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TitleInjection Molding Handbook
TagsInjection Molding
LanguageEnglish
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http://www.hanser.de/978-3-446-40781-7

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tions for injection molded PS parts are pharmaceutical and cosmetic cases, radio and
television housings, drawing instruments, clothes hangers, toys, and so on.

Polyvinylchloride (PVC)

Polyvinylchloride comes either unplasticized (PVC-U) or plasticized (PVC-P).
Unplasticized PVC is known for its high strength rigidity and hardness; however,
PVC-U is also known for its low impact strength at low temperatures. In the plasti-
cized form, the �exibility of PVC will vary over a wide range. Its toughness will be
higher at low temperatures. When injection molding PVC-U pellets, the melt tem-
perature should be between 180 and 210°C, and the mold temperature should be at
least 30°C. For PVC-U powder the injection temperatures should 10°C lower, and
the mold temperatures at least 50°C. When injection molding PVC-P pellets, the melt
temperature should be between 170 and 200°C, and the mold temperature should be
at least 15°C. For PVC-P powder the injection temperatures should 5°C lower, and
the mold temperatures at least 50°C. Typical applications for injection molded plas-
ticized PVC parts are shoe soles, sandals, and some toys. Typical applications for injec-
tion molded unplasticized polyvinylchloride parts are pipe�ttings.

2.6 Thermosetting Polymers

Thermosetting polymers solidify by a chemical cure. Here, the long macromolecules
cross-link during cure, resulting in a network. The original molecules can no longer
slide past each other. These networks prevent ��ow� even after reheating. The high
density of cross-linking between the molecules makes thermosetting materials stiff
and brittle. The cross-linking causes the material to become resistant to heat after it
has solidi�ed; however, thermosets also exhibit glass transition temperatures that
sometimes exceed thermal degradation temperatures. A more in-depth explanation
of the cross-linking chemical reaction that occurs during solidi�cation is in Chap. 3.

2.6.1 Cross-Linking Reaction

The cross-linking is usually a result of the presence of double bonds that break, allow-
ing the molecules to link with their neighbors. One of the oldest thermosetting poly-
mers is phenol-formaldehyde, or phenolic. Figure 2.25 shows the chemical symbol
representation of the reaction, and Fig. 2.26 shows a schematic of the reaction. The
phenol molecules react with formaldehyde molecules to create a three-dimensional
cross-linked network that is stiff and strong. The by-product of this chemical reac-
tion is water.

46 Injection Molding Materials [Refs. on pp. 61�62]

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Curing Systems A B C D

Sulphur 1.5 1.5 0.5 �
TMTD 0.5 � 3.0 �
MBTS 1.0 1.5 � 3.0

52 Injection Molding Materials [Refs. on pp. 61�62]

A and B are conventional curing systems which may be adequate where aging
resistance is not a particular problem. C is a low sulphur system giving much im-
proved aging but its scorch time is usually suf�cient only for ram-type injection. D
combines excellent ageing with a scorch time long enough for most applications.

2.9 Efficient Vulcanizing Systems

Ef�cient vulcanizing (EV) systems are de�ned as those where a high proportion of
the sulphur is used for cross-linking purpose. These systems have two main advan-
tages over conventional systems, giving vulcanizates with reduced reversion and
better aging characteristics. In addition to these advantages, EV systems based on
dithiodimorpholine (DTM) are very versatile, enabling a wide range of scorch times,
cure rates, and states of cure to be chosen at will.

It is particularly important to avoid reversion for injection molding of thick
sections, and EV systems give the complete answer to this problem. The conventional
system (sulphur/MBTS/DPG) shows reversion immediately after the maximum
modulus is reached, whereas the EV system (DTM/MBTS/TMTD) shows no rever-
sion even after three times the optimum cure time. EV systems can be developed to
give equivalent cure propoerties with much improved aging as compared with a
conventional cure, even when antioxidants are omitted.

Accelerator systems for injection molding should be chosen to give adequate
scorch time, fast cure without reversion, and appropriate product properties.

When molding thick scctions from polymers which revert (e.g., NR) EV systems
should be used to minimize reversion. Combinations of a sulphenamide, dithiodi-
morpholine and TMTD are ideal, and the ratios can be varied to meet precise
machine operating conditions and product requirements.

Where reversion is not a problem conventional sulphur/accclerator systems can
be used and the following accelerators will give the best cure rates for each scorch
time requirement:

MOR Decreasing
TBBS scorch
CBS/TMTD time

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Accelerator loadings may be increased to give improved product properties or to
counter the effect of oil addition.

2.10 Thermoplastic Elastomers*

Thermoplastic elastomers are a series of synthetic polymers that combine the prop-
erties of vulcanized rubber with the processing advantages of conventional thermo-
plastics. In other words, they allow the production of rubberlike articles using the fast
processing equipment developed by the thermoplastics industry.

There are many different of thermoplastic elastomers, and details of their com-
position, properties, and applications have been extensively covered in the literature
[22�29]. The commercially available materials used in injection molding can be clas-
si�ed into 10 types (Table 2.6). The commonly used abbreviations are listed in Table
2.7. The various themoplastic elastomers are discussed in more detail later in this
chapter.

Before dealing with each type individually, we can consider some features that
thermoplastic elastomers have in common. Most thermoplastic elastomers listed in
Table 2.6 have one feature in common: They are phase-separated systems (i.e., the
chlorinated ole�n interpolymer alloys are the exception). One phase is hard and solid
at room temperature in these phase-separated systems. The polymer forming the hard
phase is the one listed �rst in this table. Another phase is an elastomer and �uid. The
hard phase gives these thermoplastic elastomers their strength. Without it, the elas-
tomer phase would be free to �ow under stress and the polymers would be unusable.
When the hard phase is heated, it becomes �uid. Flow can then take place, so the
thermoplastic elastomer can be molded. Thus, the temperature at which the hard
phase becomes �uid determines the processing temperature required for molding.

2.10 Thermoplastic Elastomers 53

* Contributed by G. Holden.

Table 2.6 Thermoplastic Elastomers Used in Injection
Molding

1. Polystyrene/(S-B-S + Oil) Blends
2. Polypropylene/(S-EB-S + Oil) Blends
3. Polypropylene/(EPR + Oil) Blends
4. Polypropylene/(Rubber + Oil) Dynamic Vulcanizates
5. Polyethylene/(Polyle�n Rubber) Block Copolymers
6. PVC/(NBR + Plasticizer) Blends
7. Chlorinated Ole�n Interpolymer Alloys
8. Polyurethane/Elastomer Block Copolymers
9. Polyester/Elastomer Block Copolymers

10. Polyamide/Elastomer Block Copolymers

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must not be allowed to sit in the hot barrel of the injection molder and the machine
should be purged at shutdown. Corrosion-resistant barrel liners and screws should
be used. The user can buy precompounded products or make his own custom com-
pounded material. In this case the PVC and the plasticizer (typically DOP) are mixed
together in a tumble blender or high shear mixer until the plasticizer is completely
absorbed, after which the powered NBR is added. Compounding can then be com-
pleted on the injection molding machine. Special types of NBR, with properties opti-
mized for this end use, are commercially available.

Because they are polar materials, these blends cannot be overmolded against non-
polar hard thermoplastics such as polypropylene. Instead, they are recommended for
use with polar thermoplastics such as PVC, SAN, ABS, and SMA. Gradual migration
of the plasticizer may present a problem, and overmolded products should be
checked for this after a period of storage. The molded parts should not be allowed
to come into prolonged contact with polystyrene or other materials whose proper-
ties are affected by DOP.

Chlorinated Ole�n Interpolymer Alloys

The exact composition of these materials has not been disclosed. They are claimed
to be single-phase systems, and in this they differ from the other thermoplastic elas-
tomers. Like the PVC/(NBR + plasticizer) blends described earlier, they have limited
thermal stability; thus, molding temperatures must be carefully controlled to avoid
degradation, and the material must not be allowed to sit in the hot machine for
extended times. Corrosion resistant barrel liners and screws similarly should be used
and shear heating must be avoided. Melt temperatures should be less than 190°C.
Resistance to �ow is not much affected by temperature within the recommended
range, so molding is not usually improved by increasing the processing temperatures.
These interpolymer alloys have highly non-Newtonian �ow characteristics, so they
are also molded at high injection rates through small gates. They can be overmolded
against polar thermoplastics such as PVC and polycarbonate/ABS blends, as well as
against thermoplastic elastomers based on polyurethanes or polyesters, discussed
later. Grades with adhesion to other hard thermoplastics are under development [35].

Polyurethane/Elastomer Block Copolymers

These polymers (as well as those discussed later) are structurally similar to the
polyethylene/(polyle�n rubber) block copolymers described in Section 2.3.5. Their
morphology is shown in Fig. 2.34. In this case the crystalline hard segments are
polyurethanes, and the soft segments usually are polyesters or polyethers. Melting
temperatures for the polyurethane segments are high (typically greater than 180°C),
so melt temperatures for injection molding must be more than this value. They are
generally a little higher for the harder products. These polymers are quite suscepti-
ble to thermal degradation, which starts above 230°C, and gives foaming or bubbles
in the �nal part. They should not be allowed to sit in the hot injection molding
machine, and the machine should be purged with polystyrene or ABS before shut-

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down. Polyurethane/elastomer block copolymers readily absorb water, which causes
severe degradation of the molten material. Both virgin polymer pellets and
reprocessed scrap must be thoroughly dried (down less than 0.1% moisture) before
being molded. Injection rates should be moderate, and fairly large gates with rounded
edges are recommended. Further crystallization and crystal rearrangement will take
place after molding is completed, and the properties of the product will improve with
time for a period of about 6 weeks. The process can be speeded up by overnight
annealing at about 115°C.

Polyester/Elastomer Block Copolymers

In these polymers the crystalline hard segments are polyesters and the soft segments
are polyethers. They are more thermally stable than the polyurethane analogs
described carlier, so they can be processed at somewhat higher temperatures, up to
about 260°C. Again, processing temperatures should be higher for the harder prod-
ucts The melt viscosity changes signi�cantly with temperature, so processing can be
improved by increasing the melt temperature, although this will also increase the
cycle time. The molten polymer can be allowed to sit in the hot injection molding
machine for an hour or two. Purging before shutdown is generally not necessary, but
if desired, it is best to use a thermally stable material such as polyethylene. As with
the polyurethane/elastomer block copolymers, both virgin polymer pellets and
reprocessed scrap must be thoroughly dried (less than 0.1% moisture) before use.

Polyamide/Elastomer Block Copolymers

In these polymers the crystalline hard segments are polyamides and the soft segments
are usually polyethers. Processing temperatures depend on the melting temperature
of the polyamide (e.g., nylon-6,6, nylon-6, nylon-11) and should be higher for the
harder grades. Other processing conditions are essentially similar to the polyester
analogs described earlier. Processing detail is given in Chapter 3.

References

1. Stern, H. J., Rubber: Natural and Synthetic, Maclaren and Sons LDT (1967), London.
2. de la Condamine, C. M., Relation Abregee D’un Voyage Fait Dans l’interieur de l’Amerique Merid-

ionale, Academie des Sciences (1745), Paris.
3. DuBois, J. H., Plastics History U.S.A., Cahners Publishing Co., Inc. (1972), Boston.
4. Tadmor, Z., Gogos, C. G., Principles of Polymer Processing, John Wiley & Sons (1979), New York.
5. McPherson, A. T., Klemin, A., Engineering Uses of Rubber, Reinhold Publishing Corporation

(1956), New York.
6. Sonntag, R., Kunststoffe (1985), 75, 4.
7. Herrmann, H., Kunststoffe (1985), 75, 2.
8. Regnault, H. V. Liebigs Ann. (1835), 14, 22.
9. Ulrich, H., Introduction to Industrial Polymers, 2nd ed., Hanser Publishers (1993), Munich.

10. Rauwendaal, C., Polymer Extrusion, 2nd ed., Hanser Publishers (1990), Munich.

References 61

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