Understanding Injection Mold Cavities and Cores

1. What are Cavity and Core?

Cavity

The cavity refers to the concave hollow section on the fixed half / cavity side of the mold. When molten plastic fills in, it adheres to the inner surface of the cavity and forms the outer surface of the molded part.

Simple analogy: The cavity replicates the outer profile of the product. Cosmetic surfaces, textures, grain finishes and gloss levels are all determined by the cavity surface. Curved contours on phonecase housings and engraved LOGOs are machined directly onto the cavity.

Core

The core is the protruding forming component generally located on the moving half / core side. Molten plastic wraps around the core to create the inner surfaces, internal ribs, screw bosses and throughhole features of the part.

Simple analogy: The core replicates the internal geometry of the product. Internal screw posts, reinforcing ribs, snapfits and hollow structures are all formed by the core. After mold opening, the molded part shrinks and clings to the core, from which it is ejected by the ejection system.

Fundamental principle: Molten plastic fills the gap between cavity and core, and the thickness of this gap equals the wall thickness of the plastic component.

2. Key Differences Between Cavity and Core

• Formed Surfaces

• Cavity: forms the outer / cosmetic surfaces of the part.
• Core: forms the inner surfaces, mostly noncosmetic areas.

• SurfaceTreatment Focus

• Cavity: Demands highgrade polishing and texturing. Any surface imperfection will be duplicated onto the visible part surface.
• Core: Prioritizes dimensional accuracy and draft angles. Polishing requirements are generally lower; minor machining marks are acceptable.

• Ejection Behavior Upon mold opening, plastic shrinkage causes the part to grip the core and travel backward together with the moving half, while the part separates from the cavity side. Insufficient draft may result in the part sticking to the cavity, leading to whitening or tearing of the component.
• SteelGrade Selection

• Cavity: Highpolishperformance mold steels such as S136 and NAK80 are preferred for corrosion resistance and superior polishing, ideal for cosmetic parts.
• Core: Materials like P20 or 718H are commonly used. Higherhardness steels are selected for heavyduty conditions or parts with dense rib features.

3. Critical Design Guidelines for Cavity & Core

3.1 Appropriate Draft Angles

Draft angles must be implemented for both cavity and core.

• Cavity (outer surface): Insufficient draft increases risk of cavity sticking and cosmetic surface scuffing.
• Core (inner surface): Cooling shrinkage creates strong wrapping force onto the core; inadequate draft easily causes scratching or ejection whitening. For highgloss cosmetic parts, moderately increased cavity draft is recommended to avoid surface damage during ejection.

3.2 Consistent WallThickness Control

Wall thickness is defined by the clearance between cavity and core. Misalignment between cavity and core produces variable wall thickness, triggering sink marks, depressions, warpage and bubbles. Precise alignment is required during mold machining and assembly.

3.3 Venting Design

Trapped air easily accumulates in cavity corners and around core ribs. Venting slots should be arranged along cavity edges and insert gaps of the core to prevent burning, short shots and flow lines.

3.4 CoolingChannel Layout

Cooling channels should be arranged for both cavity and core, maintaining uniform distance from mold surfaces. Uneven cooling leads to part warpage. Many molds only emphasize cavity cooling while neglecting core cooling, which results in slow internal cooling, longer cycle time and aggravated deformation.

3.5 Insert Strategy

Deep ribs, sharp bosses and undercut features should not be machined integrally into the main cavity or core. Adopt splitinsert construction for easier machining, polishing, maintenance and replacement, lowering future modification costs.

4. Typical Defects Related to Cavity and Core

• Surface scratches and pitting: Cavity surface wear, incomplete polishing, tool marks or surface contaminants.
• Sticking to cavity (fixed half): Insufficient cavity draft, rough cavity surface, or insufficient friction on the core side.
• Ejection whitening / cracking: Excessive wrapping force from inadequate draft on core ribs and bosses, creating concentrated ejection stress.
• Local sink marks: Uneven cavitytocore clearance leading to localized overthick sections.
• Unstable dimensional variation: Differential thermal expansion of cavity and core steels, or inconsistent cooling causing temperaturedriven dimension drift.

5. ProjectSelection Recommendations

• Cosmeticappearance parts: Prioritize cavity steel grade and surface finish requirements; confirm polishing / texture specifications in advance.
• Parts with dense internal ribs: Focus on core structure, insert solutions and core cooling performance.
• When evaluating mold quotations: Do not only consider mold base; verify cavitycore material and heattreatment hardness, which directly determine mold service life and production yield.
• If stickingmold or scuffing frequently occurs during initial trial runs, first inspect draft angles and surface roughness of cavity and core.