By: Brad Bill ITDPT 303 Ball State University
Introduction:Overmolding is a process that is used in many manufacturing forms to create parts and improve product ef
ficiency. Without the process of overmolding we would not have the many consumer products that everyone wants and needs. Examples of overmolded products would be cell phones, clock radios, computer monitors. So how exactly does the overmolding process work and how can it be broken down?
(Sun Plastics 2005)
Definition:The technical definitionof over molding is "Overmolding, or two-shot molding, results in parts in which it is clearly evident that more than one material is being used. In these processes, only part of a product is molded in one material, and that molded piece is manipulated so the second material can be molded around, over, under, or through it to complete the final part. This method is sometimes referred to as inmold assembly, since the resulting part effectively acts as an assembly of two materials rather than as a layered structure." (Kerouac and Grelle 2005) Process:"Overmolding is the an injection molding process using two separate molds of which you mold one material over another to create or touch appeal such as a handle or knob." (Mastermolding 2005) Overmolding is typically used when creating an outer shell for electronics or different appliances. This is done using certain plastics and polymers. Typically research has shown that polyethylene is the most widely used plastic in the over molding process. It is used because it can be drawn out into thin sheets which makes the overmolding process easier and more efficient.
Industrial Processes:There are two basic types of overmolding used in industrial processes today. The first is the insert molding process which "the rigid substrate is molded first and transferred to a second mold, where a thermoplastic elastomer (TPE) is shot around the insert to create the finished part. The process uses standard injection molding machines and relatively simple, low-cost tools. Insert molding is best suited to applications involving relatively low volumes and manufacturing locations where labor costs are low". (Hudacek 2005) The second process involves a term called multi-shot molding which basically works off of the same principles that insert molding does but uses many insert molders to shot a compound into a mold. this process is typically used when there is more production being done and manufactures want to be more economically cautious. New processes are being examined in making the overmolding process better. Researchers are using different polymers and trying new temperatures to use when overmolding products in an industrial setting. According to the plastics technology website, new changes are being made in the overmolding process. "Driving this change is the increasingly diverse range of (TPE) materials. In overmolding, a TPE is injection molded over or around a compatible substrate using either insert or multi-shot processes. The resulting hard-soft structures are quite effective in comfortable, non-slip, and abrasion-resistant handles, grips, and buttons. A critical challenge for designers and producers of these parts is poor adhesion of TPE to substrate, which reveals itself in peeling, curling, fraying, or delamination of the material layers. Initially, this was a fairly straightforward issue since the most common approach was to combine an olefin-based TPE with a compatible rigid PP substrate." (Hudacek 2005) There are different factors along with which components to use when overmolding.
Quality Control and Potential Problems: Temperature is one of them and probably second most important to the components. Temperature affects the thermoplastics greatly and if these temperature are not exactly correct then the whole process and overmolding line can be ruined, wasting product materials as well as energy and time for the
consumer. "The relationship between temperature of the TPE melt and adhesion strength is reflected in the example of a 65 Shore A TPE overmolded on a PC substrate. As the melt temperature increases from 370 F to 400 F, a notable improvement in adhesion strength is evident. But a further increase to 430 F actually reduces adhesion strength. In this instance, the optimum melt temperature is somewhat less than 400 F. Molders must balance desired adhesion strength against the possible adverse effects of elevated melt temperature, e.g., thermal degradation and ejection difficulty." (Hudacek 2005)
Ten Tips on Overmolding
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Match compatibility of TPE and substrate. Minimize peeling with sharp transitions in shut-off design. Avoid trapping air in cavities via appropriate venting. Balance TPE thickness with Shore A hardness for desired “feel.” Maintain TPE melt temperature at the level that optimizes adhesion. Dry moisture-sensitive materials. Select color-concentrate carriers that are compatible with both the TPE and substrate. Be aware of liabilities of smooth surface textures. Keep the TPE flow-length/part- thickness ratio below 150:1. Design gating with good adhesion
(Hudacek 2005) History of Overmolding:Molding in general has been around for almost one hundred years. Different parts have been made over the course of the century using molding processes but overmolding has not been very popular until recently. The past thirty years has given over molding a chance to break though as the "high-tech" molding process. Almost all of our little gadgets today have been overmolded some how.
Anything from toaster ovens back in the late sixties to the new PDA's and cell phones of today.
Summary:As it can be seen above overmolding is a highly useful process in which one can combine two different molds for either appearance reasons or durability. By overmolding products such as plastic materials, these materials can be formed into one solid mold which can be seen in many cases in the consumer market today. Without the overmolding process the items and consumer parts that we use everyday such as cell phones, toasters, PDA's, automobiles and thousands of other parts would not exist. The overmolding process is constantly changing and the processes and components that are being used are constantly being tweaked to extend performance, durability, and life cycle. Sources and References 1) Hudacek, L (2004). How to Optimize Adhesion in Hard-Soft Overmolding. Plastics Technology, 50(2), 62-68. 2) (2003). Super-Soft TPVs for Grips And TPEs for Overmolding.. Plastics Technology, 49(10), 43-43. 3) Ogando, J (2002). Overmolding eases electronics assembly..Design News, 58(16), 34-34. 4) Caamano,J & Hoffman, J (2002). Hard rules for soft-touch overmolding. Machine Design, 74(9), 60-60. 5) (1996). Overmolded parts made from single plastic..Advanced Materials & Processes, 150(5), 4-4.