Insert molding typically involves inserting pre-fabricated parts, often made of metals like copper or stainless steel, into an injection mold. These parts can also be made from other materials such as glass, wood, or fiber.

It’s crucial that the overmolding material has a melting point equal to or lower than the substrate’s. Using the same material for both is possible, but it requires careful adjustment of the injection molding temperature to prevent the substrate from partially melting.

Overmolding usually involves a single overlay, requiring two injection molds. The final product is a combination of two materials or two color variations of the same material.

As plastic melt is injected into the steel cavity of the mold, heat is transferred into the steel and the plastic begins to cool.  If the resin cools (“freezes off”) before reaching end of fill for the cavity, you will have a “short shot”.  Increasing the temperature of the mold will help the plastic material flow better and require less pressure and thereby less clamping force to hold the mold closed during fill.

In this article, we have not listed every factor, but these represent the main considerations for selecting the proper injection molding machine clamp tonnage.  Texas Injection Molding is one of the largest custom injection molding companies in Texas and has made the strategic decision to position our company to offer a broad range of capabilities at a single location.   Texas Injection Molding works closely with our customers during initial stages of product development to ensure part performance and efficient manufacturing. We are certified to the standards of ISO 9001-2015 and have a full-service tool and die shop to complete routine maintenance, engineering changes and repairs in a cost-effective and timely manner.  In addition to our fleet of small to medium tonnage machines, we have several large tonnage machines and have planned our building, utilities, material handling and lifting capabilities to meet these broad manufacturing requirements.

Poor Adhesion: This issue often arises due to incompatibility between materials. For different materials, compatible overmolding materials must be selected. Additionally, a low temperature of the substrate’s surface can lead to poor adhesion. On the other hand, excessively strong adhesion can create recycling challenges for the product.

A large tonnage injection molding machine requires a large amount of resources.  The footprint of a large tonnage machine is approximately 40’ x 8’ x 8’.  The capital cost of a large machine can start at $250,000 and can exceed $1MM.  The injection molds that are processed in these machines exceed 10,000# and require special lifting equipment.  The facility foundations must be suitable for machines exceeding 100,000 lbs.  The utilities required to operate a large machine are great and are expensive to bring to the building.  Large tonnage machines typically run larger parts.  Handling and storing large quantities of large parts requires considerable space for staging, packaging and storing parts.  As a result of these considerations, there are fewer companies that choose to offer machines >500 tons.

It’s also possible to use hard plastics for both the substrate and the overmold, usually the same material, often employed in multi-color combinations for aesthetic appeal.

In conclusion, overmolding is a versatile and widely used plastic molding technique that enhances product functionality and aesthetics. Despite its complexity, with proper material selection, precise mold design, and attention to key issues like adhesion, short shots, and flash, overmolding offers significant benefits.

This makes 2-shot molding more efficient and labor-saving. Additionally, since the substrate remains hot during overmolding in 2-shot processes, it often results in a stronger bond between the substrate and overmolding material.

From this distinction, it’s evident that insert molding usually requires just one injection mold, whereas overmolding necessitates at least two, and occasionally three, molds.

Injection pressure is a measure of pounds per square inch (PSI) required to inject the melted plastic resin to displace the air and fill the cavity under adequate pressure for form a quality part.  The clamp on an injection molding machine “clamps” the two halves of the mold together during injection, packing and cooling process of manufacturing a plastic part.  The greater the injection pressure pushing plastic melt into the cavities the greater the clamp force required to hold the mold together.

2-shot molding combines both substrate molding and overmolding in a single injection molding machine. Here, after the substrate is molded, it’s rotated to another station within the same machine for overmolding, eliminating the need to manually transfer the substrate.

In traditional overmolding, the substrate is first injection molded, removed from the mold, and then manually placed into another mold for overmolding.

Creating molds for overmolding requires higher precision and a thorough understanding of the overmolding materials. The following are essential aspects to consider in overmolding mold design:

A general rule of thumb is 2-8 tons per square inch.  For example, a 10” x 10” part (100 square inches x 2 tons/sq. in.) = 200 tons.  The larger the part the more clamp force is required to hold the two halves of the mold together during the injection process.  There are several factors that influence the amount of clamping force required including wall thickness, geometry, gating of part, viscosity of resin and temperature of injection mold.

To mitigate adhesion issues, it’s common to design holes or grooves at the edges of the overmolded area. These features, being larger internally, help to lock the overmold in place and reduce the reliance on adhesive strength, making it easier for the overmold to bond with the substrate.

Gate Design in Overmolding: The design of the gates in overmolding molds is highly critical. They must ensure complete filling while maintaining sufficient adhesive force. The key is to ensure that all areas to be overmolded are not too far from the gates. This is to avoid insufficient injection pressure or too low temperatures in those areas, which could affect the quality of the over molding.

Although insert molding and 2-shot molding fall under overmolding, they are often referred to separately. In a narrower definition, overmolding specifically involves a plastic substrate manually placed into the mold.

There are many different grades of plastic resin.  Plastic resins are generally specified by the physical, environmental and chemical resistance properties required of the finished parts.  Each grade of resin has an ideal processing temperature range without degrading the properties during manufacturing.  Each different grade of resin will have different rate at which the plastic melt flows.  The Melt Flow Index (MFI) is a measure of the ease of flow of the melt of a thermoplastic polymer.  It is defined as the mass of polymer, in grams, flowing in ten minutes through a capillary of a specific diameter and length by a pressure applied via prescribed alternative gravimetric weights for alternative prescribed temperatures.  Thermoplastic resin manufacturers can formulate the thermoplastic resin to have a range of MFI within a specific grade. The higher the MFI, the easier the melt flows.  The higher the MFI the less injection pressure is required to fill the cavity and less clamp tonnage required by the molding machine.

Overmolding, also called plastic overmolding, molding over molding, or injection overmolding,  is a crucial injection molding technique widely used in everyday items like toothbrushes, power tool housings, and car light panels. This article will explain overmolding, highlighting its common challenges and essential precautions.

Conversely, a gap that is too small or negative, such as -0.01 to -0.02mm, can create indentations on the substrate surface. Therefore, the mold must be manufactured with high accuracy.

In summary, while overmolding technology offers enhanced functionality and aesthetics, it also brings challenges in cost, design, and material selection.

Understanding the nuances between overmolding, insert molding, and 2-shot molding is crucial for selecting the right process for specific applications, balancing efficiency, cost, and production volume.

Flash: Excessive gaps between the mold cavity and the substrate can lead to flash (excess plastic). Moreover, if the gate design is improper, leading to excessive injection pressure (to avoid short shots), it can also result in flash.

However, 2-shot molding requires highly precise molds and thus involves greater initial investment costs. Therefore, the choice between these methods depends on the production volume:

While theoretically, overmolding can be repeated multiple times, such practices are rare in real-world applications and are not the focus of this discussion.

Commonly, the substrate in overmolding is a hard plastic like PA or PC. The overmolded material, however, can be a flexible plastic such as TPE or TPU, or even another hard material.

The substrates in over-molding often include materials like PA, PC, ABS/PC, and ABS, while TPE, TPU, TPR, and TPO are common choices for the overmolding layer. This blend of soft and hard plastics represents the most typical form of overmolding.

Precision Fit Between Substrate and Mold: The fit between the substrate and the mold cavity in overmolding needs to be very precise. The gap between the substrate and the mold cavity after inserting the substrate should be between 0.01 to 0.03mm. This precision is crucial because materials like TPE, known for their fluidity, can easily cause flash if the gap is too large.

Theoretically, the possibilities extend beyond these mentioned materials. Essentially, any plastic can be used for over-molding. For specific requirements, please feel free to contact us for more information.

Short Shot: Even a slight inadequacy in the injection can cause aesthetic defects, leading to product rejection. It’s crucial to design the gate location thoughtfully to prevent short shots. A Design for Manufacturability (DFM) assessment can be used to evaluate the suitability of gate placement, focusing mainly on the required injection pressure.

Additionally, in scenarios where the maximum thickness of an injection-molded product is exceeded, a two-shot injection molding process may be used. This approach ensures each individual molding stays within the maximum allowable thickness.

During the injection molding process hydraulic cylinders push the screw forward injecting melted plastic resin into the cavity of the mold at extremely high pressures.  Pressures exceeding 20,000 p.s.i. during injection push the two halves of the mold apart.  Injection molding machines are rated by “clamping force” in tonnage.  The tonnage of force clamping the two halves of the mold together.  The larger the surface area of the molded part, the greater the tonnage is required.  Generally “large tonnage” machines are machines rated with 500 or more tons of clamping force.

The more tortuous the path and further the distance the plastic melt travels, the greater volume of melt is being pushed and greater transfer of heat from the melt flow to the steel cooling the plastic.  The additional material and resistance to flow as travel distance increases requires additional injection pressure and clamp tonnage.

Overmolding is a widely used plastic molding process with many advantages but also some limitations. Here are the main pros and cons of overmolding:

There are instances of double overmolding as well, which necessitates three molds, resulting in a product composed of three materials or color variations.

Overmolding is a process where a substrate, typically a pre-formed plastic part, is placed inside a mold. During the process, molten plastic is injected into the mold cavity, enveloping the substrate in the empty areas. Once cooled, the two materials fuse into a single product.

Thicker wall sections provide more volume for the plastic melt to flow.  As plastic melt travels to the end of the mold cavity, the steel is removing heat.  Thicker wall sections provide insulation for the melt and allow it to travel further with less pressure.  Similarly, the thinner the wall the faster heat is transferred from the plastic melt and more pressure is required to push the melt flow into thin sections.