What is Two Shot Injection Molding?
Since it first existed, plastic injection molding technologies have continued evolving. Two-shot injection molding is the successor among different plastic injection technologies.
Two-shot injection Molding also refers to multi-color, 2k, or double-shot injection molding. This process uses two shots of injection with two plastic materials to produce one part/component.
Plastic Materials for Two-Shot Injection Molding
Several plastic types exist that you can use for two-shot injection molding. The key ones are here for your consideration:
PEEK reflects flame retardance, better stability and strength, and higher chemical resistance for seamless functionality.
Featuring high stiffness, toughness, stiffness, and hardness, along with good lubricity and elasticity, makes it a must-have for some applications.
Higher gloss, optical characteristics, low shrinkage, and scratch resistance make PMMA a good plastic material.
The notable perks of PEI for various applications include high strength, chemical resistance, temperature resistivity, and flame resistivity.
Incredible strength, resistance to temperature, abrasion resistance, and better hardness are some of the desirables for PPA.
The notable properties of PP are impact resistance, high flexibility, and acid/base resistivity for two-shot injection molding.
Step-by-Step Two-Shot Injection Molding Process
Wondering how two-shot injection molding works? Get insights on step by step process right below:
Material Feeding – The process begins with insertion of specific material within the hopper. There are two hoppers for inserting two different materials in two-shot injection molding.
Heating and Melting – The material projects towards the heating section from the hopper under gravitational forces. Here the heating device melts the raw plastic material. An option exists to vary the temperature of the heating element as per the melting point of the specific material.
Conveying – After the melting completes, the screw conveyor conveys the molten plastic material to the injection device.
First Shot – Substrate Formation – At first, the machine seals the gap between the injection device and the mould by pressing. The hydraulic cylinder then pushes the material from the injection device into the mould. In the first shot, the hard plastic material (for instance, polypropylene) forms the foundation of the product.
Second Shot – Over mold Component Formation – The press opens up to isolate the mould from the first injecting device. Then mould rotates by 180 degrees. The press closes the gap between the mould and the second injection device for sealing.
Meanwhile, the plastic material of the first shot gets cooled and solidifies. After this, the plastic material from the second injection device fills the overmold component area. The over mould plastic material in the second shot is elastomer such as TPE or other, usually less rigid. Similar to the first shot, the hydraulic cylinder here forces the material within the mould.
Cooling and Ejection – After this step, the injected material gets cooled and solidifies properly. In the end, ejection removes the final product from the machine.
Two-Shot Injection Molding vs. Other Injection Moldings Processes
The superior advantages of two-shot injection molding over other methods make it stand out. Some key benefits are:
Complex Design Possibilities – Two-shot injection molding makes possible the creation of complex designs. It allows you to create components with two different materials, which is impossible with a single shot. This way, you can have parts featuring multiple properties. For example, when your goal is to achieve double layers having multi colors in the product.
Higher Quality – In two-shot injection molding, you can combine the properties of different materials in a single product. This way, you can increase the product strength and overall life, contributing to higher quality.
Eliminates Secondary Operations – In injection molding, where you need to join two products, the process involves multiple steps. The two-shot injection molding has eliminated these steps. Now no more effort goes into joining or product formation. Two-shot injection molding machine do all the necessities alone.
Saves Time and Cost – Two-shot injection molding simultaneously processes two injecting shots, first and second, collectively. This means you can create more product units in less time. You don’t need to hire labor for manual processing. The machine automatically does the task. It saves not only your precious time but also substantial costs.
Incredible Product Aesthetics – Apart from better quality functioning, you can achieve incredible aesthetics in the end product. You don’t need to use glue or welding to join the products, which affects the product’s appeal as well as appearance. Two-shot molding results in seamless joining.
Minimal Waste Generation – With two-shot injection molding, you can save much material. The machine injects only the required amount with the help of sensors and metering devices without exceeding the limit. This ensures minimum scrap or waste material production.
Considerations for Two-Shot Injection Molding
When you are going to deal with the two-shot injection molding. Consider the following things before you start:
Material Compatibility- Material compatibility is of key concern; before starting, conduct the adhesion test of two different materials. This will ensure the perfect bonding in the end product. If no proper adhesion exists, you can utilize grooves, undercuts or shoulder to achieve optimum bonding.
Material Selection – Before selecting a specific material for two-shot injection molding, you need to consider multiple things, such as:
- UV resistance
- Temperature Compatibility
- Chemical Resistivity
- Total Strength
- Stiffness
- Age
- Compression Strength
- Conductivity
These parameters will give you a clear idea of the product’s features and characteristics. The features, in the end, decide the overall product quality.
Chemical Bonding – Make sure to determine the chemical bonding of the materials. You need to ensure that the joint between the materials has a maximum surface area and the longest leg. This ensures that the removal of the second shot is nearly impossible.
Gate Type – Gate refers to the course through which melted material enters the mould. The gate design, type, and location influence the dimensions and appearance. Choose the gate very carefully considering these factors.
Gate Location – Gate location controls mold uniformity as well as solidification. Some sites allow the material to enter the thicker area and then into the narrow space and vice versa.
Parts Design Requirements in Two-Shot Injection Molding
Multiple design considerations need your attention. Some of the key considerations are as follows:
Internal Bonding Surface Design – The bonding between the two materials determines the overall strength. Factors affecting surface bonding include; processing temperature, material compatibility, interlocking structure, and molding sequence.
The maximum surface area between the bonding materials ensures high bond strength. If the surface area is low, you can roughen the bonding surface.
Mating Surface – The mating surface is very challenging, especially in the first and second shots. If not gone well, then it leads to the formation of flakes, burrs, and rolled edges. You can use fluted and stepped designs to avoid this phenomenon.
Wall Thickness and Corners
Two-shot injection molding wall thickness and corners considerations include
- No sharp corners
- Uniformity in wall thickness
- Even transitions
You need to ensure the existence of these in your molded design.
Weight Difference – You must ensure that the weight difference between the foundation material and over mould component is very low. On the other hand, this will adversely affect the injection pressure and molding cycle.
Application of Two-Shot Injection Molding
Various garden accessories use soft rubber seals bonded with solid foundations. It is beneficial because of the waterproof nature of plastic. Two-shot injection molding is responsible for this. Examples include nozzles, garden boots, etc.
With two-shot injection molding, you can make various toilet accessories, such as toothbrushes. The rigid plastic act as a foundation, while soft plastic is over mould component that offers better coloring and grip.
Using two-shot injection molding, you can produce bone saws. It improves cleanliness by preventing direct contact and ensuring optimum grip. This will lead to efficient and effective operations.
In the electrical sector, two-shot injection molding makes electric plastic items. Coupling the transparent part with the rigid base results in effective light distribution.
Two-shot injection molding can lead to the formation of containers. The rubber edges play their part in securing lids or preventing contents inside. An option exists to add multiple colors for better aesthetics.
Yes, two-shot injection molding is truly environmentally friendly. It leads to less waste generation and consumes less energy compared to traditional molding methods. You can recycle the injection molded products and turn them into new products.
Some of the key differences between two-shot injection molding and over-molding include:
- You can use over-molding for two shots and a single shot alone. On the other hand, two shots cannot be used for a single shot injection molding alone.
- Two shot serves the purpose where the larger volume of products you need to accomplish. Overmoulding helps low product volume needs.
You can use the following formulae to calculate the two-shot injection molding costs:
Material or component costs contribute to 30% of the total cost. At the same time, profit and process costs contribute 50% of the total cost. Considering these factors, a single cavity mould costs around $1000 up to $5000. Complex or Larger moulds have a cost of about $80,000 or more. Such a higher price is associated with the tools, while injection molding production has low fees.