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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.
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