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Venting of Molds for Rotational Molding
1.0 Introduction 2.0 Why
do we use Vents? 3.0 What
are the most common problems with standard vents? 4.0 What
possible solutions exist for improving venting? 4.1 Analysis of the Venting
Process 4.2 Guidelines
from Molders and Mold-Makers 4.3 Vent Size
vs Mold Volume - Theory 4.4 Molding
Trials 4.5 Industry
Developments 5.0 Choosing
a Vent and Defining the Ideal Vent 5.1 Vent Assessment 5.2 An Ideal Vent 6.0
Alternative Concepts References Acknowledgments
1.0 Introduction
Rotational
molding is a growing industry. As it grows, it attracts attention from an
increasing base of potential users. The versatility and usability of the
process are questioned by more and more people with experiences in other
industries, very often people who expect high quality, repeatability and
automation as necessities, not added extras. In recent years, research and
development in rotational molding has been examining many areas; machines with
computer controls; mold construction; process control and diagnosis; new
materials. Some of the basics for the process, however, still need to be
examined. Areas such as consistency of release for parts, powder properties,
improved control of heating and cooling cycles, comprehensive shrinkage design
parameters, automation of the loading / demolding stages, controlled venting.
The attraction of large potential markets will drive molders to search for the
answers to all these areas eventually - this paper aims to shed some light on
one of them. In
looking at the subject of venting, four questions arise: a)
Why do
we use vents? b)
What
are the most common problems encountered with standard vents? c)
What
possible solutions exist for improving venting? d)
Can we
define the ideal vent as a guide for development? 2.0 Why do we use Vents?
The
answer to the first question is simple - to
maintain equal pressure between the inside and outside of a mold during the
processing cycle. This will prevent blowholes in parts, eliminate pressure
induced flashing at parting lines, reduce the possibility of warpage, prevent
damage to molds and help avoid injury to personnel. With
such a vital role to play in producing acceptable parts, it is surprising to
find that venting can very often be a neglected area in rotational molding
design and production. A survey of papers and journals on rotational molding
produced a handful of brief references on the subject [1-8]. One reason for a
lack of hard and fast rules or guidelines may be that the approach to venting
must inevitably vary with the design of almost every part. 3.0 What are the Most common Problems with Standard Vents?
The
most common problems relating to a standard vent are: 1.
Powder (polymer) build-up at the end of the vent and the need to
constantly clear this or use filling material 2.
Polymer build-up around the base of the vent 3.
Blowholes at parting lines (as a result of 1) 4.
Warpage of parts and molds (as a result of 1) 5.
Ingress of water through the vent during cooling 6.
Wear of the vent material surface - particularly Teflon 7.
Difficulty in removing the vent from the mold and part due to shrinkage
and location 8.
Material build-up on the vent due to residual heat in the vent tube 9.
Contamination of parts due to filling material falling out during
molding 10.
Vents require cleaning or changing constantly Items
3 and 4 are not problems with a vent but problems that arise in molded parts
as direct results of the main problem - blockages. They are included because
they shed light on the intended operation of a vent, which is to provide a
path for air to flow between the inside and outside of a mold whilst
preventing powder from doing so. This is usually achieved by locating the end
of the vent close to the actual center of the mold (or center of the largest
free space) and filling the end of the vent with either glass or wire wool.
The filling at the end of the vent actually counteracts the intended operation
but is required in many molds because the powder at some point in the rotation
actually covers the end of the vent tube. Even in those situations that can
place a vent at a point beyond the powder pool (large containers, for
example), powder floating in the air inside the mold can line the inside of
the vent and cause blockages after a number of moldings. Wear
and tear on a vent can be a problem, particularly for Teflon vents which have
to be removed every cycle. Of the common materials used, Teflon is most
susceptible to the effects of shrinkage of the part. If the vent is not in a
central location, distortion of the vent or part can occur as the part cools
and drags the vent. The Teflon itself will also tend to shrink and deform due
to the heating cycle. Smaller,
more complex molds tend to have more venting problems than larger, simpler
molds due to the fact that large openings in the molds do not exist and powder
will usually cover the end of the vent at some point in the rotation. It is
not unusual for some vents to be buried in powder for a considerable portion
of the cycle. Small molds in general, therefore, represent more of a challenge
than large. The
use of soft, porous materials to seal the vent can actually contribute to
clogging. The filling does not need to be hot to attract and hold powder. When
it does become sufficiently hot to make the powder sticky (which occurs at a
point below the powder’s melting point), the melted powder acts as a glue to
attract more material and therefore accelerate the clogging process. The end
of long vent tubes can very often stay below the temperature at which powder
will stick to them but will clog due to powder trapped on the insulation used
to fill them. Previous ARM surveys have shown that the number one
problem for molders were blowholes due to vents clogging or contamination of
parts due to material falling out of the vent. Since
molds can vary in size from 2-3” (50-75mm) spheres up to 7 000+ gallon (30
000+ liters) tanks, there is a wide range of diameter and lengths of vents
that are used. Some of the problems listed above are more prevalent for small
vents which are used in cavities which are full of powder than for large vents
in cavities in which the powder has a lot of free room to flow. 4.0 What Possible Solutions Exist for Improving Venting?
The
answer to this question requires a more involved analysis. The following
sections examine a number of areas: 1.
The basic processes occurring inside a mold during molding 2.
A survey conducted with molders and mold-makers for their experience 3.
A theoretical look at expansion and contraction of gas inside the mold
during molding 4.
A series of simple experiments to compare a range of vent
configurations 5.
A look at some of the developments taking place in the industry today. 4.1 Analysis of the Venting Process
If
we look at the processes occurring inside the mold, we can develop an
understanding of when and how a vent should operate. The solid line in Figure
1 shows a typical temperature curve for polyethylene measured inside a mold
during a molding cycle [9]. The dotted line in Figure 1 shows a measurement of
pressure during the cycle inside a mold which had no vent (ie: a completely
blocked vent). Up
to point A on the curve, powder flows freely inside the mold. At point A
material starts to build-up on the inner surface of the mold. Between points A
and B, all the material in the mold fuses to the mold surface. Pressure
build-up in the mold cannot usually occur until a layer of material has
covered the parting line, ie: somewhere between A and B. It
is therefore only prior to point B that a vent must prevent powder from
exiting the mold and after point B that it must act as a conduit for air flow. For
molds that have very well sealed parting lines, pressure can build-up from the
start of the heating process. However, without a blocked vent and average
parting lines, pressure will typically start to increase after the temperature
has reached around 110-120oC (230 - 248oF for
polyethylene. This is when the first layers of polymer seal the parting line.
The pressure will increase to a peak value just after the part leaves the oven
(when the peak internal temperature occurs). In a mold with good parting
lines, blowholes from the parting line through to the inner surface (inward
blowholes) are more common than blowholes from the inside out through the
parting line (outward blowholes). Part of the explanation for this is that
during the heating cycle, the material blocking the vent tube is molten and
can usually perforate to allow air to expand and therefore relieve pressure on
the parting line. Also, the forces acting on the polymer push material into
the parting line making it more difficult for air behind to penetrate and
cause a outward blowhole. To produce inward blowholes without outward
blowholes, the vent must have allowed air to pass through during the heating
cycle and then have created a back pressure during the cooling cycle. This may
occur as air is drawn into the mold through the vent which then freezes the
material blocking it or simply be due to restricted air flow through narrow
orifices. Since blowholes can
only occur at the parting line whilst the material is molten (ie: above the
crystallization plateau D), the maximum temperature to which the material
rises comes into play. As the temperature rises, the viscosity of the material
falls and the ability to deform under localized pressure increases - higher
melt index materials will therefore be more prone to blowholes. Note
that during the cooling process, the part will usually be cooled to a point
below 212oF which means that the total potential for generating a
pressure is greater for the cooling side than on the heating side (although
the pressure potential whilst the material is molten on the heating side and
on the cooling side is approximately the same). Higher pressure means that
warpage of the part and the tool are more likely and may also extend the
cooling cycle by pulling the part away from the mold wall [10]. 4.2 Guidelines from Molders and Mold-makers
As
part of this paper a number of mold-makers, molders and designers/consultants
were contacted regarding the guidelines that they use when dealing with
venting in a mold. The response rate was good (above 80% at the time of
writing). The questions asked were basic and related to the simple practical
issues for vents. The questions and most common responses were as follows: 1.
In your
experience is venting typically included in the part design process from the
beginning or simply added at the end prior to production. Do molders rely more
on mold-makers for advice or use their own practical experience? Responses:
Molders Mold-Makers Design/Consult. Normally
included in design
6
2
3 Added
after mold is made
3
5
2 2.
What
guidelines do you use/recommend to assess the cross-sectional area for a vent
tube vs mold-size? Responses:
Molders Mold-Makers Design/Consult. Molding
experience
5
2
1 Standard
guideline (0.5”/yd3) 3
2
2 As
large as possible
1
-
1 As
small as possible
1
-
- 3.
How do
you assess the position and length of a vent tube? Responses:
Molders Mold-Makers Design/Consult. Dictated
by design
7
5
2 Center
of cavity
4
4
2 Away
from wall
1
-
1 4.
What
materials do you typically use for the vent tube and what criteria are used
for this choice? Responses:
Molders Mold-Makers Design/Consult. Mild
Steel (Teflon coated)
4(1)
4(2)
1 Stainless
1
-
1 Teflon
8
6
4 Aluminum
1
-
- Silicone
1
-
- 5.
Do you
typically fill the end of your vents to prevent clogging with material - if
so, which materials do you use? Responses:
Molders Mold-Makers Design/Consult. Steel
wool
8
2
3 Brass
wool
1
-
- Glass
fiber
7
2
4 Cotton
1
-
- 6.
Apart
from PVC molding, have you ever molded deliberately parts without a vent
(blocked vents do not count)? Responses:
Molders Mold-Makers Design/Consult. Yes
(up to 250 cubic inches)
5
2
3 No
4
1
1 7.
Do you
use vents for pressurizing mold or for inert gases via the arm of the machine?
Do you use concentric vents for this or two separate vents? Responses:
Molders Mold-Makers Design/Consult. Use
pressurization
6
1
4 Don’t
use pressurization 2
3
- Use
concentric vents
1
-
3 Use
separate vents
4
-
4 8.
Are you
currently working on improving vent design (are you interested in this)? Responses:
Molders Mold-Makers Design/Consult. Interested
8
5
4 Working
on development 3
2
- It is interesting to note from this group that the
majority of molders include the vent location in their design process, yet
most mold-makers do not receive instructions (or receive them as an
afterthought). Kelch and Plasticast report that only a few years ago almost no
molders gave instructions on vent location and that today around 50% of molds
have vents specified. Also, there are no universal guidelines used to
determine the cross-section of a vent tube according to the mold volume. Most
molders assess this based on prior experience and ‘what looks right’ since
the location and length of a vent tend to be very specific to a mold and often
it is not possible to allow the end of the vent to reach a center point or
penetrate very far beyond the powder pool. In most cases they prefer to err on
as as large a size as possible to reduce the possibility of blockages. 4.3 Vent Size vs Mold Volume - TheoryA
series of simple calculations on the heating of molds was carried out to
examine the potential increase in pressure during a standard heating cycle.
These used the standard gas equation P1V1/T1
= P2V2/T2. Effects due to pipe losses are not
considered at this time although it is recognized that the length and diameter
of a pipe will have an effect on the back pressure created. A peak internal
temperature of 200ºC (392ºF)
is chosen as a point which will produce 80-90% of a polyethylene materials
impact strength. Assumptions 1.
Internal pressure begins to increase at 120ºC (212ºF)
and rises to 200ºC
(392ºF)
2.
Calculation 1: Gas can expand unrestricted at a constant pressure, ie: V1/T1
= V2/T2 3.
Calculation 2: Gas is trapped in mold under constant volume, ie: P1/T1
= P2/T2
Table 1 Constant Pressure Volume calculation for a range of mold sizes Results: 1.
For an 80ºC
(144ºF)
temperature rise, the internal pressure rise for constant volume is 19.8kN/m2 (2.8 psi) independent of the size of
the mold 2.
For an 80ºC
(144ºF)
temperature rise, the internal volume increase for unrestricted expansion is 20.3% independent of the size of the
mold This
means that as molds increase in size, the size of the vent must obviously
increase to allow the volume of air inside to firstly escape during heating
and then to be drawn back in during cooling. The thickness of the part to be
made will also determine vent size as increasing the cycle time will mean that
the time over which the internal temperature rise occurs increases and thus
the rate of expansion reduces. To
compare mold and relative vent sizes, it is necessary to consider the back
pressure inside the mold and the rate of air moving out of the vent. In a
given situation, vent diameter, vent length, vent material and the rate at
which the air moves through it will dictate the pressure loss that occurs and
therefore the pressure that builds up in the mold. The pressure will determine
whether or not blow-holes occur. Too small a vent will restrict air flow and
cause a build up of pressure - the optimum will allow the air to move quickly
enough to ensure that the pressure does not rise above the critical point. The
standard guideline given for vent sizing is that a vent should have a diameter
of 0.5 inches for every 1 cubic yard of volume (12.0mm per cubic meter). This
stems from practical experience and appears to work well for molds around 1
cubic yard and upwards. For smaller molds, however, this guideline does not
give practical sizes. [
From temperature measurements inside parts during molding, average cycle times
for a
range of wall thicknesses have been estimated as follows (LLDPE, mild
steel mold at
340ºC
(650ºF)
oven temperature):
2mm (0.078”) - 8 min; 4mm (0.157”)
- 12 min; 6mm (0.236”) - 16 min;
8mm (0.314”) - 20 min; 10mm (0.394”) - 24 min; 12mm (0.472”) - 28
min ] If
this guideline may be used as a starting point for calculation, the rate at
which air flows out of the vent is a guide to the rate for other sizes of
molds. For a 1 m3 (35.3 ft3) mold producing a 6mm
(0.236”) part, a 12.0 mm (0.5”) ID vent will allow 0.203 m3
(7.1 ft3) of expanding air to pass through it over approximately an
8 minute period. The average air speed through the vent is therefore 3.7 m/s
(10.5ft/s). This seems quite high. A more common diameter for a mold of this
size would be a 25mm (1”) vent. This would produce an average air speed of
0.86 m/s (2.75 ft/s). This is a guide for air speed in Table 2 below.
Table 2
Estimated vent sizes for a range of mold sizes and part thicknesses based on
standard guidelines (pipe losses not included) These
figures are based only on a constant rate of flow out of the mold over a
temperature range of 120-200ºC.
They do not account for pipe losses. Pipe losses would have an effect, for
example, where a very long thin vent was being used. The flow of air through
the vent will reduce as the length increases and therefore a larger diameter
should be used. A more comprehensive set of guidelines for selecting vent
diameter could be extended to include: vent
length - vent material - part thickness - part material - peak internal
temperature - cycle times - use of vent filling material (human variable!). Figure
2 shows the data in graphical form. The map indicates the range of vent size
which might be considered for each part thickness. The starting point for this
set of curves is a 25mm (1”) diameter vent for a 1m3 (35.3ft3)mold
which is a guideline from practical experience. More detailed testing might
evaluate the actual limits for the back pressure which can exist in a part
prior to blowholes being formed and develop the map from there this is beyond
this study at present. For
smaller molds, where the table indicates very fine vents, a practical minimum
is probably around 6.25mm (0.25”). The upper necessary limit for large molds
is probably around 75mm (3”). Remember, these are figures calculated for
completely open vents, the effect of filling the end of the vent with glass
wool will be to drastically reduce the level of air that can flow. Wherever
possible, erring on the side of too much venting is better than having too
little. 4.4 Molding Trials
4.4.1
Vent wall thickness comparison A
simple, almost cubic mold 28 x 28 x 30” in dimension, was used to conduct a
series of tests to examine the heat transfer effect of different vent
materials. Eight vent tubes of the following sizes were mounted on the upper
face of the mold extending half way into the mold. Vent
OD
Wall thickness
Vent ID
Material (inches)
(inches/mm)
(inches/mm) 1
7/8” (47mm)
1/16” (1.6mm)
1 3/4” (44.4mm)
Mild steel 1
7/8” (47mm)
1/8” (3.2mm)
1 5/8” (41.2mm)
Mild steel 1
7/8” (47mm)
3/16” (4.7mm)
1 ½” (38.1mm)
Mild steel 1
7/8” (47mm)
1 /4” (6.2mm)
1 3/8” (34.9mm)
Mild steel 1
7/8” (47mm)
1/8” (3.2mm)
1 5/8” (41.2mm)
Stainless steel 1”
(25mm)
1/8” (3.2mm)
¾” (19.0mm)
Mild steel 1”
(25mm)
1/8” (3.2mm)
¾” (19.0mm)
Stainless steel 1”
(25mm)
3/16” (4.7mm)
5/8” (15.8mm)
Teflon The
purpose of this test was to compare the relative heat transfer rates through
the mold along the various vents by measuring the build up of material around
the base of the vent. Running all the vents simultaneously helps to keep the
number of tests required at a manageable level and ensure that similar levels
of heat are transferred to each vent. Table
3 below shows the thickness of material which formed around each vent at a
distance of 5/8” away from the mold surface. The parts were molded at
0.125”, 0.250”, 0.375” and 0.500” on a Ferry 280 machine at 650ºF.
Table 3
Material build-up at the bases of a range of vent tubes Location
of the vents plays an important factor since flow of material around each vent
contributes to the ability of material to form there. As the mold was mounted
centrally on the arm of the machine, the four vents in the corners are most
easily compared. It is also possible to directly compare the 1” vents
mounted between the corner vents. Comparisons between differing locations must
take account of this. Figure
3 shows data in graphical form. The thickness build-up on each vent obviously
increases as the part wall thickness increases. Difficulty in measuring
thickness at comparable point on each vent does not produce a consistent trend
relating heat transfer to build-up. The stainless vent is consistently lower
than the steel vents although this is partially due to it’s central
location. The thermal properties of the polyethylene contribute to the trend.
For a thin molded part (1/8”), the thinner vents pick up more material than
the thicker vents. As the part thickness increases beyond this, data for the
thicker moldings tends to converge which indicates that the heat transfer
properties of the polymer become the a larger factor in distribution of
thickness beyond 0.25 - 0.3” (6.25 - 7.6mm). Figure
4 shows temperature data measured at the end of four different vents (1/16”,
1/8” and ¼” mild steel and 1/8” stainless steel). The difference in
temperature build-up is approximately 50-60ºF
between the 1/16” and the ¼” vent. Figure
5 shows how temperature is retained from one cycle to the next by the 1/16”
and ¼” vents. This graph shows that, although the ¼” vent is hotter than
the thinner vent during the
cooling and demolding stages, it does not return to a much higher temperature
during the next cycle. The
recommendation from this series of tests is that a thin wall stainless steel
vent is the best from the perspective of heat build-up and keeping the vent
cool during the demolding and early parts of the cycle where steel vents are
required (typically for larger parts). 4.4.2 Vent
diameter The
same cubic mold with a new lid section was used to measure the effect of
varying the diameter of the vent on part appearance and blowholes. A single
2.5” vent was located in the center of the mold and threaded at the end to
allow threaded covers to be fitted. A series of covers were used to produce
openings at the end of the vent tube of 0” (closed completely), ¼”,
3/8”, ½”, 1”, 1.5” and 1 ¾”. The purpose of this test was to
examine the point at which the part would experience blowholes or parting line
flash. Parting
line flash appeared at the lower range of openings. However, the part did not
exhibit blowholes until the vent was completely closed over. An opening of
only ¼” was required to keep the part free of blowholes and ensure that it
did not suffer from warpage. Note that this cannot be compared directly to
having a ¼” vent of the same length since the pressure drop produced by the
air passing through a hole at the end of a much wider (2.1” ID) vent is much
lower than for a long narrow vent. Using a cover on the end of the vent with a
small hole is attractive where the end of the vent stays cool and is far from
the powder pool. 4.4.3 Vent
material Comparing
the effect of heat transfer along a mild steel, stainless steel and a PTFE
vent tube can be carried out by examining the data for the 1” diameter vent
tubes in Table 3. The mild steel is a better conductor of heat than the
stainless steel and therefore attracts more material. The Teflon has a very
low conductivity and attracts almost no material on the outside. However, it
exhibits the same tendencies to build-up on the inside of the vent as steel
and stainless vents when no insulation plug is used - material which falls
into the vent does not melt and stick at the end furthest into the mold but
falls into the vent and builds-up at the outer hot end.. 4.5 Industry Developments
VentSure Kelch
Corporation are currently assessing a new gravity operated venting system
which they have called Vent-Sure [11]. Kelch
have been working with a number of molders as a focus group to assess
potential new ways of venting. They have been working on this system for the
most difficult small to intermediate size molds. The devices have been tested
in the field and are constantly being updated as molders provide feedback . The
main benefit of the Vent-Sure system is that there is no porous filling
material to attract powder and clog. The smooth surface of the Teflon end plug
does not retain powder and is therefore more difficult to block - only powder
which floats in the air or which completely covers the end of the tube can
cause it to block. The
gravity vent is an all Teflon mechanism which opens and closes as the mold
rotates. Some molders report excellent results - primarily when placed in a
central location with thin wall parts. Others have had difficulties due to
clogging at the end of the closing mechanism and distortion of the tube during
removal. In situations where the vents have been used for long periods, the
only maintenance required has been to remove a thin layer of plastic from the
end of the mechanism between cycles. Kelch have made the latest units easier
to disassemble which makes them easier to clean and maintain. Some
possible improvements to the system include: increasing the clearance between
the rod and the vent tube to reduce the possibility of build-up; sharper
contact at the point where powder is to be kept out of the vent; ensuring that
the Teflon surface is smooth after machining; adding a metal sleeve partially
inside the vent to improve durability when removing or when being used in high
shrinkage locations. Dual
Passage Air Systems Ferry
Industries Inc. [12] provide machinery with through the arm systems for
delivering two separate lines of gas for controlling remote devices in the
oven or cooler during the rotational molding cycle. The basic capability of
passing a controlling air pulse or feed through the arm of the machine will be
essential for development of controlled venting capabilities where the machine
can actually switch venting on or off during the process as required. The
added ability to provide two lines provides excellent possibilities running
such systems side by side with air movers or drop-boxes for multiple layer
systems. Remcon/Queen’s Work
between Remcon Plastics Inc. and Queen’s University on systems aimed at
improving the process have raised some interesting possibilities for a venting
mechanism. These are active control mechanisms which react to the state of the
molded part and could be modified to provide pressure relief during the cycle. 5.0 Choosing a vent and defining an ideal vent
5.1
Vent assessment The
following list of questions form the basis of a decision making process for
determining the correct vent for a particular mold. 1.
Location Can it be located in an area of the part which can be
removed? If the area cannot be removed, can the hole created
be filled with a spin-weld? Can it be located in the center of a side where
shrinkage will not pull on the vent? Can it be located in a hidden or unobtrusive area? For ease of removal can it be mounted in the line of
draw? Should it be fixed or removable? Fixed
vents can reduce operator involvement but must be located for easy release of
the part. Can the vent hole be used to form a useful feature? 2.
Size Check the volume of the mold. Assess expansion and size vent accordingly. Table 2 is an approximate
guide. What thickness is the part to be made? How long is
the cycle? Longer cycles may need
smaller vents but the end of the vent inside will become hotter. Do you require a single vent or more than one to
achieve the cross-section required? Is
the mold large and flat - does venting need to be distributed? Make the vent as large as possible when space and
form allow. 3.
Length Assess the depth of the powder pool. Ensure
whenever possible that the vent exceeds this depth. If possible, locate the end of the vent at the center
of the largest cavity in the mold. Consider vent material when setting length - long
Teflon vents become flexible, short metal vents become very hot and attract
material. 4.
Material For smaller molds use Teflon tubing whenever
possible. For longer vents consider stainless steel to reduce
heat transfer. Teflon coating the vent will help in cleaning. Steel vents are robust. This is the most common material for long vents or large applications. Aluminum will conduct heat much more rapidly than
steels. Consider the vent wall thickness. Larger
Teflon vents should be thick (0.125 - 0.25”(3.1 - 6.2mm)) to help maintain
their rigidity. Steel vents should be thin (1/16” (1.6mm)) to reduce the
build-up of heat from cycle to cycle. 5.
Mounting How will the vent be fixed or supported in the mold? - hole
drilled in surface? - bulk
head fitting? - boss
in mold? (preferred for aluminum molds) -
bushing? (preferred for steel molds) - does
the vent require handles or threads to assist with removal? - is a
quick disconnect system for connecting pressurization required? 6.
Protection If possible leave end of the vent clear. If the end of the vent will be close to the powder
during rotation, choose the type of material to use to fill the end of the
vent tube; glass fiber, steel wool,
brass wool, cotton, nylon pad. Consider an active vent such as Vent-Sure for small
molds with narrow cavities. Consider using solid materials such as plastic film
rather than porous material for the covering.
Does the outer end of the vent require a cover to prevent the ingress
of water? 5.2 An Ideal Vent
Examining
the question of what would make an ideal vent produces the following list of
features: 1.
It will stop
all powder and remain clear at all times 2.
It will not
require constant maintenance and requires minimum operator contact 3.
It will ensure
flow of air in and out but not allow water into the mold 4.
It does not
promote build-up of material around it’s base or end 5.
It is simple,
low-cost and easily interchangeable Powder
entering the vent is the problem that causes most difficulties. Even if there
is no filling material (insulation) at the end of the vent to attract and
concentrate the powder, powder falling through the vent will stick to the
hotter regions close to the outside of the mold and may eventually build-up to
clog the vent. Stopping the powder altogether is what is required. Possible
solutions to this problem involve covering the end of the vent tube in a
sacrificial material which is non-porous - one upon which free flowing powder
does not rest. Sheeting such as aluminum or polyethylene which perforates or
melts during heating can be used to deflect the powder. However, setting the
correct perforation/failure point or melting point for the polymer can be
difficult. The covers must also be renewed each cycle. The
only certain way to alleviate the problems associated with venting is to use a
controlled vent which is activated by the machine. This may seem like an
expensive idea to many rotational molders until they add-up the costs of
traditional vents in terms of scrap and reworked parts. A
controlled venting mechanism is a spring loaded device with a light mechanism
which can be opened at both ends by the application of a pressure pulse
(continuous or intermittent). In operation, this is closed completely during
the early stages of the heating cycle to avoid powder entering the vent and is
only opened when the internal temperature has risen beyond point B on the
curve in Figure 1, ie: when all the material has adhered to the mold. Where
possible, the vent will be long enough to ensure that it remains cool enough
at the internal end to prevent polymer build-up. The fact that the mechanism
is closed during the stages when powder would clog a standard vent, the vent
may be shorter than normal and could pass through the powder pool. Machine
controls can be set to activate the vent by sending a pressure pulse through
the arm of the machine at the critical point in the cycle and either hold it
open continuously or release to close the vent whenever water is being applied
to the mold. It would be re-opened when the water is finished. Ideally the
mold would be mounted so that the vent can remain fixed to the mold whenever
the part is removed. To prevent build-up of material, the vent will either be
made of Teflon on the outside or be actively cooled. Piping air to the vent
can be done through flexible hoses with a quick connect system for easy
demolding in operation. The active section of the device does not require
components which are resistant to temperature other than the internal spring. Controlled
venting in this manner has the added possibility of using the internal
atmosphere of the mold to pressurize the part and therefore control surface
porosity, reduce cycle times and improve impact strength. 6.0 Alternative Concepts
The
following are some alternative methods of venting that may have potential in a
number of applications: a)
Use a pressure sensitive device which activates a trigger or switch to
relieve it b)
Use a temperature sensitive device to activate a trigger or switch c)
Develop a standard range of sacrificial vent covers d)
A concentric pipe system which allows constant airflow into the mold to
keep the end of the inner tube clear e)
Injecting a needle into the wall of a part to release pressure and
withdraw just prior to crystallization - requires control from the molding
machine f)
A rotating mechanism attached to the end of the vent which moves with
the motion of the arm and releases a spring loaded catch at a pre-determined
number of revolutions g)
A rotating mechanism which rolls with the motion of the arm inside the
vent to keep it clear during use h)
A multi-station oven with mechanical operation of a vent at an
appropriate point in the cycle References
1.
Sclair Polyethylene for Rotational Molding Manual, DuPont, Canada, May
1978. 2.
“Rotational Molding of Polyethylene Powders”, Borealis (formerly
Neste Chemicals) guide. 3.
Rotational Molding of Microthene Polyethylene Powder, pp 4, 1965,
Quantum Chemicals 4.
Carrow, G.E., “Here’s what you should know about rotomolding
crosslinkable polyethylene”, Oct. 1982, ARM, Chicago. 5.
Titus, J., Engineering Design Handbook: Rotational Molding of Plastics
Powders, April 1975, Dept. of Army Headquarters, United States Army Material
Command, AMC Pamphlet No. 706-312. 6.
Beall, G., “The Engineer’s Guide to Designing Rotationally Molded
Plastic Parts”, ARM, Chicago, Oct. 1982. 7.
ARM Trouble Shooting Manual for Rotational Molding. 8.
Taylor, T., “Mold Construction”, Rotational Moulding Training
Seminar, Trowbridge College, 1992. 9.
Nugent, P.J., “A Study of Heat Transfer and Process Control in the
Rotational Moulding of Polyethylene Powders”, Queen’s University, Belfast,
Sept. 1990. 10.
Crawford, R.J., “ARM Cooling Study”, ARM, Dallas, Oct. 1995. 11.
Guzikowski, G., Details on Kelch Vent-Sure system and test results on
flow experiments, Personal Communication, Kelch Corp., January 1996. 12.
Gillian, T., Details on Ferry dual passage inner air system, Personal
Communication, Ferry Industries, February 1996. | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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