Qu'est-ce que la bave dans le moulage par injection ? Quelles en sont les causes et comment les prévenir ?

In plastic injection molding, drooling refers to the leakage of excess molten plastic from the nozzle after the mold is opened. This can cause uneven textures, surface defects, and other quality issues. So, what exactly causes this drooling effect? And what can we do to prevent it? As a professional fabricant de moulage par injection de matières plastiques, I will answer these questions in this article.

What is Drooling in Injection Molding?

Drooling in injection molding refers to the excess molten plastic that leaks from the nozzle when the molds open after an injection molding cycle. It leaves behind strands or droplets of plastic that can harden and cause molding defects. Drooling typically occurs when factors like temperature, pressure, moisture content, or viscosity are not optimized.

What Causes Drooling in Injection Molding?

There are several potential root causes of drooling in injection mold processes:

High Nozzle Temperature

If the nozzle temperature is set too high, the plastic becomes overly fluid and is more likely to drip from the nozzle after injection. Reducing the nozzle temp can help avoid this issue. Around 400-500°F is typical.

Excessive Residual Pressure

Even after injection, residual pressure remains inside the barrel. If too high, this can push plastic out of the nozzle once the molds separate. Optimizing injection pressure, duration, and pack/hold times helps prevent excessive residual pressure.

Poor Temperature Control

Fluctuating barrel or mold temperatures can alter the plastic’s viscosity unexpectedly, contributing to drooling. Consistent, stable temps are key. Barrel temps of 350-600°F are common depending on the material.

Contaminants

Foreign particles like dirt, degraded plastic, colorants, or moisture can reduce the material’s viscosity and molecular weight, making it more prone to drooling. Proper material drying and filtering helps avoid this.

Worn Machine Parts

Over time, injection nozzles and screws wear down. Gaps, channels, and uneven surfaces can allow excess plastic to escape during mold open. Replacing worn parts reduces drool.

Incorrect Screw Position

If the screw moves too far forward in the barrel between shots, more space is left for molten plastic. This unused plastic then drools out once the pressure releases after injection.

Excessive Back Pressure

Too much back pressure inside the barrel can force plastic out of the nozzle once the molds separate. Reducing back pressure, possibly by adjusting screw speed, lowers this risk.

Gate Seal Issues

If the gate where plastic enters the mold cavity isn’t properly sealed, excess material can ooze out. Ensuring proper gate alignment and replacing worn parts improves the seal.

Hot Runner Temperature

For hot runner systems, incorrect manifold, gate, or hot tip temperatures also influence drooling. Keeping temperatures consistent prevents the material from becoming too fluid.

What Are the Consequences of Drooling?

Though it may seem harmless, even minor drooling can negatively impact injection molding processes and products:

Mold Fouling

Hardened plastic left on mold components can transfer to subsequent parts. Cleaning the molds more frequently helps avoid this scrap.

Part Defects

Excess material sticking to parts leads to flow marks, knit lines, blushing, and surface defects requiring additional finishing work.

Lost Productivity

Taking time to clean molds, finish parts, or resolve underlying issues cuts into production capacity. Even small amounts of drool add up over thousands of cycles.

Wasted Material

With sprues and runners accounting for roughly 15% of scrap plastic as is, drooling piles on unnecessary waste costs.

Risk of Damage

Dried plastic drool falling into moving machinery can lead to obstructed components and downtime for repairs.

How Can You Prevent Drooling?

Thankfully, drooling can be minimized or avoided entirely through proper injection molding techniques:

Monitor Temperatures

Use temperature probes and gauges to monitor barrel, nozzle, mold, and hot runner temps. Adjust as needed to maintain viscosities that prevent drooling.

Reduce Injection Pressure

Lowering injection pressure, speed, and duration prevents excessive residual pressure post-injection when molds separate. Start on the low end and increase as needed.

Balance Back Pressure

Too much or too little back pressure can cause drooling in different ways. Finding the right equilibrium stabilizes viscosity through injection and mold open.

Check for Wear

Inspect machine nozzles, screws, barrels, and gate seals for wear or damage regularly. Replace parts as preventative maintenance rather than waiting for issues.

Adjust Cycle Times

If plastic is remaining in the barrel too long between injections, its properties can shift and become prone to drooling. Reducing cycle times helps.

Improve Material Handling

Drying materials properly and filtering contaminants lowers the chances of viscosity changes that lead to drooling.

Following these best practices for balancing pressures, maintaining stable temperatures, and handling materials properly will help minimize drooling. But some amount of tweaking is always needed to optimize a particular molding process.

Careful observation after adjusting any parameters helps zero in on the sweet spot. And keeping detailed process logs makes it easier to troubleshoot any future drooling issues that arise.

With a quality injection molding machine, a well-designed mold, consistent plastic properties, and proper setup technique, you can achieve minimal to zero drooling in most applications.

Key Takeaways on Drooling Prevention:

• Monitor nozzle temps, barrel temps, mold temps, etc. to maintain ideal viscosity
• Équilibrer la pression et la vitesse d'injection avec des temps de remplissage et de maintien appropriés
• Inspecter les pièces de la machine, comme les buses, et les remplacer si elles sont usées.
• Réglage fin de la contre-pression pour éviter une pression résiduelle excessive
• Raccourcir les temps de cycle pour utiliser les matériaux avant que leurs propriétés ne changent
• Manipuler correctement le plastique - séchage, filtrage, etc.

La maîtrise de ces paramètres et la manipulation appropriée des matériaux contribueront à réduire la formation de bave. Mais de petits ajustements sont toujours nécessaires pour équilibrer au mieux un processus de moulage par injection.

FAQs About Drooling in Injection Molding

What is the main cause of drooling during injection molding?

Les causes les plus courantes de la formation de gouttes sont les températures non contrôlées entraînant une faible viscosité du plastique, l'augmentation de la pression résiduelle après l'injection, les matériaux contaminés et l'usure des composants de la machine.

Does drooling always create injection molding defects?

Pas nécessairement, mais de petites quantités de bave peuvent entraîner des marques d'écoulement, des rougissements et d'autres problèmes de surface, même si les pièces semblent en bon état par ailleurs. Une certaine quantité de bave est généralement inévitable, mais il est essentiel d'en contrôler la quantité.

How can I identify the factors causing drooling?

L'observation méticuleuse du moment et de l'endroit où la bave se produit et l'enregistrement de tous les paramètres de pression, de vitesse, de température et de synchronisation fournissent des indices sur ce qu'il convient d'ajuster. La comparaison des facteurs entre les bons et les mauvais cycles met en évidence les corrélations.

Is drooling just an unavoidable part of injection molding?

S'il n'est pas toujours possible d'éliminer complètement les petites quantités, une bave importante indique que les paramètres du processus doivent être ajustés. L'ajustement des pressions, des températures, des temps et de la manipulation des matériaux peut réduire considérablement la formation de gouttes.

En résumé

Le contrôle de facteurs clés tels que la température, la pression, le temps et l'état des matériaux permet aux opérateurs de moules d'injection de minimiser la formation de gouttes de plastique et de produire des composants cohérents, de haute qualité et exempts de défauts. En optimisant les paramètres du processus, la plupart des applications peuvent atteindre un niveau de bave quasi nul.