Cold runner system is the most basic runner system in injection molding. Unlike hot runner, it relies on natural cooling of mold to solidify plastic in runner to form waste (called "runner condensate"), which needs to be separated and processed later. Although technology is traditional, it is still irreplaceable in many scenarios due to its simplicity, reliability and low cost.
Core components
Sprue: channel entrance connecting injection molding machine nozzle and mold.
Runner: "branch-like" channel that distributes melt to different cavities.
Gate: narrow opening where melt enters cavity to control filling speed and direction.
Cold slug well: collects plastic cooled at the front end to avoid clogging gate.
Working principle: Molten plastic is injected into cold runner through injection molding machine, naturally cools and solidifies in runner to form a condensate separated from product, which needs to be removed manually or mechanically.

Advantages
Low cost: Simple structure, no heating elements and temperature control system required, mold manufacturing cost is only 1/3~1/2 of hot runner.
Easy maintenance: No high-temperature parts, low failure rate, and convenient maintenance.
Wide applicability: Almost all plastic materials can be used, especially suitable for heat-sensitive materials (such as PVC).
Limitations
Material waste: Runner condensate accounts for 30%~40% of material consumption and needs to be recycled or discarded.
Long molding cycle: Need to wait for runner to cool, low production efficiency.
Appearance limitation: Gate marks are obvious, not friendly to high-gloss and transparent parts.

Economic efficiency of cold runner makes it still the first choice in specific scenarios. Selection should pay attention to:
1. Product requirements
Low appearance requirements: such as tool handles, daily necessities;
Simple structure: no complex thin wall or multi-point injection requirements.
2. Production scale
Small batch: cold runner mold cost is low, suitable for trial production or unstable order scenarios;
Multi-variety switching: cold runner mold modification is flexible and adapts to frequent production changes.
3. Material properties
Thermosensitive materials: cold runners have no continuous heating to reduce risk of decomposition;
High viscosity materials: such as PC, PMMA, cold runners can avoid excessive pressure loss in hot runners.
4. Budget constraints
Low initial investment: suitable for small enterprises or start-up teams with limited funds.

1. Runner layout optimization
Natural balance type: runner length of each cavity is consistent to ensure uniform filling (suitable for circular layout);
Unbalanced type: compensate for pressure loss by adjusting runner section (suitable for complex layout).
2. Cross-sectional shape of runner
Circle: high cooling efficiency, but high processing cost;
Trapezoidal: easy demoulding, suitable for manual material removal;
Semicircular: taking into account cooling and processing convenience.
3. Gate design
Side gate: strong versatility, easy to cut off;
Point gate: small traces, but requires a three-plate mold structure;
Latent gate: automatic cut-off, suitable for automated production.
4. Cooling system coordination
Runner area needs to be cooled to shorten solidification time of condensate;
Avoid uneven cooling causing product deformation or residual stress.

 

Cold runners are still widely used in following fields due to their economy and reliability:
Daily necessities: hangers, buckets, and toys (low cost and low appearance requirements);
Industrial parts: gears, housings, connectors (small batches, multiple materials);
Medical consumables: test tubes, medicine bottle caps (adapted to heat-sensitive materials);
Packaging industry: bottle caps, trays (medium and low batch production).
Case: A toy factory uses cold runner to produce plastic building blocks, saving 50% of mold costs. Although 20% of waste is generated, it can be recycled through recycling and crushing.

1. Daily maintenance
Regularly clean runner residues to prevent blockage;
Check gate wear to avoid uneven filling;
Lubricate mold ejection mechanism to ensure smooth demolding of runner condensate.
2. Troubleshooting
Runner blockage: Check whether cold well is invalid, or increase injection pressure;
Gate wire drawing: Optimize gate size or reduce holding time;
Product material shortage: Adjust runner section or add exhaust slots.

Despite increasing popularity of hot runner technology, cold runners have not been eliminated, but have been rejuvenated through innovation:
Modular design: Quickly replace runner components to adapt to multi-variety production;
Mixed use with hot runners: Local heating of key areas to balance cost and efficiency;
3D printing runners: Conformal cooling runners improve cooling efficiency and reduce waste;
Intelligent recycling: Integrated crusher directly recycles runner condensate to achieve "zero waste".
Conclusion
Cold runner may not have halo of hot runner, but it has always occupied a place in injection molding field with its "basic skills" of low cost and high reliability. Whether it is a start-up or a traditional factory, essence of choosing a cold runner or a hot runner is to find the best balance between efficiency, cost and quality.