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 plastic injection molding manufacturer, 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
- Balance injection pressure/speed with proper pack & hold times
- Inspect machine parts like nozzle tips and replace if worn
- Fine-tune back pressure to prevent excess residual pressure
- Shorten cycle times to use material before properties shift
- Handle plastic properly – drying, filtering, etc.
Staying on top of these parameters and handling materials appropriately will help reduce drooling. But some small adjustments are always needed to balance an injection molding process just right.
FAQs About Drooling in Injection Molding
What is the main cause of drooling during injection molding?
The most common causes of drooling are uncontrolled temperatures leading to low viscosity plastic, residual pressure buildup after injection, contaminated material, and worn machine components.
Does drooling always create injection molding defects?
Not necessarily, but small amounts of drooling can lead to flow marks, blushing, and other surface issues even if parts seem fine otherwise. Some drool is generally unavoidable but controlling the quantity is key.
How can I identify the factors causing drooling?
Meticulously observing exactly when/where drooling occurs and logging all pressure, speed, temp, and timing parameters provides clues about what to adjust. Comparing factors across good cycles vs bad highlights correlations.
Is drooling just an unavoidable part of injection molding?
While tiny amounts simply can’t always be eliminated completely, significant drooling indicates process parameters need adjustment. Getting pressures, temps, timings, and material handling dialed in can drastically reduce drooling.
In Summary
Monitoring key factors like temperature, pressure, timing, and material condition allows injecting mold operators to minimize plastic drooling and produce consistent, high-quality components free of defects. With practice optimizing process parameters, most applications can achieve near-zero nozzle drooling.