The number of cavities of the injection mold (single cavity or family mold)) can be selected by the customer when start design the mold. However, the optimal number of cavities to choose is a critical decision that requires comprehensive consideration. There is no absolute "good" or "bad" choice, only the one that is "most suitable" for the current situation. The main determining factors are as follows:

1. The part demand  (the most important factor)
The number of cavities in an injection mold directly determines its production capacity, so when choosing the number of cavities, the part demand of the product must be considered.
Large scale production (millions or more): usually requires multi cavity molds, such as 16 cavity, 32 cavity, 64 cavity or even more, commonly used in bottle caps, preforms, small electronic components, etc. The more cavities there are, the higher the output per molding cycle, and the lower the unit cost (shared mold and machine labor costs).
Medium batch production (several hundred thousand level): commonly used 4-chamber, 8-chamber, 16 chamber, etc. A balance needs to be struck between mold cost, production efficiency, and mold replacement frequency.
Small batch production or trial production (tens of thousands or lower): usually choose 1 cavity (single cavity) or 2 cavities, 4 cavities. The mold cost is low, the manufacturing cycle is short, and the flexibility is high. Even if the order quantity is small, the mold cost can be quickly shared.
Extremely low yield or oversized complex items: single hole (1 out of 1) is the most common.
2. Part size and complexity
The size and complexity of a product mainly depend on whether it is suitable for making multi cavity molds.
Large size: A single product occupies a large mold space, and the size and locking force of the injection molding machine template limit the number of cavities. Large products such as car bumpers, instrument panels, and large home appliance casings are usually 1-in-1 or 1-in-2.
Small size: Small parts such as buttons, connectors, and bottle caps can easily achieve dozens or even hundreds of cavities.
Complex structure: For products with complex structures (such as multiple sliders, sloping tops, and precision mating surfaces), increasing the number of cavities will significantly increase the difficulty and cost of mold design, manufacturing, and maintenance, as well as the difficulty of controlling the consistency of quality in each cavity. The number of cavities should be cautiously increased.
3. Mold cost and budget
The cost significantly increases with the increase of the number of cavities: when the number of cavities doubles, the mold cost usually does not double, because the processing volume, steel consumption, hot runner system cost, cooling system complexity, exhaust system, etc. will all increase, so it is necessary to evaluate the return on investment.
Shared cost: The single piece cost (shared mold fee) of multi cavity molds is lower, but this requires sufficient production to share the high initial investment. Using multi cavity molds in small batches may actually result in higher unit costs (as mold costs cannot be fully shared).
4. Injection molding machine capability
Locking force: With an increase in the number of cavities and projected area, the required locking force increases. It is necessary to ensure that the selected injection molding machine has sufficient locking force to prevent burrs.
Injection volume: The total weight of all cavity products plus the weight of the flow channel should not exceed the theoretical value of the maximum injection volume of the injection molding machine (recommended to be below 60% -80% in practice).
Template size and pull rod spacing: The external dimensions of the mold must be able to fit into the injection molding machine template and smoothly open and close the mold.
5. Product quality requirements
The balance of multiple cavities: ensuring that all cavities are simultaneously filled and fully pressurized at almost the same pressure, temperature, and velocity is a huge challenge. Channel design (cold or hot channels), cooling system design, and exhaust design must be very precise. Otherwise, it will result in differences in size, weight, appearance, and strength among products with different cavities (known as "cavity differences").
Hot runner system: For multi cavity molds (especially those with 16 or more cavities), hot runners are almost standard. They can reduce waste, shorten cycles, and improve filling balance, but the cost is higher and maintenance is more complex.
Maintenance and troubleshooting: If there is a problem with one cavity (such as damaged inserts or stuck pins), the entire mold may need to be shut down for repair, affecting the production of all cavities. The more cavities there are, the more potential fault points there are.
6. Production flexibility and mold changing frequency
Multiple varieties and small batches: If there are multiple product types and the order volume for a single product is not large, frequent mold changes are required. Therefore, using molds with fewer cavities (such as 1-in-1, 1-in-2) is more flexible, with relatively shorter mold change times and less inventory pressure.
Single product in large quantities: The efficiency advantage of multi cavity molds is obvious, reducing the number of mold changes.
7. Mold manufacturing cycle and delivery time
The more cavities there are, the longer the mold manufacturing cycle: processing, polishing, assembly, trial molding, and debugging all take longer. If the project time is tight, the number of cavities needs to be appropriately controlled.
8. Product design maturity and modification risk
New product/design not fully frozen: If there is a high possibility of modification in the later stage of product design, it is recommended to first use a mold with fewer cavities (such as 1-in-1) for trial production and verification. Otherwise, the cost and time of modifying multi cavity molds will be very high.
Mature product: Stable design, reliable use of multi cavity molds to improve efficiency.