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What are the four most poorly
designed elements of an injection molding machine? The purging setup,
the nozzle and sprue bushing hemispheres, the shutoff nozzle, and the
decompression or pullback setup—all of which complicate the molding
process and cost productivity.
1. The purging arrangement.
Regardless of the machine’s size or price, to remove the air and
any possible contaminants, we normally place a piece of cardboard in front
of the sprue bushing in the stationary platen. Then we close the protective
doors before shooting resin to avoid getting splattered with the hot resin.
This is never an easy task and requires at best a cumbersome, nasty, and
potentially dangerous procedure. Many molders have at one time or another
been splattered with hot resin.
If the cardboard is not placed properly, resin can get into the sprue
bushing and freeze or flow onto the bottom guard. Cleaning the bottom
guard is easy compared to removing frozen resin from the sprue bushing.
Assuming all goes well and the resin is projected to the cardboard, this
resin becomes scrap.
2. The nozzle and sprue bushing hemispheres. Most, if
not all, molders have had radii on the nozzle and sprue bushing that do
not match. When this happens the result is resin all over the stationary
platen and perhaps a plugged sprue bushing. With an insulated runner this
requires removal, which is time consuming. Nicked or damaged hemispheres
also produce the same problems or worse—such as a small leak that
is not initially detected. This can be nasty to find and can produce shorted
parts, as well as accumulated resin if the leak is not found quickly.
3. The shutoff nozzle. Shutoff nozzles are used to prevent
drooling and to rotate the screw during clamp movement to increase recovery
capabilities. There are a variety of designs that have different problems.
The most common problems are leaking, maintenance requirements, excessive
pressure drop, dead spots, fire hazard, and high cost. Not all shutoff
nozzles suffer from all of these problems but if you are using a shutoff
nozzle, you can undoubtedly select a few from this list.
4. Decompression or pullback. As shutoff nozzles have
not solved the drooling problem, decompression is generally used. It is
accomplished by moving the screw rearward for a distance—velocity
tends to pull the resin upstream to eliminate drooling. The normal problem
is splay, trapped air, or gas that is introduced into the resin.
Addressing the Problems
R. Dray Mfg. Inc. has developed a device called the Coupler, which is
designed to address the above problems. A summary of the technology is
presented below. Since the device is patented (U.S. #6413076), more information
is available online from the U.S. Patent and Trade Office (USPTO).
The Coupler improves all of the above lacking designs at about the cost
of a sprue bushing and nozzle set. As most injection units have sprue-break
capability, machine changes are not necessary.
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Robert F. Dray is the founder and owner of R. Dray Mfg.
Inc.
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- The Coupler purging: The machine carriage
moves to the sprue break position. When the Coupler nozzle is
inserted into the Coupler extended sprue bushing, it remains inserted
until mold change. The purge is contained at all times, eliminating
the need for protective guards. Additional barrel support is provided,
also eliminating the need for barrel supports. Leakage is eliminated
by clearance and length; seals are not required. If shutoff is
not required, then purge is the only position needed for sprue
break or rearward carriage movement. A second position is needed
if shutoff is required. In machines without a linear transducer
on the carriage, a second limit switch is required for both purge
and shutoff.
The purge configuration shown is only the canister type; other
configurations can form the purge into a regrind that is easily
fed into the grinder or directly into the hopper. The purge savings
alone can easily pay for the unit in a short period of time.
- The Coupler injecting, fully open: This is
the normal inject position. The carriage is in the full forward
position. This is the same with any standard injection unit setup.
The Coupler eliminates the costly problems associated with present
nozzles and tips and operates with normal machine function. The
sprue break positions of purge and shutoff are only used when
required. There are no hemispheres to cause leakage. Also, there
can never be short shots caused by damaged nozzle tips because
nozzle tips are eliminated.
In relatively new injection machines equipped with linear transducers
on the carriage, decreasing the entry clearance can effectively
control viscosity. This viscosity can also be programmed in the
control on newer machines to lower viscosity (or raise melt temperature),
initially by partially closing and then moving to the fully open
position. This profiling of viscosity can be used to assist in the
filling of molds that have a difficult configuration.
- The Coupler injecting, partially closed (decreased
viscosity); and in a closed or shutoff position: This
is the normal sprue break. In this setting the injection unit
carriage rearward movement, instead of breaking off the sprue,
initially decompresses the resin to eliminate drool and close
(or shut off) any resin downstream flow, allowing for recovery
to start if desired. This movement is not required, only available
if needed. This shutoff is essentially an added feature that
costs nothing. It does not leak and does not add resistance
to flow as the flow channels are designed to eliminate pressure
drop and material hang-up.
If machine cycle time is increased by recovery time, the Coupler
shutoff can be easily initiated. One problem is that many machines
do not have the hydraulic oil flow capacity to move the clamp and
rotate the screw at the same time. The addition of a pump to provide
this additional hydraulic oil capacity is normally not difficult.
In electric machines, simultaneous movement is not a problem.
Adding a longer screw rotate time is desirable in any injection
machine using maximum rpm. Reducing rpm lowers melt temperature
and can decrease cycle time. If this decrease of rpm causes fill
problems, it is easy to increase the rpm and decrease rotate time.
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Contact Information
R. Dray Mfg. Inc., Hamilton, TX
William J. “Bill” Tobin
rdraysr@aol.com |
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