Two Shot Injection Molding – On the Double!
What is two shot injection molding?
The plastic injection molding industry has come a long way since its beginning. It has grown a lot in terms of its efficiency and cost effectiveness, but also in new and innovative ways to manufacture products. One of these innovative ways is the process of two-shot molding. This process is sometimes known as multi material injection molding, 2 colour injection molding, 2k injection molding or double shot injection molding.
In two shot injection molding, a product is formed by injecting two different materials into the same mold. These materials are not injected simultaneously, but rather in succession. It is, however, done in one manufacturing step in which the product is not ejected from the mold between the two shots.
Two shot injection molding offers freedom to the designer, as it negates the use of multiple steps in manufacturing. It provides many advantages such as lower manufacturing cost, better aesthetics and ergonomics, superior mechanical bonds and lower production times. Many products, such as toys, automotive parts, medical equipment, containers and toiletries can be manufactured using this method.
Two shot injection molding process
As the name suggests, this process makes use of two injection shots instead of just one as in conventional injection molding. A special mold is used in this process that accommodates the requirements of these two shots. The mold and its supporting systems are designed to perform two shots without having to remove the partially finished product in between steps.
The process starts similar to normal injection molding with the mold closing to form the first cavity. In this initial setting, the runner for the first cavity is aligned with the sprue of the base material, while the runner for the secondary step is sealed off. The first material is then injected and fills the first cavity.
After the first step, the mold opens and the platen containing the product rotates 180 degrees on a horizontal axis to align with the sprue of the second step. In this step, the first cavity is sealed off from any injection. As the sprue of the second material is now aligned with the runner for the second cavity, the second shot now commences to mold the remainder of the product.
In no instance during this process is the product ever removed, shifted or moved in the mold. The final product is therefore manufactured as accurate as the mold itself, barring any post process imperfections due to elements such as irregular cooling.
Examples of two shot injection molding
Plastic toys lend themselves especially well to this process. Multiple colours and textures can be molded into the item.
Two shot injection molding work well here due to the desirable properties of plastic in a water environment. Hard wearing base areas can be combined with softer seal surfaces.
Items such as toothbrushes can be perfectly molded with two-shot molding. A rigid base plastic can be used with a softer rubber second shot for better grip and colouration.
Plastic storage containers can be manufactured with this process. Different colours can be added, along with rubbery edges to seal lids, or soft container surfaces to protect contents.
Plastic electronic products with LED’s incorporated into the design can be effectively produced with two-shot molding. A translucent insert can be molded into the design to spread or colour the LED light.
Medical equipment such as bone saws can be manufactured using two-shot molding. Rubber grips are very important in the function of this equipment. Two-shot molded items also improve hygiene as there are fewer crevices where sub parts interface, as compared to conventionally assembled items.
Advantages of two-shot injection molding
Products manufactured using two shot injection molding exhibit great mechanical properties. During the molding process, different levels of the product form a molecular bond given good material selection. This bond is often stronger than conventional molding and assembly, and even stronger than other advanced methods such as over- and insert molding.
Products that are manufactured in a two-shot process can look very good while also retaining the structural properties that are required from them. In this process, the load bearing structural part can be molded in the first step while the finishing touches can be added using a more appealing material in the second step.
The process of two-shot injection molding can manufacture ergonomically designed products to perfection. Structural plastic can easily be covered or added on using materials that are more suited for human interface. Soft, rubbery material is better equipped for the human hand as it provides better grip and a softer touch.
A deciding benefit of two shot injection molding is the saving on production time. As multiple levels of a product can be added in this process, there is often no need to do secondary assembly of the item. What would have been a complete secondary step in manufacturing with conventional injection molding, can now be done in one combined molding process.
This is where two shot injection molding really come to its own. Two-shot molding enables a production line to maufacture more complex products, with less production steps than ever before. Consider the conventional injection molding process: If a certain product was to consist of two different materials, they would have had to be molded seperately, in two different molds, followed by a manual assembly step. In two-shot injection molding, these two constituents are not only molded in one mold setup, but they are also fused together, eliminating the need for any assembly.
When compared to conventional molding and assembly, or even with modern processes such as insert molding, two-shot molding provides very accurate manufacturing. As the semi finished product does not need to be removed from the mold in between steps, there is no chance of material misalignment. If thermal effects are properly controlled through good product design, the final result can truly be as accurate as the mold itself.
Efficient material usage
Two shot molding enables the designer to utilize the best of multiple materials. Engineering plastics may have many good, but vastly different properties. The designer can combine good structural properties of a hard plastic with the soft and appealing nature of a different material to achieve the desired results accurately.
Two shot injection mold and product design
The molds used in two-shot molding consist of different tool sets. In a rotating platen setup, the tools share the same core, but not the same cavities. The runners must be designed as such to be shut off during the molding steps in which it is not active.
When designing parts, the wall thickness is an important design consideration for various reasons:
- Due to molding pressure from the second step, the substrate walls should be of sufficient thickness to withstand the forces without collapsing
- Wall should not exceed conventional thickness guidelines to create deffects attributed to irregular cooling
- In order to ensure accurate and thorough material flow into all parts of the cavity, wall thickness should not be lower than a certain minimum
Materials shoul ideally be injected in a certain sequence, taking the following into consideration:
- The first shot should be the harder plastic of the two
- If a transparent and opaque materials are used, inject the transparent material first
- In general, first inject the material with the higher molding temperature
For the mold base and inserts design:
- Ensure the mold is rotatable through 180 degrees
- Any inserts should be symmetrical
- The push rod plate and ejector pin should be kept stable with a return spring to avoid damage during the rotation of the mold
- The cooling system must be able to perform sufficiently for both the first and second shots
When considering materials to be used for two-shot molding, it is important to take the chemistry into account. Not all materials bond perfectly, but some may from a very strong molecular bond. An adhesion compatibility chart is given below.
In cases where adhesion combinations are unkown, and adhesion test needs to be done before two-shot molding can commence. In cases where the inherent bonding ability is not sufficient, undercuts, shoulders or grooves may be added into the design to lock the two components together.
Things to consider before using two-shot molding
- Do a flow simulation analysis to verify the intended design before starting mold construction
- Ensure that the structural part of the design is created in the first shot
- Identify all possible defects in the process and eliminate them before mold construction
- Consider the shrinkage of both materials. In genereal, the shrinkage is dictated by the material of the first (or innermost) shot
- Keep in mind that the sub gate must enable the respective runners to be shut off during parts of the process
- Consider the position of any molded in logos to prevent damage thereof during any part of the process
- The design may allow marginal oversizing of the first shot component. This ensures a higher pressure during the second shot, and stronger bonds as a result
- Ensure that the parameters of the injection setup is able to provide the required pressure, flow rate and cooling abilities
- Verify that the structural strength of the first shot is able to withstand the injection pressure of the second shot
Two shot molding vs. Overmolding
When investigating manufacturing methods for a future product, the designer will quickly find that overmolding and two shot molding are very similar processes. In both processes, molding is done in multiple steps, using multiple materials. Both processes have similar benefits as well, such as savings in time and costs on secondary assembly steps. They do, however, have enough differences to give each merit in its respective field.
In general, two shot injection molding requires a more complicated, and therefore expensive setup. Two-shot molds typically have a rotating platen that holds the semi finished product. While this is a more complicated mold design, it does provide the process with higher manufacturing precision. As the product does not need to be removed and handled between steps, this eliminates human or machine error.
Overmolding utilizes a simpler mold setup, but can not provide the same repeatability, precision and process stability as two shot molding does. The unit is cost is however usually lower than that of a two-shot operation. Overmolding does provide the designer the ability to mold over a metal substrate (e.g. scissor blades), which makes it ideal for certain applications.
It is really situation specific to decide which process suits a product better. Two shot injection molding is usually a more expensive process, but provides the designer with higher manufacturing precision and a better mechanical bond.