Two-Shot, Multi-Shot molding, Overmolding?

Plastic injection molding produces plastic parts by injecting molten plastic into a mold cavity. In two-shot molding, two different plastics are injected in sequence into one mold to create a two-material or two-color part. Multi-shot molding injects three or more plastic shots into a single mold to create parts with multiple materials or colors. Overmolding involves injecting one material onto a pre-molded part already placed in the mold, combining two materials into one part. Multi-shot and overmolding combine different properties like flexibility, strength, and texture into one part. They increase design freedom but require more complex tooling than standard injection molding.

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Injection Molding

Injection molding is a typical manufacturing process for producing plastic parts in high volumes. The primary process involves melting plastic material into a liquid and injecting it under high pressure into a mold cavity. The plastic cools and solidifies inside the mold, taking its shape.

The process begins by feeding small plastic feedstocks into a hopper that feeds the injection molding machine. The pellets are melted inside a heating barrel using heaters and rotating screws. Once molten, the plastic is injected through a nozzle into the mold cavity with a clamping unit keeping the mold closed under pressure.

Inside the mold, channels cut into the mold allow coolant to circulate and solidify the plastic. Once cooled, the mold opens, and the finished plastic part is ejected. The mold then closes, and the cycle repeats.

Injection molding is known for its speed, repeatability, and ability to produce complex geometries efficiently. Parts have tight tolerances and low scrap rates. Although injection molds have high initial costs, injection molding has a low per-part cost at high volumes, making it ideal for mass production.

Two-shot injection molding

Two-shot injection molding, also known as 2K molding or two-color injection molding, is a plastic molding process that combines two different plastic materials into a single part.

In this process, two separate plastic materials are injected sequentially into the same mold cavity. The first shot forms the core part; the second shot molds a second material onto the core. The two materials bond through thermal and molecular diffusion, creating a two-material or two-color component with unique properties.

To keep the two melts separate, the two-shot molding requires specialized injection molding machines and molds with two separate barrel and nozzle systems. It combines the advantages of two plastics, such as flexible and rigid polymers, into one part.

Benefits of two-shot molding include design flexibility, consolidated part assembly, cost-effectiveness, and achieving a combination of characteristics like soft-touch grips or rubber seals bonded to rigid plastic. Two-shot molding is ideal for complex products with decorative effects like control panels, automotive trim, medical devices, and consumer goods.

Pros and Cons of Two-shot Injection Molding

Two-shot molding provides unique advantages but requires more investment and process expertise than conventional single-shot injection molding.

Pros:

• Combines two materials into one part, eliminating assembly
• Achieves multi-color and multi-material parts
• Lowers costs compared to assembling two parts
• Provides design flexibility not possible with one material
• Automates the overmolding process
• Strong bonding between two materials
• Allows mixing different properties like rigid/flexible
• Hides visible joints between materials
• Consistent quality and reliability

Cons:

• Higher tooling costs than single-shot molding
• More complex processes requiring specialized equipment
• Needs extensive expertise to design and produce
• Limitations on material combinations based on bonding and viscosity
• Higher pressure requirements than single-shot
• Slower cycle times due to multiple injections
• Higher scrap rate if the process is not optimized
• Challenging to combine more than two materials
• Material separation can occur if bonding is poor
• Need to balance shrinkage rates of two plastics

Multi-Shot Injection Molding

Multi-shot injection molding involves injecting two or more different plastic materials sequentially into a single mold cavity to produce complex, multi-material plastic parts in one shot.

In a multi-shot process, the mold has multiple nozzles injected into the same cavity at different times. The first shot of molten plastic is injected and allowed to partially or fully harden. Next, a second shot of a different material is injected against the first, followed by more injections as needed.

Multi-shot molding requires specialized machines with additional injection units and complicated hot runner systems to deliver the separate melts. Each material must be compatible with the other and have proper bonding characteristics.

The multi-shot process allows the combining of three or more plastic materials into one part. It provides design flexibility to integrate different properties, colors, finishes, and functions into one component. Multi-material parts can integrate soft touch grips, seals, gaskets, hinges, and clear window lenses into a single molded part.

Multi-shot molding consolidates assembly processes, reduces labor, and improves durability over multi-part assemblies. It is ideal for complex products like auto interior trim, consumer electronics, medical devices, and industrial parts. However, the molds and equipment are complex, requiring significant expertise.

Pros and Cons of Multi-Shot Injection Molding

Multi-shot molding can produce innovative multi-material parts but requires very complex processing and high initial costs compared to standard injection molding.

Pros:

• Combines three or more materials into one part
• Achieves complex multi-material and multi-color effects
• Consolidates complex assemblies into one component
• Lowers manufacturing costs compared to assembling multiple parts
• Provides unique design flexibility and product innovation
• Allows combining incompatible materials in one product
• Improves bond strength between different materials
• Automates overmolding and assembly operations
• Higher quality and consistency versus assembling separately

Cons:

• Very high initial investment in complex molds and equipment
• Requires advanced expertise in process and mold design
• Significant process optimization is needed for material compatibility
• Slower cycle times due to multiple injections
• Higher scrap rates if not optimized properly
• Limitations on material combinations based on bonding
• Challenging to combine more than five different materials
• Very costly to change or modify molds
• Higher maintenance costs for complex systems
• Need to balance shrinkage rates of all materials
• Material separation can occur if bonding is poor

Overmolding

Overmolding is a plastic molding process where one material is molded onto a second pre-formed material. It combines two different components into a single part.

The process starts with the first material being molded to create the core component. It is known as the substrate or substrate part.

The substrate is then loaded into a second mold cavity where the second material is injection molded to form an outer layer around the substrate. This is known as overmolding.

The two materials bond together to create the final over-molded part. The substrate provides the core strength while the overmold provides a gripping surface, seal, gasket, or adds aesthetic appeal.

Overmolding allows the combination of different properties, like rigid and flexible polymers, into one part. It eliminates the assembly of separate components with fasteners or adhesives.

Some examples of over-molded parts include toothbrushes with soft grips, rubber seals molded onto metal components, and handles molded onto tools. The substrate can be plastic, metal, wood, or pre-assembled sub-components.

Overmolding is ideal for cost-effectively combining the benefits of disparate materials into one part with enhanced functionality. It simplifies assembly and improves reliability.

Overmolding provides unique advantages but requires material selection, mold design, and process optimization expertise.

Pros and Cons of Overmolding

Pros:

• Combines different materials into one part
• Consolidates assembly of separate components
• Provides design flexibility not possible with one material
• Allows combining incompatible materials
• Achieves multi-material and multi-color effects
• Lowers manufacturing cost vs. assembling multiple parts
• Automates assembly operations like adding grips or seals
• Adds functionality by combining properties like rigid/flexible
• Improves ergonomics and aesthetics
• Simplifies designs by reducing fasteners and joints
• Improves bond strength between different materials
• Consistent quality and reliability

Cons:

• Requires two distinct manufacturing processes
• Needs expertise in selecting compatible materials
• Material shrinkage rates must be compatible
• Limitations on a substrate and overmold materials
• The substrate part must be designed for overmolding
• Cycle time longer than standard molding
• Higher initial mold costs
• Potential for over-molded layer separation over time
• Assembly may still be required if the substrate is complex
• Substrate imperfections and knit lines may show
• Higher scrap rate if process not optimized

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Two-Shot and Multi-Shot Injection Molding Vs. Overmolding

Two-shot and overmolding combine two materials using sequential molding, while multi-shot allows complex combinations of three or more materials through multiple injections into one mold. Overmolding also differs by using a pre-formed substrate part. All provide unique part consolidation and property integration, but multi-shot is the most complex process.

Two-Shot Molding:

• Combines only two different plastic materials into one part
• Two materials inject sequentially into one mold
• Achieves two-color and rigid/flexible material parts
• Lower complexity and cost vs. multi-shot molding

Multi-Shot Molding:

• Combines three or more different plastic materials
• Multiple injection units inject into one mold sequentially
• Produces complex multi-material and multi-color parts
• The highest complexity, initial cost, and process expertise needed

Overmolding:

• Second material molded over a pre-formed substrate material
• Substrates can be plastic but also metal, wood, etc.
• Automates assembly of grips, seals, etc., onto a substrate
• Lower complexity than multi-shot, only two materials
• The substrate must be explicitly designed for overmolding

How to Choose the Right Injection Process?

The optimal choice depends on product requirements, volumes, budget, design needs, and production capabilities.

• The number of materials: Two-shot combines 2 materials, multi-shot can combine 3+ materials, and overmolding usually uses just 2 materials.
• Part complexity: Multi-shot can make the most complex multi-material parts. Overmolding is limited if the substrate is complex.
• Tooling cost: Multi-shot molds are the most expensive. Overmolds need two tool sets, and two-shot tools fall in between.
• Production volumes: Two-shot is better for medium-high volumes. Overmolding is better for low-volume production or plastic prototyping. Multi-shot is only viable for high volumes to amortize tooling.
• Material selection: Material compatibility and bonding are crucial, especially for multi-shot with more materials.
• Process expertise: Multi-shot requires the most extensive process know-how. Two-shot is less complex.
• Equipment cost: Multi-shot machines are the most expensive. Standard presses can do two-shot and overmolding.
• Cycle time: Multi-shot has the longest cycle times due to multiple injections. Two-shot is faster than overmolding.
• Initial investment: Multi-shot requires the highest investment in equipment and tooling. Two-shot and overmolding have lower startup costs.
• Design flexibility: Multi-shot offers the most innovative design possibilities with multiple materials.
• Assembly consolidation: Multi-shot and two-shot integrate most assembly steps. Overmolding depends on the substrate.

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33 Suggestions for Design of Two-shot Mold

Conventional Design

No.1

First determine the direction of the parallel nozzle of the injection molding machine as the x-axis or y-axis. In this way, the product placement is determined.

No.2

The mold guide post and guide sleeve must be matched.

No.3

The spacing of the products must be based on the spacing of the nozzles of the injection molding machine. The nozzle spacing of the two-color injection molding machine is somewhat adjustable, and some are not adjustable.

No.4

Note on the parting surface. The rear mold parting surface is the parting surface obtained after the combination of the two products. The front mold parting surface should be taken from a single product.

No.5

Carefully review the other reference data for the injection molding machine. For example, the maximum die thickness, the minimum die thickness, and the top hole spacing.

No.6

When designing the cavity of the second injection molding, to prevent the cavity from scratching the first molded product, you can design a partial avoidance. However, carefully consider the strength of each sealing position. For example: in injection molding, the plastic deforms under large injection pressure. Incurring a second injection may result in burrs.

No.7

When clamping the a, b plates, pay attention to whether the slider is reset before clamping. This will crush the product. To avoid this situation.

No.8

The water lines of the two chambers and cores should be as full as possible and balanced.

No.9

In 99% of cases, the hard plastic part of the product is formed first. The soft plastic part is molded a second time because the soft rubber is easy to deform.

No.10

Pay attention to whether the first molded product will be displaced by the impact of plastic during the second injection molding. This causes a change in the plastic wall.

Rotary Die Design

No.11

Two separate ejector systems, two top sticks.

No.12

The direction of water in and out must be on the upper and lower sides. Every circulating water must go in and out on the same side. Do not enter water from the upper side and exit from the lower side. Ensure the size of the mold blank should not exceed the height of the sink of the injection molding machine. Otherwise, the water line cannot be connected.

No.13

Ensure that the side locks match the two front side locks after rotation. The front and rear molds should match each other. Otherwise, when the rear mold is rotated by 180°, it may interfere with the front mold and cause the mold to fail to close.

No.14

The ejector plate can only be reset with a spring and cannot be forced to reset with a screw.

No.15

After the mold is designed, simulate it rotating 180°. The mold should not interfere with the injection molding machine Gelin column.

No.16

The first injection molded product should be placed on the non-operating side. Because the product is rotated 180 degrees for the second injection after the first injection, it should go to the operating side to remove the product. To ensure the mold is not reversed, introduce a large C angle on the outside of the mold.

No.17

After rotating 180°, the front and rear molds must be matched. This check must be done during design.