Multi-Shot Injection Molding (2K & 3K)

Tooling Kinematics

Rotary Platen and Transfer Kinematics

Without multi shot tooling, adding a soft grip, a seal or a second color means molding the housing, moving it to a second mold or press, overmolding it and often assembling the pieces by hand. A 2K mold does all of that in one press. On a rotary mold, the moving half turns 180 degrees every cycle. While the first unit molds a new substrate at station one, the second unit overmolds the substrate from the last cycle at station two, so both shots fill at the same time and a finished part comes out every cycle.

Figure 1: Automated 180-Degree Rotary Platen Kinematic Cycle

One 2K cycle: the first shot fills the rigid substrate while the second shot overmolds the part from the last cycle, the mold opens, the finished part leaves station two, and the rotary half turns 180 degrees.

What happens in every rotary 2K cycle

1
Close and clampOne clamp force holds both stations shut
2
Both shots fillUnit one molds the substrate, unit two overmolds the last one
3
Pack and coolBoth materials cool together on the core
4
Open and ejectOnly the finished part leaves, from station two
5
Turn 180°The new substrate moves to station two for its second shot

Benefits of Integrated 2K Tooling

Fewer assembly steps: seals, soft grips, dampers and second colors are molded in place instead of glued or snapped on later.
Better parts: soft hard bonding adds grip, sealing and a finished look, and each part comes out the same every cycle.
Sealed assemblies: TK’s precise insert molding of metal, glass, ceramic and PCB inserts has reached waterproof performance up to 7 bar, equal to IP68.
One press, any layout: TK builds molds for parallel barrel and “L” type 2K presses, and multi shot solutions with an autonomous injection unit (AUI) that adds a second shot to a press.

Every 2K mold is tried in TK’s mold testing center on presses with rotation tables for the second shot, from a 180 ton JSW to a 1,500 ton Engel, plus a 300 ton Netstal set up for 3K. TK has run a specialized tool room for 2K molds since 2016.

Two shot mold halves for a large automotive part, built by TK for multi material molding.

Material Science

Polymer Chemical Compatibility and Thermal Bonding

A 2K bond forms when the hot second shot melts the skin of the first shot enough for the two materials to fuse. That only works when the materials are compatible, which is why the material pair is chosen first and the tool is designed around it. A standard SEBS TPE fuses well to PP, for example, but needs an adhesion modified grade to bond to PC, ABS or nylon, and a TPU bonds well to PC and ABS but poorly to PP. TK brings first, second and third shot material selection experience to every program and checks the bond with peel tests at trial.

First shot (rigid)	Second shot	Typical bond	Typical uses
PP	SEBS TPE or PP based TPV	Good, fuses during molding	Caps, closures, handles, soft grips
PC, ABS or PC/ABS	TPU or adhesion modified TPE	Good	Electronic housings, buttons, tool grips
PA 6 or PA 66	Adhesion modified TPE or TPU	Fair to good, grade dependent	Industrial and automotive housings
PBT or PA	Self adhesive LSR	Good with the right grades	Seals, gaskets, wearable parts
PP	TPU	Poor, plan an interlock	Use only with a mechanical lock
Any resin	Same resin, second color	Fuses, same polymer	Two color knobs, lenses, logos

Typical results from TPE and silicone supplier guidance. Bond strength depends on the exact grades, the substrate temperature and the overlap area, so it is confirmed with the grade’s data sheet and peel tests at trial.

Two shot personal care housings with soft touch overmolding, molded by TK.

Mechanical Interlocks for Critical Joints

When the materials do not bond, or the joint takes high peel or torque loads, the first shot is designed with features the second shot flows through and around: through holes, undercut grooves and dovetail channels. The second shot then locks in mechanically and stays put even if the chemical bond is weak. TK offers interlock feature design as a service, and reviews gate location, wall thickness and the shutoff on the first shot during our DFM and Moldflow engineering review.

Soft touch 2K parts: rigid and soft materials molded together in one cycle.

Silicone Engineering

Liquid Silicone Rubber (LSR) Multi-Shot Molding

Cold Runner, Hot Mold

LSR runs the opposite way from a thermoplastic. It is a two part liquid that is pumped in cool and cures with heat, so the runner has to stay cold while the cavity stays hot. As typical silicone supplier values, LSR molds run at about 150 to 200 °C. TK’s LSR molds use a cold runner system for the silicone and a hot runner system for the engineering plastic, and the first shot resin has to hold its shape at the silicone cure temperature, which is why PBT and nylon are common substrates.

TK Techo, TK’s silicone rubber business, molds single shot LSR, LSR overmolded onto engineering plastic, metal or silicone, and double shot 2K LSR: LSR plus LSR, LSR plus engineering plastic, or LSR plus glass. Liquid compression silicone molding is also available.

LSR molding presses at TK

0.01 mm

Parting line on TK’s flash free LSR molds

>99.8%

Vacuum in the mold from TK’s own sealing system

2 weeks

LSR prototype service on a TK standard mold base

LSR flows like water when it is injected, so it finds any gap at the parting line. TK’s flash free LSR molds pull more than 99.8% vacuum in the mold with a dedicated sealing system and hold the parting line to 0.01 mm. For fast starts, a single shot LSR prototype can run in about 2 weeks on a TK standard mold base with a cold runner.

Tooling Methods

Core-Back vs Rotary Tables

There is more than one way to get the first shot to the second material. The right choice depends on the part shape, where the second material goes, the volume and the press. TK’s engineers pick the method during DFM, before the mold design starts.

Method	How the substrate moves	Best for	Trade offs
Rotary platen or rotary mold	The moving half turns 180° (or 120° for 3K) to the next station	High volume parts, both shots fill at once	Needs a 2K press with a rotation table; two sets of cores
Core back	It stays put; a core or slide retracts to open space for the second shot	Second material on one face, simple add on layers	Shots run one after the other, so the cycle is longer; limited shapes
Robot or index transfer	A robot or index plate moves it to a second cavity or mold	Parts that must flip, or need inserts loaded	More handling time and automation
Vertical rotary table	The lower mold halves rotate between the load and mold stations	Insert molding and overmolding of metal, PCB or film inserts	Parts and inserts are loaded at the open station

Lower mold halves on the rotary table of a vertical press in a TK molding cell.

Core Back, Foaming and Combined Processes

TK’s automotive team also uses core back motion with chemical foaming, and combines 2K with other processes in one press: a 1,500 ton Engel runs 2K with MuCell and coining, and a 2,000 ton press runs 2K with spin form. TK also combines 2K with in mold labeling, molding a PA first shot and a TPE second shot onto a PET film insert.

Insert molded electronic housings with metal inserts molded in place.

Proven Programs

2K Molds Built by TK

A selection of TK multi shot programs. In a 2K mold, 12+12 means 12 first shot cavities and 12 second shot cavities, so 12 finished parts come out every cycle. Data is from TK’s project records.

Part	Cavities	Materials, Weight	Press	Cycle	Notes
2K pull ring flip top cap, ø34 x 27 mm	12+12	PP + TPE, 6.75 g	420 T	18 s	10 weeks from PO to shipment; conformal cooling on the critical insert cut the cycle 22%
2K child resistant outer closure, ø56 x 36 mm	18+18	PP + TPE, 7.0 g	450 T	10 s	One common hot half with 3 cold half versions for different sizes; Moldmax HH core inserts shorten the cycle
Automotive air intake, upper and lower	2K	Not listed	1,500 T Engel	Not listed	2K combined with MuCell and coining
2K part with in mold label (IML)	2K + IML	PET film, PA, TPE	Not listed	Not listed	IML replaced a PVD step after 2K molding; yield rose from about 50% to over 70%

Material Selection

Interactive Polymer Compatibility Selector

Pick your first and second shot materials to see the typical bond and whether to design in an interlock. Then enter the part size and volume to get a rule of thumb clamp tonnage for a rotary 2K mold, the TK 2K trial press that fits, parts per hour and how many molds your volume needs. Open “Show the math” to see every step.

Typical bondGood

PoorFairGood

Clamp tonnage, rule of thumb

TK 2K trial presses with rotation tables

The press also has to fit the mold: platen size, tie bar spacing, the rotation table and the shot size of each unit often call for a bigger press than clamp alone. TK’s 12+12 2K cap mold runs on 420 T.

Show the math

Send my 2K program to Brandon

Bond ratings are typical results from TPE and silicone supplier guidance and are confirmed with the actual grades and peel tests at trial. Clamp assumes a rotary mold with both stations filling each cycle and hot runners, using rule of thumb factors by resin; real values are confirmed with Moldflow. Trial press list is TK’s mold testing center. Default numbers are an example only.

Frequently Asked Questions

Technical Sourcing FAQ: Multi-Shot Tooling

Can standard injection molding machines run multi-shot tooling?+

A rotary 2K mold needs a press with two injection units, set up parallel or in an “L” layout, and a rotation table or rotary platen. A standard press can also run multi shot work with an autonomous injection unit (AUI) added for the second shot. TK builds molds for all of these, runs 60 2K and 3 3K presses in its molding plants, and tries 2K molds on rotation table presses from 180 to 1,500 tons.

What shutoff precision is required for 2K multi-shot tooling?+

In the second station the steel shuts off against the first shot plastic, not against steel, so the shutoff is designed with a small, controlled interference on the substrate that is checked and tuned at trial. Because the rotating half meets two different cavity sides every cycle, both sets of cores and cavities have to match closely. TK machines molding inserts to a tolerance of 0.002 mm.

What are the lead times for custom 2K rotary molds?+

TK’s typical build is 3 to 6 months, depending on size and complexity. A 12+12 two shot pull ring flip top cap mold shipped 10 weeks after the purchase order.

Which material pairs bond in two shot molding?+

Typical good pairs are PP with SEBS TPE or a PP based TPV, PC or ABS with TPU or an adhesion modified TPE, PBT or nylon with self adhesive LSR, and any resin with itself in a second color. Pairs that do not bond, like PP with TPU, need mechanical interlocks. TK confirms the bond with the actual grades and peel tests at trial, and the planner above shows the typical result for common pairs.

Can TK mold the parts as well as build the mold?+

Yes. TK builds the mold and can mold production parts in its own plants on 60 2K presses, 3 3K presses and 32 LSR presses, with painting, printing and assembly lines. See our contract injection molding page for the full molding capacity.

Developing a Two-Shot or Overmolded Assembly?

Send your part files, materials and annual volumes to Brandon Henderson, Technical Sales Support Manager for the Americas. He will review the material pair, interlocks, rotary or core back tooling and lead time with TK’s 2K engineers.

Request 2K Tooling Quotation & DFM Call Brandon: (336) 953-9664