21
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

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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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

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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26
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

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

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
1281han02.pmd 28.11.2005, 10:4828
29
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.

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