What is Mold Cavity?

1. Introduction

The mold cavity is a critical component that largely determines the shape, dimensions and surface finish quality of injection molded parts. This article will provide an overview of what a mold cavity is, different cavity types and designs, key cavity features, temperature control, surface treatments, fabrication methods and quality inspection.

2. Definition of Mold Cavity

The cavity refers to the impression or void space within a mold where the molten plastic is injected and forms the desired shape of the part. The cavity has the identical geometry as the part, just in reverse.

3. Types of Mold Cavities

There are a few main types of mold cavities used in injection molding:

Single cavity mold - This is the most basic type with just one cavity to produce one part at a time. It's good for low volume production or prototype runs.

Multiple cavity mold - Contains two or more cavities to manufacture multiple parts simultaneously. This significantly increases productivity for higher volume production.

Stacked cavity mold - Cavities are stacked vertically on top of each other to maximize machine clamp tonnage utilization. Great for large, heavy parts.

Slide mold - One cavity slides or moves to the side after each cycle to inject into the next area. Used for long, continuous parts like tubing.

Insert molding - A cavity within a cavity to mold over another pre-formed component like inserts or connectors.

Gas assist injection cavity - Includes auxiliary gas injection points to help expand or shape parts, especially thin shells.

Co-injection cavity - Injects two or more materials simultaneously into the same cavity for multi-material components.

Micro mold cavities - Tiny cavities on the scale of millimeters for micro-molding of miniature parts like electronics.

Some other specialized cavities include side-gated, hot runner, family mold, and sandwich plate types optimized for different product and manufacturing requirements. Proper cavity selection depends on production needs, part design, and process efficiency goals.

4. Components of a Mold Cavity

The main components of a mold cavity include:

Cavity block - This forms the base of the mold cavity shape. It is precisely machined to the exact dimensions and tolerances required for the part design. High-grade alloys like tool steel or carbide are often used.

Core pins - Removable round or shaped pins inserted into the cavity block that create undercuts, holes, or interior features in complex parts. Made of the same alloys as the block.

Ejector system - Consists of ejector pins and sleeves placed strategically in the cavity block. During ejection, these push the solidified part out of the cavity to be removed from the mold.

Guide pins - Shorter pins that help accurately locate and align moving core pins within the stationary cavity block.

Vents - Small channels machined into the cavity that allow air or gas to escape during filling to avoid defects.

Cooling channels - Internally machined passageways through which a cooling liquid like water is circulated to properly set the plastic part layer-by-layer.

Sprue bushing - Connects the cavity(ies) to the injection unit and distributes melt to the gate(s).

Gate - Feeding point(s) where plastic first enters the cavity from the sprue bushing or runner system.

Screw ports - Used to securely hold the cavity block half of the mold in place using clamp screws.

Locating pins - Align corresponding blocks of multi-cavity molds to ensure precise replication.

Correct design and machining of these essential cavity components is vital for optimal injection molding production.

5. Cavity Design Considerations

Several factors are considered in cavity design:

Shape – The cavity geometry mirrors the shape of the part being molded.

Dimensions – Cavity dimensions account for plastic shrinkage and achieve required part tolerances.

Surface finish – A highly polished cavity finish is needed for glossy appearance.

Venting – Adequate venting prevents defects from trapped gases.

Ejection – Sufficient draft angles and ejector pins for easy part removal.

Strength – Cavity must withstand molding pressure without distortion.

Cooling – Effective cooling layout to prevent hot spots and warpage.

6. Multi-Cavity Molds

For high volume production, molds have multiple cavities to increase part output per cycle. But extra care is needed to ensure:

Balanced filling of all cavities, through proper runner and gate design.

Even cooling around each cavity to prevent uneven shrinkage.

Sufficient mold clamping force to avoid flash.

Consistent performance between cavities over long runs.

7. Cavity Temperature Control

Precise temperature control is critical for part quality:

Cooling channels and cycling ensure the cavity doesn’t overheat during prolonged runs.

Heating elements can be added to heat up the cavity for difficult materials.

Each cavity is individually controlled for optimal conditions.

8. Cavity Surface Treatments

Special coatings are applied to the cavity walls to enhance performance:

Chrome or nitride coatings reduce friction and improve release.

Teflon or titanium films provide a lubricious surface for abrasive resins.

Anti-corrosion coatings protect against chemical attack from reactive materials.

Conformal cooling layers improve heat transfer and cooling efficiency.

9. Cavity Structural Strength

The cavity must maintain dimensional stability under repeated injection pressure loading without distortion or fatigue cracking. Common measures to improve strength include:

Using pre-hardened tool steel substrates with good fatigue resistance.

Adding strengthening ribs to increase rigidity.

Finite element analysis to optimize the cavity thickness.

Shot peening to induce beneficial compressive stresses on the surface.

10. Cavity Fabrication Methods

Common techniques to fabricate mold cavities include:

CNC Machining – For large cavities, machined from tool steel blocks.

EDM – Electrical discharge machining used for complex geometries and cavities with small details.

Laser machining – Laser ablation to selectively remove material with minimal heat input.

Additive manufacturing – Building the cavity layer-by-layer from metal powder using 3D printing.

11. Cavity Quality Inspection

To ensure correct performance, finished cavities must pass several quality checks:

Dimensional accuracy – CMM inspection of critical dimensions.

Surface finish – Measurement of surface roughness.

Leak testing – Checking for coolant leaks under pressure.

Bench sampling – Test molding parts and evaluating key attributes.

12. Conclusion

The mold cavity is the most important mold component determining the final part geometry and quality. A well-designed cavity with optimized temperature control, surface coatings and structural integrity will produce consistent, high-quality parts through extended production runs. Continued advances in machining methods, materials and mold engineering will further improve cavity performance and capabilities.