Splay defects can seriously undermine the quality of injection molded parts. These streaks and discolorations on the surface not only hurt aesthetics but also compromise strength. So how do you get rid of splay in injection molding?
As a professional plastic injection molding manufacturer, I’ve battled my share of splay defects. And I can tell you that while tricky, it is possible to eliminate splay through proper troubleshooting and process adjustments.
In this comprehensive guide, I’ll walk you through the various causes of splay and proven solutions to banish it for good.
What Causes Splay in Injection Molding?
Before jumping into fixes, it’s important to understand what’s behind those pesky silver streaks in the first place.
Splay defects occur when tiny gas bubbles get dragged along the surface as the mold cavity fills. Most often, moisture absorbed within the plastic resin causes splay.
As the granules are heated in the barrel, this moisture turns to steam. The steam bubbles migrate to the surface when the mold fills, causing those streaky defects.
Apart from moisture, splay can also arise from:
Heat-Related Causes
Excessive barrel temperatures can overheat the plastic, releasing gases that lead to splay. Back pressure set too high can also overheat.
Long screw residence times are another culprit, with material sitting too long in the barrel before filling.
Shear-Related Causes
High screw RPMs generate shear heat, degrading plastic into gases. Small gates can cause shear too.
Gate obstructions like nozzle tip burrs disrupt flow, as do internal mold details like sharp corners.
Contamination
Regrind plastic or other contaminants introduce volatile elements or moisture into the resin.
Material Issues
Hygroscopic materials like nylon and PC absorb ambient moisture readily. Insufficient drying leaves water to become steam.
Excessive use of blowing agents or moisture-laden additives can also liberate gases.
So in essence, any source of contamination, volatiles or gases within the resin or mold can be released as bubbles that mar the surface – i.e. splay.
How to Get Rid of Splay in Injection Molding
Now that you know what’s behind those pesky silver streaks, let’s get into the business of actually eliminating splay defects from your injection molded components.
Step 1: Inspect Parts for Defect Patterns
As a first step, carefully check your molded pieces for patterns in the splay defects.
- If marks occur randomly all over, moisture is likely the culprit.
- Defects mainly at the gates point to shearing.
- Consistent marks indicate a mold-based issue like lubricant bleeding out of lifters.
So pay close attention to exactly how and where the splay manifests before jumping into fixes. The defect pattern will guide appropriate solutions.
Step 2: Address Moisture-Related Causes
Since moisture is the most common root cause of splay, let’s go through a systematic process to tackle it:
A. Check Hygroscopic Materials
Scan your material types and data sheets to see if any resins are hygroscopic like nylon or PC. Even small amounts of moisture in these materials can cause major splay.
B. Verify Drying Temperature & Time
Check drying temperature and duration against resin supplier recommendations. For example, nylons need 8 hours at 180°F. Make no assumptions here.
C. Inspect Dryers
Physically check that supply air lines are hot and returns are warm. Use probes to confirm temperatures match setpoints. Faulty dryers mean wet plastic!
D. Adjust Throughputs
If using undersized dryers, materials may not spend sufficient time inside to fully dry out. Increase drying time by reducing throughput rates.
E. Drain Loaders
Moisture can re-absorb into cooled plastic sitting in loaders. Always drain out initial portions before molding from any given dryer load.
Follow this methodology to systematically rule out or address moisture-related causes. This tackles the majority of splay issues.
Step 3: Tackle Heat-Related Splay
If moisture is definitively not the culprit, excessive heat is the next likely suspect. Here’s how to resolve thermal splay defects:
A. Compare Melt Temps
Check recorded melt temperatures against material supplier guidelines. If too high, incrementally drop barrel temp and retest parts.
B. Reduce Back Pressures
Excessive back pressure risks overheating and degrading resins. Gradually decrease back pressure until eliminating splay.
C. Lower Feed Zone Temps
Cooler feed zones reduce the time resin sits in the hot barrel, minimizing degradation from prolonged heat exposure.
D. Delay Screw Recovery
If cooling times are long, delay screw rotation to finish recovery just before mold open. This prevents overheating from residency.
Methodically addressing heat-related factors will resolve many remaining splay issues.
Step 4: Address Shear-Related Causes
If heating and moisture are conclusively not factors, then shear heat is likely contributing to those silver streak woes. Here are tips for countering shear:
A. Lower Screw RPM
High screw speeds generate excess shear heat, degrading plastic. Gradually decrease RPMs until splay disappears.
B. Widen Gates
Undersized gates cause shearing of material. Widen gates or use fan gates to minimize shear heating.
C. Slow Injection
The fastest way to resolve gate shear is to slow injection speeds. This gives resin more time to flow through.
D. Eliminate Obstructions
Check nozzle tips and mold venting channels for signs of debris or burrs that could disrupt smooth melts flow.
As a last resort, addressing shear causes should tackle residual splay defects not resolved by other fixes.
Step 5: Address Contamination Factors
If splay still persists after addressing heat, moisture and shear, contamination may be the sneaky culprit. Here’s how to tackle contaminated plastic and molds:
A. Eliminate Regrind
If regrind or recycled plastic is used, purge it all out and exclude for testing purposes. Contaminants in reused resin frequently cause splay.
B. Increase Purging
Purge extensively with specialty compounds to fully clean screw, barrel, nozzle and hot runners of debris or accumulated oils, waxes and contaminants.
C. Clean Molds
Tear down molds and meticulously clean out any accumulated lubricants, rust preventatives or other foreign matter on cavity surfaces using suitable solvents.
Thoroughly addressing contamination leaves no stone left unturned in your fight against splay foes.
Conclusion
And there you have it – a comprehensive roadmap to decisively deal with those pesky splay defects!
The key is methodically working through the most likely factors, starting with the most common moisture problems and progressively addressing heat, shear and contamination in turn.
While it may seem daunting, proper attention and dedication to each step will ultimately yield huge dividends in the form of beautiful, splay-free surfaces. No more silver streak blues with this battle plan.
The knowledge to not only tackle defects, but also prevent them in the future is what transforms injection molders from technicians into true experts. I hope this guide has contributed one more weapon into your process quality arsenal!
Now go forth and fear no splay foe! Your parts await flawless and pristine once more.