The screw provides for comparatively rapid heat exchange between the “hot” barrel
and the relatively “colder” material by continuous exchange and rearrangement of material in
the screw channel.
It usually is a screw with three zones and has a length to diameter ratio of 20:1+ 10%.
Shorter screws do not provide an adequate quality of the melt. With longer screws 24:1 as the
The utmost one has to anticipate the degradation of a number of engineering plastics from too long residence time.
This is the reason for using screws with an L/D ratio of 22:1 to 26:1 only in fast running
molding machines e.g. in the packaging sector. Then they are called “packaging screws”. They
are often equipped with mixing sections or with a combination of shear and mixing sections and eventually with an efficient feeding zone.
Another screw design known from extrusion is now introduced to the injection molding
technique as a barrier screw. The melted material is separated from solid pellets beginning at the point of melting and both are carried in two parallel channels.
Table of Contents
Standard Screws for Thermoplastics
Modern screws for thermoplastics are generally designed as the one depicted. The terminology connected with screw design is presented and explained with essential dimensions are flight depth and the channel depth ratios.
Guidance for flight depth values. Smaller flight depths are applicable for crystalline thermoplastics. They are not suited for rigid PVC. An increase in the length of the
standard screw cannot be expected. Such a development would be faced with distinct
disadvantages such as the hazard of degradation from extended residence time and intense
exposure to shear and heat. Modern screws with an L/D ratio of 20:1 provide an adequate
output in almost all cases.
The standard three-zone screw is not designed for efficient mixing. It is only conditionally qualified for compounding or dry coloring of plastics. If such duties are desired in
addition to plasticating and injection, then shear and mixing elements must be employed,
which causes a significant improvement in dispersing additives.
A shear element should always be placed near the end of the compression section and the mixing elements within the range of the metering section.
Assuming that
L/D = L o / D o and p/D = p o / D o
and melt temperature, dynamic pressure, and heat-flux density at the inside barrel wall are
constant, then
flight-depth ratio and residence-time ratio are
h/h o = (D / D o ) 0.74 = t r / t ro
the circumferential speed ratio is
n /n o = (D / D o ) 0.74
The molding with large screws and consequently the time needed for feeding and
plasticization have created a frequent bottleneck. This is a result of the steady development of
plastic materials and processing techniques. Presently the simplest and most practical solution is the use of a larger screw with a sufficient diameter. The nomograph of Fig. 45 can assist in the selection of such a screw.
With feeding travel of more than three times the diameter, the homogeneity of the melt
is incompatible with high-quality demands. There is an additional hazard of getting air into the
melt, which results in unavoidable surface blemishes.
Special Screws for Thermoplastics
Screws with a relatively shallow flight depth are utilized for processing Nylon, PBT,
PET, and POM (flight depth from Fig. 44) while screws for CA and CAB can eventually be
modified in the opposite manner. Heavy-duty screws for processing especially PS, PE, and PP
for packaging parts are extremely long screws with L/D 25:1.
Barrier Screws
The barrier is a design known from extrusion, which separates pellets and melt and
conveys them in two parallel channels beginning at the point of melting.
Starting at the theoretical point of melting a second, at the first narrow channel is added.
The melted material flows through a small gap between flight and barrel wall into this channel
and is collected there. After the whole material has been melted the channel that has conveyed
the solid pellets disappears. After this, the melt is intensely mixed by a shear and mixer
combination, which consists of also called the “Maddock” section and a diamond-shaped pin
mixer.
Striking, however, is the gained homogeneity of the melt. It exceeds by far that of the
standard three-zone screw. This opens up new prospects for the molder concerning quality and
economics. Especially if recycled material is added, the compounding can immediately be
done by the molding machine and the intermediate step of separately compounding reused
material is saved. Thus, not only costs are cut but the material is preserved from an additional
processing step.
Screws for Rigid PVC
While plasticized PVC is commonly processed with standard screws, special screws are
generally used for rigid PVC. The sections along the screw remain the same as but
the total length and flight depths are different.
For processing PVC, it is customary to plate the screw and screw tip – occasionally also the barrel and the barrel head if no bimetallic lining is preferred – as protection against corrosion. Suitable materials are chromium and nickel.
Steel grades have to be selected which provide good adhesion of the protective plating. Heating is accomplished with electronically-controlled band heaters as it is with other thermoplastics. It is normally not necessary to heat the screw.
Screw for vented Barrels
Vented barrels have been employed with Acrylics, CA, CAB, and ABS to extract water that
is adsorbed or absorbed in the molecular structure. Usually, no vacuum is used and the water
escapes as vapor through the vent into the open. A number of difficulties in practical
operations led to a retreat from degassing. The problems are of the following kind:
- degraded depositions in the pressure less degassing region leading to visible flaws in brightly colored moldings.
- difficult change of materials, products of degradation hard to remove.
So far, degassing screws for vented barrels have been built with diameters between 25 to
170mm.
There are three different concepts:
- Two standard screws of medium length are arranged in tandem, L s /D = 2 × (13 to 16):1 = 26 to 32:1;
- Screws with starve feeding (with decompression zone);
- Short screws L s /D = 20:1 (Fig. 47) (L s = effective screw length)
Long degassing screws result in too long residence time of the material in the
plasticating unit and with this to partial degradation of the plastic (e.g. Nylon, PC, PET,
PBT). They are not suitable for these materials and have disappeared from the market.
Degassing units with starving feeding makes it possible to adjust the flow of material in the first screw to the one in the melt-conveying second one by an appropriate setting of the metering device e.g. a vibrator.
It feeds the screw only with as much material as the second screw can handle. This ensures that no melt can escape through the vent. Because of its manufacturing costs, this solution is not offered anymore on the market.
Degassing screws exhibit higher wear than standard screws because of their
unfavorable design in this respect. In the open area, there is an additional strain on the material
from water vapor and volatile ingredients. For this reason, these screws should have worn-
resistant surface. In some cases, a chrome-plated surface has proved particularly suited.
Screws for Thermosets
Screws for processing thermosets have less flight depth and a smaller channel-depth
ratio than screws for thermoplastics. They are used without a nonreturn valve. All
commercially available screws show similar features.
Their design should prevent heating the curable material unduly by shear to avoid a reaction in the flights of the screw. Therefore screws with the flight-depth ratio of 1:1 to 1:1.3 are employed most of the time.
Screws for thermosets are generally shorter than those for thermoplastics. The L/D ratios for
free-flowing materials are 12:1 to 15:1 for already plastic polyesters (BMC, DMC, etc.) a ratio
of about 10:1 is sufficient.
Because these screws must operate without non-return valves, there is a comparatively
large back flow of material into the rear section during injection and holding pressure stage.
Therefore flight depth and width are of particular importance.
The flight is wider than that of screws for thermoplastics. It may be 0.15 to 0.2 times the screw diameter. The remaining smaller channel cross-section impedes the backflow of the plastic and the wear resistance of the thicker flight is relatively good.
Screws with a slight increase in flight depth towards the tip exhibit little backflow and
therefore they have a lesser tendency to wear.
Wear and wear protection
Causes of wear
When PVC was processed, however, shortcomings in the service life of parts of the
plasticating unit were observed. When the number of new plastics and those with an increasing variety of fillers grew, difficulties became clearly more severe.
At the same time, the efficiency of injection molding machines was steadily increased by raising screw speed and torque. All effects combined resulted in more and more in the breakdown of plasticating units. Standards steels have to be suitable for processing standard plastics.
Local weak points in the system are:
- Non-return valve (primarily subject to wear),
- Nozzle (subject to wear),
- Connections between barrel, barrel head, and nozzle (sealing surfaces),
- Screw,
- Barrel.
Wear protection of plasticating units
- Selection of suitable materials for plasticating components and their appropriate treatment,
- Favorable design of components.
Temperature injection speed, pressure and residence time have to be chosen or
optimized according to criteria different from those of wear resistance. Therefore it is
important to create a safety margin against wear as great as possible by means of inventive
design and skillful material selection.
Preventive wear protection by design is particularly related to screw geometry and configuration of barrel head, non-return valve, and nozzle. The planeness of sealing faces at the head is an absolute must.
Screws for thermosets without non-return valve are manufactured with angles at the tip of
about 60-90º.
Open nozzle for processing thermosets are made of alloy steel because of their wear.
For larger nozzle, inserts of cemented carbides are even used.
Nitrided through-hardening steels for barrel without bimetallic lining have to have a
the high amount of alloying constituents – 12 to 17% chromium as a rule – to provide adequate
corrosion resistance besides hardness. Although higher carbon content increases hardness and
abrasion resistance, the formation of chromium carbide ties up the effective portion of free
chromium needed for corrosion resistance.
Merely a compromise is usually attainable. Such version are employed only occasionally because of their tendency to fracture and their use is limited to small diameters.
Bimetallic barrels are gaining increased significance. They are a combination of an outer steel structure with a uniformly hard internal alloy lining which is typically 1.5 to 2mm thick. It contains carbides embedded in a tough matrix.
Three different materials employed for bimetallic barrels, tungsten carbide composites, chromium-modified iron-boron alloy and nickel alloys. Tungsten carbide provides the best overall resistance to abrasion and corrosion.
Screws are commonly made of medium-carbon alloy steel such as the ANSI 4140 steel
or its foreign equivalent. It is hardened to about 30Rc.
Screws with improved resistance to abrasion can be made of through-hardened
vanadium-containing tool steel with about 56Rc. Its use is limited to screws of less than 40mm
diameter because of costs and brittleness.
Barrel heads are less exposed to abrasion than screws. Ion-nitrided high-alloy
chromium steels or chrome-plated nitriding steel offer sufficient protection.
Non-return valves again are more imperiled especially by the wear between the sliding ring
and the fins of the tip or by the axial motion of the sliding ring in the barrel. Chromium-alloy
steels provide the necessary resistance to corrosion.
An ion-nitriding is rarely needed. The sliding ring itself should have high hardness, wear-resistance and still be sufficiently tough.
Wear protection by repair:
Worn-out units of “standard” nitriding steel can be used again by providing them with a
liner. Rotational- cast liners are shrink-fitted into turned-out old barrels. The old barrel part
remains a weak point, in which the gaps at the sealing surfaces, where plastic can degrade and corrosion take place.
Wear protection by design:
Deposits of plastic in the barrel and friction of steel against steel or against plastic
under high pressure should be avoided. Necessary steps are:
- Producing contact pressure between barrel and barrel head by using a flanged ring with bolts that permit contact pressure of 400 MPa after tightening with a torque wrench.
- Avoiding dead corners in the barrelhead;
- Designing an open cross-section in the non-return valve of 80 to 120% of the free annular area of the front end of the screw;
- Applying armored flights on the screw tip
- Allowing for an adequate length of the valve ring (about one D for screws up to 70mm diameter and 0.7 D for screws with larger diameter);
- Providing a length of the feeding section preferably 10 to 12 times the diameter that no unmelted or few partially melted pellets are conveyed into a section with increasing pressure.
Screw Tips
The highest pressure occurs at the screw tip, which is the front end of the screw.
Therefore it makes sense from processing viewpoint to prevent the back flow of the material
into the rear flights by means of closing elements. This is especially important during the
injection and holding pressure stages.
It is advisable to use a screwed-on tip with a larger diameter than that of the screw root
at the foremost end of the screw. The narrow gap with the barrel causes a pressure
rise that restrains the back flow. Such tips with an angle between 60 and 90º permit particular
careful processing. A complete shutoff, however, cannot be achieved with such tips.
Rigid PVC without plasticizer calls for an open design of the screw tip.
It should promote good flow of the melt and impede backflow during injection and
holding pressure stages.
In output-promoting designs, such helixes prevent deposits and restrain backflow
during the injection. Such screw tips should be protected against corrosion just like the screw.
Non-return Valve
A non-return valve is a component at the foremost section of the screw that prevents the
backflow of the plasticated material during the injection and holding pressure stages. It can
accomplish this duty best if it produces a high-pressure loss of small free cross-sections which
can be closed rapidly. illustrates the principle of the ring-type non-return valve, which is
most common.
Generally non-return valves should be dimensioned in such a way that the free cross
section is not smaller than 80 % (120% if possible) of the free annular area at the foremost end
of the screw.
In the closed position the ring rests tightly on the seats, the conical contact surfaces
forming an angle of 45 to 60 with the axis.
During feeding, the ring is in the open position and rests against three or four ribs
which are attached to the tip like fins.
The following criteria have to be considered for the design:
- Refraining of sharp turns of the melt flow, sufficient length of the ring for its sealing function (about one D; 0.6 to 0.8D for diameters more than 70mm)
- Rapid closing to avoid leakage flow;
- Eventual hard facing the flights at the tip, which are in sliding contact with the ring.
Barrel
The barrel is a metallic tube that surrounds the screw. It forms the outer boundary of the
screw channel as in an extruder. For injection molding one can assume that the major portion
of the heat that must be applied to the plastic is supplied by the barrel.
To do this, the barrel is equipped on the outside with electrical band heaters. The watt density of commonly used ceramic heating elements is about 6 to 8 W/cm². In the nozzle and barrel head section, mica-insulated band heaters are employed to accommodate smaller diameters; they have a watt density of about 4 W/cm².
Since it is desirable to avoid melting the plastic prematurely in the feeding throat and the first few flights of the screw, this section has to have provisions for cooling.
The barrel should be easily disassembled for a rapid change of screw or cleaning procedures. It
is important for assembly or disassembly that only few bolts are used.
The geometry of the throat has a distinct effect on the output. The opening should have
a length of one to two diameters and, if possible, leave the screw partly covered opposite to the direction of rotation. This covering increases the feeding rate. Occasionally barrels, mostly
with a rectangular throat, are supplied with a recess (feeding pocket) adjacent to the throat in
the direction of rotation.
The barrel head is considered part of the barrel. The assembly of both is of some
importance. The connection and transition contour to the nozzle must be streamlined to avoid
hang-up of material in this section.
The mating surfaces have to remain leak-proof, even under maximum pressure. A common solution is shown in. The nozzle is either screwed into or, less often, flanged onto the cylinder head.
Metallic contaminants in the material can cause breakdowns of production and damage
to screws, barrel, non-return valves, nozzles, runner systems, hot runners, mold surfaces, or
directly to the molding. Metals arresters (magnets) mounted at the transition from the feed
hopper to the feeding throat offer a high degree of safety against getting the contamination into the screw.
Conclusion
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