¿Cuál es la causa del deslizamiento en frío en el moldeo por inyección?

Cold slug is one of those moldeo por inyección¹ defects that sneaks up on you. You set the temperatures, you run a few good shots, and then a solidified chunk of plastic appears in your part, right at the gate or in the runner. It ruins the cosmetic surface, weakens structural integrity, and can scrap an entire batch before you notice.

I have seen cold slug happen with everything from ABS housings to glass-filled nylon gears. In most cases, the root cause traces back to one of three things: inadequate nozzle temperature, a poorly designed cold slug well, or excessive barrel decompression. The frustrating part is that cold slug does not always show up consistently — it can appear only during cold starts, or only on certain cavities in a multi-cavity mold.

This guide covers every angle of cold slug formation — from the physics of melt solidification in the nozzle and sprue, to the diseño de moldes² features that catch or fail to catch these frozen plugs. Whether you are troubleshooting an existing defect or designing a new mold from scratch, you will find actionable steps grounded in real production experience from our moldeo por inyección operaciones.

What Is a Cold Slug in Injection Molding?

A cold slug is a solidified chunk of plastic that forms when melt temperature drops below the freezing point before the cavity fills. It is not a short shot or sink mark — cold slugs are prematurely frozen material carried into the part by subsequent injection cycles. They typically appear as raised bumps, discolored spots, or small pits near the gate area.

For a broader look at diseño de moldes de inyección, our pillar guide covers tooling structure, thermal control, and manufacturability tradeoffs.

In technical terms, cold slugs typically form at three locations: (1) at the moldeo por inyección machine nozzle tip, where heat loss is fastest between shots; (2) in the sprue and runner system, where the melt travels through cold steel channels; and (3) at the gate, where the cross-section narrows and the flow velocity changes dramatically. Each location has different root causes and different fixes.

On our shop floor, we see cold slug most often during cold starts, when the mold has not reached thermal equilibrium yet, or when production switches from a high-melt-temperature material (like PEEK at 370 °C) to something cooler (like PP at 220 °C) and the operator does not purge the barrel completely. The residual high-temp material solidifies in the nozzle and gets injected as a cold slug into the first few shots of the new run.

What Causes Cold Slug During Injection Molding?

The root cause is always the same: the melt loses too much heat before it reaches the cavity. But the reasons behind that heat loss vary. Here are the four most common culprits we encounter in production.

Nozzle Temperature Too Low or Unstable

The nozzle is the last point where you can control melt temperature before it enters the mold. If the nozzle heater band is undersized, poorly controlled, or simply set too low, the melt at the tip cools between shots. When the next injection cycle starts, that cooled plug of plastic gets pushed into the sprue as a cold slug. This is especially common with materials that have a narrow processing window, such as polycarbonate or POM.

We once traced a recurring cold slug problem on a medical device housing back to a worn thermocouple on the nozzle — the controller showed 260 °C, but the actual tip temperature was cycling between 230 °C and 270 °C. Replacing the thermocouple and adding an insulation jacket solved it immediately.

Mold Temperature Below Optimal Range

When the mold steel is too cold, the melt solidifies on contact with the cavity walls. If the frozen layer builds up faster than the cavity fills, cold slugs appear in the part. This is particularly problematic for thin-wall molding, where the flow channel is already narrow.

The fix is not always to increase mold temperature — that can increase cycle time and cause warpage. Instead, you need to optimize the cooling circuit layout so that the temperature is uniform across the mold face, and ensure that the areas near the gate are warm enough to prevent premature freeze-off.

Excessive Barrel Decompression (Suck-Back)

Decompression — also called suck-back — pulls the melt away from the nozzle tip after holding pressure ends. If you overdo it, you pull air into the nozzle, and the melt at the tip oxidizes and cools rapidly. On the next shot, that degraded, cooled material enters the cavity as a cold slug. This is one of the most overlooked causes, because operators often add decompression to prevent drooling without realizing the side effect.

Long or Narrow Runner Systems

Every millimeter of runner length is an opportunity for the melt to lose heat. Long, thin runners with high surface-area-to-volume ratios cause rapid cooling. By the time the melt reaches the gate, its temperature may have dropped below the flow threshold, and the leading edge solidifies into a cold slug. This is why multi-cavity molds with balanced runner layouts are so important — they minimize the runner length to each cavity.

How Does Mold Design Contribute to Cold Slug?

Diseño de moldes is probably the single biggest factor in whether cold slug becomes a chronic problem or a non-issue. A well-designed mold accounts for heat loss at every stage and includes features specifically meant to catch or prevent cold slugs.

The Cold Slug Well

The cold slug well (also called a cold slug pocket or catch pad) is a small cavity placed directly opposite the sprue entrance in the runner system. Its job is to catch the cold slug that naturally forms at the nozzle tip between shots. When the injection cycle starts, the first material to enter the mold is the coldest — that plug gets pushed straight into the cold slug well instead of into the runner and cavity. If your mold does not have cold slug wells, or if they are too small, cold slugs will travel downstream.

A properly sized cold slug well should have a volume at least 1.5 times the volume of the nozzle tip channel. It should be easy to eject and clean during maintenance. In multi-cavity molds, every runner branch should have its own cold slug well.

Gate Design and Location

The gate is the narrowest point in the flow path, and it is where the melt undergoes the highest shear and the fastest cooling. Small gate diameters (especially sub-gates or pinpoint gates) create high shear heating but also restrict flow, which can cause the melt to freeze off prematurely. Edge gates and fan gates provide a larger cross-section and are less prone to cold slug formation.

Gate location also matters. If the gate is far from the sprue, the melt has to travel a longer runner, losing more heat along the way. Placing gates closer to the sprue — or using a hot runner drop directly into the cavity — eliminates most of the runner heat loss.

Runner Cross-Section Shape

Round runners have the lowest surface-area-to-volume ratio, meaning the least heat loss per unit of melt flow. Full-round runners are the gold standard for cold slug prevention. Trapezoidal runners are a common compromise because they are easier to machine, but they have about 20% more surface area than equivalent round runners, which translates to faster heat loss. Half-round runners should be avoided entirely for any material prone to cold slug.

What Process Parameters Lead to Cold Slug Formation?

Los cuatro parámetros del proceso que más probablemente causan babas frías son la velocidad de inyección, la presión de compactación, el tiempo de refrigeración y la decompresión del cilindro. Incluso un molde perfecto producirá babas frías con configuraciones incorrectas. Aquí se explica cómo ajustar cada parámetro.

Velocidad de inyección

Una velocidad de inyección baja da al fundido más tiempo para enfriarse mientras fluye por el canal. Para materiales con velocidades de cristalización rápidas (como POM o PA66), una velocidad de llenado baja casi garantiza la producción de babas frías en la entrada. Incrementar la velocidad de inyección impulsa el fundido más rápido por el canal, reduciendo el tiempo de residencia y la pérdida de calor. Sin embargo, una velocidad excesiva puede causar rebabas y chorreo, por lo que necesita encontrar el punto óptimo.

Holding Pressure and Time

Una presión de mantenimiento insuficiente significa que la cavidad no está completamente compactada. La capa congelada en las paredes crece hacia el interior, y el material fundido restante en el centro puede solidificarse en una baba fría antes que la entrada se congele. Si observa marcas de hundimiento combinadas con babas frías, aumentar la presión de mantenimiento y extender el tiempo de mantenimiento generalmente soluciona ambos problemas simultáneamente.

Tiempo de enfriamiento

Contraintuitivamente, un tiempo de refrigeración excesivo entre disparos puede hacer que la baba fría sea peor en el siguiente ciclo. Cuando el molde está inactivo con agua de refrigeración circulando, la boquilla de entrada y la área de la entrada continúan enfriándose. Cuando se dispara el siguiente ciclo, esas superficies están más frías que durante la producción en estado estable, y la parte frontal del nuevo material fundido se solidifica al contacto. Optimizar el tiempo de refrigeración para la sección más gruesa de la pieza — no para la entrada — ayuda a evitar esto.

How Can You Detect and Identify Cold Slugs?

Las babas frías se detectan mediante inspección visual, radiografía o TC, o análisis térmico (DSC/TGA). La detección temprana evita que las piezas defectuosas lleguen a los clientes. En nuestra planta, el equipo de QC relaciona el defecto con la etapa de llenado, compactación, refrigeración o expulsión, luego rastrea el material frío hasta su origen. Los ingenieros también registran la temperatura del boquilla, la distancia de decompresión y la tasa de desperdicio del primer disparo para que la acción correctiva esté vinculada a evidencia del proceso, no a suposiciones.

Inspección visual

Las babas frías aparecen como defectos visibles en la superficie de la pieza — normalmente una protuberancia elevada, una zona decolorada o un pequeño hoyo cerca de la entrada. En piezas transparentes (como lentes de PMMA), las babas frías aparecen como inclusiones turbias o opacas. Este es el método de detección más rápido y funciona para la mayoría de piezas cosmética, pero no detectará babas frías internas.

Rayos X y Tomografía Computarizada

Para aplicaciones críticas — dispositivos médicos, componentes de seguridad automotriz, piezas aeroespaciales — no se puede depender únicamente de la inspección visual. La radiografía y la tomografía computarizada (TC) pueden detectar babas internas que son completamente invisibles desde el exterior. La TC es especialmente valiosa porque proporciona un mapa 3D de la ubicación exacta, tamaño y forma del defecto.

Análisis térmico (DSC/TGA)

Cuando las babas frías son causadas por degradación del material o contaminación (no solo por problemas de temperatura), herramientas de análisis térmico como la Calorimetría de Barrido Diferencial (DSC) ayudan a identificar el problema. La DSC puede detectar si el material de la baba fría tiene un punto de fusión diferente al de la resina base, lo cual indica contaminación o material degradado.

How Do You Prevent and Eliminate Cold Slugs?

La prevención siempre es más económica que la detección. Aquí presentamos un enfoque sistemático para eliminar las babas frías, organizado desde los cambios más sencillos hasta las modificaciones más complejas.

Soluciones rápidas (No se requieren cambios en las herramientas)

Estos cambios pueden realizarse en la máquina sin modificar el molde:

Aumentar la temperatura de la boquilla por 5–10 °C — suele ser suficiente para mantener el material fundido en la punta por encima del punto de solidificación entre disparos. Monitorear la formación de fibras o baboseo como efectos secundarios.|||Reducir la distancia de decompresión — minimizar la retracción para evitar que el aire entre en la boquilla. Si ocurre baboseo, use una boquilla de cierre en lugar de decompresión.|||Increase injection speed — un llenado más rápido reduce el tiempo que el material fundido permanece en el distribuidor frío. Incrementar la velocidad gradualmente mientras se observa la formación de rebabas.|||Optimizar la temperatura del molde — aumentar la temperatura del refrigerante cerca del área de la entrada en incrementos de 5 °C. Utilizar refrigeración zonificada si el molde lo permite.

Modificaciones del molde

Si los cambios en el proceso no solucionan el problema, el molde necesita atención:

Agregar o ampliar pozos de babas frías — cada rama del canal debe tener un pozo de babas frías dimensionado al menos a 1.5× el volumen de la punta de la boquilla. Esta es una modificación de bajo costo que puede realizarse durante una ventana de mantenimiento regular del molde.Cambiar de canales trapezoidales a canales completamente redondos — reduce la pérdida de calor aproximadamente 20%. Requiere recortar los canales del distribuidor en ambas mitades A y B del molde.|||Instalar una boquilla de entrada caliente — mantiene la entrada a temperatura de fusión, previniendo el punto más común de formación de babas frías. Esta es una modificación de costo medio que se amortiza rápidamente en producciones de alto volumen.

Actualizaciones de equipos

Para problemas persistentes de babas en producción de alto valor:

Sistema de canal caliente — el estándar de oro para la eliminación de babas frías. El material fundido permanece a temperatura dentro del distribuidor, por lo que no hay pérdida de calor en el distribuidor frío. Los distribuidores calientes añaden $5,000–$20,000+ al costo del molde dependiendo del número de salidas, pero eliminan el desperdicio del distribuidor y prácticamente eliminan las babas frías.|||Boquilla de cierre — una válvula con resorte o accionada hidráulicamente en la punta del boquilla que sella el material fundido entre disparos. Previene tanto el baboseo como la formación de una baba fría en la punta.|||Buje caliente aislado — un compromiso entre un canal completamente caliente y un canal frío. La entrada se calienta mientras el resto del canal permanece frío.

Menor costo que un sistema caliente completo pero aún aborda el punto principal de formación de babas.

What Materials Are Most Susceptible to Cold Slug?

No todos los materiales son igualmente propensos a formar babas frías. El riesgo depende de tres factores: temperatura de fusión, velocidad de cristalización y viscosidad del fundido. Los materiales con puntos de fusión altos, velocidades de cristalización rápidas o viscosidad alta son los más susceptibles.

Materiales de alto riesgo: PEEK (343–399 °C melt), LCP (280–350 °C), PPS (280–330 °C), and glass-filled nylons. These materials need very high barrel and nozzle temperatures, and even a small temperature drop can cause premature solidification.|||Medium-risk materials: PC (260–310 °C), POM (175–225 °C), and PA66 (260–290 °C). POM (acetal) is particularly tricky because it crystallizes very quickly — the window between molten and solid is narrow.|||Low-risk materials: PP, PE, PS, and ABS. These amorphous or slow-crystallizing materials have wide processing windows and tolerate temperature variations better.

If you are molding a high-risk material and cold slug is a recurring issue, consider whether a material with better flow characteristics (higher Melt Flow Index) could work for your application. Sometimes switching from a standard-grade PA66 to a high-flow grade with an MFI of 60+ g/10 min eliminates the problem entirely without any tooling changes.

Conclusión

Cold slug in injection molding is ultimately a heat management problem. The melt loses too much heat before it reaches the cavity, and the result is a solidified chunk of plastic embedded in your part. The fix can be as simple as raising the nozzle temperature 5 degrees, or as involved as retrofitting a hot runner system. When the symptom tracks melt recovery or barrel wear, our process engineers also inspect the screw injection molding machine setup before changing tooling. The key is to diagnose where the heat loss is happening — at the nozzle, in the runner, or at the gate — and target your solution accordingly.

From two decades of running injection molding production at ZetarMold, a leading proveedor de moldeo por inyección based in Shanghai, we have found that the most effective cold slug prevention strategy is a combination of proper mold design (cold slug wells, round runners, heated sprue bushings) and disciplined process control (nozzle temperature, injection speed, minimal decompression). Get these fundamentals right, and cold slug becomes a rare exception rather than a chronic headache.

Preguntas frecuentes

Preguntas frecuentes

What is the difference between a cold slug and a short shot?

A cold slug is a solidified piece of plastic that forms when the melt cools prematurely and gets trapped in the part or runner. A short shot occurs when the mold cavity is not completely filled with plastic — the part is missing material. They have different root causes: cold slugs stem from premature solidification at the nozzle, runner, or gate, while short shots usually result from insufficient injection pressure, inadequate venting, or an incorrect shot size setting on the machine.

Can cold slugs cause structural failure in molded parts?

Yes, cold slugs can absolutely cause structural failure in molded parts. A cold slug embedded inside a part wall creates a stress concentrator, acting as a microscopic notch that significantly reduces both impact strength and long-term fatigue resistance. In load-bearing applications such as automotive brackets, medical device housings, and consumer electronics enclosures, an internal cold slug can lead to premature crack initiation and catastrophic failure under repeated stress cycles. This is precisely why X-ray or CT inspection is considered essential for all structural and safety-critical plastic components.

How do I know if my cold slug well is large enough?

A properly sized cold slug well should have a volume at least 1.5 times the volume of the nozzle tip channel itself. You can verify this in production by inspecting the sprue puller after each cycle: if cold slug material overflows the well and enters the main runner, the well is clearly undersized for your application. Another reliable indicator is if you still observe cold slug defects in the finished part despite having a well installed — in that case, enlarge the well by approximately 50 percent and re-test during your next production run.

Does a hot runner system completely eliminate cold slugs?

Hot runners eliminate cold slugs caused by runner heat loss, which is the most common source in cold runner molds. The melt stays at temperature inside the heated manifold, so there is no opportunity for premature cooling in the runner. However, cold slugs can still form at the nozzle-to-manifold transition or at the gate tip if the thermal balance is incorrect. Proper hot runner design, consistent temperature control, and regular maintenance of heater zones are essential for achieving near-complete elimination.

What injection speed is best to prevent cold slugs?

Faster injection speeds reduce the melt residence time in the cold runner system, minimizing heat loss and the chance of premature solidification during the filling phase. The ideal speed depends on the specific material and part geometry — generally, you should use the fastest speed that does not cause flash, jetting, or burn marks on the part. For cold-slug-prone materials like POM or PA66, fill speeds of 80 to 120 mm per second are typical starting points. Always validate any speed changes with a small trial run before committing to full-scale production.

Why do cold slugs appear more often at the start of a production run?

During startup, the mold steel has not yet reached its thermal equilibrium — the cavity surfaces and runner channels are significantly cooler than their steady-state operating temperature. The first few shots lose heat rapidly to these cold steel surfaces, causing the leading melt front to solidify into cold slugs before the cavity fills completely. Running five to ten purge shots before starting production, and gradually ramping up to full cycle speed over the first twenty shots, helps the mold reach steady-state temperature and effectively minimizes startup cold slug defects.

1. moldeo por inyección: Injection molding is a manufacturing process that injects molten plastic into a mold cavity to produce parts with precise geometry and repeatable quality. ↩

2. diseño de moldes: Mold design is a structured engineering process for defining gate layout, runner geometry, cooling, ejection, venting, steel choice, and tolerances so an injection molded part can run reliably. ↩

3. canal caliente: hot runner refers to a hot runner system uses heated channels inside the mold to keep plastic molten from the nozzle to the gate, eliminating runner waste and reducing cold slug risk. ↩