Tài liệu The Plastic Product P2 pdf

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2.3.2 Are Special Fits with Matching Products
Required?
Often, certain dimensions of a product are specified with unnecessary close
tolerances, when all the designer wanted to convey is that the product should
fit suitably on another product (tightly or loosely), typically, a container and
a matching lid. This requirement must be clear. Especially, when molding
plastics with high shrinkage factors (e.g., PP or PE), it can be difficult to
arrive at the proper “steel” dimensions, and some experimenting may be
required to achieve the required fit. Specifying the matching diameters with
standard, loose tolerances may yield pieces correct in size, but wrong because
the fit is not as desired. The alternative – providing closer tolerances – could
be unreasonable, because the dimension of the molded product depend not
solely on the steel dimensions of the stack parts but also on the molding
parameters. In such cases, it is of advantage to complete the more complicated
mold first and test it in actual molding conditions until the best cycle time is
established. The critical mold parts of the matching product (e.g., the lid)
should be finish-machined only after having established what the actual
molded container dimensions are. This could require completing the lid mold
with only one cavity, using assumed suitable dimensions, testing the un-
finished mold until the best cycle is achieved, and then adjusting the assumed
dimensions so that the proper fit can be achieved. All lid mold parts can then
be finished. For more information on this subject see [5].
2.3.3 Tolerances for the Filling Volume
This applies specifically – but is not restricted – to containers into which a
more or less viscous product will be filled by volume to within closely specified
limits (typically, containers for margarine, paint, etc.). In their end use, it is
important for the seller that a minimum amount must be filled into the
package without shortchanging the buyer, but also they should not be over-
filled, which would mean a loss for the seller. There should be clearly defined
fill lines (usually inside the container) to mark the minimum and maximum

volumes. This can be a problem with plastics with large shrinkage factors
such as PE and PP. It requires special consideration when dimensioning the
cavity and core because of the unavoidable variations in shrinkage values, as
the plastic flows away from the gate and slowly cools and as the injection
pressure within the mold decreases. The same considerations apply to
measuring cups or vials which have the various levels (or volumes) indicated
by lines on the sides of the product. It may be necessary to first test the mold
to find the best cycle times, and then establish the location of the measuring
lines.
Prototyping is often used to verify
the required dimensions or fits of a
part after shrinkage
2.3 Accuracy and Tolerances Required
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2 The Plastic Product
2.3.4 Stacking of Products and Free Dispensing
Any product stacked for shipping must have a clearly defined stacking height,
which is usually created by resting the outside or the bottom of one piece on
the inside stacking provision of the following piece. These provisions for
stacking can be “stacking lugs”, or clearly defined steps in the product. The
purpose of these lugs (or steps) is
 The products must not jam when pushed together, which would make it
difficult to separate them where required by the user, and
 They will ensure a total stack height of a certain, specified number (e.g.,
20, 25, 40, etc.) of the products when stacked. The stack height should be
suitable for the size of boxes or containers (preferably, standard size
cartons) in which stacks will be shipped.
 If special cartons are to be provided, it may be necessary to investigate if

their size will suitable for standard rail or sea shipping containers, for
best use of the available space inside these containers.
Stacking is more difficult if the angle of the sidewalls is small. Obviously, a
cylindrical container (0° draft) cannot be stacked at all. A typical disposable
drinking cup has approx. a 7° angle. Larger angles stack easily.
Problems can also arise when parts are used in an assembly line or in a
dispensing mechanism (e.g., vending machine) where it is important that
the parts will release easily, without fail, from the stack, i.e., not being “hung
up” by vacuum or by friction because the gap between two stacked con-
tainers is too small, even though they are properly stacked as designed. When
the gap between two sidewalls is very close, static electric charges may also
prevent the lowest part from falling from the stack when desired. Some
dispensers have mechanical separators and don’t depend on gravity, but it
is preferable not to depend on having such separators (added costs). It is
highly recommended to make sure that any stacking height dimensions are
carefully checked before beginning to build a mold. If they are wrong, the
mold has to be changed after finishing, or the packaging (carton size) has
to be redesigned after the height of the stack was not as originally planned.
Occasionally, a mold maker may decide to make slots in the mold cavity
for the stacking lugs by EDM into the core only after the mold is finished,
rather than do it before and then have to increase the height later. The
disadvantage of this method is that the mold has to be dismantled to be
able to machine the cores (costs!). The advantage is that a minimum stacking
height can be achieved. Also refer to Appendix 12 for more advice for mold
designers.
Figure 2.24 View of stacked lids
Figure 2.25 View of stacked products
Figure 2.26 View of products stacked on
lugs
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2.3.5 Mismatch (Deliberate)
Trying to produce a “perfect match” between two surfaces is not only difficult
to achieve but also very costly. Designers often create deliberate mismatches
for ease of manufacturing.
There are two areas of “deliberate mismatch” to consider, and two typical
examples are shown. There are many variations of matching parting lines,
or between lids and covers, but the basic principle applies to all of them.
Mismatch at the Parting Line, Between Cavity and Core
First, it must be clarified, whether a rounded edge is really necessary for the
product. In many cases, the product designer may not be aware of the possible
additional cost involved to produce a round edge as in Fig. 2.27, (a) or (c),
and will often agree that a simple, “sharp” edge (b) or (d) would be just as
acceptable for the application.
Figure 2.28 shows just one of several designs of a round edge, with the ideal
case (1) having a perfect match at the parting line. However, due to the build-
up of manufacturing tolerances of the mold parts, such ideal case is not
practical. In reality, the nominal diameter D of the cavity, or of the core, will
be either larger or smaller than the matching one, and create either a hook
(2), which is generally not tolerable, or small step (3), which in most cases is
perfectly acceptable. Note that the actual differences caused by the tolerances
of the diameters are small, usually less than 0.1 mm (0.004 in.), so that a step
would not be more than about half this amount. However, a step is much
less noticeable than a hook.
In fact, a mismatch can be corrected by very time consuming handwork, by
grinding or stoning (polishing), but this should be avoided because of the
high cost.
The suggested proper (and most economical) approach is to dimension the
matching diameters so that there is always a step, as shown in Fig. 2.28, item
(3), of a magnitude between 0 and 0.1 mm (0–0.004 in.).

Mismatch Between Two Matching Pieces, such as Box and Lid
The conditions are similar when designing and building molds for “matching”
boxes and lids. Here, deliberate mismatch (2) is even more important, because
the products may come from different cavities and even molds, made under
varying molding conditions, and the mismatch due to build up of many
tolerances (in cavities for both products) could be much larger.
Figure 2.29 shows the ideal condition (1), which is difficult to achieve, and a
way to minimize the effect of a mismatch between matching parts (2). There
is also another way shown by adding a “decorative” band to the larger part
(3).
Always consider:
1. Is the rounded edge really
necessary?
2. Is the sharp edge really
necessary?
2.3 Accuracy and Tolerances Required
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2 The Plastic Product
Figure 2.28 Round edge: ideal (1),
with “hook” (2), and with “step” (3)
Figure 2.27 Typical round edges where
a “sharp” edge could be considered
Figure 2.29 Mismatch
avoidance between box and lid
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2.4 Tolerances, Mold Alignment,
and Mold Costs
The relationship between: (1) product tolerances, (2) machining tolerances

of mold parts, (3) resulting requirements for alignment of the mold halves
(cavities and cores), and (4) the mold cost could be the subject of another
book. Here, we will try to condense the subject, by outlining some major
points when making the decision of which method of alignment to select.
The main reason for any alignment method between cavity and core is to
keep the centerlines of cavities and cores in line. Any deviation from the
actual centerlines of cavity and core from the “true” centerline will result in
thickness variations of the sidewalls of the product. This is true for any cup-
shaped product. With flat products, including lids, usually we do not have
this concern; in such cases, alignment of the mold halves using only the
machine tie bars could be sufficient, even without leader pins. But don’t forget:
leader pins (even if not used for alignment) on the core side are also meant
to protect the (projecting) cores from damage. They should always be higher
than the cores.
There are, basically, four methods of alignment used:
1. Use only the machine tie bars to align cavity and core. This can be done
in some cases where the alignment between cavity and core is not very
important; it can be used for experimental and prototype molds, or even
for limited-production molds. This case will not further be discussed
here.
2. Alignment of the mold plates with leader pins. This is the oldest and
most common method used, for any size of mold, and for any number
of cavities. This is the lowest cost method of alignment.
3. Alignment with taper pins between mold plates, and occasionally between
cavities and cores, and taper locks between the individual sets of mold
stacks, whether in single- or multi-cavity molds. This method usually
also requires at least two or more “loosely” fitting leader pins (with or
without bushings), not for real alignment purposes, but to protect the
core(s) from damage and to facilitate handling of the mold outside of
the machine. This method is more expensive than leader pin alignment.

With taper locks, we also have to chose between
(a) round tapers (less expensive), or
(b) wedges (adjustable)
4. There are also combinations of these two methods of alignment, such as
where the mold plates are “loosely” aligned with usually 2 (sometimes 3,
rarely 4) leader pins, but the final alignment is achieved with tapers
between each cavity and core stack, in single- or in multi-cavity molds.
Figure 2.30 1+1 cavity mold requires only
leader pin alignment to keep mismatch to
an acceptable level (Courtesy: Stackteck)
Figure 2.31 This lid mold has leader pins
and round taper lock alignment, while the
modules have no alignment mechanism.
This works well for shallow parts
(Courtesy: Husky)
Figure 2.32 Lid stack module with flat (no)
alignment on the stack. Mold alignment is
typically accomplished with round taper
locks on plates
2.4 Tolerances, Mold Alignment, and Mold Costs
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2 The Plastic Product
Figure 2.33 shows an example of a modular mold for a container with circular
alignment tapers (A). Note that the cavity (B) is set into the cavity retainer
plate (not shown), while the core (C) is mounted on top of the core backing
plate (not shown) to ensure proper alignment. Note the absence of a stripper
ring: this product is air-ejected from the core, making for a much simpler
mold. For best cooling efficiency, there is a beryllium-copper alloy (BeCu)
core cap (D), and a BeCu gate insert (E) in the cavity bottom plate (F). Note

the intricate venting channels in both cavity and core to ensure fast filling of
the cavities. This type of stack usually produces at 6.0 s or less.
Figure 2.34 shows an 8-cavity modular mold for rectangular containers. The
cavities (A) are set into the hot runner plate (B), the cores (C) are mounted
with float on the core backing plate (D). Each core is aligned with its cavity
with wedges (E); the containers are ejected by air.
Figure 2.34 8-cavity mold for rectangular
containers (Photo: Courtesy Dollins Tool
Corp., USA)
A
A
B
C
D
E
F
Figure 2.33 Modular mold for container
(round taper interlock)
(Courtesy: Husky)
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Figure 2.35 shows a modular single-cavity mold for a very large, thin-wall
container, with wedge-lock alignment. This mold too operates with air
ejection only, and is simple in construction. For the most effective cooling,
there is a BeCu core cap (A) and a BeCu gate insert (B). The jaws (C) for the
wedge lock are easily adjustable. The two leader pins (D) are basically only
for mold handling and for protection of the cores and fit only loosely in the
leader pin bushings (E).
If the sidewall tolerances are large, which is often the case with heavy-walled
products, a possible misalignment between cavity and core is usually insignifi-

cant, and alignment with leader pins is perfectly viable. The average clearance
between leader pins and leader pin bushings (standard hardware) is about
0.04 mm (0.001 in.). If, e.g., a wall is 1.5 mm thick (0.060 in.) and the tolerance
is ± 0.1 mm (± 0.004 in.), any misalignment falls within the permissible limits,
and leader pins are perfectly acceptable for the mold. Note: In theory, only two
leader pins are ever required to ensure proper alignment. The fact that many
molds use 4 pins is mainly to protect the cores during servicing the molds.
If the walls are thinner than in the above example, say, in the order of 1.0 mm
or less, and the tolerances are tighter, alignment with leader pins may not
be good enough to ensure that the variations fall between the allowable
limits. In these cases, individual ”taper locks” (of various designs) are
required.
Round tapers are relatively easy to manufacture, but require high accuracy to
ensure concentricity with the center of the cavity, and to ensure that the
proper preload is achieved. The basic requirement of any taper fit is the
preload between the matching faces.
A
B
C
D
E
Figure 2.35 Modular single-cavity mold
for large thin-walled container
(square lock alignment)
(Courtesy: Husky)
The tolerances of the product decide
which method of alignment to use
Without preload, a taper is useless
for alignment
2.4 Tolerances, Mold Alignment, and Mold Costs

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2 The Plastic Product
The importance of preload is discussed in detail in [5], Chapter 30.
The biggest problem with round taper fits is that the tapers wear with time,
and need to be reset or replaced, which is often quite expensive. But it is still
the most economical method of alignment.
The “wedge lock” method is a very good, efficient method, used mainly for
molds where accuracy is very important and the higher cost can be easily
justified over long periods of use. It consists of two opposing pairs of matching
wedges, at 90 degrees
The advantage is that the wedges are easily accessible and can be adjusted (by
shimming and/or grinding) or replaced with little cost.
The main disadvantage of the wedge lock design is that more space is required
to accommodate the wedge lock than the space required for a round taper
lock, thereby making the mold larger and more expensive.
2.5 Heat Expansion, Alignment,
and Mold Cost
Heat expansion [6, Chapter 14], is another area that must be taken into
account. It is always necessary to have both mold halves at the same tempera-
ture; particularly the mold plates carrying the alignment elements. The plates
on the cavity side in a hot runner mold can easily become hotter than the
plates on the core side of the mold. For example, a temperature difference of
20 °C between two plates, on a distance of 400 mm, causes an expansion of
0.091 mm (0.004 in). This can result in a serious misalignment. If we depend
on leader pins for alignment, they will deflect and/or wear rapidly, as will the
bushings. If taper elements are used, they too will wear out rapidly and lose
their usefulness.
There are basically only two ways to avoid misalignment caused by heat and/
or manufacturing variations

1. Make sure that the cooling channels are laid out so that the temperatures
of the plates are kept the same; this has little effect on the mold cost.
2. For molds with more than one cavity, allow the cores to “float”: the cavity
side consists usually of a ”cavity retainer plate” into which the individual
cavities are set in. These locations are fixed but subject to manufacturing
variation (tolerances). The mold can be designed so that in the individual
stacks, the cores (with their taper alignment) can ”float” on their
mounting surface (plate) to “find” the matching taper in the cavity. There
are two methods commonly used to achieve this:
– The cores are screw-mounted to the backing plate, with the screws
accessible from the parting line. The mold is assembled completely,
Usually, molds are designed with
fixed cavities and floating cores
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but these screws are, at first, not tightened fully so that the mold, the
first time it is closed, will push the cores into proper relation to the
cavities. After the mold is opened again, the screws can be fully
tightened to be ready for production. This method is satisfactory as
long as the temperature difference between the two mold halves is
kept low, at about 5 °C or less.
– A better, but more expensive method is to make the cores really
floating, regardless of the temperature differences, as shown in [6].
Note that the amount of float is limited and only in the order of
0.1 mm (0.004 in.)
2.6 Surface Finish
The finish of the mold parts, the molding surfaces, and the fitting surfaces
where mold parts meet, are important cost factors. The finer the machining
finish, and the more hand finishing is required, the higher is the mold cost.
This appears to be obvious but is often overlooked or neglected.

The relationship between surface finish and costs and the relationship
between tolerances and costs (as shown in Fig. 2.23) are very similar and
apply here too.
2.6.1 Finish of Molding Surfaces
Molding surfaces (the areas in contact with the plastic product) are finished
1. To provide the required appearance or function of the product
2. To ensure that the product can be easily ejected from the mold, however:
– Occasionally, a relatively rough surface in specific areas may be
beneficial to keep the product on that side of the mold, from where it
will be ejected.
– On the other hand, sometimes, a high polish could also be detrimental
to easy ejection, depending on the design of the product.
In such cases it is the decision of an experienced mold designer to specify
the proper finish in these locations (refer to Appendix 16 for list of surface
finishes commonly used).
Especially with very thin-walled products, the surface finish of the cavity
space affects the plastic flow over the molding surfaces. Better finish results
in faster filling and shorter cycle time. In some cases (notably with PS), flash
chrome plating over a highly polished area can increase the productivity of
the mold by up to 10%.
2.6 Surface Finish
Figure 2.36 The etched cavity wall gives
this tumbler a frosted look
The costs rise exponentially with
finer finish
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2 The Plastic Product
Finishing (polishing, etc.) the mold parts is generally an expensive activity
in the mold making process because much handwork is required, and should

be limited to those areas that really require it. Most mold makers today utilize
hand-operated mechanical and some fully automatic methods to finish a
surface, but there is still much need for hand finishing wherever the shape
of the product does not allow easy access for mechanical or automatic
equipment.
The purpose of finishing, in general, is to remove the tool marks remaining
on the surface of a work piece. In many cases, the rough, “as machined”
finish after chip removing operations (turning, milling, etc.) could be quite
satisfactory for the appearance of the product, for example on the inside
surface of a technical product (enclosures, boxes, television cabinets, etc.),
but this may not always be satisfactory for the ejection of the product, because
the plastic will not easily (or not at all) slide over too rough a surface. It is
also important to consider in which direction the rough machining grooves
are lying: to be in line with the ejection could be satisfactory, but across it is
usually not acceptable. Also, the draft angle of a wall (or of the sides of a rib)
is important. With little draft (a small draft angle), the surface finish must be
much better, whereas with a large angle (approx. 5° or more), a much rougher
finish, such as “as machined”, could be permissible. With the need to design
for less and less mass, the draft angles, especially of ribs, must be kept small,
and these walls therefore need a good finish, but not necessarily a polish: a
good finish in line with the ejection motion (“draw stoning”) will usually be
good enough. If ejectors can be placed under such ribs, the finish becomes
even less of a problem. We must always consider what would happen if a
piece of plastic breaks off inside a rib: it may save time in the making of the
mold but can become expensive later, when the service personnel are
frequently required to remove some broken-off bits of plastic from the mold
causing severe delays in production.
Grinding and electric discharge machining (EDM) leave smaller tool marks
on the worked surface; such surfaces may not need any further finish, except
polishing where required for appearance. EDM finish can be from rough to

very fine, which may not require any polishing at all. Rough finish is the
result of high currents and faster cutting speed and therefore requires less
time.
In addition, with today’s new methods of finish turning and milling hardened
surfaces, the achieved finish is often as good as a ground finish and no further
polishing is required.
2.6.2 Texturing of Surfaces
There are also other surface finishes for appearance, such as texturing, to
create leather, basket weave, or other patterns. If it is a deep pattern, it should
be clear if any related dimensions apply to the highest point of the pattern or
to the base where it is applied to. A rough EDM finish is a good and
inexpensive solution for a good-looking, matte surface.
Figure 2.39 Polishing area in a shop
Be specific as to where dimensions
point to; for example, to the peaks
and valleys of the finish
Figure 2.38 PS tumblers and core/cavity
show the highly polished finishes required to
achieve the glass-like look of the molded cup
Figure 2.37 Typical PS tumblers
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