Design Considerations to Successfully Manufacture Complex … · 2017-08-10 · Xcentric Mold &...

Design Considerations to Successfully Manufacture Complex Overmolds

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In the mid 90’s, two Michigan-bred brothers working for other companies as mold makers realized that all the mold shops in the Detroit area lacked technology and relied primarily on traditional and more manual processes. In 1995, they decided that the only way to differentiate from their peers in the trade was to learn and apply technology on their own and Xcentric Mold & Engineering was born. They built a proprietary advanced technology platform focused on software, automation, systems, and custom standardization and processes – it would become the foundation that would surpass all other traditional manufacturing companies. Employees were hand-picked and trained through a rigorous

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Xcentric Mold & Engineering provides the fastest, most affordable quick-turn low-volume injection molding, CNC machining and rapid prototyping services within 15 days or less. Xcentric can manufacture complex parts without compromising the engineers’ design or time-to-market.

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By mechanically and/or chemically bonding two plastic materials,

the overmolding process can be used to help

product designers achieve additional functionality or aesthetics that would

otherwise not be practical.

Overmolding is a two-shot injection molding process that creates a single part by combining two separate but complimentary thermoplastic materials.

The first shot, called the substrate, is usually composed of a more rigid plastic and is designed to accept the second shot, called the overmold. The overmold is typically composed of a softer, more flexible plastic-like rubber.

Substrate1st shot

+ =

Overmold2nd shot

FinalAssembly

What is overmolding?

Learn how to enhance your part design functionality with overmolding, next page >



Tactile Functionality

One of the more common reasons to utilize the overmolding process is to create a soft grip. Pulling from the example used above, the green part is used functionally as a twistable nozzle that will help direct the flow of a liquid. However, in this case, the plastic chosen to facilitate the flow and to keep its chemical resistance for lasting part integrity happens to be very rigid and hard to the touch; not ideal for tightening and loosening with human hands.

The solution is to design for an overmolded rubber-like grip to aid the user in the twisting of this product.

But, tactile functionality is not limited to human grip. Overmolding can also be cleverly used to add rubber-like grips to clips designed to grab inanimate objects.

Here is another tactile example where a softer, rubber-like grip has been overmolded onto a rigid plastic part. In this case, though, the rubber-like grips are overmolded in two separate areas, which is a feasible use of the overmolding process. That is, overmolding can be utilized in more than one area of the same part to achieve even more functionality.

Overmold

Overmold

Substrate

Substrate



Another fairly common use of overmolding is for purely cosmetic reasons. Overmolding can be used to create color contrasts for a more pleasing look and feel. The part example on the right uses a red plastic overmold onto a black substrate, creating a nice looking part design...which can help with product branding or to differentiate functionality (ie a button).

Also, overmolding can be used to hide features within the substrate. In this example, there happens to be a visible parting line in the substrate of this housing panel. To hide it, the designer chose to overmold the perimeter of this part. And, by using a rubber-like material, it also functions as a mechanical grip to the base of this part when the panel is closed (acting as a bumper/shock absorbtion to prevent damage from drops, constant opening and closing, etc.). Finally, by using a contrasting color, this single overmolding design serves three different functions!

Cosmetic Details

Overmold

Overmold

Substrate

Substrate



Coupling Multiple Substrates

A third reason to use overmolding is to connect two or more substrates. In this example, a plastic button (red) is connected to a plastic housing (green) via a rubber overmold (blue).

This overmolding design allows the button to move freely within the housing substrate while providing good pressure for the button. In this case, the overmold was accessed through the rear of this part and encapsulates the button in a way that allows the actuator pin to interact with the circuit board that will be assembled within. This also doubles as a seal around the button.

The softer, rubber-like material also allows for more flexible acutation that cannot be found on a standard button assembly.

As you can see, overmolding provides design engineers with solutions to enhance a products functionality and look and feel. But, before you start your overmolding project, we’d like to comment on the best way to prepare your files.

Front

Back

Overmold

Substrate



File Preparation

Before you begin designing your part for the overmolding process, it is important to note how to properly prepare your files to pass off to your injection molder. With 20 years experience overmolding thousands of parts, we have identified the best, most expediant way to prepare your files that results in the most accurate and fastest turnaround.

Knowing a particular part design is intended for overmolding, it is always best to design them as two solid bodies so that they can be evalutated separately. At Xcentric Mold & Engineering, we will evaluate each piece separately to establish a good process as this is critical to the success of an overmolding project. Upon receiving the files, we will evaluate and consider the materials chosen to engage the DFM process. Once we establish a good process, we then evaluate both pieces together to establish a good flow using our Mold Flow Analysis. Establishing a good process and a good flow will help ensure your overmolded part can be successfully manufactured. Therefore, it is recommended that you upload three separate files. • File 1 - Substrate only• File 2 - Overmold only• File 3 - Substrate and Overmold together

This will provide us everything we need to evaluate your part. Be sure to keep this in mind before you start the project.

Now that you understand how to properly set up and submit your files, we will dive into some of the design considerations you will need to get your overmolding project right the first time.



Design Considerations

There are a fair amount of things to consider from a design perspective to help your injection molder successfully manufacture your overmolding project. Understanding these and executing them can save days of redesign work or retooling. Remember, the integrity of your design is the first critical aspect of a successful project.

In the following, we will look at:• The feature specifications you will need to concentrate on for succesful overmolding including Wall Thickness,

Radii, Shutoffs and Gate Location• How to evaluate the materials for overmolding• The surface finishes you can expect and how it will affect your project



Wall Thickness

Wall thickness is a very important part of the overmolding process because it will determine how well your overmold material will flow around or through your susbtrate.

During the overmolding process, we will run first shots to produce your substrates. We will then place the substrate back into the mold and prepare for the second shot; the overmold.

The overmold material will be flowing over a very cool substrate. And because the injection molding process requires a consistant temperature and pressure within the mold, this can cause difficulty that can lead to warp or flash or short shots (incomplete fill of overmold).

The best way to minimize problems with a hot material flowing over a cold material is to make sure your wall thickness on both the substrate and the overmold is consistent and nominal. Thin to thick sections, as well as thick to thin sections will impede the flow and can cause defective parts.

Walls should be designed to be even, consistent and nominal through the entire part.

Wall thickness is consistent

and even.

KEY POINT



Radii

Another important factor to successful overmolding is giving your design ample corner radius. As with Wall thickenss, a sharp corner radius will impede the flow of hot material, not only through the mold, but over the cold substrate within the mold. This will cause similar issues to having inconsistent wall thickness.

Imagine water flowing through a pipe. When water hits a sharp 90 degree angle within the pipe, friction is created slowing that water down. If, however, the turn within the pipe is more more gradual and radial, the flow of water is much less impeded. The same is true for any given material (though slightly more viscous). Radii will give the material a smooth, fast flow giving it enough time to reach it’s destination before it naturally cools.

Designing smooth, gradual radii will help the overmold material establish good flow through the mold and over the substrate

Radius is too sharp

Radius is gradual and

smooth

Overmold

Overmold

Substrate

Substrate

KEY POINT



Shutoffs

Another important part of the overmolding process are shutoffs. A shutoff is anywhere, against the mold or the substrate, the flow of plastic ends. You want to design your substrate and overmold with hard shutoffs; even and clean edges. The key point is to keep the nominal wall thickness all the way to the end of the part. Knife or feathered areas can be difficult to fill which can cause air trapping or gassing. To compensate, the molder will have to apply more pressure to the flow of material to get it to fill completely. Applying too much pressure, however, has the potential to go above the parameters of the material which can result in flash.

The pressure inside the mold should be consistent. High pressure caused by poor shut-offs can cause distortion in the substrate within the mold.

Design even and clean hard shutoffs by keeping the nominal wall thickness to the end of the part.

Overmold

Substrate

Shutoff is even and clean

KEY POINT



Gate Location

When designing for the overmolding process, an often overlooked step is considering the gate location for your overmold. If your overmold is designed outside of the edge of the part, off the parting line of the mold, you will need to find a different method to gate that material. Typical gate locations are on the parting line. You must account for where the mold maker is going to gain access to an inaccessable overmold area. In some cases, you can design holes within your substrate for just this purpose. These are called a subgates.

In cases where the overmold needs to fill behind the substrate, the molder can subgate from the core. If you need to connect two separate overmolding areas from the core, you can design a “landing pad” (a gate drop area) with channels as a way to connect them. A typical gate location is in the center of the core side of the tool.

Proper consideration to accessing the overmold area within the mold will save time.

Subgate

Overmold

Substrate

KEY POINT



Material Evaluation

Choosing a material for both your substrate and overmold is the final critical piece toward a successful overmold project.

First, it is highly recommended you spend some time reviewing the material data sheets to identify materials that pair well together. For your substrate, as a general rule, choosing a material with good chemical bonding properties such as ABS, PC/ABS and PC are good places to start. Otherwise, you want to choose a material that has low to no lubricity.

For your overmold (which will typically be a type of rubber material like TPE, TPU, TPV), again, refer to the material data sheets. The data sheets will tell you how it will chemically bond to your substrate material. Look at the data sheets to see what it bonds well with.

If, however, the materials you choose do not bond well chemically, you can design your substrate with a system of holes or slots that allow the overmold material to flow through and bond mechanically to your substrate. Though its also good practice to design for a mechanical bond in any case.

Always review the data sheets or call your material supplier for pairing materials and proper adhesion. Design for a mechanical bond as well.

Design holes or slots to

help interlock mechanically

KEY POINT



Surface Finishes

Stay away from polished finishes on your overmold part. Use a blast finish to ensure separation from the tool and overmolded part.

To ensure the overmold “sticks” to the substrate and not the mold itself during the second shot of this process, the surface finish options for your overmolded part are limited. It’s best (and recommended) to choose a rougher texture like a B3 or Blast finish.

A highly polished, smooth overmold finish will inherently want to stick to the mold. Using a Blast finish will break up the surface of the mold and prevent sticking.

B3 320 Paper B2 400 Paper

MT-11010Light Bead Blast

MT-11020Med. Bead Blast

A2 Polish A3 High Polish

Tooling MarksRemoved

KEY POINT



Summary

Overmolding is a great process to connect two separate parts to eliminate assembly, to provide color contrasts, to provide an ergonomic grip, to seal an area or to hide known blemishes (such as parting lines) in the substrate.

Before beginning an overmolding project, understand that you will need three files to upload: the substrate, the overmold and the substrate with the overmold.

Understanding how to design for an overmold project will help save time...

Walls should be designed to be even, consistent and nominal through the entire part. Designing smooth, gradual radii will help the overmold material establish good flow

through the mold and over the substrate. Design even and clean hard shutoffs by keeping the nominal wall thickness to the end

of the part. Proper consideration to accessing the overmold area within the mold will save time. Always review the data sheets or call your material supplier for pairing materials and

proper adhesion. Design for a mechanical bond as well. Stay away from polished finishes on your overmold part. Use a blast finish to ensure

separation from the tool and overmolded part.



Answers to Common Questions

Q: Can I overmold clear PC and get optical clarity?A: You can get optical clarity on the substrate, but because of

the surface finish limitation of the overmold, you will not get optical clarity on the overmold.

Q: Can I overmold two rigid materials together?A: It depends on the mold temperature needed for the

second material (overmold). It should not exceed the melt temperature of the substrate.

Q: Does the type of gate used effect the price or manufacturability of the mold?

A: Yes. Gate location and gate type will directly effect the mold set up and cost.

Q: Can undercuts be included on an overmolding project?A: Yes. An overmold can be considered as a separate part and

thus, can utilize undercuts within the part design.

Q: Can I completely encapsulate a substrate with an overmold material?

A: Because the substrate still must be held in place by the mold, its possible to surround a large majority of the substrate, but not all because contact must be made to keep the substrate in place.



Additional Resources

Webinar Series

What is Overmolding? An animation

White Paper: The Smart Guide to Designing for Manufacturability

Video Tour

About Injection Molding

Material Data Sheets

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Xcentric can provide you additional resources to increase your level of understanding of injection molding and manufacturability. Click the images to view in your browser.

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Gate - Referring to where the plastic enters into the cavity of the mold.

Edge Gate - The flow of material between cavity and core into the edge of part

Sub Gate - Gate into the core which allows material to flow into the part through a machined ejection pin.

Landing Pad - An area within the mold that will accept the gate location to allow material to flow into channels to connect them.

Lubricity - The amount lubrication inherently found in a material

Undercuts - The portion of the designed component where a slide or hand pull is required to create holes, windows or clips that are not in the line of draw.

Wall Thickness - This refers to how thick the cross section of the plastic part is.

Flash - Material that is forced through the parting line due to excessive mold pressure.

Glossary


Design Considerations to Successfully Manufacture Complex … · 2017-08-10 · Xcentric Mold &...

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