The injection molding process has been in use for several decades. It offers exceptional repeatability and is relatively inexpensive. However, it also has some limitations, which have opened up the space for new ways. The two-shot injection molding process has become an excellent alternative to injection molding methods.
Dual short molding is an advanced process that is becoming rapidly popular. It helps create products of complex shapes with multiple colors or materials. This manufacturing process is quickly replacing the old ways of production. This article will discuss everything related to the two-shot injection molding process. So, let’s get started!
What is Two-Shot Injection Molding?
Dual-shot, double-shot, or multi-shot – all refer to two-shot injection molding. Let’s begin with the definition of this production process.
Two-shot injection molding is an advanced manufacturing process. It uses two barrels (injection unit) and injects two materials into the mold. What makes it prominent is that it can create a product of two materials or colors using one mold during a single molding cycle.
Let’s understand this with a simple example. Have you seen products with two colors? The most common example is a blue and white toothbrush. Similarly, the brush has hard plastics and a soft grip. The manufacturing of such toothbrushes uses two materials. However, simple injection molding cannot create a product with two colors.
Moreover, it can only create products of one material, i.e., rubber or plastics. This is a significant limitation of simple injection molding where two-shot molding comes into the scene. This advanced method uses a molding machine that has two barrels and screws (injection unit). These two barrels allow the two-shot molding process to create products of two materials.
Moreover, manufacturers can add colors to those two materials to make colorful products. Interestingly, this method does not use two separate molds. Instead, only a single mold gets material from both barrels and gives us the final product. Not only this, but different materials are also injected in a single molding cycle.
Step-by-Step Process of Two-Shot Injection Molding
Do two-shot injection molding methods consume more time than single molding? The answer is NO! As I said earlier, this method completes the injection of two materials quickly in a single molding cycle. So, it produces the final product as fast as single-shot molding.
I will explain how this advanced process works in the section below. You will learn the steps involved in two-shot injection molding.
1- Materials & Color Selection
As I said, this process can use two materials in a single molding cycle. Manufacturers choose those two materials depending on their needs. This process also allows manufacturers to use multiple colors. Let’s discuss the process using the toothbrush example.
Generally, brushes consist of hard plastics and soft rubber. Plastics make the whole structure, and soft rubber makes the external grip. In this case, manufacturers will choose hard plastic as the first material and soft rubber as the second material. Both materials can have different colors, and manufacturers can add dyes to them.
2- Designing of Mold & its Cavities
The second step in this process is to design and create a mold. Remember, you don’t need two different molds. Two-shot injection molding uses one mold but with two cavities. Both materials will be filled in a single mold. The cavities are grooves in the mold where the materials are filled. The mold used in two-shot injection molding must have two cavities.
The first cavity will help create the product’s base. The second cavity will get the second material and bond two different materials together. The shape of these cavities will be similar to the desired shape of the product. The double cavities in a single mold differentiate these two shots from single-shot injection molding.
3- First Injection of Material Though Frist Barrel
As said earlier, two-shot injection molding uses two barrels and screws. Are they confused by barrels? These are tubes where materials go through and then enter the mold. Each barrel has a screw and heating element. These heating elements melt the material. The screw then pushes the molten material into the barrel.
As a result, the molten materials move and go into the mold. In two-shot molding, the barrels’ first material (suppose hard plastic) melts due to heating elements. The screw then moves the molten material. As a result, the first material goes into the first cavity of the mold. It cools and adopts the shape of the mold.
4- Second Injection of Material Through the Second Barrel
As said earlier, two-shot injection molding uses a mold with two cavities. The first barrel (injection unit) fills the first cavity with material. Once the first cavity is filled, it is allowed to cool and become semi-solid. It then adopts the shape of the mold. After that, the mold rotates at 180°. As a result of this rotation, the semi-solid material filled in the first cavity goes into the second cavity.
The second barrel (injection unit) fills the second cavity with a second material. For toothbrushes, the first material will be hard plastic, and the second will be soft rubber. This second material will enter the second cavity, where the first material is also present. The second material will combine with the first material inside the mold.
5- Cooling, Solidification & Ejection
Both materials will combine and make strong bonds in a single mold. The mold halves will be closed, and the material will be cooled. This process is common in both single-shot and double-shot injection molding processes. The combined material inside the mold will solidify due to cooling.
This solidification requires a controlled temperature and pressure. The channels inside the mold help lower the temperature. After a specific duration, the material completely dries and becomes solid. After that, the ejection pin of the mold helps eject the material out of the mold. Once ejected, you’ll have a product of the desired shape consisting of two materials.
Benefits of Two-Shot Injection Molding
At the start, I mentioned that two-shot injection molding is becoming very popular. This is due to the potential perk that this process offers. In the section below, we will discuss the benefits of this process:
- Strong Bond: Two materials combine when they are soft. So, they undergo cooling and dry together. As a result, they make a firm bond and glue together. This increases the durability of the whole product.
- Reduced Time: Two different materials combine without needing an additional assembly process. This saves time and increases productivity. In one molding cycle, you get a perfectly fused product made of two materials.
- Colorful Product: The single injection molding process product product with one color. However, the two-shot molding is very versatile. You can use different colors in this process. This increases the aesthetics and look of the product.
- Product Feel & Aesthetics: As said, this process involves the blend of two materials. The manufacturers strategically use two materials to create a unique look. For example, the base of a truth brush consists of hard plastics. The soft rubber creates a grip on the toothbrush.
- Reduced Waste: The two-shot molding process reduces the waste. It uses material that is used to manufacture the product. There is no leakage of molten materials. Moreover, the two materials fuse perfectly and adopt the shape of the mold. You won’t have to drill or machine the product. This results in zero to minimal loss of material.
Applications of Two-Shot Injection Molding
The shot injection molding process is a relatively newer manufacturing method. So, many manufacturers still treat it as a secondary option. However, with time, things are changing quickly in favor of dual-shot injection molding. Various industries have adopted this advanced process to enhance their productivity.
Here is a table showing the application of this method in various industries:
| Industry | Application of Two-Shot Molding |
| Automotive | Steering wheel grips, dashboards. |
| Aerospace | Control buttons, seat belt buckles. |
| Military & Defense | Gun grips, protective gear. |
| Agriculture | Tractor controls, sprayer nozzles. |
| Construction | Power tool casings, safety equipment. |
| Healthcare & Medical | Inhalers, surgical tools |
| Consumer Electronics | Game controllers, remote control buttons |
Remember, I mentioned only a few products for each industry. However, this molding method can create a product of any shape and geometry. This process is not suitable for those who operate on a small scale. The reason is that two-shot machines are relatively expensive. This process is cost-effective for bigger manufacturers who produce at a large scale.
Frequently Asked Questions
What is two-shot injection molding?
It is an advanced manufacturing method that uses two-barrel injection molding machines. It can produce a single product using two materials during a single molding cycle. Moreover, you can create products of different colors using this method.
What is the difference between single and double-shot injection molding?
Single shot molding product is a product of one material. Moreover, those products are not colorful. On the flip side, double-shot injection molding can create one product by using two different materials. It uses a double-shot injection machine with two injection units.
Is two-shot injection molding inexpensive?
The answer is NO! It requires 2K injection molding machines, which are very costly. This process also requires molds with dual cavities, which further increases the cost. I don’t recommend this method for small-scale manufacturers.
Conclusion
Injection molding has been used for many decades. However, this dual injection molding process is nothing short of a revolution. We should not compare it to the single-shot molding method. Instead, it expands old and conventional methods that offer better value. I’ve discussed everything from the step-by-step process to its applications in this article.
Remember, dual-injection molding produces a product made of two different materials. Moreover, its prominent feature is that you can add different dyes to make the product colorful. However, the higher initial cost of dual injection molding machines is a drawback. Therefore, I consider it suitable only for established manufacturers.