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Delrin® | Technical Guide
Molding Tips to Optimize
Production of Components Made
®
with Delrin Acetal Homopolymer
Delrin® | Technical Guide
Delrin® acetal resins are molded throughout the world in a wide variety of types and
designs of injection equipment. The guidelines below will help deliver the excellent
mechanical performance achievable with Delrin resins.
Safety Considerations
Read the Safety Data Sheet(s) for the specific product(s) being processed. While processing Delrin, all of the
potential hazards associated with thermoplastic elastomer resins must be anticipated and either eliminated
or guarded against by following established industry procedures.
Hazards include:
•Thermal burns resulting from exposure to hot molten polymer
• Fumes generated during drying, processing and regrind operations
• Formation of gaseous and liquid degradation products
Never mix or process acetal with halogenated polymers or chemicals such as PVC or flame-retardant resins.
The HCl or HBr given off will cause rapid degradation of Delrin. Safety Data Sheets include such information
as hazardous components, health hazards, emergency and first aid procedures, disposal procedures and
storage information.
Note: Adequate ventilation and proper protective equipment should be used during all aspects of the
molding process. Refer to the Delrin Ventilation Guide for more detailed information. Refer to the Delrin
Molding Guide for more information on safe handling.
Melt Quality
Delrin is stable in well-maintained and properly set injection units and hot runner systems. If a small amount
of melt degradation does occur, it will not lower molecular weight or cause brittleness (which is the case
with almost all other polymers).
Melt Temperature: Long residence times (also known as hold-up time) or too high melt temperatures will
cause degradation. Figure 1 illustrates the effect of high temperatures on the thermal stability of Delrin. The
recommended melt temperature for Delrin is 215+/-5ºC (highlighted area) at which the melt for standard
grades will remain perfectly stable for over 30 minutes.
Delrin® | Technical Guide
Nozzle/hot runner temperature: The outer skin of molten polymer remains stationary against the
inside metal of the machine nozzle or hot runner. This explains why it can take many shots to switch over to
a different color.
For best results, machine nozzles and hot runners used with Delrin® should be set to 190ºC. If, due to other
machine constraints, they need to be set higher than 190ºC in order to run, then degradation may occur after
a few hours of operation and may require more frequent cleaning. Please consult the Hot Runner Guide for
more detailed tips on optimal use of hot runner technology with Delrin.
Foaming Test: One can do a qualitative foaming test to determine the melt quality of Delrin. This test
is particularly useful to observe the effect of pigment or colorants. Collecting a molten purge in water and
observing how it behaves will give one an indicator of the thermal stability resulting from the addition of the
pigment or colorant. A good molten melt that has no gas bubbles is denser (Figure 2) and will sink when
molten. A degraded melt has gas bubbles in it and will float when molten.
When coloring Delrin with a masterbatch pigment, selection can have a big influence on melt quality and
a few pigments and additives in masterbatches are known to cause melt degradation in Delrin. Their use
should be avoided. A resin that foams when molten may quickly cause mold deposit and may also
accelerate screw deposit which can lead to black speck contamination.
Consult your Delrin representative on appropriateness of a masterbatch for use with Delrin.
Optimal Packing
The high crystallinity of Delrin requires the compensation of volume drop during solidification. To prevent the
generation of voids and control shrinkage, the gate and runner should be large enough to allow the additional
feed of resin during the packing time.
Theory: Delrin is one of the most crystalline of the semi-crystalline thermoplastics (at almost 60%
crystallinity). As a consequence, as shown in Figure 3, the difference between its melt density (1.17 g/cc)
and its solid density (1.42 g/cc) is relatively high (18%). In other words, the crystallization (solidification) of
the polymer leads to a large volume drop. This drop should be compensated by the injection of additional
molten resin into the mold cavity, during the entire packing time to produce a solid part and without voids
and uncontrolled shrinkage. In addition, at the end of a correctly set and efficient packing phase, Delrin does
not need further cooling time as the whole part is crystallized and solid.
Delrin® | Technical Guide
Packing Time: However, care should be taken to ensure that the maximum part weight corresponds to
the optimum packing time for the part thickness at the gate (Figure 4). It is important to note that the
relationship between optimum packing time and part thickness is not linear. As the part thickness increases,
the packing time per millimeter also increases (Figure 5).
Delrin® | Technical Guide
Gate Design: To achieve maximum part weight, there are some fundamental differences in gate and
runner design versus those used for amorphous materials. While the major risk of a bad process with
amorphous materials is over-packing, with semi-crystalline materials it is the problem of under-packing,
which causes excessive shrinkage. Figure 6 below shows the difference between a tunnel gate (left) and
a conical gate (right). For semi-crystalline materials like Delrin, the tunnel gate is appropriate. In a conical
gate, which is appropriate for amorphous resins, Delrin would crystallize in the narrow tip before packing
is complete in the part. Poor packing results in poor mechanical performance (by up to 30%), warpage and
uncontrolled shrinkage.
For suitability with Delrin and most highly crystalline materials, the tunnel gate system on the left in Figure 6
illustrates the following key design criteria:
• Position gate in thickest area of the part
• Diameter of the gate d must be at least half the part thickness, T
• The length of the gate must be shorter than 0.8 mm (0.03 in) to prevent premature gate freezing
during packing
• Diameter D of the tunnel next to the gate must be at least the part thickness T+1mm
Dimensional Precision
Eject the part hot and let it cool outside of the mold. An adequate packing time will secure both part
performance and dimensional consistency from shot to shot.
Theory: Amorphous materials require short packing times but long cooling times before ejection. This can
lead to longer total cycle time compared to Delrin. As shown above, the high crystallinity of Delrin requires
continuous feeding of molten resin to fully pack-out the part during the crystallization phase (11% volume)
shown in Figure 3. The PVT diagram also shows a thermal shrinkage phase during which a further 7%
volume reduction (~2% linear dimensions) also occurs.
In order to obtain even shrinkage and avoid deformation, the part must be allowed to shrink evenly, without
constraints, during cooling—this is best achieved by ejecting the part from the cavity as soon as possible
after pack-out allowing it to cool slowly in air.
We recommend ejecting onto a conveyor before transferring into a bulk package. Ejecting into a box or bag
directly will result in a temperature gradient through the height of the container and may result in
dimensional differences. This sequence also leads to a shorter overall cycle time versus amorphous materials.
Note, that in the event of different thicknesses throughout the part, if crystallization is compensated for by
an efficient packing phase, the level of thermal shrinkage will be independent of the thickness of the part.
Delrin® | Technical Guide
Example: The part shown in Figure 7 is used for wear and friction testing, for which it must be as
dimensionally precise as possible. The part was molded and measured under varying conditions. An
optimum packing time of 15 seconds (highest part weight) and the shortest cooling time are shown to
result in the least warpage.
As shown in Table 1, similar results were obtained when seeking the optimum conditions for highest
dimensional precision (outer diameter, OD).
Once more, best results are with an optimized packing time and no additional cooling time beyond screw
retraction time. Note that when the part is constrained in the mold, such as when using a typical amorphous
cooling time of 15 seconds, after optimum packing, an increase in variability of the OD by +/- 0.07 mm
results. The constraints of mold lead to uneven shrinkage with visible and measureable symptoms of
warpage and deformation.
Table 1. Dimensional precision vs. packing and cooling times
Trial 1
Trial 2
Trial 3
Trial 4
Packing time, sec
Cooling time, sec
Overall cycle time, sec
Outer diameter, mm
49.48+/-0.2
49.48+/-0.7
49.61+/-0.02
49.76+/-0.07
Best Practices
• Maintain melt at optimum temperature for stability
• Ensure gate design meets the requirements of semi-crystalline materials
• Run process with adequate hold pressure times for maximum part weight
• Eject parts hot and let them cool outside of mold
• If dimensions do not fit required tolerance, check gate design and process conditions
• If dimensions still out of tolerance, correct cavity
• Do not compromise process settings to achieve dimensional precision/production consistency
Delrin® | Technical Guide
To learn more, contact your Delrin representative or visit Delrin.com.
The information and recommendations contained herein are offered as a service to our customers but are not intended to relieve the user from its responsibility to investigate and
understand other pertinent sources of information and to comply with all laws and procedures applicable to the safe handling and use of these materials. The information and
recommendations provided herein were believed by Delrin® to be accurate at the time of preparation or obtained from sources believed to be generally reliable. However, Delrin®
makes no warranty concerning their accuracy and Delrin® will not be liable for claims resulting from any party’s use of or reliance on information or recommendations contained
herein regardless of whether it is claimed that the information or recommendations are inaccurate, incomplete or otherwise misleading. Caution: For medical applications, discuss
with a Delrin customer representative and consult the Delrin® Medical Caution Statement.
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reliability, supply and product performance. The iconic Delrin brand, coupled with proprietary technology and deep application expertise make us a leader in the high-end
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