The main considerations governing insert selection are: type of plastic, installation method and insert performance. The tables below will assist the specifier in choosing the best insert for a given application. Additionally, custom inserts can be designed to suit specific requirements. Contact PENCOM to speak with a technical representative.
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Commercial plastics can be categorized into four main types: thermoplastics, thermosetting plastics, foams, and elastomers. Thermoplatics and thermosetting plastics are most suitable to insert installation and will be summarily explained. Thermoplastics soften and melt at elevated temperatures. They can be divided into amorphous or semi–crystalline polymer sub-types depending on their structure at room temperature. Amorphous polymers have a random molecular structure and soften gradually with rising temperatures. They are resistant to creep (deformation due to prolonged stress) and impact, but sensitive to stress failure and have limited chemical/solvent resistance. Common amorphous polymers include ABS (acrylonitrile butadiene styrene), PVC (polyvinyl chloride) and PC (polycarbonate). Semicrystalline polymers have a more ordered molecular structure with a distinct and limited melting point range that is generally above amorphous thermoplastics. They are more resistant to chemicals, fatigue, stress cracking and wear but have a tendency to creep under sustained loads. PET (polyethylene terephthalate) and PEEK (Polyetheretherketone) are typical examples. Polyamide, or nylon, can be either amorphous or semi–crystalline depending on the blending. During formation, thermoset plastics experience an irreverisble chemical change and cannot be softened with heat. They are durable and resistant to heat. Examples include phenolic, urea and epoxy resins. The physical characteristics of plastics can be enhanced by the addition of fillers and plasticizers depending on the application. They are used to increase strength and resistance to creep, minimize shrinkage, modify conductive and thermal properties and reduce cost. However, these additives can increase sensitivity to stress and influence the installation and performance of inserts as well.
Ultrasonic is the most common insert installation method for thermoplastics. A molded or drilled hole receives an insert that has a slightly larger diameter to create a small amount of interference and guide the insert into place. A “horn” contacts the top of the insert and imparts ultrasonic vibrations which travel through the insert. Frictional heat is generated at the insert/ plastic interface causing localized temporary melting of the plastic. The insert is pressed into place with the horn allowing the molten material to flow into the external knurls, the vibrations cease and pressure is maintained until the plastic solidifies preventing back– out of the insert. Advantages of ultrasonic installation include: reduced cycle times, lower induced stress as compared to mold–in or press–in inserts, ability to install multiple inserts simultaneously, suitability for automated operations, and repeatable and consistent results as compared with heat–only installation. Because the method requires temporary melting of the plastic, ultrasonic installation is not recommended for thermosetting plastics.
Similar to ultrasonic installation, inserts installed using heat begin with a molded or drilled hole that receives an insert with a slightly larger diameter to create a small amount of interference and guide the insert into place. The heated tip of an insertion press contacts the top surface of the insert, as well as, the internal threads. The insert is heated via thermal conduction and pressed into place once the proper melting temperature of the plastic is reached. Once installed, the heated press tip is retracted and the plastic solidifies locking the insert in place. Since the entire insert is heated, it takes longer to cool after installation thus providing a natural stress relief for the plastic. However, a small amount of back– out of the insert may occur. While somewhat slower than ultrasonic, advantages of heat installation include: excellent insert performance, ability to simultaneously install multiple inserts on different levels, more quiet and less expensive equipment required compared to ultrasonic, and more favorable results with larger inserts. Because the method requires temporary melting of the plastic, heat installation is not recommended for thermosetting plastics.
Designed for non–critical applications, expansion– type inserts sacrifice strength for ease of installation. They may be simply pressed into a molded or drilled hole using hand tools or standard press. Installation of the mating screw expands the insert and forces the knurls or fins into the sides of the mating hole creating torque–out and pull–out resistance and somewhat of a thread locking effect. Because heat or ultrasonic vibration is not required for installation, the diamond–knurled and fin versions are popular for use with hard thermosetting plastics.
For applications where ease of installation and reduced cost are more important than torque–out and pull–out performance, press–in inserts with a free–running thread are cost–effective solutions. The insert diameter is slightly larger than the hole diameter but with a pilot end that’s marginally smaller than the hole to guide the insert. Pressing the insert into cold plastic creates undesirable stress so increased boss wall thickness is usually necessary. Performance can be improved if the insert installation is done while the mating plastic is still warm from molding.
Self–tapping inserts for post–molding installation are manufactured with external threads to create the maximum shear surface area while minimizing induced stress with the mating plastic component. A thread–cutting groove makes these inserts suitable for thermoset and brittle materials. A tapping head attaches to the insert and transfers the torque to install the insert. The thread friction between the insert and plastic component is greater than the internal thread so that tapping tool is easily removed, as well as, any mating fastener without worry of insert back–out. The self–tapping design is suitable for weak materials with low core strengths and where jack–out may be unavoidable.
While having the largest overall installation cost, mold– in inserts provide the best performance. When the mold is open, the inserts are placed on guide pins in the cavity which hold the inserts in place. The inserts have a reduced-tolerance minor diameter to maintain a good fit with the pins and alignment with the plastic component. After encapsulation by the plastic, the mold opens and the pins are retracted exposing only the insert threads. Because the inserts must be loaded on the core pins, total molding time is increased, as well as, down time to repair mold damage caused when an insert in improperly loaded. Plastic sink marks and internal stresses are sometimes a concern because of the different cooling rates of the plastic and inserts. Mold–in inserts are popular for use with thermosetting plastics because of the limited post–mold insert options and inherent strength.
Rotational force acting to pull the insert out of the host material. The condition results from mating component not bearing directly on the insert.
Inserts develop their strength by having the host plastic form around integral knurl bands, recesses and vanes (fins). In general, knurls increase an insert’s resistance to torque while recesses and vanes increase pull–out resistance. The greater the insert length the greater the performance due to the increase size or number of insert features. Straight knurls offer the greatest torque resistance while helical knurls offer a compromise between torque and pull–out resistance. Ease of installation and host material type are additional design considerations. Therefore, the goal of the insert design is to achieve the greatest performance for a specific application.
Some inserts are offered in a headed configuration. This option:
Regardless of the insert design or installation method, installing an insert straight in the hole in critical. Although inserts have a taper and/or lead–in to facilitate self– alignment, failure to maintain axial integrity with the hole can result in boss side loads which may cause cracking.
After installation, the end of the insert should be flush or within .005″ above the host plastic to achieve maximum performance. With heat installation, the insert may back out somewhat and is considered normal. An insert should never be installed below the surface as this can lead to jack– out.
To achieve maximum performance, the boss must be correctly sized according to diameter and taper. Hole sizes shown in the bulletin are for post–mold conditions because as plastic cools it shrinks and hole sizes may change. Oversized holes result in decreased insert performance while undersized holes lead to stresses in the boss walls and possibly flash at the hole edge after insert installation.
If fillers are used, the hole sizes may need the be adjusted as follows: increase hole diameters .003″ for filler contents greater than or equal to 15%; increase hole diameters .006″ for filler contents greater than or equal to 35%; interpolate hole diameter increases for intermediate filler contents; filler contents greater than 40% may result in problems with installation and/or performance. PENCOM recommends pre-production testing to verify the correct boss hole size. All inserts in this bulletin require boss holes with a 0.5° to 8° total inclusive taper depending on the insert type. Inserts are designed to fit a particular boss hole configuration and should not be interchange with other boss designs. Greater boss hole tapers are preferred by molders due to an easier release from insert locating core pins.
Boss hole depth is critical to achieving a flush insert installation. For ultrasonic/heat installed inserts the hole depth should be at least .039″ (1.00mm) greater than than the length of the insert to allow space for forward displaced material that may otherwise be forced into and contaminate internal threads. Hole depth should also be sufficient to prevent the assembly screw from bottoming out in the hole and causing jack–out.
Minimum boss wall thicknesses shown are for reference and may need to be increased to avoid bulging and remain strong enough to resist assembly torque. Post-mold quality is important as poor knit lines can lead to failures. Cold pressed inserts require larger wall thicknesses due to the greater stresses imposed. Installing these inserts while the plastic is still warm reduces boss wall internal stresses.
Countersinks and counterbores should be avoided on all post–mold installed inserts except self–tapping inserts. This hole treatment could interfere with the lead–in features of an insert and influence the self–aligning characteristics.
When installing a studded or blind threaded insert, a small vent should be added to the bottom of a blind hole to allow trapped air to escape. Otherwise, the pressure buildup may distort the plastic surface around the insert and make consistent installation results difficult.
To prevent jack–out, it is very important that the clearance hole of the mating component is sized correctly. The clearance hole should be larger than the assembly screw yet smaller than the outside diameter of the insert so that the insert, not the host plastic, carries the compressive load. If the clearance hole must be oversized for misalignment purposes, a headed insert is recommended to increase the insert bearing area surface.
In bolted assemblies where the mating component is also plastic, creep or stress relaxation resulting from sustained compressive loads may be prevented by using a compression limiter. A compression limiter maintains joint integrity by absorbing the load between the fastener and insert and prevents joint loosening due to creep. The compression limiter should be large enough to provide clearance for the mating fastener yet small enough to bear directly on the end of the insert. A headed insert may be required for compression limiters with large thru-holes. Additionally, the length of the compression limiter must be equal to or slightly larger than the thickness of the mating component to prevent plastic creep. As most requirements are different, compression limiters are designed and manufactured for each specific application.