I.Bakelite
Bakelite is also called thermosetting . It refers to a class of polymer
materials whose molecular structures undergo a change during
processing, which means, their molecules are linear ones before the
processing and network after that. As they having taken shapes they
can't soften or flow. Bakelites differs a lot from thermoplastics in
properties. Except for recycling, they have merits such as high
strengths, creep resistance, heat resistance, high the precision of
specification processing and electric arc resistance.
Currently
common bakelites include phenolic resins, amino resins, epoxy resins
and unsaturated polyesters. Other ordinary bakelites include
urea/formaldehyde resin, melamine/formaldehyde resin, organic silicon
resins, etc. All of them are thermosets. Bakelites are used in
conditions such as high temperature, attrition, insulation, high voltage
and other harsh conditions. The most common usage for them is serving
as panhandle and high or low pressure Electric Appliance.
We take phenolic resin, the most common bakelite, for example.
Phenolic resins are cheap. Their raw material are abundant. Their
properties are excellent. Therefore phenolic resins progress rapidly.
Now their output ranks the sixth among (and up to 5% of) all plastics
and ranks first among all thermosets. Because they can't be recycle,
phenolic resins get restrictions in manufacturing in the developed
countries of Europe, America and Japan. This makes them occupy a great
portion of Chinese market.
The
modeling of bakelites includes compression and injection, thus most
bakelites are categorized as compressing materials and injecting
materials. Generally the cost of compression molding is lower, but it
needs high intensity of human labor, and the precision and the yield of
products is low. Injection is comparably more costly. But the intensity
of human labor is lower, and the precision and the yield of products is
higher. Therefore it has been gradually replacing compression molding.
II.Molding Machines for Bakelites
Borch
Machinery Corp. has exploited a series of injection molding machine for
bakelites. We have sufficiently taken into account the systematization
and universality of machines. Because the major axis of the screw of
cylinder.is relevantly short, our design is to relocate the the blanking
mouth forwardly to the top of cylinder., as illustrated by Pic 1.
Although the appearance looks a bit more ungainly, this design gains
more universality for the machines comparing to the common molding
machines for thermoplastics. Their structures are as same as the latter.
These two kinds of machines are interchangeable as the cylinder.and
screws are replaced, thus the universality is.
Because the processing of phonemic resins results corrosiveness to the
screws of cylinder and hence shorten the lives of screws and nozzles
made my ordinary materials, our company unanimously employs alloys for
screws and nozzles in order to lengthen the lives of them.
Furthermore, because processing of phenolic resin needs very precise
temperature control (the margin of error should be no mo than
5%),Heating by cylinder.isothermal system and by ordinary mold
temperature controller are generally adopted in our country. The former
is generally less costly. The latter can realize more precise
temperature control but is more costly, because this mold temperature
controller requires bi-section control therefore equates to two
controller.，Currently
the heaters of more advanced controllers of other countries are cooled
by way of heated water. The molding machines series for Bakelites made
by our company adopt the heating by mold temperature controller.
Comparing to ordinary molding machines for thermoplastics, the moving or
fixing template of the molding machines for Bakelites need insulating
boards for molds, as illustrated in Pic 2.
Generally
there is an insulating board at the base, In order to protect the
board, an iron board collocates outside it, which is the configuration
adopted by our company. This is mainly because when molding proceeds,
the temperature is between 160-180℃. The heat should be prevented from
other components of the machine or else it will waste the energy and
impair the performance of the machine. At present there are other
companies in our country which adopt insulating boards made by
high-strength fiber composites and stop the use of iron boards. This can
make machines more good-looking but more costly. The power of heaters
must be enough high, or else it will cost much longer time in heating or
fail to get the temperature desired and thus disable the machine.
Because
the curing process of bakelites produces corrosive gases, there are
exhausting grooves in molds and their inner sides plate hard chrome. But
for some deep cavity products, the gases can't be sufficiently
exhausted through grooves, it is necessary to open the mold to exhaust.
The configuration for opening and exhausting is alternative when the
situation and techniques require. Because there are exhausting grooves
on the bisected final type face of molds, burrs are seen on the bisected
final type face of products. They are thin and can be remove with hands
or air blow.
During injection, it takes a relatively long time for bakelite
materials to be in the state of pressing and curing. Our company's
machines have advantage of being more energy-saving than the machines
made by others because of the use of variable displacement pump system.
During the long time of pressing and curing (it can be regarded as the
colling time for ordinary molding machines for thermoplastics), the
output power of variable displacement pump is relatively low, So
generally molding machines for bakelites are evidently more
energy-saving than ordinary molding machines for thermoplastics.
Comparing to ordinary PQhydraulic system machines, Borch's machines can
save more 40-60% energy, therefore more competent in the market.
III、injection molding process conditions
1、temperature
Melt
temperature The material achieves the melting temperature which is
controled by the barrel temperature, when the material plasticizing's
temperature meets the requirements, the material has good fluidity, the
material should not have solidification phenomenon in cylinder.The
barrel stage control setting should according to the material’s
characteristic to satisfy requirement
Mold
temperature The mold temperature is the main factor of affect the
bakelite product solidification moulding , low mold temperature,long
solidification time, high mold temperature , too quick solidification
reaction rate, the gas which is different to vent, all these reason will
cause the product ‘s organization to be loose, bubble and dark flaws
and so on.
2 、Pressure and speed
Injection
pressure and speed The bakelite contains too many filling component,
the viscosity is big, so there needs high injection pressure , usually
about at 100-170MPa. According to the experience, the injection speed is
about at 3m/min-4.5m/min for best.
（2） Holding pressure and pressing time Holding pressure 、holding
time with product’s density and shrink has great relationship. Usually
,we setting holding time at 5~20S, holding pressure at 30-70MPa.
（3）Backpressure and
Screw speed Screw backpressure should not be too big , in the majority
situations does not use the backpressure .But should pay attention to
that low backpressure may cause measure not steady , plasticizing not
equally When plasticizing ,the screw rod rotational speed cannot be too
high, may be located in 30-70r/min.
3、Molding
cycle The moulding cycle including the closing mole time, injection
and holding time, solidification time (including charging time ),
solidification time is the important time factor.in the entire moulding
cycle .
4、The
representative moulding method for some common bakelite The next table
show bakelite’s injection molding craft that serval kinds of bakelite
we commonly used , because of that the injection moulding craft will
decided by the bakelite grade and produced company , the followings is
just for your reference .
http://www.borch-machinery.com/EN/Article.aspx?id=353
