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Tips for insert molding of plastic parts.

Insert over-molding or metal over-molding can reduce the labor and assembly costs in plastic part manufacturing, while improving a product's strength and wear resistance as well. This look at the technique provides an overview of its advantages, disadvantages, and its effects on cost, product quality and safety.

Engineers and designers will use plastic as their material of choice due to many factors such as cost, light weight and design freedom. Often, a portion of the newly designed part needs to meet design goals that only metal can satisfy. Threads may have pull requirements that cannot be met by the preferred plastic choice. A portion of the part may need to conduct electricity. A sensor, hinge, clutch, or torque insert may need to be positioned in the middle of the part. Plastic can be over-molded to these metal components to meet all of the design requirements.

Inserts can be placed into plastic parts using other methods such as gluing or press fitting. Gluing relies on a chemical bond and a press fit relies only on friction to hold the insert in place. If over-molding is used though, the plastic can encapsulate features on the insert that will mechanically lock it into place. By over-molding, you also eliminate the secondary operations and labor of gluing or pressing the insert into place. The over-molding process is done within the injection molding cycle utilizing the labor that is already running the injection molding machine.


Over-molding metal can increase engineers' and designers' options. In many industries, including medical manufacturing, companies want to replace large, heavy metal components with plastic, or replace complex multipart assemblies with one plastic component. Over-molding pins or threaded inserts can allow one part to replace multiple parts and yet still be able to interface with its mating components. Over-molding will also allow a metal part to be replaced by a plastic part while retaining the key metal properties achieved with the over-molded insert.


Over-molded inserts can also simplify mold construction. Some parts require threads but do not have a lot of mechanical stresses on them. These parts could successfully use plastic threads that are molded into the plastic part. The requirements on the mold builder to add these features often involve slides or unscrewing mechanisms in the mold. For high volume parts, the added mold cost may justify the added tooling costs, but for low volume parts it may be more cost effective to over mold a metal insert that has the threads.

Most metals can be used in the over-molding process. As the plastic surrounds the metal insert, it will see relatively high temperatures of the molten plastic, but since the mold is also designed to remove the heat of the plastic, it will only see the elevated temperatures for a short period of time. There are some highly engineered resins that process at extremely high temperatures and require the molds to be at temperatures above 225 degrees F. Inserts used with these resins may have to be preheated before being placed in the mold and will be hot to the touch when the part is ejected from the mold.

Most plastics can be used in the over-molding process but the properties of the plastics will dictate how easily an insert could be removed. Plastics such as polypropylene or polyethylene will not hold a threaded insert in place under load as well as nylon or polycarbonate.

The mold will also have to be designed specifically for the over-molding process. The inserts are generally put into a hole or on a post designed to hold it in place. Once the inserts are in the mold, they must stay in place and not move once the plastic flows around it during the injection molding process. The plastic will be flowing into the mold at very high pressures and speeds. If the insert is not secured in place, it could move and create a reject. If a hole is used in the mold to hold an insert, a detent or spring-loaded device can be used to hold it in place. If the insert has an internal hole, a post can be used to hold it in place. Due to the manufacturing tolerances of inserts, rarely is the fit on a post firm enough to securely hold it in place. Sometimes all it takes is temporary adhesion between the mold surface and the insert while the plastic envelopes it. Application of water or petroleum jelly can provide enough adhesion to the mold face to dramatically reduce rejects due to inserts moving during the injection molding process.


The process used for placing the metal inserts is driven by the volume of parts being run. For low to medium volumes, it may be more cost efficient to have a machine operator place the parts into the mold and run the injection molding machine in semi-automatic. This method does have some negatives however. A lot of variability is added to the process. The cycle will no longer be consistent since it is controlled by the speed of the operator opening the gate, putting the inserts into the mold, and closing the gate. The process also relies on the operator placing the insert in the right location every time with very low deviation. To eliminate the process variability, the mold can move from a horizontal injection molding machine to a vertical injection molding machine. Vertical machines generally operate with either a shuttle table or a rotary table. The operator is now working in a station that is not in the injection molding process. The operator can still add some placement variability and could affect the cycle time, but not to the same degree as a horizontal injection molding machine.

Once the volumes increase to provide an acceptable return on investment, automation can be purchased to orient the inserts and have a robot place them into the mold. Most inserts can be oriented for automated handling utilizing vibratory bowls. The bowls will then present the inserts to a robot that will grab the inserts and place them into the mold after it removes the finished part. This leads to a very consistent cycle time and stable process, and therefore a higher quality level of finished products.

Insert over-molding has potential issues. If the inserts fall out during the molding process, they can be caught in the mold and damage the mold as the injection molding machine applies clamp pressure to the mold. At best, , the safeties in the machine stop this and may just leave a small indentation in the mold. At worst, the mold crushes the insert as it closes and damages major components of the mold. If the inserts move a little during plastic injection, it could cause the part to be unusable. Some molds also have to be run at very hot temperatures depending on the type of resin. If the operator is putting the inserts into the mold, they could get burned or at least become very uncomfortable as they perform this operation over and over during their shift.

Although the many benefits of insert over-molding outweigh the negatives in most cases, consideration should be given to each specific application before deeming it appropriate for a project. Just as potential issues that can compromise the product quality should be evaluated, so should all safety concerns for production personnel.

Joe Vest--VP (and engineer), Bright Plastics
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Title Annotation:Emphasis On: Insert Molding
Author:Vest, Joe
Publication:Medical Design Technology
Date:Sep 1, 2016
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