Views: 0 Author: Site Editor Publish Time: 2026-06-05 Origin: Site
One of the first questions you'll face when designing an injection mold is: "How many cavities should this mold have?"
The answer isn't random. It's a calculated trade-off between production volume, equipment capacity, mold cost, and part quality.
And here's something that confuses many people: there's a big difference between a multi-cavity mold (identical parts) and a family mold (different parts in one mold).
Let me break it all down for you.
Term | What It Means | Example |
|---|---|---|
Multi-cavity mold | One mold with multiple identical cavities, producing the same part simultaneously | 4 identical water bottles per shot |
Family mold (co-mold) | One mold with different cavity shapes, producing related but different parts in one shot | 1 lid + 1 cup body per shot |
The key difference: Multi-cavity gives you more of the same thing. Family mold gives you a complete set.
The family mold challenge: Different parts fill at different rates. The larger or easier-flowing cavity will "steal" material from the smaller one. Solutions include flow control pins or cavity-specific hot runner nozzles.
Choosing the right number of cavities isn't guesswork. You need to run the numbers through these five filters.
Start with your annual demand. How many parts do you need per day? Per shift?
Annual Volume | Recommended Cavities |
|---|---|
< 50,000 | 1-2 cavities |
50,000 - 250,000 | 2-4 cavities |
250,000 - 1,000,000 | 4-8 cavities |
> 1,000,000 | 8+ cavities (or multiple molds) |
Quick formula: Required cavities ≥ (Annual demand × Cycle time in seconds) / (Available hours × 3600 × Machine efficiency)
Typical efficiency: 85-95%
Every cavity needs clamping force to keep the mold closed during injection.
Total clamp force = Projected area × Cavity pressure × Safety factor
Material | Typical Cavity Pressure |
|---|---|
PP, PE | 20-30 MPa |
ABS, PS | 30-40 MPa |
PC, Tritan™ | 40-70 MPa |
PA+GF (nylon with glass fiber) | 50-80 MPa |
The rule: Total clamp force cannot exceed your machine's rating.
Your injection unit must be able to melt enough material for all cavities plus the runner system.
Best practice: Shot volume should be 50-70% of the machine's theoretical capacity.
Shot Size (% of machine capacity) | Problem |
|---|---|
< 30% | Material degrades from long residence time |
30-50% | Acceptable, not ideal |
50-70% | Ideal range |
70-80% | Acceptable near limit |
> 80% | Risk of short shots |
For high-cavity molds (16, 32, 48 cavities), the screw's ability to melt material becomes the bottleneck.
The machine must melt material faster than the mold can consume it.
The mold must physically fit in your machine. This means checking:
Maximum mold height (daylight opening)
Minimum mold height
Tie bar spacing
Platent dimensions
Spacing rule of thumb: Leave at least 2-3× wall thickness between cavities for cooling lines.
Layout | Best For | Pros | Cons |
|---|---|---|---|
Linear | 2-4 small cavities | Simple, short runners | Uneven filling |
Symmetrical | 2-4 cavities | Good balance | Wastes some space |
Circular | 8-16 small parts (caps, connectors) | Perfect flow balance | Complex hot runner |
H-pattern | 4-8 medium parts | Good space use | Requires flow analysis |
Nested | Family molds | Produces complete sets | Filling imbalance risk |
Here's the economic reality of choosing cavity count:
Cavities | Mold Cost | Part Cost | Machine Size | Risk |
|---|---|---|---|---|
1 | Lowest | Highest | Smallest | Production bottleneck |
2-4 | Low | Medium | Standard | Low |
8-16 | High | Low | Large | Cavity-to-cavity variation |
32+ | Highest | Lowest | Very large | One failure stops all |
The trade-off: More cavities lower your per-part cost but increase your upfront mold investment and risk.
A typical mold represents 10-30% of a project's total tooling cost.
Family molds deserve extra attention because they're trickier to get right.
Imagine a mold with:
Cavity A: Large cup body (100g)
Cavity B: Small lid (20g)
The large cavity offers less flow resistance. Melt prefers to go there, leaving the small cavity short.
Solution | How It Works | Cost Impact |
|---|---|---|
Flow control pins | Restrict flow to the fast-filling cavity | Low |
Size the runner | Make the runner to the small part longer/thinner | None (just design) |
Individual hot runner nozzles | Control each cavity independently | High |
Valve gates | Sequence the fill order | High |
Best practice for family molds: Use flow simulation (Moldflow) before cutting steel. It's much cheaper to fix on screen than in hardened steel.
Problem | What Happens | Solution |
|---|---|---|
Filling imbalance | Cavities fill at different rates → dimension variation | Optimize runner layout |
Pressure drop | Far cavities don't pack properly | Balance runner lengths |
Uneven cooling | Warpage, longer cycle times | Design balanced cooling circuits |
One bad cavity | Entire mold stops for repair | Use replaceable inserts |
Start
│
▼
1. Calculate required cavities from annual demand
│
▼
2. Check clamping force ── Too high? ──► Reduce cavities or get larger machine
│ OK
▼
3. Check shot capacity ── Too low/high? ──► Adjust cavities or machine
│ OK
▼
4. Check plasticizing (for high cavity counts)
│ OK
▼
5. Evaluate mold cost vs. per-part savings
│
▼
6. Apply experience-based guidelines (see table below)
│
▼
Final cavity numberPart Type | Typical Size | Common Cavities | Mold Type |
|---|---|---|---|
Large housing (TV, appliance) | >300 mm | 1 | Single cavity |
Medium part (water bottle, food container) | 100-300 mm | 1-2 | Single or two-cavity |
Small part (toothbrush handle) | 50-100 mm | 4-8 | Symmetrical or circular |
Tiny part (cap, connector) | <50 mm | 16-48 | Circular hot runner |
Precision gear | <30 mm | 8-32 | Full hot runner |
Assembly set (lid + body) | 50-200 mm | Family mold (2) | Nested |
Start conservative. High cavitation introduces:
Cavity-to-cavity variation in clarity
Stress differences between cavities
Pressure drop that affects surface finish
Recommended: 1-2 cavities for optical-quality parts; 4 cavities max for general transparent goods.
Start with a 2-4 cavity mold if volume is uncertain. You can always build a second high-cavity mold after validation. This approach:
Reduces upfront risk
Allows process optimization on fewer cavities
Gives you a backup mold
Use flow simulation. Period. The imbalance problem is real, and guessing leads to expensive rework.
Your Situation | Recommended Approach |
|---|---|
Low volume (<50k/year) | 1 cavity |
Medium volume, simple part | 2-4 cavities |
High volume, small part | 8-16 cavities |
Ultra-high volume (caps, connectors) | 32+ cavities |
Complete assembly (lid + body) | Family mold with flow control |
Transparent/optical parts | 1-2 cavities (quality over quantity) |
Uncertain demand | Start small (2-4), add high-cavity mold later |
There's no single "right" number of cavities. The best choice balances:
Production volume (how many you need)
Machine capability (what your press can handle)
Part quality (especially for transparent or precision parts)
Budget (upfront mold cost vs. ongoing part cost)
For critical applications — especially with high-quality materials like Tritan — conservative cavity counts often yield better long-term value than maximizing output at all costs.
