enables the packing force (which compensates for differential shrinkage) to be transmitted directly to those areas of the molding which require attention. This dramatically reduces differentials in shrinkage and thus the sinkage. In addition, the internal stresses are kept to a minimum, considerably reducing any distortion that may otherwise have taken place. Maximum clamp pressures are normally required during the packing phase of a molding cycle. This is due to the force which has to be exerted at the polymer gate in order to pack melt into the extremities of the mold cavity in an effort to compensate for the volumetric shrinkage of the solidifying melt. In comparison to compact injection molding, gas injection molding typically has considerably shorter distance over which the solidifying melt is required to be packed because of the gas core. This means that proportionally lower packing pressures are required to achieve the same results and in turn, lower machine clamp forces are required. Gas injection allows cost effective production of components with: Thick section geometry. No sink marks. Minimal internal stresses. Reduced warpage. Low clamp pressures. 4 Molding processes
Gas-assisted Fill+Pack analysis benefits Gas-assisted Fill+Pack analysis provides you with the ability to study polymer and gas flow behavior within a part model and examine the influence that design modifications make on both the polymer and gas flow paths. Using this information, the design engineer will be able to optimize product design and accurately position polymer and gas injection points. Also to ensure that the product specifications are met, utilizing the full capabilities of the gas injection molding process. Expensive tool modifications, long lead times and trial and error will also be kept to a minimum. The process engineer will benefit from the program's capacity to examine the effects that varying processing conditions will have on the component and enable optimum processing conditions to be established prior to mold commissioning. Gas-assisted injection molding analysis types and analysis technologies The following table shows the available analysis technologies for a Gas-assisted injection molding analysis type. Table 3: Gas-assisted Injection Molding process and analysis types Analysis Type Analysis Technology Fill Fill+Pack Fiber Fill+Pack Cool Warp Stress Gas entrances A gas entrance is the position where compressed gas is injected into the mold cavity. The gas injection stage of the software is an integral component of the filling phase, and allows you to specify single or multiple gas entrances directly into the cavity, or through the polymer injection location(s), that is, machine nozzle or in-runner. Gas entrance considerations Gas, like the molten polymer, always flows toward the point of lowest pressure. Therefore, select the gas entrance to ensure that the gas stays in Molding processes 5
the gas channel, and that the area of lowest pressure is near the end of the gas channel. Some of the more important questions to consider when setting gas entrances on your model are: Polymer injection location(s). Along which route will the gas flow? How far will the gas penetrate? Will gas penetrate into the thin wall section? What will the channel and wall thickness be? Is the optimum part weight being achieved? Will sink marks be avoided? As with all polymer melt flow analyses, geometry changes in one area can have an effect on the flow characteristics in another section. This is even more significant with gas-assisted injection molding due to the sensitivity with which the pressurized gas searches out and flows through the route of least resistance until such a time as the cumulative resistance of the melt exceeds the pressure of the gas. For this reason, changes to component geometries must not be looked at in isolation to one another. Due to the complexity of the problem this is only feasible with the aid of a computer based simulation process. Gas injection methods During Gas-assisted injection molding, the gas can be injected into the polymer melt either through the nozzle of the molding machine, or by direct injection into the mold or into a runner. This help topic outlines the advantages and disadvantages of each using each method. 6 Molding processes
Gas injection through the nozzle The molding machine is fitted with a special polymer/gas injection nozzle through which firstly polymer and then gas is injected into the tool. Advantages Disadvantages Requires no tool modifications or tool specific components, thus implementation of gas injection molding is relatively cheap. Gas and polymer are injected through a single location, resulting in less process control. Cannot be used with hot runner systems. Gas control unit is often tied to a particular molding machine. Location and number of gas injection points are determined by the runner system, which is a constraint for mold design. Runner system affects the gas flow. Very difficult to use with multi-cavity (family) molds, due to difficulties in control of gas and polymer flow paths. Gas injection directly into the mold or runner The gas is introduced directly into the runner or cavity by a needle device built into the tool. Advantages Gas is introduced directly into the cavity, so it can be injected where most appropriate. Independent gas injection times and pressure profiles may be set for each injection location. Hot runner systems may be used. Gas control unit is not molding machine specific. Can be used with multi-cavity (family) molds. Molding processes 7
Disadvantages Higher tooling costs. Overflow wells gas injection molding Ideally, the placement and extent of gas channels should be controlled by suitable modifications to the part geometry. In cases where this does not provide sufficient control, overflow wells can be used to increase gas penetration, or direct the gas into specific areas of the part. An overflow well is a secondary cavity into which the gas can displace polymer and thereby penetrate further into the part. Overflow wells provide paths of least resistance along which the gas will preferentially travel. Further control over gas flow can be achieved by opening and closing the overflow wells at specific times by means of valve gates. The following figure illustrates the typical application of overflow wells in gas-assisted injection. The passage to the overflow well is generally closed during the injection phase while the plastic is filling the rest of the cavity, as illustrated in a) below. At the end of polymer injection, there is an optional delay time which allows the polymer over the thin sections to solidify. Immediately before gas injection is triggered, the passage to the overflow well is opened, creating an additional volume to accommodate the resin that is displaced by the incoming gas, as illustrated in b) below. After the part is ejected from the mold, the overflow can be trimmed off if it is undesirable. 1) Control valve closed during polymer injection. 2) Gas penetration. 3) Gas entrance. 4) Control valve open during gas injection. 8 Molding processes
Figure 1: Legend: Overflow wells must be modeled with a defined volume. For Midplane models only, an overflow well with infinite volume can be simulated by setting a venting location at the end node of the overflow well. Co-injection molding overview Co-injection molding involves injection of two dissimilar materials. Because of this, co-injection has some special advantages, as well as some potential molding problems. The Co-injection analysis helps you overcome the potential problems and leverage the advantages, by helping to optimize process control strategies and enhance part quality. Co-Injection analysis simulates the sequential injection of skin and core plastic materials. Sequential co-injection processes have two barrels and one nozzle in an injection molding machine. (a) The skin plastic is injected into the mold first. (b) The core plastic then is injected. (c) finally, the skin plastic is injected again, to purge the core material from the sprue. Molding processes 9
Figure 2: Co-injection process The skin plastic is the material that is expected to be deposited on the cavity wall over the entire surface of the part. The core plastic displaces the skin plastic at the hot core, pushing it to fill the rest of the cavity. The end product is a sandwich-like structure, with the core plastic in the middle and the skin plastic on the surfaces of the part. Co-injection molding takes advantage of a characteristic of injection molding called fountain flow. As the cavity is filled, the plastic at the melt front moves from the center line of the stream to the cavity walls. Because the wall temperature is below the transition temperature (freeze temperature) of the melt, the material that touches the walls cools rapidly and freezes in place. This provides insulating layers on each wall, through which new melt makes its way to the melt front. Advantages and applications The advantages of this process are: The combination of two material properties into one part. The maximization of the overall performance/cost ratio. Examples of co-injection applications include: The use of plastic regrind as the core material, while maintaining surface finish quality by using virgin plastic as the skin material. The use of a core material that is thermally more stable, to increase the thermal resistance of a part. The use of a high melt-flow index plastic as the core material, to reduce the overall clamp force. Co-injection molding analysis types and analysis technologies The following table shows the available analysis technologies for a Co-injection molding analysis type. 10 Molding processes
Table 4: Co-injection Molding process and analysis types Analysis Type Analysis Technology Fill+Pack Fiber Fill+Pack Cool Warp Stress Overcoming Co-injection molding problems Because Co-injection molding involves the injection of two dissimilar materials, there are some potential molding problems that may need to be overcome. The biggest challenges in Co-injection molding are: To determine the optimal ratio of skin material to core material. To determine the optimal time to switch from injection of skin material to injection of core material. The theoretical maximum amount of core plastic in a part is about sixty-seven percent by volume. However, it is very difficult to accomplish this in an actual application, with complex part geometry. In practice, about thirty percent core plastic by volume can be achieved. With an improper mold design or an insufficient amount of skin plastic, the core plastic may eventually deplete all the skin plastic injected ahead of it, and appear on the part surface. This undesirable core surfacing typically occurs at areas that fill last (where plastic has the longest flow length). Skin Polymer, Core Polymer Molding processes 11
Figure 3: Correct flow (above), Core Surfacing (below) Using Co-injection to overcome potential molding problems Co-injection analysis traces the spatial distribution of the skin and core plastics throughout the cavity during the filling process. The analysis accounts for the differences in material properties and in processing temperatures of the skin and core plastics, as well as the mass, heat, and momentum interactions between them. Co-injection provides information that designers and engineers use to qualitatively predict part performance, improve mold designs, and optimize process controls. In particular, it is an efficient tool for determining the best combination of skin and core plastics, and the most appropriate switch-over time. Injection-compression analysis The Injection-compression molding process is an extension of conventional injection molding. After a pre-set amount of plastic melt is fed into an open cavity, it is compressed. The primary advantage of this process is the ability to produce dimensionally stable, relatively stress-free parts, at a low clamp force. Injection-compression molding is sometimes called coining, stamping, compressive-fill, or hybrid molding. Injection-compression simulates the following special characteristics of the Injection-compression molding process: Injection phase Compression phase During this stage, the mold cavity thickness is designed to be larger than the target part thickness, in order to allow plastic to flow easily to the extremities of the cavity. Because the plastic flows easily, it can do so under relatively low pressure and stress. During or after filling, a compressive force reduces the mold cavity thickness, forcing the resin into the unfilled portions of the cavity. This produces a more uniform packing pressure across the cavity. This results in more homogeneous physical properties and less molded-in stresses compared to conventional injection molding. 12 Molding processes
Figure 4: Injection phase (above) Compression phase (below) Advantages and applications Injection-compression is advantageous for production of precision parts that require low residual stresses, such as optical discs, and high-precision moldings. Conventional injection molding may not be able to meet product design requirements for these parts because thermoplastics are inherently difficult to process due to their pvt characteristics and high viscosity. Sequential or simultaneous An Injection-compression process can be sequential or simultaneous. Sequential Simultaneous The injection unit (filling, packing) and the compression unit (speed control, force control) do not work at the same time. Compression begins only after filling and packing are completed. The injection unit and the compression unit can be working at the same time. In each of the diagrams below: The top line represents the injection unit, with F = Fill Time, P = Pack Time, C = Cool Time, and O = Mold Open Time. The bottom line represents the compression unit, with W = Press Waiting Time, and PC = Press Compression Time. Molding processes 13
Figure 5: Sequential Injection-Compression Figure 6: Simultaneous Injection Compression There are no constraints in Injection-compression for a sequential or simultaneous process in which the compression unit is under speed control. However, for a simultaneous process in which the compression unit is under force control, the Injection-compression analysis may take longer. Even when the press force exceeds the press force used, the analysis will continue. Injection-compression molding analysis types and analysis technologies The following table shows the available analysis technologies for a Injection-compression molding analysis type. Table 5: Injection-compression molding process and analysis types Analysis Type Analysis Technology Fill+Pack Fiber Fill+Pack Cool Warp Stress Injection-compression analysis process An Injection-compression analysis process involves injecting a volume of plasticated melt into an open cavity and subsequently compressed. Process Details 1 Oversized cavity. For Injection-compression molding, the cavity thickness is initially oversized by 0.5 to 10 mm more than the target thickness; later the thickness will be reduced. 2 Injection. Plastic melt is injected into the cavity. 14 Molding processes
3 Press waiting. Meanwhile, the press is moved to a pre-determined position. It will be stationary and stay in this position for a period of time. This time period is called the press waiting time. The waiting time starts when the melt injection begins and ends when the press begins to move. 4 Compression. The compression stage starts when the press begins to move. (The press is sometimes called the plunger, moving mold half, or moving platen.) The total press compression time includes the time during which compression is under speed control plus the time during which the press is under force control. 5 Compression under speed control. In the press compression stage, the movement of the press in initially under speed control, specified by the press compression speed at incremental distances profile. For each incremental distance, the press moves at a constant speed. This type of movement continues until it reaches a pre-set press compression force. 6 Compression under force control. After the press force reaches the pre-set force, the press switches from speed control to force control. The press can keep moving forward, however, it will be in a constant force control mode (pre-set force). 7 Stationary press. After the press compression phase is complete, the press will stay in that position and remain stationary thereafter. 8 Press returns. When packing and cooling end and mold-opening begins, the press begins to move backward. Features Injection Compression provides: Independent control of the injection and compression units. Two-stage press control: speed control and force control. Open and close control for the polymer injection location and valve gate. Injection-compression helps you to: Minimize the press force (clamp force) for compression. Minimize the injection pressure. Minimize shrinkage, warpage and residual stress. NOTE: In this version of Injection-compression, the compression effect is applied only to the triangular elements. In general, press force control needs more computational time than press speed control. Molding processes 15
Reactive Injection-compression Molding analysis types and analysis technologies The following table shows the available analysis technologies for a Reactive Injection-compression Molding analysis type. Table 6: Reactive Injection-compression process and analysis types Analysis Type Analysis Technology Fill+Pack Reactive Molding analysis types Reactive Molding processes, also called thermoset molding processes, use thermoset materials. Thermosets, unlike thermoplastics, are characterized by the following: A chemical reaction during the molding process Cross-linked polymer structure Simultaneous polymerization and shaping during the molding process. Processes The Reactive Molding processes include the following: Reaction Injection Molding (RIM) Structural Reaction Injection Molding (SRIM) Resin Transfer Molding (RTM) Multiple-barrel reactive molding (RIM-MBI) Thermoset injection molding Rubber injection molding Microchip Encapsulation Underfill Encapsulation Advantages The Reactive Molding analysis offers the following advantages: Thermosets' cross-linked polymer structure generally imparts improved mechanical properties and greater heat and environmental resistance. Thermosets' typically low viscosity permits large and complex parts to be molded with relatively lower pressure and clamp force than required for thermoplastics molding. Thermosets can be used in composite processes. For example, RTM and SRIM processes, which use a preform made of long fibers, offer a way 16 Molding processes
to make high-strength, low-volume, large parts. Fillers and reinforcing materials can enhance shrinkage control, chemical and shock resistance, electrical and thermal insulation, and/or reduce cost. Reactive Molding analysis types and analysis technologies The following table shows the available analysis technologies for a Reactive Molding analysis type. Table 7: Reactive Molding process and analysis types Analysis Type Analysis Technology Fill+Pack Warp 1 Runner Balance Venting (Process Settings option) Reactive Molding analysis Reactive Molding provides useful information to detect various molding problems and to optimize part, mold, and process design in an efficient and cost-effective way. Reactive Molding can be applied to various processes that use reactive (thermoset) materials, including Reaction Injection Molding (RIM), Structural Reaction Injection Molding (SRIM), Resin Transfer Molding for fiber reinforced plastic (RTM), thermoset injection molding, rubber compound injection molding, Microchip Encapsulation and Underfill Encapsulation. Reactive Molding analyses help to predict how the mold will fill with or without the presence of fiber reinforced pre-forms, avoid short shots due to pre-gelation of the resin, highlight potential air trap or weld line problems, balance runner systems, select the proper molding machine size, and evaluate different reactive resins. The Reactive Molding analyses are integrated with the Autodesk material database, which offers more than 50 grades of lab-tested reactive molding materials. Specifically, Autodesk Moldflow Insight's Reactive Molding analyses can: Predict the melt front pattern to aid in part design and gate placement to optimize cavity filling for most reactive processes. 1 To complete a Warp analysis for Midplane or Dual Domain analysis technology, you must select an analysis sequence that includes the Compressible solver. Molding processes 17
Calculate the conversion (extent of cure) versus time at any location within the mold during filling and post-filling. Determine injection pressure and clamp force requirements for proper molding machine selection. Display injection pressure at any point within the cavity at any time during the filling stage. Graphically display the temperature change as a result of the reaction kinetics inside the mold at any point in time. Detect short shots due to pre-gelation conditions. Accurately identify weld (knit) lines based on part design and gate placement. Accurately identify air traps for proper mold venting. For RTM and SRIM analyses: allow users to define multiple anisotropic fiber mats with different orientations in the cavity. For Reactive Molding and Microchip Encapsulation analyses, predict part warpage. 2 Reactive Molding analysis process Thermosets are usually purchased as liquid monomer-polymer mixtures or as a partially polymerized molding compound. Starting from this uncured condition, they can be formed to the final shape in the cavity by polymerization. The polymerization is activated either by heat or by chemical mixing, with or without pressure. In the Reactive Molding process, the temperature in the feed mechanism (the barrel) is only slightly increased, however, the cavity is usually hot enough to initiate chemical cross-linking. As the warm pre-polymer is forced into the cavity, heat is added from the cavity wall, from viscous (shear) heating of the flow, and from the heat released by the reacting components. The temperature of the part often exceeds the temperature of the mold. When the reaction is sufficiently advanced for the part to be rigid (even at a high temperature), the cycle is complete and the part is ejected. Molding problems The chemical reactions that occur during filling and curing add complexity to mold and process design for Reactive Molding processes. For example, slow filling may cause premature gelling, resulting in a short shot. Fast filling may induce turbulent flow, creating internal porosity. Improper control of the mold-wall temperature and/or inadequate part thickness will result in either moldability problems or scorching of the materials. Reactive Molding analyses can help you avoid such problems, without costly and time-consuming trial and error debugging. 2 To complete a Warp analysis for Midplane or Dual Domain analysis technology, you must select an analysis sequence that includes the Compressible solver. 18 Molding processes
Reactive Molding analysis technical information This topic lists some useful technical details related to a Reactive Molding analysis. Mold filling is modeled by a generalized Hele-Shaw flow model for areas without reinforcement and by Darcy's Law for areas with fiber mat reinforcement. The numerical solution is based on a hybrid finite-element/finite-difference method for solving pressure, flow, and temperature, and a control-volume method to track moving melt fronts. Material viscosity is calculated as a function of temperature, conversion (extent of cure) and shear rate. The effect of induction time is included in flow calculations for rubbers and polyester resins. Special numerical methods are used to track the curing history of material at the melt front (fountain region). Curing kinetics are included in the calculations of both flow dynamics and temperature. Microchip Encapsulation analysis The Microchip Encapsulation analysis simulates the encapsulation of semiconductor chips with reactive resins, which provides protection from hostile environments, facilitates heat dissipation, and enables electrical interconnection of the chips. The reactive nature of the thermosetting encapsulants (typically epoxy molding compounds), and the complex geometries (with molded-in silicon chip paddles, connecting wires and leadframes) pose problems for product design, material selection, tool making, and process control. Microchip Encapsulation helps solve these problems by providing the tools to design the encapsulation package, tool, leadframe and wires, and to select optimal processing conditions, including mold temperature, filling time, ram-speed profile, and curing time. Microchip Encapsulation analysis can: Simulate the mold filling and curing of the microelectronic device, including the effect of polymer preconditioning within the transfer pot, to predict the impact of processing on the encapsulated device. Predict wire sweep, the deformation of the bonding wires within the cavity. Predict paddle shift, the shifting of the leadframe due to pressure imbalances. Molding processes 19
Predict part warpage. 3 Microchip Encapsulation analysis types and analysis technologies The following table shows the available analysis technologies for a Microchip Encapsulation analysis type. Table 8: Microchip Encapsulation process and analysis types Analysis Type Analysis Technology Fill+Pack Wire Sweep Wire Sweep Detail Paddle Shift Dynamic Paddle Shift Warp 4 Runner Balance Venting (Process Settings option) Microchip Encapsulation analysis types A Microchip Encapsulation analysis can be one of several different types. The pre-conditioning analysis calculates the temperature and degree of cure distribution of the epoxy molding compound (EMC) preform after it is placed in the pot and before it is transferred into the mold. Typically, the temperature of the transfer pot is higher than the temperature of the preheated preform, and the preform experiences changes in temperature and degree of cure while it is in the pot. The pre-conditioning analysis calculates the average temperature and degree of cure of the preform and passes the data to the subsequent filling and curing analysis. Optionally, you can omit the pre-conditioning analysis if you do not have data for it. The mold-filling and curing analysis is performed by Reactive Molding. 3 To complete a Warp analysis for Midplane or Dual Domain analysis technology, you must select an analysis sequence that includes the Compressible solver. 4 To complete a Warp analysis for Midplane or Dual Domain analysis technology, you must select an analysis sequence that includes the Compressible solver. 20 Molding processes
The Wire Sweep analysis calculates the deformation of the bonding wires (connecting the chip to the leadframe) that occurs during encapsulation. This calculation enables you to improve the mold design and process conditions to prevent wire-sweep from occurring during encapsulation. The wire deformation can be calculated either internally in Autodesk Moldflow Insight using the Warp module or externally using Abaqus. 3D Microchip Encapsulation also supports a Wire Sweep Detail analysis, which accounts for the effects of the wires on the fluid flow as well as the effects of the fluid on the wires. The 3D chip cavity model must include the wire cavities as well as the wires themselves. The Wire Sweep Detail analysis takes more time to complete compared to the regular Wire Sweep analysis, but the calculation of deformation can be more accurate. The Paddle Shift analysis calculates the deformation of the paddle due to the pressure difference in the two sub-cavities separated by the leadframe. Microchip Encapsulation calculates the pressure in the cavity. The leadframe deformation due to pressure differences in the cavity can be calculated either internally in Autodesk Moldflow Insight using the Warp module or externally using Abaqus. 3D Microchip Encapsulation also supports a Dynamic Paddle Shift simulation where the paddle shift is recalculated several times during filling. This analysis can provide a more accurate prediction of the final paddle shift when large deformations occur. The dynamic paddle shift analysis includes an option to perform Core shift analysis during pressure iteration, which is valid if the paddle has been modeled using 3D elements. However, if the paddle has been modeled using shell elements, the additional Core shift analysis cannot be performed. Microcellular Injection Molding analysis A Microcellular Injection Molding analysis simulates the development of cells in the melt during injection molding. NOTE: Not supported for 3D. The microcellular foam process, often referred to as MuCell process (developed by Trexel. inc ), works by heating and pressurizing a non-flammable gas such as nitrogen or carbon dioxide to a supercritical state, as illustrated in (a) below, which has characteristics similar to a fluid and produces a foaming agent, as illustrated in (b) below. Once this process has taken place within the barrel, the foaming agent is then injected into the plastic melt as indicated below: a) SCF is injected into the barrel/melt through the control valves. b) gas is dissolved in polymer melt to form single phase solution. Molding processes 21
Figure 7: Microcellular molding process Through the injection of the tiny uniform cell structure, higher properties at lower densities is retained more so than is seen with conventionally foamed parts. This allows molding of thin, light parts, producing parts that are not brittle. The reduced amount of resin in the parts also helps reduce cycle times. In addition, the process reduces clamp force requirements for molding. This is because the super-critical foaming agent acts as a solvent, which lowers the viscosity of the material by 40-60%, so lower pressure is required to push the material into the mold cavity. The process can be run at temperatures as much as 140 F below normal. The Process The microcellular processing consists of three main steps: 1 Gas dissolution In the plastication section of the injection molding process, a supercritical fluid SCF of blowing agent (CO2 or N2) is injected in to the polymer to form a single-phase solution. The gas is dissolved in the polymer melt due to applied high pressure. 2 Nucleation and bubble growth (foaming). There are two types of processes, either short shot or full shot: Short-shot process The mixture is injected into the mold cavity as a short shot to fill only part of the cavity. Due to the substantial and rapid pressure drop, the solution of the gas in the melt becomes supersaturated and a large number of bubbles nucleate and grow to fill the rest of the cavity. Full-short process The mixture is injected to fill the mold cavity completely. After the cavity is volumetrically filled, it is pressurized and the nozzle is shut off. The material within the cavity then shrinks and foams as the pressure falls. 3 Solidification During the foaming process the mold is continuously cooled down, creating the internal cellular structure. The result is a foamed material with cell size of around 5-100 microns, the actual size depending on processing conditions. The lack of a formal packing phase 22 Molding processes
reduces the residual stress in the material and results in extremely low warpage. Capabilities Microcellular Injection Molding analyses can help to: Reduce manufacturing costs. Improve material processability: reducing the amount of resin in the parts. Reduce cycle time. Low warpage, and eliminate sink marks. Provide useful information to detect various molding problems and to optimize part, mold, and process design in an efficient and cost-effective way. Lower the viscosity of the material by 40-60%. Mold thin, light parts, producing parts that are not brittle. Microcellular injection molding analysis types and analysis technologies The following table shows the available analysis technologies for a Microcellular injection molding analysis type. NOTE: The Microcellular molding analysis is not supported for 3D analysis technology. Table 9: Microcellular Injection Molding process and analysis types Analysis Type Analysis Technology Fill Fill+Pack Fiber Fill+Pack Cool Warp Resin Transfer Molding Resin Transfer Molding (RTM) is a liquid composite molding process. Unlike materials used in RIM or SRIM processes, where the chemical reaction is activated by mixing the reactants, the chemical reaction for resins used in RTM are thermally activated by heat from the mold wall and fiber mat (preform). The reaction rate in RTM processes is typically much slower than that in SRIM, allowing a longer fill time at lower injection pressure. Molding processes 23
The RTM process RTM is a process for the manufacture of fiber-reinforced composites. The resulting light-weight, high strength parts are attractive for many applications. Examples are chairs, automobile parts and aircraft components. RTM is of interest to the aerospace industry because it promises cost savings and performance improvements over traditional methods. In the RTM process, dry fiber reinforcement, or fiber preform, is packed into a mold cavity which has the shape of the desired part. The mold is then closed and resin is injected under pressure into the mold where it impregnates the preform. After the fill cycle, the cure cycle begins, during which the mold is heated and resin polymerizes to become rigid plastic. Benefits of RTM The greatest benefit of RTM relative to other polymer composite manufacturing techniques is the separation of the injection and cure stages from the fiber preform stage. Liquid molding also enables high levels of microstructural control and part complexity compared with processes like injection molding and compression molding. Other benefits afforded by RTM include: Low capital investment. Good surface quality. Tooling flexibility. Large, complex shapes. Ribs, cores and inserts. Range of reinforcements. RTM/SRIM analysis types and analysis technologies The following table shows the available analysis technologies for an RTM/SRIM analysis type. Table 10: RTM/SRIM process and analysis types Analysis Type Analysis Technology Fill+Pack Runner Balance Underfill Encapsulation analysis The Underfill Encapsulation analysis is used to analyze the flow of the encapsulant material in the cavity, between the chip and the substrate during the underfill encapsulation process. 24 Molding processes
NOTE: For analysis of a pressurized underfill process, use Microchip Encapsulation. Flip chips are sometimes used for high-density electronic packaging. Flip chips offer the advantage of an area array which interconnects the chip and substrate. A weakness in the flip chip process is the solders that typically connect the chip to the board. During service, the solders can be damaged, mainly from the stresses associated with the temperature change of the package. Underfill encapsulation is the process of filling the cavity between the chip and the substrate with a thermoset encapsulant. This helps to protect the solders during service. In most cases, underfill encapsulation is done by dispensing the encapsulant material along the periphery of the chip. Capillary force drives the encapsulant through the space between the chip and the board. During Underfill Encapsulation, each injection location is open at a different time because of the time delay when the dispensing head moves around the edge of the chip for the dispensing. It is often common to find that dispensing is done over several passes to avoid excessive spreading of encapsulant in the dispensing area when a large amount of encapsulant is dispensed all at once. This is known as dynamic dispensing. The Dynamic Dispensing analysis option in the Process Settings Wizard enables you to simulate this situation. Underfill Encapsulation analysis types and analysis technologies The following table shows the available analysis technologies for an Underfill Encapsulation analysis. Table 11: Underfill Encapsulation process and analysis types Analysis Type Analysis Technology Fill+Pack Shape factor calculation for underfill encapsulation There are different shape factor methods that you can use for an Underfill Encapsulation analysis, depending on whether the region is modeled away from the solders or near the solders in the model. Using shape factors To model the region away from the solders, use a shape factor of 1. To model the region near the solders, you can calculate a shape factor by using the following equation: Molding processes 25
Shape factor = (actual area of solid wall in thickness or planar direction) / (area of solid wall used in the simulation) For example, consider the following case shown below, where one cell is taken around a solder from the array of solders to consider the surface area around that solder: There are three areas related to this cell: A area around the solder in the upper region (not including the solder region). B area around the solder in the lower region (not including the solder region). C surface area of the solder. For this case, the shape factor is = (A + B + C) / (A+B) If the length of the cell (assuming square cell) is L and the diameter of the solder is D and the cavity thickness (which is the same as the solder height) is T, then: A = L2 πd 2 /4 B = L2 πd 2 /4 26 Molding processes
C = πdt Multiple-barrel Reactive Molding analysis types and analysis technologies The following table shows the available analysis technologies for a Multiple-barrel Reactive Molding analysis type. Table 12: Multiple-barrel Reactive Molding process and analysis types Analysis Type Analysis Technology Fill+Pack Molding processes 27