Plastic Molding Enhancements

Melt Flow Analysis for Injection Molding, Reaction Injection Molding, and Structural Foam Molding Processes

aPriori now performs a basic material flow analysis for injection molding processes that determines the minimum number of gates required to fill a cavity and more accurately estimates cavity pressure and required clamp force. The improved clamp force calculation results in better machine selection and thus more accurate direct overhead costs.

aPriori 2023 R1 further extends the improvements to estimates of cavity pressure and required machine clamping force that were provided in release 2022 R1 SP1. The cavity pressure is calculated from various inputs including the part wall thickness distribution across the part and the maximum flow length(s) that the melt must travel from the gating locations to the extremities of the part. The 2022 R1 SP1 enhancements improved estimates of the flow length, based on part dimensions and consideration of gate locations. However, aPriori still relied on user input on the number of gates used in the mold and the number of gates was assumed to be 1 by default. If the number of gates was not specified correctly for large parts and other parts that are molded with multiple gates, aPriori clamp force estimates were higher than required. In turn, aPriori could select a larger machine with higher Overhead Rates than required.

The Geometry engine in aPriori 2023 R1 performs a Flow Appraisal, which is a basic melt-flow simulation that uses the geometry of the part and flow properties of the selected material to automatically determine the minimum number of gates required to fill the cavity, the corresponding cavity pressure, and the required clamp force. As a result of this enhancement, estimates of required clamp force generally will decrease for many parts, particularly larger parts or those with higher length-to-width ratios, compared to previous releases of aPriori. This in turn results in more accurate selection of the lowest-overhead machine that can produce the part, potentially reducing Overhead costs.

To support this enhancement, materials in the Plastic Molding process group now contain a new field, Flow Length Ratio, which is used in the melt flow simulation. The Flow Length Ratio is an experimental index that indicates the distance to which the leading edge of the flow can reach when a specific plastic is made to flow inside a cavity of uniform unit thickness and at a fixed pressure. Materials that flow easily, such as LDPE or PP, have values of ~300 or higher, while more viscous materials have lower values. The aPriori Regional Data Libraries 2023-02 provide values for this field for all the baseline materials. Customers should specify this value for any custom materials in their Digital Factories.

If the Flow Length Ratio is not specified, aPriori uses a conservative default value of 150. If this value is too conservative for the part, aPriori could underestimate the material’s ability to flow and therefore overestimate the number of gates, the cavity pressure, and the required clamp force. The default value for Flow Length Ratio is controlled by the site variable flowLengthRatioDefault.

Flow Appraisal analysis is iterative. It evaluates possible gate locations and determines the minimal number of gates needed to guarantee that all portions of the part can be reached by the melt material before it solidifies. The analysis considers both the Flow Length Ratio of the selected material and the geometry of the part. The analysis does not prioritize the gate placement in terms of manufacturability or cosmetic appearance. As such, the gate locations are not displayed in the diagram.

By default, the Flow Appraisal analysis is performed whenever a new part that uses the Plastic Molding process group is analyzed. Flow Appraisal analysis is leveraged in the Injection Molding, Reaction Injection Molding (RIM) and Structural Foam Molding (SFM) routings. These three potential tooling configurations are always analyzed and captured as Flow Appraisal GCDs:

• Flow Appraisal:1 – considers Gate(s) freely placed within the cavity

• Flow Appraisal:2 – considers Gate(s) placed on the long edge of the cavity

• Flow Appraisal:3 – considers Gate(s) placed on the short edge of the cavity

Note: The aPriori Flow Appraisal is a basic melt flow analysis and is not intended to replace specialized mold tool design and analysis software. However, to validate this enhancement aPriori, compared its clamp force estimates to those produced by an industry-leading mold simulation application. A set of ten parts of varying size and geometric complexity were assessed across 16 different baseline materials (Polycarbonate, PC+ABS, Acrylic (PMMA), TPE, ABS, PPS, Acetal, PBT, Nylon (PA6), Nylon (PA66), Nylon (PA46), Polystyrene, Polypropylene, TPO, TPU and PET). For these parts and materials, the average difference in clamp force predicted by aPriori versus the specialized software was only 7%, with a low standard deviation.

Configuration

This table shows the default settings and process setup options that control the tooling configuration that aPriori uses to calculate manufacturability and cost estimates.

The settings were available prior to 2023 R1.

Tooling Configuration	Default Value	Alternative Value	Controlling Cost Model Variable
Gating Runner System	Cold Runner	Hot Runner	defaultRunnerSystem
Gating Type for Cold Runner System	Edge	Center	defaultGatingType
Edge Gate Location	Long	Short	defaultRunnerAttachmentSide
Number of Plates in the Mold	2	3	–

To review or edit the tooling configuration:

1. in the Geometric Cost Driver pane, in the menu bar, select Edit > Process Setup Options.

   

2. Then, in the Process Options Editor, expand the relevant nodes and select the molding process.

   

   Tip: The calculated values for the Minimum Number of Gates per Cavity and Selected Flow Appraisal are also displayed in the Process Options Editor.

Previous releases included the Number of Gates per Cavity as an editable process setup option for inputting the number of gates. This process setup option is no longer editable in the Process Options Editor. However, you can override the displayed calculated minimum required number of gates:

1. In the Geometric Cost Drivers pane, navigate to and select the Flow Appraisal GCD that aPriori selected.

2. Double click the row that contains the Number of Sites and then edit the value.

   

   For example, if aPriori calculates that a minimum of 5 gates are required for the specified tooling configuration but you know that 6 will be used in practice, updating the value and recosting the part results in updated estimates of required clamping force, which in turn can influence machine selection and overhead cost estimates.

   Caution: If you override the Number of Sites value with a number that is lower than calculated value, it is likely that the costing will fail due to an expected short shot condition. If the costing fails but you still want to evaluate the scenario for a lower number of sites, you can change the feasibility rules for the molding process via the routing editor and then recost the part.

Results

The Flow Appraisal analysis in aPriori Professional generates a 3-D heat map (color-coded representation) of the part that allows you to visualize how material fill the mold for each potential tooling configuration and assumed gate locations.

As this flow chart shows, the heat map results depend on the selected tooling values for the Flow Appraisal.

To open a heat map, in the Geometric Cost Drivers pane, in the Name Column, expand the for Component and Flow Appraisals nodes, and then select the Flow Appraisal that you are interested in.

The blue regions of the part fill first and are less than 10% of the material maximum flow ratio from a gate location. The red regions of the part fill last and are between 90% and 100% of the maximum flow ratio from a gate location.

You can also see the effects of a short shot condition in the Flow Appraisal heat map. For example, the black regions in this figure are for short shot condition that resulted from a Cold Runner system with Edge gates on the short side. Regardless of the number of gates placed along that short edge, the material flow is not sufficient to travel across the length of the part.

In addition to reviewing these results in the Process Options Editor, as described in Configuration, you can see the calculated values for the MinimumNumber of Gates per Cavity and Selected Flow Appraisal in the Parts Details tab:

1. Open the Parts Details tab.

2. In the Cost Object column, in the Name sub-column, expand the relevant nodes to reveal the molding process.

3. In the Custom Outputs column, expand the nodes for the Output Name and the Output Value sub-columns.

   

To view the clamp force:

1. in the Manufacturing Process pane, in the menu bar, select View > Process-Based Manufacturing View.

   

2. In the Process Step column, expand the relevant nodes to reveal the molding process.

   

If aPriori calculates that the cavity cannot be filled for the specified tooling configuration regardless of the number of gates, then the costing fails and a warning message indicates the reason for the failure.

A short shot condition is likely to be the reason for costing failures for cold runner systems.

aPriori provides Design Guidance that indicates how to avoid short shot conditions. For example you could change the tool configuration, material, or part geometry.

Impact

In a set of test parts, estimates of required clamp force decreased significantly compared to the previous release of the manufacturing process model, with an average reduction of 39% for both hot and cold runner systems. This reduction sometimes led to the selection of a lower-clamp force, lower-overhead machine.

On average the resulting machine overhead rate decreased by 18% when hot runner systems were specified and 13% for cold runner systems. The resulting decreases in Piece Part Cost were approximately 6.5% and 3.75%, respectively.

Machine selection did not always change compared to the previous release even when required clamp force decreased, either due to other machine requirements or the specific clamp force capabilities available across the various machines in the Digital Factories.

More significant differences in required clamp force estimates were seen for longer, thinner parts and for parts made with higher viscosity materials.

Limitations

The Flow Appraisal analysis is not yet included in the Assembly Molding and Assembly Plastic Molding process groups for insert-molded/over-molded and multi-shot parts, respectively.

Upgrade Considerations

To leverage the Flow Appraisal enhancement, you must upgrade your manufacturing process models to version CMV:220 or higher. To then use Flow Appraisal for a scenario that was cost in previous release or cost model version, the scenario must be connected to the CAD file and the geometry must be re-extracted.

If you do not want to upgrade your cost model at this time, to avoid an unnecessary 9% increase in extraction time, you can disable the Melt Flow simulation by setting the site variable isFlowAppraisalEnabled to False.

Average Material Injection Pressure for Cavity Pressure and Required Clamping Force Estimates

Cavity Pressure and Required Clamping Force calculations now better represent typical manufacturing conditions by using the average maximum and minimum injection pressure values for the material.

In previous releases, Cavity Pressure and Required Clamping Force calculations used the maximum injection pressure value for the material. To better represent typical manufacturing conditions, both calculations now use the average maximum and minimum injection pressure values.

The new process setup option (PSO) Maximum Material Injection Pressure allows you to select or specify an alternative value for the maximum material injection pressure in cavity pressure and clamp force calculations.

As the figure of the Maximum Material Injection Pressure PSO shows, you can now choose between these options:

• Material Recommended Max

• Material Recommended Average (default option)

• Material Recommended Min

• User Override – Enter a value that is greater than or equal to the recommended minimum value.

Impact

Due to this enhancement (and to material flow analysis enhancement described earlier in these release notes), estimates of cavity pressure and required clamping force will generally decrease compared to the previous version of the manufacturing process model. This decrease, in turn, may result in the selection of a machine that has lower-clamp-force and lower overhead rates than the machine that was selected for some parts in the previous release.

Industry Standard Values for Material Injection Pressure Boundary Values

The minimum and maximum injection pressure values for several Plastic Molding materials were updated to correspond to standard values used in industry-leading material flow analysis tools.

The Injection Pressure Min and Injection Pressure Max values were updated for several Plastic Molding materials. The updated values are included in the aPriori 2023 R1 starting point Digital Factories and the aPriori 2023-02 Regional Data Libraries.

This table below shows molding material families and indicates those for which the minimum or maximum injection pressure value was updated. Arrows indicate whether the new value is higher (▲) or lower (▼) than the value in the previous release. A dash (–) indicates that the value is unchanged and therefore is not shown.

Material Type	Injection Pressure Min (MPa)	Injection Pressure Max (MPa)
ABS	–	–
Acetal	40 ▼	–
Acrylic (PMMA)	50 ▼	–
Nylon (PA46)	40 ▼	150 ▲
Nylon (PAL)	55 ▲	–
Nylon (PA66)	55 ▲	–
PBT	70 ▲	–
PC+ABS	50 ▼	130 ▲
Polycarbonate	70 ▲	–
Polypropylene	–	–
Polystyrene	30 ▼	130 ▲
PPS	35 ▲	110 ▲
TPA	–	–
TPE	40 ▲	140 ▲
TPO	–	120 ▼
TPS	–	–
TPU	–	100 ▼
TPV	–	–

Explicitly Calculated Plasticizing Times for Injection Molding Cycle Time Analysis

aPriori cycle time estimates now explicitly consider whether plasticizing time exceeds cooling time rather than assuming the opposite.

Plasticizing time is the time required to convey the plastic pellet material down the barrel of the machine and melt it in preparation for the next injection cycle. Plasticizing occurs in parallel with cooling time. While one part is cooling, the molding machine begins plasticizing the material in the barrel for the next injection cycle. Previously, cooling time was always assumed to exceed plasticizing time, and cycle time was computed as the sum of injection time, cooling time, and eject time. Now aPriori computes the plasticizing time explicitly; if plasticizing time exceeds the calculated cooling time then plasticizing time replaces cooling time in the total cycle time calculation.

To support this enhancement, a new machine property, Plasticizing Rate (GPPS), was added. The new property specifies the machine's plasticizing rate for general purpose polystyrene, which is the industry-standard benchmark. To calculate plasticizing time, the manufacturing process model adjusts the Plasticizing Rate (GPPS) to account for the part’s actual material. If no value is specified for Plasticizing Rate (GPPS), the cost model retrieves the information from the plasticizingRateByClampForce lookup table.

Impact

For most parts, this enhancement will not change cycle time estimates compared to the previous release because plasticizing time rarely exceeds cool time. However, plasticizing times may exceed cooling times for parts that are:

• Made of glass-filled materials

• Thin-walled parts and made from a material that has a very short cooling time

• Processed by a machine that has an insufficient injection barrel capacity

Lower Ejection Velocity for Injection Molding, Reaction Injection Molding, and Structural Foam Molding Cycle Time Estimates

aPriori now assumes that the ejection velocity is 50% of the machine’s maximum velocity rather than 100%, thereby increasing ejection time and total cycle time estimates.

CM3-390

In the previous release, aPriori overestimated eject velocity and therefore underestimated eject time for Time for Injection Molding, Reaction Injection Molding, and Structural Foam Molding. Eject velocity was assumed to be the machine's maximum velocity (estimated based on the machine's dry cycle time and tie bar distances).

Now, by default, eject velocity is assumed to be 50% of the machine's maximum velocity. The resulting ejection and cycle time estimates for Time for Injection Molding, Reaction Injection Molding, and Structural Foam Molding are slightly higher than they were in the previous release.

The cost model variable ejectorPercentMaxMachineVelocity controls the percent of the machine's maximum eject velocity that aPriori uses for ejection time calculations. Digital Factory managers can customize the variable value which is set to 50 in starting point Digital Factories.

Note: You can restore the behavior of the previous release by setting the cost model variable ejectorPercentMaxMachineVelocity to 100.

This release also introduces the cost model variable numEjectorCycles, which allows aPriori to make better estimates for processes that use multiple ejector cycles. In starting point Digital Factories, numEjectorCycles is set to 1. Digital Factory managers can increase the value.

Expanded Cavity Finish Options and Improved Estimation of Mold Texturing for Tooling Cost Estimates

aPriori now provides more accurate Tooling Cost estimates because it estimates mold texturing costs more mechanistically and uses updated values for cavity finish options and polishing times.

Expanded Cavity Finish Options

CM3-522

In the previous release, the Cavity Finish process setup option (PSO) allowed you to choose between these four finishing grades:

• A (High-Gloss)

• B (Semi- Gloss)

• C (Matte)

• D (Textured).

In this release each of the four grades was expanded into three sub-grades (1, 2, 3), so the PSO now provides 12 finishing grades:

• SPI A1 - Super High Gloss

• SPI A2- High Gloss

• SPI A3 - Normal Gloss

• SPI B1 - Normal Semi-Gloss (default)

• SPI B2 - Medium Semi-Gloss

• SPI B3 - Normal Semi-Gloss

• SPI C1 - Fine Matte

• SPI C2 - Medium Matte

• SPI C3 - Normal Matte

• SPI D1 - Satin

• SPI D2 - Dull

• SPI D3 - Rough

The polishing hours associated with each grade were also updated for 2023 R1.

As a result of this change, relative to the previous release, parts with a higher surface area generally have a larger change in tool finishing cost. With respect to the four finishing groups, tool finishing costs will:

• Increase somewhat for Grade A.

• Increase or decrease for Grade B depending somewhat on cavity surface area.

• Decrease somewhat for Grade C

Note: If a scenario from a prior release includes an override for the default cavity finish, aPriori 2023 R1 uses the current default value instead of the override value. The defaults are specified by the cost model variables defaultCavityFinishIM, defaultCavityFinishRIM, and defaultCavityFinishSFM (set to SPI B3 - Normal Semi-Gloss in starting point Digital Factories).

Mold Texturing Estimation Improvements

Error Checking for Tooling Configurations

aPriori now checks for incompatible process setup options that result in an invalid plastic molding tooling configuration.

If you specify an invalid combination of plastic molding process setup options (PSOs) for the PSOs that are shown in this figure, aPriori now fails to cost the part.

An invalid combination for a part that uses the plastic molding process group is:

• Either the Number of Cavities is greater than 1 or the Minimum Number of Gates Per Cavity is greater than 1.

• Gating Runner System is set to Cold Runner.

• Gating Type for Cold Runner System is set to Center.

• Number of Plates in Mold is set to 2.