Inventor for Mould Design: Creating Core and Cavity Tooling

Injection mould tooling sits at the intersection of precision engineering and applied materials science. A mould tool for a consumer plastic part must hold dimensional tolerances of a few hundredths of a millimetre, withstand cyclic injection pressures exceeding 1,000 bar, dissipate heat rapidly through a cooling circuit, and eject the solidified part without marking or distorting it — all while running hundreds of thousands of cycles without significant wear. Designing that level of precision requires capable tooling CAD, and Inventor’s Mould Design environment provides a structured workflow for generating core, cavity, and slider geometry directly from a plastic part model.

This guide covers the complete mould design workflow in Inventor: analysing the plastic part for mouldability, generating the parting surface, creating core and cavity inserts, designing the mould base, adding cooling circuits and ejector systems, and extracting tooling drawings.

Autodesk Inventor Professional is available from GetRenewedTech at £39.99, with the Mould Design environment included in the Professional edition.

Understanding the Mould Design Workflow

Inventor’s Mould Design module operates as an environment within the assembly structure. The workflow follows a logical sequence that mirrors how toolmakers approach the problem:

1. Part analysis: Review the plastic part for draft angles, undercuts, wall thickness uniformity, and gate location suitability
2. Parting definition: Define the parting line and parting surface that separates the core from the cavity
3. Core and cavity generation: Use the plastic part geometry to subtract the part shape from the tool inserts
4. Mould base: Select and configure a standard mould base from the catalogue
5. Cooling system: Route cooling channels through the core and cavity inserts
6. Ejection system: Place ejector pins and define the ejector plate travel
7. Gate and runner system: Define the sprue, runners, and gates that deliver plastic to the cavity
8. Documentation: Generate tooling drawings and BOM

Plastic Part Analysis

Before creating any tooling, Inventor’s part analysis tools help identify potential problems in the plastic part design that would make it difficult or impossible to mould.

Draft Angle Analysis

Draft angle — the taper applied to vertical surfaces to allow the part to release from the tool — is fundamental to injection moulding. Surfaces parallel to the direction of ejection cannot release; they must be tapered by at least 0.5° (and typically 1–3° for textured surfaces). Inventor’s Draft Analysis tool colour-codes the part surfaces based on their draft angle relative to the specified pull direction:

• Green: Sufficient positive draft (core side)
• Blue: Sufficient positive draft (cavity side)
• Red: Insufficient draft — likely to cause ejection problems
• Yellow: Near-zero draft — borderline, depends on surface finish and material

Red surfaces must be addressed before proceeding. Options include adding draft to the part design (if you’re also the part designer), designing a lifter or slider to allow undercut features to release, or splitting the part differently to change the pull direction.

Undercut Analysis

Undercuts are features that would prevent the part from being ejected in the main pull direction — interior threads, lateral holes, clips that extend beyond the parting surface. Inventor identifies these automatically and highlights them for tooling consideration. Each undercut requires either a side action (slider or lifter) in the tool or a design change to the part.

Thickness Analysis

Wall thickness uniformity affects both the quality of the moulded part and the cooling time. Very thick sections create sink marks; very thin sections may not fill. The thickness analysis tool colours the part by wall thickness, making hot spots (areas significantly thicker than the average wall) immediately visible. Flagging these to the part designer early prevents expensive tool modifications later.

Defining the Parting Line and Parting Surface

The parting line is the line around the perimeter of the part where the core and cavity meet. Inventor’s Parting Line tool detects candidate parting lines automatically based on the pull direction you specify. For simple parts with a clear flat parting, this works immediately. For complex parts with stepped parting geometries, you may need to manually adjust which edges are included in the parting line loop.

Once the parting line is defined, the Parting Surface tool generates the surface that extends from the parting line outward to fill the split plane between core and cavity. For a flat parting, this is straightforward. For a complex contoured parting — common in automotive and consumer product tooling — Inventor can generate swept or lofted parting surfaces, though these sometimes require manual NURBS surface editing for particularly complex geometries.

Generating Core and Cavity Inserts

With parting geometry defined, the Core/Cavity tool splits the plastic part (with appropriate shrinkage scaling applied) into core and cavity regions and uses these to create the two mould inserts. The process involves:

1. Specifying the shrinkage factor for the plastic material being moulded (typically 0.3–2.5% depending on material; Inventor includes a material database with standard shrinkage values)
2. Defining the insert block sizes (length, width, depth of the core and cavity inserts)
3. Selecting which surfaces form the cavity side and which form the core side

Inventor then creates two solid insert models: a cavity insert with the female impression of the part (including the shrinkage scaling), and a core insert with the male impression. These are parametrically linked to the original plastic part — if the plastic part changes, the inserts update. This link is invaluable during the iterative early stages of a project when part design is still being refined.

Applying Shrinkage

Shrinkage scaling is critical — if the mould is made to the nominal part dimensions, the moulded part will be slightly undersized because the plastic contracts as it cools. Inventor applies a uniform or directional shrinkage factor to the part geometry before using it to generate the inserts. For a part with a nominal dimension of 100mm and a material shrinkage of 1.5%, the insert cavity is machined to 101.5mm so that the shrunk part measures 100mm.

Some materials (e.g., glass-filled nylons, long-fibre composites) exhibit anisotropic shrinkage — different in the flow direction versus transverse. For these, directional shrinkage values can be specified for each axis independently.

Selecting and Configuring the Mould Base

Inventor’s Mould Design module includes a catalogue of standard mould bases from major suppliers — DME, HASCO, Futaba, and LKM among others. Rather than designing the mould base from scratch, you select the appropriate series and size, and Inventor places the complete base assembly (including A and B plates, support pillars, leader pins and bushings, and return pins) into the design.

Mould base selection is driven by the insert sizes. A DN series HASCO base in 396×396mm provides standard hole patterns for cooling circuits and ejector pins that match industry-standard toolmaker expectations, making it straightforward for any toolroom to work with the design.

Designing the Cooling System

Cooling circuit design is often where the greatest opportunity for cycle time optimisation lies. An efficient cooling system removes heat uniformly from the cavity and core, minimising the time the tool must remain closed before the part is solid enough to eject. Inventor’s cooling tools allow you to route cylindrical cooling channels through the insert geometry and check for interference.

Routing Cooling Channels

Cooling channels are typically drilled holes (6–12mm diameter for most tools) routed close to the cavity surface. Inventor places these as sketch-driven cylindrical features within the core and cavity inserts. Best practice guidelines for channel placement include:

• Distance from channel centreline to cavity surface: 1.0–1.5× channel diameter
• Distance between adjacent channels: 3–5× channel diameter
• Baffle or bubbler fittings in areas where straight-through channels cannot reach (e.g., tall bosses or deep cores)

After routing channels, run interference detection to ensure channels don’t penetrate the cavity surface or collide with ejector pin holes, leader pin bushings, or other features within the insert block.

Cooling Analysis Integration

For comprehensive thermal analysis of the cooling system, the mould design can be exported to Autodesk Moldflow (included in the PDMC Collection) for simulation of heat transfer, cycle time, and potential warpage. The feedback from Moldflow can then drive changes to the cooling circuit layout in Inventor before the tool is cut, avoiding expensive EDM or drilling modifications after machining.

Ejector System Design

Ejector pins push the solidified part off the core when the mould opens. Inventor’s Mould Design module allows you to place standard ejector pin sizes (from HASCO, DME, or custom dimensions) at specified locations on the core face, then defines the ejector retainer plate that drives them.

Ejector pin placement requires judgment: pins should be located on robust features of the part (ribs, bosses, thick sections) where the ejection force won’t mark or distort the visible surfaces. The number and diameter of pins must be sufficient to distribute the ejection force across the part without stress-whitening or distortion, particularly for deep-draw parts where vacuum can create significant resistance.

Inventor automatically trims ejector pins to the correct length based on their position in the insert and models the ejector retainer and backing plate assemblies.

Gate and Runner System

The gate is the point through which molten plastic enters the cavity. Gate location affects fill pattern, weld line position, and the ease of gate removal from the finished part. Common gate types modelled in Inventor include:

• Edge gate: Simple, on the parting line, leaves a visible vestige — used for non-cosmetic parts
• Submarine gate: Enters below the parting line through a tapered tunnel, auto-degates on ejection — good for fully automated production
• Pin gate (three-plate tool): Gates from above into the cavity, requires a three-plate mould base but produces minimal gate marks
• Hot runner gate: No cold runner — plastic is kept molten in a manifold and injected directly into the cavity. Requires a hot runner system controller and heated manifold components modelled separately

Runners connecting the sprue to the gates are modelled as trapezoidal or circular cross-section channels in the parting surface. Full round runners are hydraulically efficient but require machining in both halves of the tool; trapezoidal runners can be machined in one half only.

Generating Tooling Drawings

The Mould Design assembly feeds directly into Inventor’s drawing environment. Standard tooling drawing views include:

• Plan view of cavity insert with gate, runner, and cooling channel positions dimensioned
• Plan view of core insert with ejector pin positions and cooling channel positions
• Section views through the parting line showing insert depth and overall tool height
• Exploded view of the complete mould assembly identifying all components
• BOM listing all purchased components (mould base plates, pins, fittings, cooling connectors) and machined inserts

For UK toolmakers, drawing standards typically follow BS 8888 (technical product documentation), and tolerancing for insert machining surfaces follows ISO 2768 or tighter tolerances specified by the tool designer based on functional requirements.

Summary

Inventor’s Mould Design environment provides a structured, parametrically driven approach to injection mould tooling that integrates directly with the plastic part model. From draft analysis and parting definition through to core/cavity generation, cooling circuit design, and ejector system layout, the tools guide the tooling engineer through a logical sequence that mirrors real toolroom practice. The parametric link to the plastic part means that design changes propagate correctly rather than requiring the tooling to be redesigned from scratch, and the output feeds both machining documentation and downstream simulation tools.

For engineers ready to explore Inventor’s tooling capabilities, Autodesk Inventor Professional is available from GetRenewedTech at £39.99. For broader product development workflows including Moldflow simulation, the Autodesk PDMC Collection at £149.99 provides the complete toolchain.