Ultimate Guide to Overmolding

Ultimate Guide to Overmolding

In injection molding, overmolding is a versatile injection technique used generally for part creations.  It is one of the many injection techniques used today in the manufacturing industries. However, not every injection molder can perform as an overmolder. This technique requires extensive training, specialized equipment, and precise control of multiple variables throughout the production process. This is because the slightest of all variations can affect the quality of production. In a bid to avoid unforeseen circumstances in the production process, this guide provides comprehensive information about Overmolding. This information may include and is not limited to advantages, types of overmolding techniques, material considerations in overmolding.

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    CHAPTER 1:
    Overmolding Introduction

    1.1 What is Overmolding?

    Overmolding is an injection technique in which a single part is manufactured using two or more combinations of materials. It involves the creation of an assembled products in a process where a layer of material is molded over another component. This is done by placing the base component in the injection mold, and the hot molding materials fill the area around the inserted part. This technique is generally referred to as two-part or two times injection molding. This process may be done chemically or mechanically.

    For example, to enable a consumer to turn a screw effectively, a T-handle is needed on the screw. Then, the T-handle can be incorporated into the screw head either chemically or mechanically. For the chemical attachment process, before adding the overmolding materials, the initial part is coated with adhesives. On the other hand (mechanically), the primary piece is either scored or slightly altered with recessions or projection to better attach the overmolding materials.

    Free Tips: A substrate and the overmolding materials should interlock in a mechanical capacity. This action will not only make the two materials bond chemically, they will be held together physically.

    1.2 Advantages of Overmolding?

    Advantages of Overmolding

    When we think of overmolding, what comes to mind is the fact that it offers a wide variety of aesthetic benefits. However, the advantages it offers are not limited to aesthetics but in many ways.

    The use of overmolding is advantageous in several ways, they include:

    • Improved product Performance
    • Reduced Production Cost
    • Improved shelf appeal
    • Reduces Human Error
    • Durability & Toughness
    • Simplified process flow
    • Increased user ergonomics and experience
    • Design Flexibility
    • Fast Automated Process
    • Increase IP Rating
    • Elimination of Fatteners and Adhesives

    Improved product Performance

    One of the primary reasons for designing and manufacturing a product or parts using the overmolding technique is better product performance. For example, the TPE is an ideal material for manufacturing a soft, non-slip grip for a variety of products. This is because tactile sensitivity is considered for comfortability in the use of those products examples include medical products such as surgical instruments.

    Reduced Production Cost

    Overmolding is versatile and it adds value in a variety of ways such as reducing the cost of production. The question is how does overmolding reduce the cost of production? Overmolding technique helps to reduce cost because it reduces the number of manufacturing steps involved in the production. This is evident via the elimination of injection molding and assembly for a component such as bumpers and gaskets. Another way is through the elimination of some secondary operations such as painting, priming, and coating. Labor cost is also reduced in the process.

    Improved shelf appeal

    Overmolding is known to help a product stand out from the competition on the market shelf. This is because it can be used to create a visually attractive surface for varieties of products. For example, TPE can be used and it comes in a variety of color ranges. It also exhibits the ability to be engineered for different finishes. In addition, overmolding makes products stand out because corporate logos, messages, or operation instructions can be overmolded on customized or patterned substrates.  

    Reduces Human Error

    Overmolding processes also make it possible to reduce human errors in production to the barest minimum. This is because successful overmolding techniques involve tremendous precision during production. This is to ensure that there is no gap between the two materials when they are joined together as a unified body. This specialized technique gives no room for error. This is because any misalignment can result in bad shaped product or stresses that can cause fractures.

    Durability & Toughness

    Another advantage the overmolding technique offers is the toughness of products. Products that are manufactured using overmolding are shock-resistant, waterproof, and airtight. For example, the use of LSR as an overmolding material confer unto the end product with impressive properties. These properties include and are not limited to rigidity, toughness, lightweight, chemical resistance. Overmolding is used for products such as knives, toothbrushes, cell phones, flashlights, and many more.

    Impressive Texture Feel

    Another impressive advantage of parts created using overmolding is the texture feel. These textures feel range from soft to firm and can also be produced based on the customer’s preferences and needs. Overmolding also makes it possible to insulate products from electrical shock, sharp edges, and corners of products can be eliminated and minimized.

    Simplified process flow

    Products manufactured with the use of overmolding can also be produced by other methods. The difference is that overmolding makes it easy to produce in about two operations. This has thereby simplified the process flow compared to other production techniques.

    Increased user ergonomics and experience

    In production processes, overmolding produces products with multiple colors that fascinate and appeal to attractions.  It also offers soft-touch, better grip, and many more features to enhance the end user’s experience and ergonomics.

    Design Flexibility

    Another advantage of overmolding is its flexibility in design. It can be used in a situation where there are complex assembly designs. Instead of assembling multiple parts, the overmolding techniques can help to create a single part having all those features at the same time. An example is in the car production part such as car headlight, car dashboard, and many more.

    Fast Automated Process

    With the help of overmolding, the overmolding process becomes simple and automated. It allows molders in the industries to overmold two connections much faster than alternative connection protection methods that can be applied. The machines automate the overmolding process without interference from human labor. The implication of this is that the cost of labor is reduced thereby leading to a quick turnaround and increased output.

    Increase IP Rating

    The process of overmolding helps to create a material bond around wire/cables, PCBs, and connectors that provide watertight seals. This helps to protect electrical components from dust, dirt, and debris. As a result, the overmolded electrical assembly can achieve IP67, IP68, and IP69K standards.

    Elimination of Fasteners and Adhesive

    In overmolding, two or more components are molded together in a single or multiple molding shots. As a result, overmolding has made it possible to eliminate the use of fasteners such as screws or adhesives to couple parts together.

    1.3 When to use overmolding?

    When it comes to the use of overmolding, there are lots of unique requirements to be considered while designing parts. An example is the substrate’s wall thickness which determines how well it allows free flow of overmold materials. The flow of the overmold materials may be inward, on the material, or around the materials. This means that the wall thickness for both substrate and the overmold material must be consistent and even. This will help to reduce warping, incomplete fills, and an increase in a flash that may arise due to temperature differentials.

    Another situation that makes manufacturers use overmolding is in a situation where a large number of parts are required. This is done when identical parts are required to be produced at high speed. Overmolding is also used because it helps to minimize the cost of production especially when large production is required.

    1.4 Overmolding Applications

    Overmolding usage is popular across a large variety of manufacturing industries ranging from consumer products to medical devices. While overmolding seems to enhance the aesthetics of some products at first glance, the use of overmolding is highly essential for critical use in highly technical and highly regulated products. It is has a wide variety of applications including:

    • Surgical Devices & Instruments
    • AED Units
    • Electrical Applications
    • Navigation and Communication Devices
    • Automobile
    • Personal care – Toothbrushes, Razors, Pens, Sports-Grips, Cosmetics, Handles, Knobs, Flaps,
    • Power & Hand Tools – Handles, Grips, Caster Wheels
    • Housewares & Toys
    • Garden & Lawn Tools
    • Sporting equipment
    • Business equipment and many more.

    Surgical Devices & Instruments

    Surgical Devices & Instruments

    Over molding has a wide variety of applications in the health industries especially in the production of surgical devices and instruments. The handles of surgical instruments used by surgeons are produced using overmolding techniques. It helps to give comfort and ergonomic grips for optimum and proper precision during operations.

    Overmolded items such as portable monitors and many items and surgical suites must be sterile. Also, they must be able to withstand harsh environmental conditions like the radiations from scans, harsh cleaning agents, and autoclaving. Overmolding makes it possible to achieve all this as well as meeting the FDA medical requirements.

    AED Units

    AED Units

    AED Unit also known as Automated External Defibrillators are life-saving portable units and must be protected from weather, impact, and other factors. The use of an overmolded TPE layer helps to provide a watertight seal, reduce shock, and provide UV resistance to this unit. Also, it helps to insulate the interior unit from abuses that may arise in the fast-paced emergency environment.  

    To ensure compliance with the FDA regulatory review for AEDs in recent years, a qualified injection overmolder with cleanroom capabilities and deep plastic engineering experience must be used.

    Electrical Applications

    In situations where electrical components need to withstand the climate elements the TPE – an overmolding material is ideal. An example is the outdoor electrical outlet covers that must be weatherproof for years of abuse in unfavorable weather conditions. Different overmolding materials are useful in achieving these functions. The specially manufactured TPE gasket can exhibit expansion to provide a seal. This process restricts moisture from having direct contact with electrical connections. In the long run, electrical shock and fire can be prevented.

    Navigation & Communication Devices

    Two-way radio and GPS units offer end users impressive convenience. However, when these devices are used for military operations and emergency operations their reliability becomes critical. As a result, it is important to create lightweight hand-held devices for EMTs or soldiers in the field. This is done especially for those who are weighed down by personal protective gear.  In this case, material selection becomes paramount to minimize weight as well as protecting the delicate inner of the device against fine dust particles, rain, shock, and many more. To achieve all these, overmolding is used because gives reliability and durability.

    Automotive Industries

    Automotive Industries

    The automotive industries make use of plastic parts that are generally made through the injection overmolding process. This involves the production of a significant portion of the car’s exterior and interior designs such as car headlights, car dashboards, and many more. The use of overmolding has revolutionized the production of automobiles. Without such technology, weight, safety, cost, and production rates of today’s cars won’t be achievable.

    1.5 Things to consider when choosing overmolding

    The options created by overmolding are endless. It can be added to change the surface of a product or parts. This is done by adding varieties of color texture, functionality, and many more. However, before embarking on overmolding, here are quite important things to consider for effective and successful projects they include:

    • Selection of Machine
    • Handling and Preparation of Materials
    • Condition of Overmolding Process

    1.5.1 Selection of Machine

    When considering a molding machine for overmolding, machines with reciprocating screws are recommended. For better control of the molding parameters, newer computer-controlled machines are preferred over multi-cavity tools. Also, machines that come with programmable pressures and injection rates helps to produce better quality parts. Another preferred overmolding machine is the one that controls shot size by position. They are preferred to those machines that control process pressure and or time.

    1.5.2 Handling and Preparation of Materials

    Another consideration is the handling and preparation of materials for effective overmolding. Things to consider include:

    • Coloring
    • Drying
    • Purging
    • Regrinding
    Coloring

    Before embarking on overmolding the color masterbatch carrier must be compatible with the selected overmold. It must be noted that a high concentration of waxes in color concentrate can adversely affect adhesion. However, most PE carrier negatively affects adhesion. To avoid this, there must be strict compliance with the color carrier recommendation on the individual Technical Data Sheets. Lastly, to achieve easier dispersion, the color concentrate must have a slightly lower viscosity than the base TPE compound.

    Drying

    In the case of drying, molders must consider desiccant or vacuum dryers. To be on the right page, follow the individual Product Technical Date Sheet. This will give you the right guidance into having a successful project.

    Purging

    Whenever the press is down for over 10 minutes, it is highly important to purge it before restarting production. To avoid flashing, the machine should be restarted using a shot size with a gradual increase to the previous setting.

    Regrind

    Although regrind may not be general, it’s why you have to do considerations to know if the product will require regrind. It is required in about 20% of overall overmolding productions. It used when there is clean under graded TPE scrap generated during the production process. It should be noted that higher levels of regrind are tolerated in black materials.

    For a natural product, clear compounds or light-colored may show discoloration or contamination unless they are properly controlled. Use consistent regrind levels whenever possible. Use of high regrind level may burn out organic pigments used to produces yellow, blue, green, and red colors.

    1.5.3 Condition of Overmolding Process

    For the overmolding process, there are varieties of things to consider to help achieve a successful process. If proper care is not taken in observing these factors, it may adversely affect the overmolding process. Things to consider include:

    • Injection Pressure & Speed (200 – 600 psi)
    • Melt Temperature (Consult Product Technical Date Sheet)
    • Substrate Temperature (Consult Product Technical Date Sheet)
    • Mold Temperature (70-120oF are typical)
    • Cooling time (about 35-45 minutes for every 0.100 in)

    CHAPTER 2:
    Overmolding Process Techniques

    2.1 Overmolding Process Techniques

    The process of overmolding is widely known in the manufacturing industry. In this article, we will emphasize the three widely used overmolding processes techniques in the industry. They include:

    • Multiple Material Molding
    • Insert Molding
    • Rotary and shuttle table molding

    2.1.1 Multiple Material Molding

    Multiple material molding is otherwise known as Two-Shot Molding or 2K molding. In this process, the overmolded parts form a bond that enables the substrate to endure the environmental adverse effect. It involves two sets of cavities including cavity 1 and cavity 2. The first cavity allows the setting of the substrate while the other cavity molds the overmolding material. Products manufactured with this technique can be manufactured using insert molding and vice – versa. However, this technique has the advantage of superior part quality, low cost, and low cycle time. The following are options available for the Multiple Material Molding:

    • Elastomer over Thermoplastic Substrate
    • Thermoplastic over Thermoplastic Substrate

    Steps involved in Multiple Material Molding

    Step 1: The injection of molten plastic into the injection mold from Barrel A

    Step 2: The injected molten plastic is cooled followed by the separation of the core and cavities

    Step 3: The injection mold undergoes rotation to align in position with injection Barrel B

    Step 4: In this step, the mold is closed and the molten plastic is injected from Barrel B

    Step 5: After cooling, the material, the part is then ejected out of the injection mold

    2.1.2 Insert Molding

    Insert Molding

    For insert molding, a single-shot injection molding machine is used. The tools for insert molding is less expensive compared to the cost of two-shot and co-injection molding.  

    Step involved in Insert Molding

    Step 1: The substrate is placed and positioned inside the injection mold. An example is a screw that needs the “T” head. The screw without the “T” head is positioned in the injection mold

    Step 2: The secondary materials such as the elastomer or thermoplastic is injected into the injection mold. This helps to form the “T” head needed for the screw

    Step 3: The part is cooled and it is injected out of the injection mold

    2.1.3 Rotary and shuttle table molding

    Rotary and shuttle table molding

    This type of overmolding involves molding or insert of a substrate. The substrate is then placed in the B cavity using a horizontal injection unit or robot in the first position. The table is rotated or shuttled into the next station. At this station, the TPE is injected using a vertical or horizontal injection unit. At this level, a hot sprue or a runner can be placed at the parting line. At the rotatory unit, the third rotation moves the table to an “off-load” station where the ejection of two-component part occurs.

    2.2 Overmolding Process Selection

    As explained in our article, we presented the 3 most widely used overmolding methods. For an effective and successful project, each of these methods has a specific use. Each method can’t be used for all overmolding processes. As a result, there is a need to make decisions to plan and select the right overmolding process. To do this, the following factors are considered:

    • Selected Materials
    • Required quantities of production
    • Funds available for tooling
    • Cost of localized labor
    • Available methods at the location of productions

    Considerations

    • If the substrate insert is not a thermoplastic, then insert molding technique is advisable
    • In a situation where production volume is low, hand insertion of plastic or metals is advisable. In this process, it helps to lower cost.
    • For higher production volume, a shuttle press may be used. Also, where production levels may justify the expense, then a robotically placed insert or rotary table machine is advisable.
    • Two material molding methods can be used when production volume is high and or local labor is high.
    • For higher production volume with the best product aesthetic, the hot runner system using a valve and the gate is advisable.
    • Two material mold that can shuttle or rotate between mold section is advisable. It is required for elastomer overmold to be on both A and B sides of the part’s material mold.
    • In a situation of forming a uniform coat on a portion or complete side of a simple part, the two-material mold using moving cores is advisable.

    CHAPTER 3:
    Overmolding Material Selection

    3.1 What materials can be used when overmolding?

    When it comes to overmolding, the materials that can be used and their respective applications are presented in the table below for reference purposes.

    Substrate Type Applications
    Polypropylene (PP) Personal Care - Toothbrushes, Flaps, Handles, Knobs, Pens, Razors, Sports-Grips, Power & Hand Tools- Handles, Grips, Caster Wheels
    Acrylonitrile Butadiene Styrene (ABS) Grips, Handles, Housewares, Knobs, Portable Electronics, Toys
    PC Business Equipment Housings, Hand and Power Tools, Hand-held Electronics, Healthcare Devices, Sporting Goods, Telecommunications and Business Machines
    PC/ABS Grips, Handles, Hand and Power Tools, Housewares, Knobs, Portable Electronics, Sports and Leisure Equipment, Toys, Telecommunications, and Business Machines
    Nylon Lawn and Garden Tools, Hardware, Power Tools
    Propionate Handles, Toothbrush, Cosmetic Packaging, Eye ware
    Copolyester Handles, Toothbrush, and Cosmetic Packaging

    3.2 Overmold material selection tips

    For the selection of materials in overmolding, there are several factors to be considered. These factors include:

    • Adhesion requirements
    • Material Flow Behavior
    • Hardness

    3.2.1 Adhesion Requirements

    For the adhesion requirements, the end-user environment and application must be considered. However, not all overmold grades of material bonds effectively with all substrates. It is important to note the compatibility of the overmold material with the substrate. Factors such as oil or organic chemicals and temperature play a role in influencing the selection of grades to be used. For automotive under-hood, microwave safe, and dishwasher applications, special grades of overmold material are required. The service life expectancy of the products to be produced is quite important when considering the adhesion requirements. The following factors affect adhesion:

    • Color concentrate type
    • Conditions of the process (pressure, cooling time, melt temperature, injection speed)
    • Mold & part design
    • Overmold and substrate’s moisture level
    • Preparation & Preheating of substrate
    • Regrind level
    • Substrate grade (mineral-filled, glass-filled, lubricated, heat stabilized)

    3.2.2 Material Flow Behavior

    Overmolding compounds should be examined for viscosity level. The level of viscosity of an overmolding compound may vary from low to high. The lower the better for the effectiveness of overmolding. At the high shear rate of processing, the sheer responsiveness and viscosity of the overmolding compound are reduced.  As a result, there is easy flow behavior into and filling of the thin-walled section that commonly occurred in overmolding.

    3.2.3 Hardness

    The hardness of the overmolding compound is another important factor to consider. For overmolding applications, the hardness typically ranges between 30 Shore A to 70 Shore A. This range gives the best balance between set-up rates, flow properties, bonding, and ergonomic feel. However, in overmolded parts, apparent hardness or soft feel is dependent on the thickness of the overmold. Examples include ultrasoft gels in the Shore OO scale, TPE compounds with a broad range of hardness from 2 Shore A to about 45 Shore D, and many more.

    CHAPTER 4:
    Common Overmolding Problems and Solutions

    Overmolding is a sensitive process and it can come with one problem or the other. The most commonly encountered problems in overmolding include flashing of overmold, incomplete filling of overmold or substrate, poor adhesion, and many more. Below are the common overmolding observations and their possible solutions.

    Shorts

    Shorts may be caused by any or combination of the following; insufficient material, insufficient fill speed, insufficient injection pressure, too cold melt, and poor venting. To correct these actions the following can be done; increasing shot size, injection pressure, injection speed, and melt temperature. Another way to solve this issue is by decreasing the clamp tonnage and recut vents.

    Flash on the substrate or peripheral part

    This may occur due to poor mold fit, shrinkage of the substrate, inadequate molding machine tonnage, and poor shut off of design. It can be corrected by bluing the tool, recutting to obtain a complete shutoff. An increase in tonnage and decrease injection pressure can also help to resolve the issue.

    Overmold Breaks or Impinges through Hollow Substrate

    This occurs due to high injection temperature, pressure, wrong location of the gate, and improperly supported substrate. This may be resolved by giving full support to the substrate which can help to resist hydraulic injection. It can also be resolved by relocating the gate to the right position and lowering injection pressure and temperature.

    Poor Quality of Knit Lines

    Another setback of overmolding is the poor quality of knit lines. This is caused by low temperature or gas trapped between polymer fronts. It can be corrected by improving the vents and increasing the speed of injection and melt/mold temperature.

    Poor Adhesion

    One of the commonly encountered problems in overmolding is poor adhesion. Poor adhesion in overmolding can be caused due to incompatible materials, inadequate material temperature, and contamination. It may be resolved by using the right material grade and increase in process and mold temperature. Also, the color concentrate compatibility or the use of lubricated grade should be checked.

    Non-Uniform Color

    Non-uniform color in part produced may be attributed to incompatible color concentrate, inadequate melting, and dispersion of color concentrate. It can be solved by increasing rear zone temperature, screw RPM, and backpressure.

    Sticking of Part in “A” side of the tool

    This is caused by insufficient drafts, too shiny and too hot “A” cavity, vacuum formation in “A” cavity, and many more. This can be resolved by increasing the draft, sandblasting the cavity, providing air-assist release, and running “A” half cooler.

    Warped Parts

    This is can be caused by post-mold shrinkage. To solve this, lowering the molded and stress for both the substrate and elastomer. An increase in the stiffness of the substrate by adding glass or increasing thickness or ribs can also resolve this issue.

    Sticky or Smelling Elastomer

    Sticky or smelling elastomer is caused by material degradation. To check this issue, remember to check for the material residence time and reduce the temperature in rear zones if possible.

    Surface Sinks

    This is caused as a result of gate freezing off too early leading to non-uniform release from the tool surface. It can be resolved by increasing the pack pressure/hold time and a decrease in material temperature. An increase in gate size can also go a long way in solving this issue.

    Dimension out of specification

    This is caused by loss of control of shrinkage and improperly packed parts. This can be corrected by checking hold pressure (no drop) and maintaining cushions. Also, an increase in cool time and tool dimensions should be properly checked and implemented.

    Jetting

    Another issue that may arise in overmolding is jetting and it may be caused by different activities. The first cause may be too high viscosity flow and may be corrected by increasing the melt temperature. The second cause is attributed to too fast injection speed and can be corrected by relocating the gate so that the melt impinges off the wall as it enters the cavity.

    Folds, Flow Marks, & Back Fills

    These issues may be caused by too low melt temperature and can be corrected by increasing the temperature to the optimum level. It can also be caused by irregularity in surface texture and uneven filing of section. This can be resolved by relocating the gate to balance the flow or reduce the runner diameter and addition of surface texture to part design and steel wall cavities. Besides, filling from thin to thick section may be another possible cause while this is resolved by repositioning the gate to a thick section.

    CHAPTER 5:
    F.A.Q

    Before embarking on overmolding, it is important to consider a wide variety of factors for a successful project. These factors include and are not limited to the selection of machine handling and preparation of materials, and the condition of the overmolding process. To learn more about these processes, read chapters 1 to 4 of this article.

    Overmolding and insert molding are quite similar but there are distinct differences between the two methods. Overmolding is a process by which another layer of material is added to a polymer part. An example is the overlaying of a material on a plastic substrate to form a combined part. it may be used to create catheter connectors, lures, hubs, and more in medical industries.

    While insert molding is the combination of plastic and or metal into a single unit. For example, in medical industries insert molding is used to add metal pins and bushings to a plastic part that has been already molded. The process inserts the piece into the handle, hub, or other components.  

    Overmolding is a process by which another layer of material is added to a polymer part while double-shot molding is used to produce complicated molded parts from two different materials by molding plastic around a preformed metal or plastic insert.

    A hot runner system can be used in the overmolding injection process. A hot runner system using a valve gate is advisable in overmolding for higher production volume with the best product aesthetic.

    2K injection molding is otherwise known as 2 shot injection molding. It is an innovative manufacturing method used to produce complicated molded parts from two different materials. It is a type of overmolding in which its machine is programmed to perform two injections in one cycle.

    Yes, it is possible to overmold hundreds of different PCBs with no damages on components/board or reflowing solder.

    Parts that are overmolded typically meet or even exceed IP 67 rating for the environment they are meant to operate. It means that overmolded parts are dust-tight, and protected against the effects of immersion.

    Yes, batteries can be successfully overmolded without reducing their effectiveness. Although to achieve effectiveness, simple prototype overmolding is to be used for a particular type of battery before starting a full-scale production.

    Yes, Parts manufactured using overmolding techniques typically maintain dimensional stability after molding. This stability can be achieved if the overmolding regulations are effectively followed.

    No, there won’t be any distortion if the molder follows the proper design guidelines. For example, uneven encapsulation like a very thick material section on one side of a PCB with a thin layer can result in warping.

    Summary

    Overmolding has evolved as an injection molding process along with material advances over the past decade. This can help to produce new technical and creative opportunities for many industries such as medical, electrical, and many more. Beyond a soft touch, overmolding offers a wide range of possibilities for designing products with added functionalities, improved aesthetics, brand identification, and increased value. Also allows OEMs to take economic benefits of two or more materials by incorporating different properties on molded components

    We’d love to speak with you about overmolding, whether you ultimately choose to work with us or not. Give us a call or stop by.

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