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Delrin® Technical Guide
Delrin® Acetal Homopolymer
Hot Runner Manual
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Contents
Introduction….................................................................................................... 2
1. Gate designs…............................................................................................. 3
1.1 Indirect hot runner gating using cold sub runner…...................... 3
1.2 Direct hot runner gating of the part…........................................... 4
2. Hot runner selection…................................................................................. 4
2.1 Nozzle design….............................................................................. 4
2.1.1 Hot runners with open nozzle or torpedo…............................... 4
2.1.2 Hot runners with valve gates (shut-off nozzles)....................… 5
3. Hot runner manifold….................................................................................. 6
4. Temperature distribution inside the hot runner…....................................... 6
5. Temperature control of the cavity…............................................................ 7
6. Temperature settings for Delrin®….............................................................. 7
7. Safety considerations…............................................................................... 8
Conclusion…...................................................................................................... 8
Delrin® | Technical Guide
Introduction
Maximizing productivity with injection molding is an important factor in determining part cost. One way to
improve productivity is using a hot runner system. The decision when to use a hot runner system is mainly
influenced by “yield” as hot runners can add complexity and cost to molds and will require additional
maintenance. Due to continuous improvement in hot runner designs, a robust molding process is possible
even with traditionally more thermally sensitive thermoplastic resins.
Delrin® acetal homopolymer is a high performance semi-crystalline polymer. Versus alternative plastic types,
Delrin offers superior mechanical performance, such as higher tensile strength, stiffness, creep and fatigue
resistance, and significantly higher impact resistance, without need of glass or other reinforcements. The
combination of these properties in a single material allows for thinner, lighter-weight parts to be developed
and, subsequently, for shorter molding cycles to be achieved, delivering potential cost reductions. Typical
applications of Delrin include, but are not limited to, actuator gears, push buttons, clips, valves, springs,
handles and conveyor belts across a range of industries, including industrial products, consumer goods and
medical device components. Given the wide range of uses, Delrin is often selected for components that are
manufactured by use of hot runner systems.
The hot runner design requirements for semi-crystalline polymers, such as Delrin® (POM), differs from
amorphous polymers given the difference in their softening, melting and freezing behaviors. Amorphous
materials gradually soften with slowly decreasing viscosity from the solid state (Tg) to the processing
temperature. This behavior provides a wide temperature range to control viscosity when melting or freezing
the resin. Whereas a semi-crystalline polymer becomes fluid with a relatively low viscosity at a defined melting
temperature (Tm). In the same way, a semi-crystalline resin freezes again at a defined freezing temperature,
where no flow is possible. As a result, the processing window for the melting and freezing of semi-crystalline
resins is relatively narrow, which needs to be considered when designing a hot runner system.
This brochure provides guidance on basic gate design and hot runner selection for the robust molding of
semi-crystalline resins like Delrin® acetal homopolymer. You will learn that the best suited systems for Delrin
typically:
•
Use a small sub-runner where possible and implement a semi-crystalline gate design
•
Employ hot runner systems with valve nozzles if direct gating is needed
•
Utilize a titanium or DuPont™ Vespel® cap at the nozzle tip versus a self-insulation design
•
Ensure there is good thermal balance through the entire system to allow optimal temperature settings to
be applied
•
Minimize risk of material stagnation in all areas of the hot runner
Other design options are also described in this guide, with the understanding that certain constraints may
not allow the implementation of all these optimum design recommendations.
Delrin® | Technical Guide
1. Gate designs
When using a hot runner system in a mold, there are essentially two different gating scenarios possible:
•
Indirect hot runner gating of the part using cold sub runners
•
Direct hot runner gating of the part
The advantages and disadvantages of the different hot runner gate designs, as well as the preferred solution
based on the part design and material choice, are described in the following chapter.
1.1 Indirect hot runner gating using cold sub runner
Whenever possible, it is recommended to use a hot runner combined with a cold sub runner for molding
Delrin® and other semi crystalline resins. This combination requires less precise thermal control around
the nozzle tip and therefore contributes to a more robust process. With indirect gating, it is recommended
to move the nozzle tip back from the parting line to avoid heat loss at the nozzle tip area once the tool is
closed. Especially with parts requiring a long hold pressure time and a good packing, indirect gating is highly
recommended as the freezing time of a cold runner is easier to control versus a hot tip. For the cold subrunner, a cold slug trap in front of the hot tip should be provided to catch any frozen or degraded material,
preventing it from entering the cavity.
Cold runner and gate designs for Delrin should follow the guidelines for semi-crystalline resins (left side of
Figure 1). It is recommended that the gate diameter (d) should be at least half of the wall thickness (T) of
the part. The diameter (D) of the tunnel next to the gate should be at least 1.2 times the part thickness. The
amorphous gate design shown on the right side in Figure 1 is not recommended for semi-crystalline resins due
to the risk of an early freezing and therefore an insufficient hold pressure time. This can result in uncontrolled
shrinkage that causes voids and/or sink marks, low mechanical performance, and dimensional problems.
Using a well-designed hot runner system and optimum process settings for
Delrin® can greatly enhance productivity and reduce overall manufacturing costs.
Figure 1: Indirect hot runner gating using a cold sub runner. Tunnel gate for semi-crystalline resins (left), tunnel
gate for amorphous resins (right)
Delrin® | Technical Guide
1.2 Direct hot runner gating of the part
If it is required to gate directly onto the part, it is important to consider the choice of gate location carefully.
Try to avoid cosmetic surfaces where possible, since the material will be more stressed around the gate
location and more prone to markings. A similar approach should be taken if the area will be subject to
mechanical stresses or is needed to act as a sliding or bearing surface since mechanical properties and
surface smoothness will not be optimal around the gate location. Beyond this, the choice of nozzle design is
key in determining the success of a direct-gated system. We will cover this in the next section.
2. Hot runner selection
2.1 Nozzle design
There are two basic types of designs for hot runner nozzles that are widely used in injection molding:
•
Open nozzle (includes also open nozzle with torpedo and internally heated nozzles)
•
Valve gate nozzle (shut-off nozzle)
When molding semi-crystalline resins like Delrin®, the nozzle design should allow a precise freezing and
therefore a controlled separation between molten material in the nozzle tip and frozen material in the cavity.
Poor nozzle design often leads to freezing of the material in the nozzle or, conversely, to stringing and
drooling. Both result in production and quality issues.
To avoid this, attention needs to be given to the thermal insulation between the hot nozzle tip and the mold.
Insufficient thermal insulation between the nozzle tip and mold generally leads to unacceptably high temperature
settings of the hot runner and therefore to material degradation. Regardless of the nozzle type used, careful
attention should always be paid to ensure optimal thermal insulation is achieved. This topic will be revisited
several times throughout this guide.
2.1.1 Hot runners with open nozzle or torpedo
An open nozzle design, as shown in Figure 2, offers good flow properties and is often used when molding
highly filled and abrasive materials. This design is not recommended for unreinforced materials as the
freezing behavior of those materials limits precise separation of the bushing/runner and the molten material
at the nozzle tip. Thus, stringing in the gate area can occur during mold opening. For an open nozzle design,
it is recommended to always use indirect gating, with a cold sub runner equipped with a cold slug trap (as
covered in Chapter 1.1). For highly filled resins, it is recommended to use an exchangeable nozzle tip for
ease of maintenance.
Figure 2: Open nozzle design with short bushing (left) and long bushing (right)
If direct gating with an open nozzle design is needed, a system equipped with a flow restrictor will help to
minimize problems with stringing. A torpedo is one such example of a flow restrictor. This is an internal
element that helps to break the flow of the melt and create a more homogeneous filling of the part. However,
for direct gating of parts with high surface aspect requirements, there is a risk of uncontrollable flow marks
around the gate depending on the design of the flow restrictor being used. Various open nozzle types are
shown below in Figure 3.
Internally heated torpedo nozzles, as in Figure 3 (a), are not recommended for molding semi-crystalline
resins, like Delrin®, because of the risk of stagnation of the material on hot steel surfaces. To counter this, it
is good practice to add a separate cooling circuit around the nozzle to be more independent from the mold
temperature in controlling the temperature around the nozzle and the nozzle tip.
Delrin® | Technical Guide
Figure 3 (b) shows a nozzle design with a flow restriction at the tip. Again, this is not ideal due to concerns
with material hold-up in the tip area. A better solution is a non-heated torpedo nozzle designed, as in
Figure 3 (c), to avoid such flow restrictions.
Another key point to highlight, is that when the nozzle tip is self-insulated by the molding resin, there is a
higher risk of stagnation and subsequent purging of degraded resin into the part. This can cause surface
defects around the gate and black specks in the finished parts. To avoid the stagnation and hold-up spots,
customers have good experience using titanium or DuPont™ Vespel® caps at the nozzle tip, as shown in
Figure 3 (d). Another advantage of non-self-insulating nozzle tips is improved maintenance in the case of
abrasion and corrosion. If the part is gated on a surface which does not allow an exchangeable nozzle tip, a
self-insulating nozzle can still be used provided the above recommendations are followed.
Internally heated
torpedo nozzle
design with selfinsulating tip
Flow restriction at
self insulating tip
Torpedo with self
insulating tip
Torpedo with
titanium/DuPont™
Vespel® cap at
nozzle tip
Figure 3: Open nozzle designs
2.1.2 Hot runners with valve gates (shut-off nozzles)
Hot runner systems with valve gates are more commonly used for the molding of precision parts made from
Delrin®. Especially for multi-cavity tools, with more than one nozzle, it is strongly recommended to use a
valve gate system to ensure balanced filling of all cavities. Furthermore, when molding bigger parts where
the pressure drop for filling is too high, a valve-gated hot runner system with a selected number of nozzles
allows a segmented filling and stable packing by opening the valves in cascade.
Depending on the nozzle design and the thermal insulation between nozzle and tool, a valve-gated hot runner
nozzle may lead to a limited hold pressure time. This is due to an early freezing at the valve pin guide before
achieving the sealing time of the part. In these instances, often a cold deformation occurs when the needle
is closing, or a remaining pin is visible on the molded part. If this happens, the thermal insulation of the hot
runner nozzle needs to be improved to avoid part breakage in the gating area as well as dimensional
instability and an increased number of voids of the molded parts.
Cylindrical guidance of the valve pin is always recommended when molding Delrin and semi-crystalline
resins. With a conical shape, there is a high risk of deforming the sealing surface especially with reinforced
resins. For thermally sensitive resins, which are critical to hold up time, an adapted manifold design with an
improved purging behavior is preferred (see Figure 4). This is covered in the next chapter.
Delrin® | Technical Guide
Figure 4: Valve gate design, standard
manifold design (left), improved design
minimizes stagnation (right)
3. Hot runner manifold
If there is more than one nozzle used in the tool, the melt is transported to the hot runner nozzles by the
manifold system. In general, the channels of the manifold should be as smooth as possible to minimize melt
sticking to the tool steel. To avoid corrosion inside the hot runner, a steel with a higher chrome content
is preferred.
To achieve uniform filling of all cavities, it is recommended to use naturally balanced systems. All channel
corners should be flow optimized to avoid hold-up spots and minimize pressure drop in the manifold. Sharp
corners in manifolds result in high shear stress, potential degradation of the material and increased abrasion
for reinforced resins. Hot runner suppliers offer a wide range of flow optimized geometries. Figure 5 shows
channel corner designs which are available from hot runner suppliers.
Figure 5: Manifold channel corner design
The design shown in Figure 5 (a) is not suitable for molding semi-crystalline resins. In addition, the purging
behavior is very limited when changing the color or the material. An optimum flow design which minimizes
shear, pressure drop and the risk of hold-up spots is shown in Figure 5 (c). This solution is the most
expensive but offers the best flow properties and purging behavior. In Figure 5 (b) a compromise between
design and cost of the hot runner is shown.