Two-Shot Mold Bases: Rotary vs Flip-Stack
Two cavity systems, one position-change mechanism
The core of a two-shot mold base is two cavity systems plus one reliable position-change mechanism. We design and machine two-shot and two-material injection mold frames in both rotary and flip-stack configurations, and the choice is usually settled by the part, the press and the cycle time rather than by preference. Both routes are machined to the customer's drawings after process review sign-off.
In a rotary (indexing) structure the centre plate rotates 180° so the tool completes both injections on one machine. It suits two materials with very different processing temperatures that need independent temperature control, and the indexing mechanism is mature and stable. The cost is that it places high demands on the rotary table's positioning accuracy and load capacity, which have to be matched against the press you actually have.
What decides between the two
A flip-stack (core-back) structure retracts a moving cavity or slide to create the second-shot space. It is relatively compact with a lower mold height and suits small to medium parts and projects with demanding cycle times. Four things then decide between them: the temperature and shrinkage difference between the two materials; whether each side needs independent water circuits; press tonnage, rotary table diameter or platen distance limits; and the repeatability of the locating structure, which directly determines misalignment at the two-material interface.
Independent locating is where most of the machining difficulty sits. Our multi-cavity two-shot work, such as the medical consumables project, combines a multi-cavity layout with independent locating structures, and those locating features are among the dimensions recorded on the CMM before dispatch. If the two-material interface is a visible surface, say so at drawing stage, because it changes both the locating scheme and the accuracy we plan for.
Machining and verifying a two-shot frame
Two-shot frames concentrate hole patterns, counterbores and insert pockets into a smaller envelope than a comparable single-shot tool, so pillar holes, cooling circuits and locating features are checked against each other during review rather than sequentially on the machine. Finishing runs on vertical machining centres holding ±0.005 mm within 1200 by 600 mm, with jig grinding to ±0.001 mm for high-precision bushing bores.
Final verification uses a coordinate measuring machine rated at ±0.0015 mm within a 1200 by 800 mm envelope, and the measurement data ships with the plates. Send the 3D model of the two-shot part together with your press specifications - tonnage, rotary table diameter or platen distance - and we will return structural and cost recommendations with the free review, which comes back within 24 hours.
FAQ
What is the difference between rotary and flip-stack?
In a rotary structure the centre plate rotates 180 degrees so both injections happen on one machine; it suits materials needing independent temperature control but demands accurate, load-capable indexing. A flip-stack structure retracts a cavity or slide to create the second-shot space, giving a compact, lower mold height suited to shorter cycle times.
What should I send for a two-shot review?
The 3D model of the part and your press specifications: tonnage, rotary table diameter or platen distance limits. Also state whether each side needs independent water circuits and the acceptable misalignment at the two-material interface.
How accurate is the locating structure?
Locating feature repeatability directly determines misalignment at the two-material interface. Bushing bores are jig ground to ±0.001 mm and verified on a CMM rated at ±0.0015 mm, with the data shipped with the plates.
Do you machine multi-cavity two-shot bases?
Yes. Our medical consumables project is a multi-cavity two-shot base with independent locating for each cavity, machined complete and inspected before dispatch for clean-production requirements.
All figures above refer to standard product data sheets; final confirmation is subject to written quotation.