Overmolding and two-shot injection molding have many of the same advantages. They are ideal for quickly creating durable, reliable, and vibration-resistant parts with complex geometries, but over-molding is best suited for low-volume production runs.

(2) Try to evenly mix the recycled material and the raw material. Because the recycled pellets, as well as their density are relatively larger than the raw material, if the mixing is not uniform, the amount of the molten material is easily reduced, resulting in reduced injection volume accordingly.

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At the beginning of the over-molding process, engineers inject the substrate with a harder over-molding material. Then, the substrate is placed in an over-mold tool or an over-mold cavity within the same mold. The molten-over mold material is then sprayed into, onto, or around the substrate. After the molten material is cooled, the substrate and the over-mold are chemically or mechanically combined. The entire over-molding process only takes 30 seconds.

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During the product design process, the principle of uniform wall thickness must be followed. If it is impossible to maintain a uniform wall thickness, please change the mold design in a timely manner. Increase the number of runners or ribs in the area where the feeding is difficult, so as to avoid the defect of underfill caused by too-thin wall thickness.

In order to eliminate such defects, the melt / mold temperature, and the injection pressure / speed can be increased to allow the melt to reach the end of the cavity before solidifying. In such a case, a great plastic fluidity is of particular importance, so selecting the plastic with a better fluidity is also a solution. If the flow coverage is too long and the plastic cannot properly fill the injection molded part, it is recommended to change the gate location to reduce the plastic flow length.

However, if two less compatible plastics must be used, the team can design mechanical bonding properties for the part after the fact, although this may result in higher costs.

(1) Poor plastic fluidity – If the plastic material doesn’t feature a great fluidity, it will solidify before reaching the furthest end of the cavity or flowing to the overflow tank, which often results in a short shot.

Injection molding is a popular manufacturing process, which can quickly produce complex-shaped precise parts without wasting a lot of materials.

Many different processes belong to the category of injection molding, including over-molding and two-shot molding. The two processes are similar, but there are some key differences-here are what engineers and designers need to know:

Two-shot injection molding is efficient and economical manufacturing technology. This process also produces highly durable terminal parts and assemblies.

Engineers should know that Two Shot Mold can be accelerated or slowed down, depending on how the substrate is transferred to another cavity of the mold. Hand and robot arm transfer takes longer than the rotating plane, but the rotating platen molding is more expensive, usually, there are only high-efficiency options, mass production runs.

(1) Insufficient injection molding capacity – This is caused by an overestimation of the capacity of the injection molding machine, which may also occur due to insufficient plasticizing capacity or insufficient injection volume. Among them, the insufficient plasticizing capability can be increased by extending the heating time, increasing the screw speed and raising the backpressure. If the injection volume is not high enough, the injection machine with a larger injection capacity can be used to solved the problem.

From there, the mold opens and rotates 180° with one side of the substrate to meet the other mold chamber and injection molding nozzle. Once the substrate is in place, the second material is injected and combined with the substrate to form a firm hold. Once the second layer has cooled, the last part will be sprayed out.

Overmolding, like Two Shot Moulding, is a multi-shot injection molding process that produces a single final product from two or more different thermoplastics. This process is ideal for engineers who want to build components that are powerful, functional, beautiful, and that will not separate over time.

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In addition, it is very important that the material of the mold is easy to bond, and the mold must be aligned to prevent deformation of the parts.

From a design point of view, two-shot molding provides designers with a lot of flexibility, because this process can create complex geometric shapes and adapt to multiple colors to produce more beautiful parts.

(2) The flow range of the melt is too long, and the flow resistance is too large – The parts that hinder the flow of the melt include the nozzle, the sprue, the runner, the gate, and the thin walls of a product. The flow resistance of the nozzle can be reduced by increasing the nozzle diameter / temperature, and using a nozzle with a small flow resistance.

The product team must remember that all thermoplastics used in the over-molding process must be chemically or thermally compatible with each other. Compatibility with metal substrates is usually not a problem, because they can be used with any plastic over mold, but the product team may encounter compatibility issues when using plastic over molds. If the substrate and mold are not compatible, the final product may be deformed or poorly bound.

(3) Poor venting. When filling the cavity, air is trapped to cause counter pressure. When the melt is injected into the cavity, the cavity is often closed by the melt in the very beginning, with air trapped in the unfilled local areas. Also, because the filling speed is too fast, sometimes there is not enough time for the air to be vented through the parting surface but compressed instead, resulting in partial unfilled areas in the cavity, thus short shot in the molded part.

(3) Unstable production cycle – Frequent machine shutdown, and production inconsistent with the normal cycle make some plastics stay in the barrel for too long with reduced density and viscosity, thus resulting in plastic underfill.

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Two Shot Injection Molds, also known as dual-molds, double-shot molds, or multi-shot molds, are a subcategory of injection molding that allows engineers to create multi-material or multi-colored parts without adding additional assembly steps.

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In addition, because one machine manufactures the entire part, no post-processing is required, and engineers can greatly reduce manufacturing time, thereby reducing costs. However, it is worth noting that the initial two-shot injection molding machine may be costly, and the two-shot injection molding machine is more expensive than the standard injection molding machine. Fortunately, these costs are usually offset by labor savings and assembly costs for large-scale production runs.

(2) Oversupply of plastic – If too much plastic enters the barrel, the injection pressure is lost due to the compression of the pellets, thus reducing the pressure required to inject the melt from the nozzle, which is necessary for injection molding, resulting in insufficient injection pressure. The solution is to adjust the amount of feed, i.e., the amount of melt, and make the amount just right for the molding.

All in all, the reasons for short shot are multifaceted, which are also mutually restrictive and mutually influential. To reduce and correct such a defect, we need to make adjustments through an overall consideration of the relationship between these aspects, while making constant practice and accumulating experience continuously to quickly identify the causes of underfill, thereby reducing resource waste and improving product yield.

Short shot refers to incomplete filling of a mold cavity which results in the production of an incomplete part due to the insufficient plastic fluidity. The main causes are the inappropriate injection pressure and speed (including excessive pressure loss caused by resistance), which are mainly affected by the following aspects:

(1) Local short shot caused by the flow imbalance in each cavity of a multi-cavity mold – When the injection molding capacity of the injection molding machine is sufficient, this defect is mainly caused by the uneven flow in each gate, i.e., the mold cavities are not distributed in a balanced way.

Through the different layers of materials or colors created by the injection molding machine, the two-shot injection molding process is best understood. The first material is injected into the mold to create the substrate, and other materials or materials will be molded around the substrate. After the substrate solidifies and cools, it is transferred by hand, robotic arm, or rotating plane to another cavity of the mold.

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In terms of disadvantages, the tolerances of over-molded parts are often lower than those of two-shot injection molding. It is also important to remember that plastic compatibility requirements may constrain designers.

Compared with two-shot molding, over-mold design is also easier, because engineers can use any standard injection molding machine to carry out this process.

(1) Improper injection molding process – The low barrel temperature, slow injection speed, short injection time, and insufficient backpressure lead to a shortage of plastic.

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