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Reactive Molding OverviewChapter 1:

Aim

The aim of this chapter is for you to learn about reactive molding analysis; its capabilities and usage.

Why do it

Many products are produced using thermoset materials, specific processes are used to mold these materials. Understanding the requirements, advantages and limitations of the process is important to reduce possible problems in the final product.

Overview

In this chapter, you will be introduced to:■ Reactive molding analysis processes types.■ Objectives of running a reactive molding analysis.■ Advantages and limitations of reactive molding processes.■ Similarities and differences of thermoplastics and thermoset injection molding analysis.■ Supported analysis types for thermosets and other capabilities.

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About reactive molding processes

Reactive molding processes use thermoset materials. Thermosets, unlike thermoplastics, are characterized by: ■ A chemical reaction during the molding process. ■ A cross-linked polymer structure. ■ Simultaneous polymerization and shaping during the molding process.■ Molded materials that cannot be recycled by re-grinding and re-melting.

Reactive processes simulated by Autodesk® Moldflow® InsightMajor reactive molding processes include thermoset molding, reactive injection molding (RIM), and composites processing, such as resin transfer molding (RTM) and structural reactive injection molding (SRIM). The typically low viscosity of the reactive materials permits large and complex parts to be molded with relatively lower pressure and clamp tonnage than required for thermoplastics molding. Reactive resins can also be used in the composite processes. For example, to make high-strength and low-volume large parts, RTM and SRIM can be used to include a preform made of long fibers. Reactive molding is also used for the encapsulation of microelectronic integrated circuit chips.

The reactive molding processes that Autodesk Moldflow Insight simulates are: ■ Thermoset injection molding ■ Reaction injection molding (RIM) ■ Structural reactive injection molding (SRIM) ■ Resin transfer molding (RTM) ■ Rubber compound injection molding ■ Reactive injection-compression molding■ Multiple-barrel reactive molding■ Microchip encapsulation (see Autodesk Moldflow Insight/Microchip Encapsulation, customized

for this process)■ Underfill encapsulation (see Autodesk Moldflow Insight/Underfill Encapsulation, customized for

this process)

How reactive molding worksA thermoset material is usually purchased as a liquid monomer-polymer mixture or as a partially polymerized molding compound. Starting from this uncured condition, a thermoset can be formed to the final shape in the cavity by polymerization. The polymerization is activated by heat or by chemical mixing, with or without pressure.

In reactive molding, 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 mold wall, from viscous (frictional) 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.

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Advantages of reactive moldingReactive molding processes offer the following advantages: ■ The cross-linked polymer structure of molded thermosets generally imparts improved mechanical

properties and greater heat resistance and environmental resistance.■ Thermosets' typically low viscosity permits large and complex parts to be molded with relatively

lower pressure and clamp tonnage 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 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.

Potential reactive molding problemsThe chemical reactions that occur during reactive molding filling and curing add complexity to mold and process design. For example, slow filling may cause premature gelation, resulting in a short shot. Fast filling may induce turbulent flow, creating internal porosity. Improper control of the mold temperature and/or inadequate part thickness can result in moldability problems or scorching. Scorching results from pre-mature gelation. The material cures too quickly, resulting in a scorched area that may be significantly degraded.

Autodesk Moldflow Insight/Reactive Molding analyses can help you avoid such problems, without costly and time-consuming trial and error debugging.

About Autodesk Moldflow Insight/Reactive Molding analysis

Autodesk Moldflow Insight/Reactive Molding generates results that help you design and optimize a reactive molding project, to achieve the objectives listed below.

Table 1: Reactive Molding results and objectives

Results generated by Autodesk Moldflow Insight /Reactive Molding

Objectives

Predicts melt-front advancement against time. ■ To aid in part design and gate placement.■ To avoid hesitation, race tracking, and

stagnation of flow.

For SRIM and RTM processes, predicts the effect of fiber-mat preform (fiber-mat) properties on melt-front advancement.

■ To aid in part design and gate placement.■ To avoid hesitation, race tracking, and

stagnation of flow.

Calculates the degree of cure at all locations. ■ To avoid short shots caused by resin that cures before the cavity fills.

■ To predict cycle time required for adequate cure (gelation).

Evaluates different reactive materials from the Moldflow material database.

To select the best material for your project.

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Similarities and differences of thermoplastic and thermoset injection molding analysis

When you analyze reactive molding processes, keep in mind that your analyses have much in common with Autodesk Moldflow Insight thermoplastics injection molding analyses, as follows. ■ Model and mesh requirements are the same. ■ There is heat transfer between the mold and the resin.■ Many of the resins show shear thinning behavior. ■ Filling is from a single source.

However, remember that thermosetting resins have other characteristics that must be treated differently from thermoplastic resins, as follows: ■ The melt temperature at the entry point is lower than the mold wall temperature. ■ Initially, heat is transferred from the mold wall to the resin.■ A chemical reaction is initiated as the temperature of the resin reaches a threshold. ■ The reaction releases heat that can raise the temperature of the resin above the mold wall

temperature. ■ The viscosity is a complex function of the temperature, shear rate, and degree of the chemical

reaction.

Graphically displays the temperature change as a result of the reaction kinetics inside the mold at any point in time.

To troubleshoot the process.

Determines injection pressure and clamp force requirements.

To select the proper molding machine size.

Highlights potential weld or meld line problems. To improve part design and gate placement.

Highlights potential air traps. To design proper mold venting.

Balances runner systems. To optimize the design and reduce cost.

Table 1: Reactive Molding results and objectives (continued)

Results generated by Autodesk Moldflow Insight /Reactive Molding

Objectives

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Supported Analysis Types in Autodesk Moldflow Insight/Reactive Molding

The analysis sequences available for Reactive Molding include:■ Flow (Incompressible)■ Runner Balance (Incompressible)■ Fill (Compressible)■ Fill + Pack (Compressible)■ Fill + Pack + Warp (Compressible)■ Runner Balance (Compressible)■ Fiber option for Fill, Pack, Warp (Compressible)

When running a Warp analysis for Reactive Molding or Microchip Encapsulation, both shrinkage and the warpage of components can be calculated. Running a Fill + Pack + Warp analysis requires PvTc (Pressure, specific volume, Temperature, and degree of cure) information. The PvTc data must be gathered from both cured and uncured states. A Warp analysis for a themoset material is done in much the same way as a thermoplastic, by using PvT and mechanical properties. During the packing/curing analysis the volumetric shrinkage is calculated. Using the PvT, coefficient of thermal expansion, and cure level, residual stresses are calculated. This information is then used in the structural analysis to determine the warpage.

When using a compressible sequence, the Process Settings Wizard is configured similar to a thermoplastics input page (Figure 1).

Figure 1: RIM settings when using a compressible material

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Other Analysis Capabilities

Mold surface temperature profiles available for 3D thermoset analysesBy default, a constant mold temperature is used for the analysis. However you can set up variable mold surface temperature profiles for Reactive Molding and Microchip Encapsulation analyses on 3D models.

Variable mold temperatures are used in molding thermosets to mainly decrease cycle times while ensuring that there is no premature curing; at the same time it reduces the scrap rate. In general, the temperature of the mold is quickly increased during the second half of the curing time if sooner is not possible; the timing will depend on the amount of swelling experienced by the resin when it is molded. The increase of temperature will result in a decrease of the curing time. Depending on the material molded, this technique could improve the surface finish of the part.

The profiles are used in analyses including Fill + Pack analysis sequences. This will allow you to examine the effect of the mold temperature variation specially during the curing stage of the material.

A mold surface temperature profile is set up by specifying part surface properties on the model. A study can have up to 2,500 individual profiles, each profile having up to 50 time and temperature data pairs.

Mold surface temperature profiles can be specified on the following element types:■ Cavity■ Cold runners■ Cold sprues■ Cold gates■ Part beamTemperature profiles cannot be set on hot runner elements.

The dialog used to set up a mold surface temperature profile is shown in Figure 2.

Figure 2: Mold surface temperature profile dialog

Temperature profiles are not available for Underfill Encapsulation analyses.

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Notice that a profile is defined by time and temperature data pairs, and the times must be entered in ascending order. You can have a combination of linear or abrupt changes in temperature. The last temperature specified in the profile will be used for the remainder of the cycle.

To set up a mold temperature profile, you need to assign a mold surface profile to an element property first, then you can modify the profile as you would like.

To assign a temperature profile:1. Select the entities on which the temperature profile is to be applied.

2. Right-click on the selected entities and select Properties.

3. If necessary select the correct Part (3D) properties from the list and click OK.

4. Select the Part Surface Properties tab.

5. From the Mold surface temperature list, select Profile.

6. Click Select.

7. Select a profile from the Mold surface temperature list. If the required profile does not appear in the list:

7.1. Click Select to browse for a predefined profile; for example, you can choose a profile that you saved in another study or from a personalized database you may have.

7.2. Click Select to confirm the profile selection and return to the Select mold temperature controller dialog.

8. If necessary, click Edit to make changes to the selected profile or define a new profile.

9. Edit the temperature profile information in the table; specify a suitable name if you are defining a new profile.

10. Click OK to save the profile and return to the Select mold temperature controller dialog.

11. Ensure the required profile is selected from the Mold surface temperature controller list and click OK twice to exit the dialog.

When a local Mold Temperature profile has been specified on an area, it overrides the constant Mold surface temperature set for the analysis on that same area.

If you modify an existing profile, all zones previously assigned with that profile will be modified as well.

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Preconditioning analysis option for all thermoset analysesThe option to perform preconditioning analysis is supported for all mesh types. This option is enabled by default and the information is used to calculate the melt temperature of the thermoset material as it starts flowing into the cavity.

You can access the option to Perform preconditioning analysis from the Profile Settings page of the Process Settings Wizard, as shown in Figure 3. You can change this setting from this menu as well.

Figure 3: Process Settings Wizard - Preconditioning analysis option

From the Edit data button you will access the Preconditioning data menu displayed in Figure 4.

Figure 4: Preconditioning data

The preconditioning data includes:■ Pellet diameter.■ Pellet length.■ Transfer pot temperature.■ Delay time in the pot.■ (Check box for) Stop flow calculation when pellet volume is smaller than filled volume.

Preconditioning analysis is available for Reactive Molding and Microchip Encapsulation molding processes only. Preconditioning analysis for Underfill Encapsulation molding analysis is not supported at this time.

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Vents can be defined with reactive molding processesWhen a Reactive Molding or Microchip Encapsulation analysis is run with a tetrahedral mesh, vents can be added to the model. The conventional assumption in a Fill + Pack analysis is that the cavity is perfectly vented; that is, the air within the cavity had means to vent completely and did not influence the flow of material. However, this assumption may not always hold true. With a venting analysis, the location and size of the vents and their influence on the filling can be determined.

To assign location and size of the vent: 1. Click Analysis > Set Venting Analysis Locations to open the Set Venting analysis location dialog

(Figure 5).

2. Setting a vent location works the same way as setting a coolant line location. Select the vent property required.

3. Click on the model to set the location at nodes.

4. Click the New or Edit buttons to view or modify the vent properties (Figure 6).

5. Using a vent size defined in the analysis Advanced Options is the default. A specific vent size can be set, as shown in Figure 7. The vent sizes include:

• Thickness, 0.03 mm (0.0012 in)

• Length, 3 mm (0.118 in)

• Width, 3 mm (0.118 in)

The vent exit pressure is also defined. The default value is 0 MPa, or atmospheric pressure. The pressure can be negative to define a vacuum or a positive pressure to 1 MPa.

To run a venting analysis1. Ensure that you have selected either Reactive molding or Microchip encapsulation as your

molding process.

2. Double-click (Analysis > Process Settings Wizard) in the Study Tasks pane.

3. Click Advanced Options.

4. Click Edit next to the Solver parameters section.

5. Select the Venting Analysis tab.

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6. Select the Perform venting analysis check-box.

Figure 5: Setting a vent location

Figure 6: Venting analysis location dialog

Figure 7: Setting the size of the vents

How the venting analysis worksWhen a venting analysis is run, the analysis calculates the pressure drop through the vents, then the pressure ahead of the flow front. Finally, it calculates the influence of the air pressure on the flow front.

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Viewing venting resultsThe result from the venting analysis is called Vent region pressure. It is an intermediate result that shows at time steps during the filling phase the pressure of the air ahead of the flow front. The pressure will typically be very low, but can become very high if there is an area that is not vented. In Figure 8, there are vents in the second rib (0.2787 [MPa]) and at the end of the part (0.0024[MPa]) but not at the rib closest to the gate. This is why the pressure is highest in the rib closest to the gate. After the rib and end of the part fills, the pressure in the first rib jumps to over 6 MPa because the air is trapped.

Figure 8: Vent region pressures

In Figure 9, the vents are different sizes. The part on the left has the default gate size, 0.03 mm x 3.0 mm x 3.0 mm. The vent on the right was reduced in size to 0.01 mm x 6.0 mm x 3.0 mm. The pressure is significantly higher in the part with the smaller vent. This also increased the pressure at the V/P switchover by nearly 6 MPa.

Figure 9: Vents of different sizes

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