Bewise Inc. www.tool-tool.com Reference source from the internet.
作者：D-M-E North America, Trevor Pruden
Considerations when thinking of incorporating a hot runner system
Market
trends are continuing to drive toward the use of hot runner systems. Like many
technologies, hot runner systems were created out of a need to do things better,
faster and at a lower cost. Cold runner systems result in a wasted runner every
time the mold opens. When molding small parts, a high-cavitation cold runner
tool produces significant material waste, cutting deep into the bottom line for
the molder.
The expected material cost savings is often the primary driver
for switching to a hot runner system. Lead time reduction is another factor.
Molders and moldmakers are becoming increasingly sophisticated in the technology
and processes they use, generating more interest in hot runners.
While hot
runner systems have significant benefits, it is always important to look at each
individual application, and consider part size, aesthetic requirements,
production quantities, material, additives and more to ensure the proper and
most cost-effective molding method.
Hot and cold runner choices and what to
expect from each
Cold runners
The benefit of a cold runner is its ease
of use. It also can be beneficial if dealing with certain aesthetic
requirements. Cold runners can reduce jetting in parts such as clear acrylic or
polycarbonate used for the transmission of light. Jetting can produce
ribbon-like effects in a part that may become visible.
In addition to
material waste, cold runners’ downsides include added steps to the process
through automation and/or labor. Cold runners must be removed from the parts
while in the mold requiring a robot or person physically separating the part
from the runner. Depending on the size of the runner, scrap material can amount
to hundreds of thousands of dollars, especially if using expensive material such
as polycarbonates. With molders’ profit margins squeezed to the max, hot runners
are often a money-saving solution.
For higher cavitation systems, cold
runners may be undesirable. In these cases, the sprue mass can exceed the
combined mass of the molded parts. When this happens, the cooling time for the
sprue exceeds the cooling time for the molded parts, leaving molders at the
mercy of sprue cooling as a cycle time driver – an undesirable processing
condition to be in. Hot runners address this issue and can effectively increase
the total number of part cavities since the sprue is kept molten throughout the
process.
Cold Runners:
•are ideal for family molds where gates remain
attached
•can have fast delivery times
•are good when molding
symmetrical parts
•can be low in cost to produce
•can be more sensitive
to running semi-crystalline materials
•have relatively low maintenance
•are flexible with gate locations
•work well with edge gate requirements
•have a limit on the maximum number of cavities
•are well understood and
easy to use
Hot Runners
Eliminating the need for ejecting a large
secondary cold runner is important when simplifying mold action as well as
reducing part waste. The key remains keeping the melt molten throughout the
entire runner system so that it doesn’t solidify until it hits the cool walls of
the part cavity.
Internally heated
Internally heated hot runner systems
were the first wave of hot runners to the market, emerging in steps.
The
first solution developed for cold runner problems was to heat the sprue bushing,
producing the most benefits for the production of small parts. The next method
developed was a distributor tube system. With this technology, major runner legs
are heated, and the molten resin moves from there to the unheated vertical legs,
called drops, prior to passing through gates into the part cavity. Around the
same time as the development of the distributor tube system, another group of
molders started placing heated probes into each drop leg, fed by an unheated
insulated runner. Since insulated runners were hard to keep running “on cycle”
due to freeze-off at the gates, the key was to keep the resin molten while
passing through the gates. These three features were later combined to create an
“internally heated” hot runner system.
The internally heated runner system
may be more complex than a cold runner system but it allows increased processing
flexibility for more difficult resins and reduces resin waste. Due to the
annular geometry of the flow paths, internally heated hot runners work well with
resins that are not overly sensitive to shear. Amorphous materials also lend
well to this method, with the exception of materials that are residence
time-sensitive, such as polycarbonate.
To this day, some molders remain
stalwart users of internally heated hot runner systems, or even cold runners,
due to the higher cost and experience needed with an externally heated hot
runner system.
Externally heated
The externally heated hot runner
started as a steel block, machined with an internal passageway for molten resin.
Heaters affixed to the outside perimeter of the steel block heat the manifold
from the outside in, enabling an even heat inside the block. Melt is then
distributed throughout the heated manifold block, fed into a heated nozzle, then
sent into the final gate well or “bubble,” just prior to passing through to the
part cavities.
The benefits of a hot runner system are clear. When designed
properly, the system will achieve maximum mold-processing capability while
effectively eliminating resin waste per injection cycle, a “win-win” solution.
At D-M-E Company, we recommend hot runners whenever possible because of the huge
material savings alone, not to mention the other efficiencies they bring to a
molder’s process.
Some tips: It’s difficult to produce a perfectly flat
thermal profile from the entrance of a hot runner system to the tip of each
nozzle assembly. Some hot runner systems get close, but there are times resins
require a drop in temperature right at the gate to prevent drooling or
stringing. Also, flow rate and injection speed have great effects on resin
processing, as well as on color changes. Hot runner system designs that work
perfectly with one resin, but not another, are not uncommon. A small tweak to
the thermal profile will usually correct these problems. No matter what hot
runner solution a designer chooses, the intended resin and molded part
application should be considered.
Hot Runners:
•lead to significant
material cost savings
•support lean manufacturing
•serve high-volume
needs
•are good for molding with multiple, identical cavities
•create
lower labor and automation costs
•increase production speeds
•increase
process efficiencies
•lower material recycling costs
•are still in the
early stages of full understanding for some molders
•are ideal for
high-cavitation molds
Each runner system, ranging from unheated “cold”
runners to externally heated “hot” runners, has its unique benefits and
drawbacks. Hot runners require a greater initial investment, but also produce
greater efficiencies. The process and speed improvements can pay for a system in
just a few months, making it an attractive option for molders.
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