Cold runner
1. There is sprue material, material waste
2. Product quality defects such as stress, deformation or weld line
3. Sprue injection and cooling lead to long injection time
4. Sprue storage, reprocessing process, high cost
5. Too much waste material for molds with more than 16 cavities, and mold is too large
Hot runner
1. No sprue material, low long-term production cost
2. Small stress, small deformation, no weld line, better quality
3. Shorten molding cycle and improve efficiency
4. Reduce sprue processing process, easy automation
5. More (24, 32, 64, 128...) cavity production
Mold runner system: "lifeline" of plastic molding
--From melt delivery to precision temperature control, reveal blood vessel network picture of injection molding
Invisible "plastic highway"
When you hold a plastic water cup, turn on car headlight, or use electronic equipment, you may not think that birth of these products depends on a set of precision systems hidden inside mold-runner system. It is like a human blood vessel, accurately delivering molten plastic to each cavity. Quality of its design directly determines product quality, cost and production efficiency. This article will deeply analyze working principle and design wisdom of this "lifeline".

1. Definition and core mission
Runner system is a channel network connecting injection molding machine nozzle and mold cavity, responsible for evenly and quickly delivering high-temperature molten plastic to each cavity. Its core missions include:
Efficient transmission: filling cavity at a speed of 100-500 mm/s;
Pressure maintenance: continuous transmission of pressure during holding phase to compensate for material shrinkage;
Temperature control: maintaining optimal fluidity of plastic by heating or cooling.
2. "Four organs" of runner system

1. Cold runner system
Working principle: Runner and product are cooled synchronously, and waste needs to be cut off after demolding;
Advantages: simple structure, low cost (mold cost reduced by 30%-50%);
Disadvantages: material waste (waste rate 30%-60%), long molding cycle;
Applicable scenarios: small batches and multiple varieties, heat-sensitive materials (such as PVC).
Typical case: Disposable tableware production, cold runner waste can be crushed and reused, and the overall cost is controllable.
2. Hot runner system
Working principle: Runner is continuously heated to keep plastic molten, and no waste is generated;
Advantages: zero waste, short cycle (efficiency increased by 20%-40%), and adaptable to complex products;
Disadvantages: high cost (mold cost increased by 2-3 times), complex maintenance;
Applicable scenarios: mass production, transparent parts, thin-walled parts (such as mobile phone shells).
Technical breakthrough: A certain car lampshade uses hot runner needle valve technology, gate mark is ≤0.05mm, and light transmittance meets standard rate of 99.9%.

1. Balance Principle
Goal: Melt reaches end of all cavities at the same time to avoid short shots or flash;
Implementation method:
Natural balance: symmetrical layout (such as H-type, radial type);
Artificial balance: adjust length/section of diverter to compensate for pressure loss.
2. Design of runner size
Diameter formula (empirical estimation):
D=0.8×V3
DD: diverter channel diameter (mm);
VV: product volume (cm³);
Reference standard:

3. Cold material handling strategy
Cold material well volume: at least 1.5 times amount of cold material at the front end of nozzle;
Position layout: set at the end of diverter channel or at a right-angle turn.
4. Essence of gate design
Gate type:

Size experience: Gate thickness ≈ 50%-80% of product wall thickness.
5. Exhaust design
Exhaust groove size: depth 0.01-0.03mm, width 5-10mm;
Key positions: end of melt flow, insert joints

 

Application example of one mold with eight cavities

1. Difference in fluidity

2. Treatment of heat-sensitive materials
PVC, POM and other easily decomposable materials: cold runner is preferred to shorten runner length;
High-temperature engineering plastics (such as PEEK): hot runner needs independent temperature control, and temperature difference is ≤±3℃.

1. Daily maintenance
Cleaning: clean runner residue every 5000 molds;
Inspection: regularly measure resistance value of hot runner heater (deviation ≤±5%);
Lubrication: apply high-temperature grease to matching surface of guide column and slider.
2. Common problems and countermeasures

3D printing conformal runners:
Freely design special-shaped cross-sections, reduce pressure loss by 30%;
Case: A medical device uses spiral runners, and filling efficiency is increased by 40%.
Intelligent hot runners:
Each nozzle is independently temperature controlled (accuracy ±0.5℃), and AI dynamically adjusts parameters;
Equipped with pressure sensors to monitor melt state in real time.
Green runner technology:
Low thermal capacity heating elements, energy consumption is reduced by 50%;
Special runner coatings for degradable plastics to avoid material degradation.
Conclusion: Runners - "silent commander" of injection molding
From a toothbrush to spacecraft parts, runner system controls flow destiny of plastics with millimeter-level precision. It is not only a physical channel, but also a cross-battlefield of thermodynamics and material science. When wave of intelligent manufacturing hits, this "plastic highway" is evolving in a more efficient, smarter and more sustainable direction, continuously pushing boundaries of human manufacturing civilization.