Two shots, one frame: what a two-color mold base has to hold

A two-color mold — also called double-shot — forms two materials in one cycle, so the part leaves the machine joined rather than coming together in a press or on a glue line afterwards. Everything in the stack is shared: one clamp, one ejector, one water supply, one machine. The second material lands on a surface that is already solid, so the frame's real job is to bring the two cavity stations back to the same place on every cycle.

Two constructions, and they ask different things of the frame. On a rotating platen the mold opens, one half indexes through half a turn, and the core carrying the first shot presents it to the second cavity: the frame becomes two stations built into one set of plates, laid out about the centre of rotation. In a core-back construction the second cavity volume opens inside the same stack and the frame carries the moving element, its guides and its stops. The two are machined as described on the two-color mold base line, and they are not the same frame with a different pocket.

Registration is a distance between two bores, not a tolerance on one. What decides whether the second material lands on the first is where station two sits relative to station one, and where the pair sits relative to the feature that locates the mold in the machine. The guide pillar bores are the physical reference both halves close on, so their positions are the registration. Boring them in separate setups adds the error of two setups — the setup question behind clamped flat, released warped; we bore them in one setup and hold ±0.02 mm on fitting features, our own callout confirmed at quotation rather than a claim of compliance with a general tolerance standard.

A frame that rotates has to be symmetric about its centre. Half a turn puts whatever was at one station at the other, so a layout that is not symmetric presents two different halves to the platen on alternate cycles. A larger cavity on one side, a slide hanging off one edge, a coupling and a manifold on one half only — the frame still closes, but the two stations no longer arrive at the same height under the same clamp. Settling symmetry while the layout is still a sketch costs a conversation; finding it at the trial costs a plate.

One clamp force, two closing planes. Where the stack has two parting lines they close on one machine tonnage, and the frame has to be built so both close together. The distance between the planes is a frame dimension machined into the rails and plates, and it is what stops the second plane running with less clamp than the first. Where the two are not held in relation to each other, the plane that closes last is the one that flashes.

The locating hardware needs a seat, and the seat is in the frame. Index pins, locating bushes, the rotation coupling, the target the position sensor reads: each needs a bore, a face or a bracket on the frame, positioned against the same datum as the cavities. A frame machined without those seats arrives square and useless, and adding them afterwards means re-setting up a finished plate — the same trap as a side action whose seat was never in the drawing.

The ejector side follows the finished part. Nothing ejects until the second shot is done, so the ejector pattern has to clear both cavity footprints and the part has to leave without dragging across the first material. On a rotating construction the ejector stays on one side, which means that half needs the clear space and the travel for the rotating element to swing past it. Clearance is a plate layout decision: cheap to draw, expensive to discover.

Two circuits, and they never share a route. The two materials run at different temperatures, so the frame carries two separate circuits. Its job is the routing: straight bores where a circuit crosses a plate or a spacer rail, deep-hole drilling where it has to travel a long way, and no circuit through the volume the rotating or sliding element occupies. Two manifolds on the outside of the frame, rather than one shared block, is usually where that layout ends up.

Shut height is not the single-color number. Two stations and an indexing element add height to the stack, and a machine has a minimum as well as a maximum. Send the model, the platen size, the tie-bar spacing and the maximum mold height with the drawing: the five press interfaces are set out in where the base meets the press, and a two-color frame is confirmed against the machine rather than assumed from the single-color mold it replaces.

What to put on the enquiry. The two materials; the machine; the construction the toolmaker has chosen; the two cavity layouts on one drawing with the centre of rotation marked and their relative position dimensioned; which faces locate the second shot; whether the frame is to be symmetric; the two water circuits; the ejector pattern; and the locating hardware that will be fitted. That is enough to quote a frame that can be built, and it is shorter than the thread that follows a frame that cannot.

Where our work stops. We machine the frame and the two cavity pockets to your layout, including the guide bores, the indexing seats and the water. The inserts and their heat treatment, the second-material feed, the texture and the trial belong to the toolmaker, and we do not claim to run a two-shot process on your behalf. What we can put behind the frame is the part that is measurable before it ships: fitting and ground faces at ±0.02 mm, ground faces at Ra 0.8 µm, and an inspection record that travels with the crate.

Why it keeps coming back to the frame. Buyers weighing a mold base from China usually compare two numbers first, price and plate thickness, and a two-color project is where that shorthand fails hardest: neither number says whether the second station is where the first one expects it. If you are shortlisting Chinese mold base suppliers, ask the same three questions of each: in how many setups the guide bores are bored, how the second station is located from the first, and whether the indexing seats are cut at the frame or added later. Ours are machined in Dongguan, China, in our own shop rather than bought in and re-crated; most run 7–10 days from confirmed order to shipment, the plant turns out 2,000+ sets and pieces a month, and the price is the factory's with no trading layer between the drawing and the machine.