Inserts

Inserts are used in plastic parts, to allow the use of fasteners such as machine screws.The advantage of this is that since these inserts are made out of metal, they are robust. Further, machine threads also allow great many cycles of assembly and disassembly

Inserts are installed using one of the following methods:

Ultrasonic insertion. The insert is vibrated using an ultrasonic transducer, called the “horn” mounted in an ultrasonic machine.

The horn has to be specially designed for each application for optimum performance. The ultrasonic energy is converted to thermal energy due to the vibrating action, which allows the insert to be melted inside the hole.
This type of insertion can be done rapidly, with short cycle times, low residual stresses. Good melt flow characteristics for the plastic are necessary for the process to be successful.The ultrasonic equipment is relatively expensive, and also needs a custom horn for optimal production rates.

 

Thermal Insertion. The inserts are heated by placing them over the hole and pressing them in with a heated tool. The tool first heats the insert, then the insert is pressed in.

 

The advantage of this method is that the special tooling necessary is relatively simple, usually a cylindrical tool, which can be easily, fabricated in the machine shop. The cycle times are usually short.

However, care has to be taken, not to overheat the insert or the plastic, or it will lead to local plastic degradation.

Press Fitting.The inserts are designed with barbs (straight knurls that are interrupted) and can be press fitted inside the hole in the boss.This process is fast and requires no special tooling. However, high hoop stresses are generated, so the boss design has to be robust. Also, the retention is strictly based on press fitting and the small amount of material that flows inside the barbs. Thus retention is not very high.

Molded-in Inserts. The inserts are placed in the mold prior to the injection of plastic. The injection of the plastic completely encases the insert on the outer diameter and provides very good retention. In fact retention of such inserts is the best compared to other process (barb/knurl design being the same).This process slows down the operation of the mold, since the inserts have to be manually placed inside the mold. Inserts can be automatically placed in a mold, but this greatly increases the complexity and cost of the mold. This can only be justified if the volume of production is very high to offset the cost savings in shorter cycle times.

 

Inserts used in plastic parts provide a place for fasteners such as machine screws. The advantage of using inserts is that they are often made of brass and are robust. They allow for a great many cycles of assembly and disassembly. Inserts are installed in Injection Molded parts using one of the following methods:

ULTRASONIC INSERTION

Ultrasonic insertion is when an insert is “vibrated” into place by using an ultrasonic transducer called the “horn” that is mounted into the ultrasonic device. For optimum performance, the horn is specially designed for each application. Ultrasonic energy is converted to thermal energy by the vibrating action, which allows the insert to be melted into the hole. This type of insertion can be done rapidly, with short cycle times, and low residual stresses. Good melt flow characteristics for the plastic is necessary for the process to be successful.

THERMAL INSERTION

This method uses a heated tool, like a soldering iron, to first heat the insert until it melts the plastic, and then presses the insert into place. As the plastic cools it shrinks around the insert, capturing it. The advantage of this method is that the special tooling is inexpensive and simple to use. Care does need to be taken not to overheat the insert or plastic, which could result in a non-secure fit and degradation of the plastic.

MOLDED-IN

To mold inserts into place during the molding cycle, core pins are used to hold the inserts. The injected plastic completely encases the insert, which provides excellent retention. This process may slow the molding cycle because inserts have to be hand loaded, but it also eliminates secondary operations such as the ultrasonic and thermal insertion methods. Finally, for high volume production runs, an automatic tool can load the inserts but this increases the complexity and cost of the mold.

 

Molded-in Inserts

Adding ribs, bosses or molded-in inserts to various
part designs can solve some problems but may create
others. Ribs may provide the desired stiffness, but
they can produce warpage. Bosses may serve as a
suitable fastening device for a self-tapping screw, but
they can cause sink marks on a surface. Molded-in
inserts may enable the part to be assembled and
disassembled many times without loss of threads.
Considering these possible problems, the appropriate
question is, when should molded-in inserts be used?
The answer is the same for ribs and bosses as well.
Inserts should be used when there is a functional need
for them and when the additional cost is justified by
improved product performance. There are four principal
reasons for using metal inserts:
• To provide threads that will be serviceable under
continuous stress or to permit frequent part disassembly.
• To meet close tolerances on female threads.

• To afford a permanent means of attaching two
highly loaded bearing parts, such as a gear to a
shaft.
• To provide electrical conductance.
Once the need for inserts has been established, alternate
means of installing them should be evaluated.
Rather than insert molding, press or snap-fitting or
ultrasonic insertion should be considered. The final
choice is usually influenced by the total production
cost. However, possible disadvantages of using
molded-in inserts other than those mentioned previously
should be considered:
• Inserts can “float,” or become dislocated, causing
damage to the mold.
• Inserts are often difficult to load, which can prolong
the molding cycle.
• Inserts may require preheating.
• Inserts in rejected parts are costly to salvage.
The most common complaint associated with insert
molding is delayed cracking of the surrounding plastic
because of molded-in hoop stress. The extent of the
stress can be determined by checking a stress/strain
diagram for the specific material. To estimate hoop
stress, assume that the strain in the material surrounding
the insert is equivalent to the mold shrinkage.
Multiply the mold shrinkage by the flexural modulus
of the material (shrinkage times modulus equals
stress). A quick comparison of the shrinkage rates for
nylon and acetal homopolymer, however, puts things
in better perspective.
Nylon, which has a nominal mold shrinkage rate of
0.015 mm/mm* (in/in) has a clear advantage over
acetal homopolymer, with a nominal mold shrinkage
rate of 0.020 mm/mm* (in/in). Cracking has not been
a problem where molded-in inserts are used in parts of
Zytel® nylon resins.
The higher rate of shrinkage for acetal homopolymer
yields a stress of approximate 52 MPa (7600 psi),
which is about 75% of the ultimate strength of the
material. The thickness of the boss material surrounding
an insert must be adequate to withstand this stress.
As thickness is increased, so is mold shrinkage. If the
useful life of the part is 100,000 hours, the 52 MPa
(7600 psi) stress will be reduced to approximately
15 MPa (2150 psi). While this normally would not
appear to be critical, long-term data on creep (derived
from data on plastic pipe) suggest the possibility that
a constant stress of 18 MPa (2600 psi) for 100,000
hours will lead to failure of the acetal homopolymer
part. If the part is exposed to elevated temperatures,
additional stress, stress risers or an adverse environment,
it could easily fracture.
* 1
⁄8″ thickness—Recommended molding conditions

Because of the possibility of such long-term failure,
designers should consider the impact grades of acetal
when such criteria as stiffness, low coefficient of
friction and spring-like properties indicate that acetal
would be the best material for the particular application.
These grades have a higher elongation, a lower
mold shrinkage and better resistance to the stress
concentration induced by the sharp edges of metal
inserts.
Since glass- and mineral-reinforced resins offer lower
mold shrinkage than their base resins, they have been
used successfully in appropriate applications. Their
lower elongation is offset by a typical mold shrinkage
range of 0.003 to 0.010 mm/mm (in/in).
Although the weld lines of heavily loaded glass or
mineral-reinforced resins may have only 60% of the
strength of an unreinforced material, the addition of a
rib can substantially increase the strength of the boss
(see Figure 3.27).
Another aspect of insert molding that the designer
should consider is the use of nonmetallic materials for
the insert. Woven-polyester-cloth filter material has
been used as a molded-in insert in a frame of glassreinforced
nylon.

Part Design for Insert Molding
Designers need to be concerned about several special
considerations when designing a part that will have
molded-in inserts:
• Inserts should have no sharp corners. They should
be round and have rounded knurling. An undercut
should be provided for pullout strength (see
Figure 3.28).
• The insert should protrude at least 0.41 mm
(0.016 in) into the mold cavity.
• The thickness of the material beneath it should be
equal to at least one-sixth of the diameter of the
insert to minimize sink marks.
• The toughened grades of the various resins should
be evaluated. These grades offer higher elongation
than standard grades and a greater resistance to
cracking.
• Inserts should be preheated before molding; 93°C
(200°F) for acetal, 121°C (250°F) for nylon. This
practice minimizes post-mold shrinkage, preexpands
the insert and improves the weld-line
strength.
• A thorough end-use test program should be conducted
to detect problems in the prototype stage of
product development. Testing should include
temperature cycling over the range of temperatures
to which the application may be exposed.

From a cost standpoint—particularly in high-volume,
fully automated applications—insert costs are comparable
to other post-molding assembly operations. To
achieve the optimum cost/performance results with
insert molding, it is essential that the designer be
aware of possible problems. Specifying molded inserts
where they serve a necessary function, along with
careful follow-up on tooling and quality control,
will contribute to the success of applications where
the combined properties of plastics and metals are
required.