Plastic Molding Cost Model Enhancements

aPriori 2019 R2 includes several enhancements to the plastic material data available in the aPriori Regional Data Libraries. Several material compositions were added, material property values were updated, and a new methodology for determining Thermal Diffusivity was introduced in order to make it easier for customers to add additional compositions to their own VPEs.

• Eight additional material compositions now are included in the Plastic Molding and Assembly Molding process groups, at the request of several customers:

o Acrylic (PMMA)

o PBT (Polybutyleneterephthalate)

o PBT GF30

o PBT GF40

o PPS (Polyphenylenesulfide)

o PPS GF30

o PPS GF40

o Polystyrene (General Purpose)

• The Materials table for the Plastic Molding and Assembly Molding, and Roto & Blow Molding process groups now includes three new fields: Specific Heat of Melt, Thermal Conductivity of Melt, and Density of Melt. These new fields are used to calculate a value for the Thermal Diffusivity of a material, which in turn is an input to Cooling Time calculations. Previously, the Material Table included a field directly for Thermal Diffusivity; however, it often was difficult to determine the value of this field directly from material specification data sheets. In contrast, Specific Heat and Thermal Conductivity values are readily available, making it much easier for customers to add materials to their configured VPEs and provide all the necessary material properties.

By default, the historical field for Thermal Diffusivity now is hidden in the baseline Material Table, as the value now is computed instead. However, this field still exists and has a value, enabling the updated Material RDLs to be used with older cost model versions if desired. Also note that the current cost model will fall back to using the value of the Thermal Diffusivity field if the values for Specific Heat, Thermal Conductivity, and Density of Melt are not populated. To ensure the most accurate cooling time estimates, customers who upgrade to the latest cost model version are advised to populate the new Specific Heat and Thermal Conductivity and Density of Melt fields for the materials in their VPEs.

Note: For some materials, the computed value of Thermal Diffusivity used in aPriori 2019 R2 is somewhat different than the material field value which was used in previous releases. As a result, the calculated Cooling Time for a part made from one of those materials also has changed somewhat. The amount of change depends on the specific material and part. See more information at the end of this section for a summary of the impact of this and other changes on plastic part estimates.

• aPriori expanded the data sources from which we gather plastic material property information in order to validate the plastic material property data available in our Regional Data Libraries. As a result, various material property values were updated for many of the material compositions included in the Plastic Molding, Assembly Molding, and Roto & Blow Molding process groups. Material fields which may have been updated for a given material include Density, Melt Density, Eject Deflection Temperature, Melting Temperature, Mold Temperature, Min Injection Pressure, and Max Injection Pressure.

In most cases, data value adjustments were small-to-moderate. In some isolated cases a more significant correction was made (for example, the Density of Nylon 66 previously was significantly too high and was reduced to an appropriate value; for some ABS compositions the Eject Deflection Temp, Melting Temperature and Mold Temperature values all were changed by a moderate amount).

Because of these various updated material property values and the new method for computing Thermal Diffusivity, calculated estimates of cooling time may change, and therefore the selected machine, labor time and cost, overhead rate, time, and cost, and/or material cost estimates will change for a given plastic part when comparing aPriori 2019 R2 results to previous releases. The amount of change depends on the molding process, material, part geometry, and production volume and can be either an increase or decrease. Change to thermal diffusivity values for ABS materials resulted in significant increases in estimated cooling time (in a set of test parts, we observed ~45 – 95% higher depending on grade) and therefore cost (~ 30 – 55% increase in our test set for parts with thickness <= 4mm, and ~ 60 – 95% increases for parts with higher thickness, often made with Structural Foam molding process ). For Nylon materials, changes to the values of Thermal Diffusivity values, Eject Deflection Temperature, Melting Temperature and Mold Temperature resulted in decreases in estimated cooling time (~ 14 – 80% lower in the test set) and therefore cost (~0 – 30% lower in the test set). For more details, see Impact to Cost Estimates When Upgrading to aPriori Professional 2019 R2 (also referred to as Version 2019 R2 of the aPriori Cost Impact Guide).