Multi-cavity molds help you make small plastic items like drawer handles and drip emitters in large quantity. But a common headache: some cavities produce good parts while others have flash, thin walls or wrong dimensions.
Most production teams spend weeks adjusting machine settings, yet the problem comes back again and again. The truth: uneven parts usually come from mold design issues, not machine setup.
SOLIDCO MOULD lists 4 main causes and practical solutions for stable, consistent multi-cavity production.
1. Unbalanced Runner System in Multi-Cavity Molds
Even if runner lengths look the same, plastic melt heats up when flowing through long runners. Shear friction lowers melt viscosity, and some cavities fill faster and pack harder than others — even with a perfectly symmetric layout.
This is the #1 hidden cause of uneven part weight and dimension variation.
- Adjust runner diameter to balance melt flow for every cavity
- Use round runner shape for smooth plastic flow and minimum shear
- Run flow simulation before steel cutting to find imbalance early
At SOLIDCO MOLD, we run full flow simulation for every multi-cavity mold project before machining, to catch and correct imbalance at the design stage. This avoids costly rework after steel is cut and keeps your production launch on schedule.
2. Temperature Variation Across Multi-Cavity Mold Plates
Only 5~10℃ temperature difference across mold plates will cause size and surface difference on finished parts. Edge cavities naturally lose more heat to the mold base than center cavities, creating uneven shrinkage.
Many operators try to fix this with pressure adjustments, but it only masks the problem.
- Use independent cooling circuits for different cavity zones
- Separate temperature control for each hot runner nozzle
- Keep mold plate thickness even to avoid local overheating
3. Mismatched Gate Sizes in Multi-Cavity Molds
Many mold makers cut all gates to the same size. But cavities at different positions see different pressure and melt flow. Edge cavities typically see higher pressure loss, so they need slightly larger gates to achieve the same fill speed and packing.
Proper gate calibration ensures every cavity sees the same shear rate and packing pressure.
- Fine tune gate size for cavities at different locations
- Make sure each cavity gets same plastic shear rate
- Keep gate mark uniform for all finished parts
4. Fix Multi-Cavity Mold Design Before Tuning Machine Parameters
Turning up injection pressure to fill slow cavities will create flash on fast cavities. Machine tuning can only cover up design defects temporarily, and will create new quality issues elsewhere.
Always fix the root cause in the mold first.
- Check runner and gate design before changing machine settings
- Use cavity pressure sensor to check real flow balance
- Test part dimensions after mold fully heats up
Frequently Asked Questions
Q: Why is my multi-cavity mold producing parts of different weights?
A: Weight variation almost always comes from runner imbalance or uneven packing across cavities. Start by checking runner balance and gate sizing before adjusting machine settings.
Q: Can I fix uneven cavity filling with machine settings?
A: Minor variations can be tuned, but major imbalance is a design issue. Trying to fix it with pressure will usually create flash in faster-filling cavities.
Q: How do I ensure consistent parts from every cavity?
A: Use flow simulation to balance runners, design independent cooling per zone, calibrate gate size by cavity position, and verify with cavity pressure sensors.
Final Thought
A well-balanced multi-cavity mold can produce identical parts shot after shot. This consistency comes from flow simulation and careful mold making from SOLIDCO MOULD.
View our household plastic mould cases for real multi-cavity project examples.
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