Система холодного канала в литье под давлением: полное инженерное руководство

If you are sourcing injection molded parts and evaluating mold options, the система холодной обкатки¹ is one of the first decisions you will face. It is the simpler, cheaper, and more widely used runner configuration — but that does not mean it is always the right choice. Understanding when a cold runner makes sense, how to design one properly, and what materials run best through it can save you thousands of dollars per mold and weeks of debugging time. In this guide, we break down everything an engineer or sourcing manager needs to know about cold runner systems — from basic mechanics to real-world design tips drawn from two decades of mold-making experience.

What Is a Cold Runner System in Injection Molding?

A cold runner system in injection molding is defined by the function, constraints, and tradeoffs explained in this section. A cold runner system is the network of unheated channels inside an injection mold that delivers molten plastic from the machine nozzle to the cavity gates, solidifying with each cycle.

In practice, the cold runner is the default configuration for most injection molds worldwide. It is mechanically simple, requires no temperature controllers or heated components, and works reliably across a wide range of thermoplastics. The trade-off is material waste: the solidified runner must be separated from the part and either discarded or reground and reprocessed. For high-volume commodity parts where resin cost is low, this waste is often economically acceptable. For engineering-grade resins or medical applications where regrind² is restricted, the economics shift toward hot runners.

How Does a Cold Runner System Work?

The working principle is straightforward. During the injection phase, the machine screw pushes molten plastic through the nozzle into the mold sprue bushing. From the sprue, the melt flows through a primary runner channel that branches into secondary runners, each terminating at a gate that leads into a cavity. Because the mold is maintained at a temperature well below the melting point of the polymer (typically 20–80 °C depending on the material), the plastic in the runner begins to cool the moment it contacts the steel walls.

After the part is packed and sufficiently cooled, the mold opens and the ejector system pushes both the finished part and the solidified runner out of the mold. In a two-plate mold, the runner is ejected on the same side as the part. In a three-plate mold, the runner is stripped separately, allowing automatic separation without manual intervention. The cycle then repeats.

The key variables controlling cold runner performance include: runner diameter (typically 3–12 mm depending on part size and material viscosity), runner length (shorter is always better for pressure drop), gate type and size (edge gates, submarine gates, and pin gates are the most common), and the number of cavities in the layout. A well-designed cold runner delivers uniform fill to every cavity with minimal pressure loss and acceptable waste volume.

What Are the Types of Cold Runner Molds?

There are two primary cold runner configurations: two-plate molds and three-plate molds. A third variant, the insulated runner mold, exists but is rare in modern production.

Two-Plate Cold Runner Mold

The two-plate mold is the simplest and most economical configuration. It has a single parting line: the mold splits into two halves — the cavity side (A-side) and the core side (B-side). The runner is machined into one or both halves of the parting line. When the mold opens, both the part and the solidified runner drop from the same side. This design is ideal for parts with gates on one face, and it works well for low- to mid-volume production where manual degating is acceptable or where robotic pickers can separate the runner automatically.

Three-Plate Cold Runner Mold

The three-plate mold adds a second parting line, creating three plates: the stripper plate, the A-plate, and the B-plate. The runner is located on the first parting line (between the stripper plate and the A-plate), while the part is formed on the second parting line (between the A-plate and B-plate). During ejection, the mold opens at both parting lines sequentially, automatically separating the runner from the parts. This allows gating on any surface of the part (not just the edge) and enables automatic degating — a significant advantage in high-volume or automated production environments.

The trade-off is complexity: three-plate molds are more expensive to build and maintain, have higher mold heights (requiring larger presses), and add moving components that increase wear. In our experience, three-plate molds are justified when the part geometry demands center gating, when production volumes exceed 100,000 cycles, or when labor costs for manual degating become prohibitive.

When Should You Choose a Cold Runner Over a Hot Runner?

This is the question we hear most often from customers evaluating mold options. The answer depends on four factors: production volume, material cost, part geometry, and color-change frequency.

Cold runners are the better choice when: (1) Production volume is low to moderate (under 500,000 parts) — the lower литьевая форма cost of a cold runner mold provides faster ROI. (2) Material cost is low — if you are molding PP, PE, or PS at a few dollars per kilogram, the runner waste penalty is minimal. (3) Color changes are frequent — cold runners purge instantly because there are no heated manifolds to flush. (4) You need multi-cavity production with simple part geometries.

Hot runners become more attractive at high volumes (over 1 million parts), with expensive engineering resins where eliminating runner waste saves significant material costs, or when cycle time optimization is critical — hot runners can reduce cycle times by eliminating the time needed to cool and eject the runner.

What Materials Work Best with Cold Runner Systems?

Most thermoplastics run well through cold runners, but some materials are better suited than others. The key material properties that affect cold runner performance are melt viscosity, thermal stability, and shrinkage behavior.

Polypropylene (PP) and polyethylene (PE) are the most common cold runner materials by volume. Their low cost, low viscosity, and wide processing window make them ideally suited for unheated channel systems. PP cold runners are found in everything from packaging caps to automotive battery cases.

Nylon (PA6, PA66) performs well in cold runners but requires careful attention to moisture content. Nylon is hygroscopic — it absorbs moisture from the air — and must be thoroughly dried before molding to prevent splay and weak weld lines. In a cold runner, the longer flow path compared to hot runners means the material must maintain sufficient temperature to avoid premature freeze-off. For glass-filled nylons (PA6-GF30, PA66-GF30), runner diameters should be increased by 20-30% to accommodate the higher viscosity.

Engineering Resins and Specialty Materials

Polycarbonate (PC) and acrylic (PMMA) are also well-suited to cold runners. These materials have higher melt temperatures (280-320 °C for PC) but good thermal stability, which means they flow well through unheated channels without premature solidification. PC in particular benefits from cold runner simplicity in optical lens and electronic housing applications, where material purity and surface finish consistency are essential. Acrylic (PMMA) cold runner molds are common in automotive lighting and display applications.

Materials that are challenging in cold runners include PVC (thermal degradation risk if the runner is too long), PEEK (very high melt temperature makes cold runners impractical for most applications), and liquid silicone rubber (LSR), which always requires a cold runner but of a completely different design. If you are working with specialty resins, talk to your литьё под давлением partner early in the design process to evaluate runner system feasibility and cost impact.

How Do You Design an Optimal Cold Runner System?

Good cold runner design is not complicated, but it demands attention to a few critical parameters. Get any of these wrong, and you will spend weeks debugging short shots, sink marks, or unbalanced fills.

Runner Diameter and Shape

The standard recommendation is a full-round runner cross-section, which provides the best surface-area-to-volume ratio for heat retention and low pressure drop. The runner diameter should be approximately 1.5× the maximum wall thickness of the part, with a practical range of 3–12 mm. Too small, and you get excessive pressure drop and premature freeze-off. Too large, and you waste material and extend cycle time. A trapezoidal runner is an acceptable alternative when both halves of the mold cannot be machined, but it increases pressure drop by 15–25% compared to a full-round design.

Runner Layout and Balancing

For multi-cavity molds, the runner layout must be balanced so that every cavity fills at the same time and pressure. An artificially balanced layout uses different runner diameters and lengths to equalize flow resistance. A naturally balanced layout arranges cavities symmetrically so that the flow path to each cavity is identical — these are always preferred because they are more robust to viscosity changes and process variations. In our mold shop, we use Moldflow simulation³ to verify fill balance before cutting steel, reducing first-shot debugging time by approximately 40%. With in-house mold manufacturing producing over 100 sets per month, we handle complex runner balancing across diverse part geometries.

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The gate is the narrowest point in the flow path and has the greatest impact on part appearance and dimensional accuracy. Common gate types for cold runners include: edge gates (simplest, used on the parting line), submarine gates (tunnel gates that are automatically sheared during ejection), and pin gates (small-diameter gates fed through a three-plate mold). Gate size should be approximately 50–80% of the part wall thickness at the gate location. Too small a gate causes high shear, which can degrade the material and create visible gate marks. Too large a gate extends packing time and makes degating difficult.

What Are Common Cold Runner Defects and How to Fix Them?

Common cold runner defects and how to fix them are the main categories or options explained in this section. Even with good design, cold runner molds can produce defects. Here are the most common issues we encounter and their solutions.

Short shots occur when the cavity does not fill completely. In cold runner molds, the most common cause is a runner that is too small or too long, creating excessive pressure drop. The fix is to increase runner diameter or reduce runner length by repositioning the cavities closer to the sprue. Increasing injection pressure or speed can also help, but this is a band-aid — the underlying geometry should be corrected.

Flow lines and weld lines appear where multiple flow fronts meet. In cold runner molds, these often occur at runner junctions or where two gates feed the same cavity. Solutions include: relocating gates to move weld lines to non-critical areas, increasing melt temperature to improve flow fusion, or using a single gate design for cosmetically critical parts.

Excessive runner waste is not a defect per se, but it is a common cost concern. If runner waste exceeds 30% of the total shot weight, consider: reducing runner diameter (within pressure constraints), switching to a hot runner for the primary manifold while keeping cold runners for the final drops (a hybrid system), or implementing a regrind program to recycle the runner material. For most thermoplastics, regrind at 15–20% blend ratios is acceptable for non-critical applications.

What Are the Most Frequently Asked Questions About Cold Runner Systems?

What is the typical runner waste percentage in a cold runner mold?

Runner waste in a cold runner mold typically ranges from 15% to 30% of the total shot weight, depending on the number of cavities, runner layout, and part size. For small, multi-cavity molds, the runner volume can actually exceed the part volume, pushing waste above 30%. For larger single-cavity parts, waste may drop to 10–15%. The good news is that for most thermoplastics, this runner material can be reground and reprocessed at blend ratios of 15–20% with virgin resin, significantly reducing effective material loss.

Can cold runner waste material be recycled?

Yes, cold runner waste can be recycled for most thermoplastic materials. The solidified runners are ground into regrind using granulators and then blended back with virgin material at typical ratios of 15–20%. This regrind process works well for commodity materials like polypropylene and polyethylene, where the cost savings are meaningful. However, for medical-grade parts, optical components, or applications with strict regulatory requirements, regrind use may be prohibited or limited. Always verify material integrity after multiple regrind cycles, as thermal degradation can reduce mechanical properties.

How do you size a cold runner channel?

A cold runner channel should be sized at approximately 1.5 times the maximum wall thickness of the molded part, with a practical diameter range of 3 to 12 millimeters. Full-round cross-sections are preferred because they offer the best surface-area-to-volume ratio for heat retention and low pressure drop. If the runner is too small, excessive pressure drop causes short shots and premature freeze-off. If the runner is too large, material waste increases and cycle times extend due to longer cooling requirements.

What is the difference between a two-plate and three-plate cold runner mold?

A two-plate mold has a single parting line where both the part and runner eject together, making it simpler and cheaper to build. A three-plate mold adds a second parting line that allows the runner to separate from the part automatically during ejection. Three-plate molds enable gating on any surface of the part, not just the parting line edge, which is essential for center-gated or multi-gated geometries. The trade-off is higher tooling cost, increased mold height, and more maintenance due to additional moving components.

When is a cold runner not recommended?

Cold runners are not recommended when molding very high-cost engineering resins where 15–30% material waste becomes economically prohibitive, when production volumes exceed one million cycles and cycle time optimization is critical, or when working with liquid silicone rubber that requires specialized cold runner designs with actively cooled channels. They are also less suitable for applications requiring absolutely zero post-molding operations, since the solidified runner must always be separated from the finished part. For medical-grade parts where regrind is restricted, hot runners eliminate waste entirely.

How does runner length affect injection pressure?

Runner length has a direct and significant impact on injection pressure requirements. Pressure drop through a cold runner is proportional to the runner length and inversely proportional to the fourth power of the runner diameter. Doubling the runner length roughly doubles the pressure drop, while doubling the diameter reduces pressure drop by a factor of sixteen. This is why mold designers always strive to minimize runner length by positioning cavities as close to the sprue as possible and using naturally balanced layouts.

Can you use cold runners for multi-material molding?

Cold runners can be used for multi-material molding in certain configurations, but they are more limited than hot runner systems. Two-shot or overmolding applications can use cold runners in the stationary mold half while the rotating core moves between stations. However, cold runners cannot selectively control material flow to different cavities the way valve-gated hot runner systems can. For complex multi-material applications with three or more materials, or where precise material placement is required, hot runner systems are generally more appropriate.

What gate types work best with cold runner systems?

The most common and effective gate types for cold runner systems are edge gates, submarine (tunnel) gates, and pin gates. Edge gates are the simplest and most economical, machined directly on the parting line. Submarine gates are automatically sheared during ejection, making them ideal for automated production where manual degating is undesirable. Pin gates, used in three-plate molds, provide small-diameter gate vestige and allow gating on any surface. Gate size should be 50–80% of the part wall thickness at the gate location to balance fill quality and degating ease.

1. cold runner system: cold runner system refers to an unheated channel system in an injection mold that conveys molten plastic from the machine nozzle to the cavity gates, solidifying with each cycle. ↩

2. regrind: regrind refers to the process of grinding solidified runners and rejected parts into reusable plastic granules, typically blended with virgin material at 15-20% ratios. ↩

3. Moldflow simulation: Moldflow simulation refers to computer-aided engineering software that simulates plastic flow, cooling, and warpage in injection molds to optimize design before manufacturing. ↩