Coremaking Feasibility and Routing for Sand Casting
The core-making routings (see Sand Casting Process-level Routing) each have 0 or more occurrences of each of the coremaking processes shown below.
See Sand Casting General Process for a description of each coremaking method.
Which processes are included in a routing depends on which features are cored features. A GCD is considered to be a cored feature if it is obscured from the draw direction (and so not assigned a moldmaking operation—see Sand Casting Operation-level Feasibility).
The stock core process has at most one occurrence, which accounts for all the features that can be handled by a stock core (because, roughly, they have a standard diameter and length—see Stock Core Feasibility). Each other coremaking process generally has one occurrence for each segment of each core that is made by the process’s associated coremaking method. (By default, each cored feature has a single-segment core, but see Customizing the Number of Core Segments.) An exception to this occurs for some groups of simple hole cores or multistep hole cores that are similar in size; for such a group, a single occurrence may handle all the holes in the +group.
The following example shows the process-operation hierarchy for a part that has three voids handled by Isocure Gas coring, and two voids handled by No Bake coring:
Note that the coremaking processes each have a component-level child operation. The component-level operation itself has a child operation assigned to a particular hole or void GCD.
Each process occurrence often has only a single operation assigned to a cored feature, so that different cored features can use different machines even if they use the same coremaking method. For example, two cored features that differ significantly in size can use cores made by different isocure gas machines.
See Coremaking Machine Selection for more information.
How aPriori Chooses a Coremaking Method
The coremaking method associated with a given feature is determined as follows:
• Stock Core handles holes that, roughly, have a standard length and diameter (see Stock Core Feasibility).
• Other cored features are handled by the most preferred method that has a feasible machine (see Coremaking Machine Selection) and can accommodate the mass of the required core. The following table lists the coremaking processes, in preference order, together with the maximum core weight that the process can accommodate:
Method (in preference order) | Max Core Weight in Starting Point VPEs | Max Core Weight Cost Model Variable |
Isocure Gas (1st choice) | 115kg | maxWeightGasCore |
CO2 Cured | 115kg | maxWeightGasCuredCore |
Shell | 5kg | maxWeightShellCore |
Oil Core | 115kg | maxWeightGasCuredCore |
Hot Box | 70kg | maxWeightHotBoxCore |
No Bake | None | None |
Manual (last choice) | None | None |
Core weight is calculated based on sand density and core volume. Sand density is looked up by sand type in the lookup table tblSand; the sand type for each coremaking process is specified by the toolshop variable coreSandType. Note also that core volume includes coreprint volume (see Coremaking Machine Selection).
Manual coremaking is chosen only if there is no feasible No Bake machine.
Coremaking Feasibility
A GCD is considered feasible for coremaking only if it has all the following characteristics:
• Voids:
o At least one opening
o Some exposed area (for core prints)
o Nonzero tool reach
o Sufficiently small aspect ratio. Aspect ratio is tool reach divided by virtual diameter, and is considered sufficiently small if it is less than the value of the site variable maxCoreApectRatio (18 by default). Virtual diameter is the square root of the ratio of exposed area to number of openings.
• Holes:
o Diameter that equals or exceeds the value of the cost model variable minSandCastableHoleDiameter (6.4mm in starting point VPEs).
o Sufficiently small aspect ratio. Aspect ratio is length divided by diameter, and is considered sufficiently small if it is not greater than the value of the site variable maxCoreApectRatio (18 by default).
Additional feasibility constraints are described in Stock Core Feasibility and Feasibility for Coremaking Except Stock Core. See also How aPriori Chooses a Coremaking Method.
Stock Core Feasibility
The Stock Core process is included in the routing if there are any Simple Hole GCDs with all the following characteristics:
• Hole is obscured (not oriented in the draw direction).
• Draft angle is 0, if the setup option Should Check Drafts is set to true.
• The lookup table tblStockCores lists a core with both the following characteristics:
o Hole diameter equals the core diameter or exceeds it by no more than the value of the cost model variable coreDiameterPlay (1.5875mm in starting point VPEs).
o Core length exceeds the length of the hole by at least the value of the cost model variable lengthCorePrint (10mm in starting point VPEs).
Feasibility for Coremaking Except Stock Core
Generally, a coremaking operation is feasible for a given GCD only if all the following hold:
• Aspect ratio is acceptable. The aspect ratio is, roughly speaking, the GCD’s diameter to length ratio:
o For Simple Holes, the geometric properties Diameter and Length are used.
o For Multistep Holes, the geometric properties Min Diameter and Total Length are used.
o For Voids and ComboVoids, diameter is considered to be the square root of the ratio of the GCD’s Exposed Area to its Number of Openings, and the length is the geometric property Tool Reach.
The threshold for acceptable aspect ratios is specified by the site variable maxCoreAspectRatio (18 in starting point VPEs).
• Roughness and tolerance (if specified) are achievable by the current routing. In starting point VPEs, roughness must be no better than 12.5 microns, and tolerance no tighter than IT 14, for routings without machining.
• Length is not too long for the selected machine, that is, the machine property Max Corebox Length is greater than or equal to the longest dimension of the corebox required for the GCD. See Coremaking Machine Selection.
• Length is not too short for the selected machine, that is, the machine property Smallest Core is less than or equal to the following:
o For Simple Holes, the geometric property Length
o For Multistep Holes, the geometric property Total Length
o For Voids and Combo Voids, the geometric property Box Length
• Size is not too small to be castable; that is, the cost model variable minSandCastableHoleDiameter (6.4mm in starting point VPEs) is less than or equal to the following:
o For Simple Holes, Diameter
o For Multistep Holes, Min Diameter
o For Voids and ComboVoids, both Box Height and Box Width
See Sand Casting Operation-level Feasibility for operation feasibility by GCD type.
Note: Sand casting coremaking processes allow you to customize the number of coring operations for a given feature. By default, each cored feature is assumed to have a single-segment core, and so has one associated coring operation. For coremaking processes, you can override the default by editing the operation sequence for a cored feature—see Customizing the Number of Core Segments for more information.