INSERTS:

Definition:

An integral part of a plastic molding consisting of metal or other material which may be molded into position or may be pressed into the part after the molding is completed.

Types:

There are basically two types of inserts:

1)      Molded in inserts: They have medium or coarse knurl to provide adequate gripping power. Some of the molded in inserts are :

·         Blind hole female insert with internal threads.

·         Open end female insert with internal threads.

·         Blind hole female insert with internal threads and counter bore.

·         Male stud with internal thread.

·         Eyelet projecting.

·         Eyelet both end projecting.

·         Projecting rivet.

·         Double projecting insert with external threads.

·         Blind hole female insert with internal threads and double sealing shoulder.

·         Metal stamped insert.

·         Female insert with undercut.

·         Drawn eyelet.

·         Rod or pin type inserts.

2)      Post molded inserts: They may be threaded or may be installed by heat or ultrasonically and may also be coarse knurled.

Selection criteria:

The insert should have a shoulder fitting into the mold to prevent entry of the molten plastic into the threads.

Design criteria:

·         If the orientation not perpendicular to the parting line is to be used, it will be difficult and expensive to mold in the inserts. Hence a post molded installation should be considered.

·         Location of an insert relative to the edge of the part or an adjacent wall is very important.

·         Sometimes a thermal stress is imposed on the plastic, in addition to the mechanical stress, due to its higher coefficient of thermal expansion i.e. when the molded part is cooled the plastic tend to shrink more than the metal, causing stresses in the plastic which can result in cracks around the insert during operation. Using a sufficient amount of plastic around the insert, either in the form of a Boss or an increased distance from walls, edges or other insert will prevent consequences of the thermal stresses.

·         Inserts with sharp corners should be avoided to minimize cracking.

·         Inserts should always be used sparingly because they are placed in mold or part by hand, unless large quantity production justifies automation. In either case their use is expensive.

Significance:

Metal inserts are used in plastic parts:

·         To take wear and tear if frequent disassembly is required.

·         To carry mechanical stresses above the limits of the plastic.

·         To act as electrical contact.

·         For decoration.