COMMERCIAL PLASTICS COMPANY
GAS ASSISTED INJECTION
A Guide in Concept, Application & Processing Basics
800 E. Allanson Rd., Mundelein, Illinois, 60060 (847) 566-1700 (Fax) (847) 566-4737
REQUEST FOR APPLICATION
1. What is the application?
2. What material will be used?
3. Any special material requirements? [UV, UL, Custom Color?]
4. What are the overall parts dimensions [L x W x H]?
5. What is the nominal wall section?
6. Is this an aesthetic part?
7. Will any secondary operations be required?
8. Is this a structural part? Explain
9. What is the estimated annual usage?
10. What are the timing requirements?
11. What is your target pricing?
PLEASE SEND PRINTS AND SPECIFICATIONS, IF AVAILABLE, ALONG WITH THIS FORM
Commercial Plastics Company, INC.
800 E. Allanson Rd.
Mundelein, IL 60060
Attn: Sales Department
or Fax to (847) 566-4737
(e-mail) bmarshall@ commercialplastics.com
GAS ASSISTED INJECTION MOLDING TECHNOLOGY
Concept, Application & Processing Basics
I. What is Gas Assisted Injection Molding?
A. Process Basics
Gas Assist Injection Molding is a process enhancement to conventional injection
molding, involving the injection of high-pressure nitrogen gas into the resin melt-stream
immediately after injection of the resin. The intent is not to cause mixture of the nitrogen
and resin, but for the nitrogen to displace resin in flow/gas channels and thicker sections
of the molded product. The process is a high speed, low pressure injection method,
enabled by “short shooting” the tool and completing the resin filling phase with a second
fluid, i.e., nitrogen, at much lower pressures than with conventional injection molding.
The three primary considerations for successful application of the technology are 1)
repeatability and control of molding parameters on the molding machine, 2) precise
control of time and pressure of the nitrogen injection phase, and 3) control of the nitrogen
penetration within the molded product. It is important to note that the flow path of the
nitrogen in the partially filled tool is primarily controlled by the partial resin fill in the tool
cavity, volume of resin, and characteristics of the resin in the tool cavity, not by the
B. Advantages of Gas-Assist Injection Molding
The primary reasons for using Gas Assist injection molding methods are 1) to improve
productivity, 2) reduce resin costs, 3) produce higher quality parts, and 4) expand design
options not previously available for injection molded products.
1. Cycle Time Reduction
Reduced cycle times are a benefit of the process for numerous reasons, the most
significant being elimination of high injection pressures in the tool, and elimination of
thick sections in the molded product that could retain heat. Low injection pressures are
realized in virtually all applications because the tool is never completely filled as it is in
conventional injection molding. After the partial filling, the injection barrel is isolated from
the gas injection point (to prevent nitrogen from entering the barrel and contaminating the
resin) and nitrogen is injected into the cavity, displacing the hotter (and more fluid) resin
in flow channels and thicker sections of the product. The displaced resin completes the
filling of the part under pressures of usually no more than 1,000 to 2,000 PSI, far lower
than “pack-out” pressures experienced in conventional injection molding.
Because high injection pressures are avoided and thicker sections are now hollow, cycle
time is reduced due to shorter cooling times and elimination of the need to “pack-out” the
part with ram injection pressure. All pressure-hold for packing of the part is maintained
with the gas pressure, not the ram. This allows the injection screw to return for the next
shot immediately after completion of resin injection, also reducing cycle time.
By eliminating high injection pressures in the tool, variations of molded-in stress are
significantly reduced. Without the stress differentials, there is no need to maintain a
longer cycle in attempt to cool the part in the desired shape (with inherent stress). With
reduced stress variation, the tendency to warp is reduced or eliminated.
2. Reduced Part Weight
Part weight reduction is accomplished through two methods. The most frequent means
of reducing part weight is through reduction of mass in heavy sections, bosses, and in
the area of intersection of ribs. Actually actually making bosses often facilitates this and
ribs heavier than would not be advisable with conventional molding. As an additional
benefit, the resulting channels can enhance strength or rigidity by removing the resin in
those areas to form hollow/tubular geometry.
Weight will be reduced to a minor degree in “nominal wall” products due to the lower
pressure fill and lower, more uniform pressures in the tool. The gas pressure provides
the hold-pressure in the tool, and, as the part cools, nitrogen gas displaces the resin that
is shrinking rather than ram injection pressure packing additional resin into the cavity.
In many structural foam applications the nominal wall thickness may be reduced, and,
with proper application of flow/gas channels, parts may have the same desired strength
and rigidity as their structural foam predecessors. This is also true for heavy-wall non-
foamed products. With the addition of flow/gas channels in the part, the tool is easier to
fill as the channels act as in-part cold runners. The resin filling the cavity, after the
channels have been filled, is primarily filling from the channel or channels, not just from
the gate itself. For example, a product with a channel located along an outboard wall will
complete the fill from the full length of the channel, not just the gate, similar to using a
fan-gate along that entire length.
When the flow channels become gas channels (after gas injection) part strength and
rigidity is enhanced, as the channels become tubular structures. With this is mind,
products designed typically with heavy wall sections may be reduced in thickness,
resulting in a weight savings.
3. Reduced Resin Costs
With the design advantages of gas assist molding, many applications have realized the
ability to reduce the grade and cost of resin being used for an application. With the
greater ability to fill a tool and the enhanced strength of rigidity enabled by structural
channels, it is often possible to use a resin of lower cost than the original design intent.
As an example, in some outdoor products PVC may be replaced by a UV stable, filled
polypropylene. In automotive applications, lower cost resins often are utilized where
stability and strength requirements previously required glass filled or higher heat resistant
4. Expanded Design Options
Molded products are no longer restricted to confining parameters such as boss and rib
size relative to wall thickness. Conventional injection molding techniques have many
restrictions that frequently inhibit or disallow design options. With Gas Assist injection
molding, many of those concerns are eliminated or greatly reduced. Heavy ribs bosses
are not only acceptable, but frequently necessary to facilitate the process. This is not to
say that all products need to include these features, but where necessary or desirable
they are part of the design and aid in the Gas Assist process, and are not characteristics
that will be detrimental to qualify products during mass production.
Every gas channel is utilized first as a flow channel and will become, after injection of the
gas, a gas channel. The channels are designed into the part to facilitate the process,
and ultimately improve product strength as well as improve the molding process itself.
The flow channels also function to frequently eliminate the need for hot runners, as the
channels allow for increased flow lengths. Often, tubular sections can be designed into
the product and eliminate the need for undercuts and lifters in the tool. This is another
cost saving feature, as costs are reduced by reducing complexity and often the size of
the tool. Clamp tonnage is seldom a consideration with gas injection, as the criterion for
machine size is not clamp pressure, but mold size and shot size. Tools may be run in
molding machines at a lower tonnage that when conventionally molded, and with less
“action” in the tool, tools for larger parts often may fit in smaller molding machines due to
the reduced complexity of the tool.
5. Enhanced Part Strength
As noted above, part strength may be increased, as tubular and hollow sections in the
part create a geometry that gives greater structural rigidity to the product being produced,
without adding mass for heavier nominal wall thickness and/or structural foam. It is not
to be implied that impact characteristics of the nominal wall thickness areas of products
will be enhanced, as that is not a feature of the gas injection process. Improvements to
product strength will be realized through reduction of weld lines, addition of tubular
structural sections, and elimination of stress, which in a conventionally molded part will
result in premature breakage and product failure.
6. Quality Improvement
Production repeatability and nonconforming part reduction are significant benefits of the
Gas Assist process. With adequate control of shot size and other molding parameters,
molded products are more uniform in size due to the elimination of molded-in stresses
which result from typically high pack-out pressures. Stress in a molded part results from
the molded product attempting to return to a physical condition other than its molded
configuration after cooling. Due to the low and more uniform pressures with Gas Assist
molding and reduced variation of pressure within the part, stress is reduced, and part-to
part size and other product attributes can be maintained. Post-molding warpage is
Weld lines, which are necessary to avoid on Class A surfaces can be significantly
reduced with gas injection. As noted before, a tool designed for Gas Assist primarily fills
from the flow/gas channel, not from multiple gates from a cold or hot runner. With fewer
gates to the molded part, weld lines are reduced and in some cases, eliminated. Where
weld lines are unavoidable, it is often possible to re-locate them to an area where they
will not be visible on the Class A surface.
II. Product Applications
A. Part Strength and Rigidity
Structural integrity and part stiffness are major advantages for conversion to Gas Assist.
Flat, relatively thin wall parts can be made rigid with the addition of a gas channel or
channels where allowable in the part, with accommodation for proper filling of the cavity.
The only revision to a part would be the channel or channels, with resin gated to the
channel and the subsequent gas injection phase displacing that resin for final tool fill.
The part would be “short shot”, with the flow/gas channel filling with resin as well as up to
99% of the remainder of the cavity. The nitrogen is injected, evacuating the flow/gas
channels and completing fill of the remainder of the cavity. Note that the cavity, in this
example, never is completely filled with resin. Resin in the channel is displaced to fill the
last-to-fill sections of the tool under reduced pressures. The channels(s) will result in a
tubular cross section, which will significantly increase rigidity of the part, while low
pressures facilitate a stress free part and reduce cycle time.
B. Long Flow Lengths
Long flow lengths are readily accomplished utilizing gas injection. A common example is
chrome plated front facia molding approximately 70 inches long, 3 inches wide, with a
.100-inch nominal wall. The example part is molded in plating grade ABS with one gate
into a post in the center of the part, of a diameter of approximately .150 inches. In this
example, the cavity would appear impossible to fill, and if accomplished, would result in a
very stressed, warped molded part with several weld lines. However, at the front edge of
the molding there is a flow/gas channel, which is approximately .220 inches in diameter.
At the time of injection of the short shot, the melt fills approximately 96% of the cavity,
including and from the flow/gas channel. At the point of gas injection, the gas displaces
the resin in the channel, completing the filling of the thin wall section of the molding.
Effectively, the flow length in the nominal wall, under any pressure at all is 3 inches, not
36 inches, as the final filling of the thin wall section is from the flow channel, not from an
adjacent thin wall area of the cavity. Consequently, the part can be molded under
minimal pressures; stress, warp and weld lines are eliminated, and close dimensional
tolerances are held.
C. Heavy/Problem Ribs and Bosses
The natural tendency of the pressurized gas is for it to flow to the path of least resistance
in the tool. This area will be where the resin is the thickest, and consequently, the most
fluid, as higher temperatures are retained in the heavier sections. Correspondingly, gas
channels are directed to locations under bosses and intersecting ribs, allowing those
locations to eventually become the gas channels. Because nitrogen vacates the resin in
the thicker areas, sink marks are eliminated, as there is no longer a thick section of resin
to cause excessive shrink. Furthermore, uniform nitrogen pressure is held on all internal
channel sections, thereby packing resin in those areas against the wall of the cavity.
This further facilitates shorter cooling cycles. Ribbed sections in a molded part will not
cause sink marks to read through to the Class A surface.
D. Structural Foam Products
Structural Foam Products frequently have the highest per-part cost reductions available.
This is because with usually minor tooling modifications on existing products, resin usage
and cycle times can be significantly reduced. This is accomplished by strategically
locating gas channels to increase part strength on the reduced wall-thickness product.
Cycle times can be reduced from as low as 180 seconds and as high as 6 minutes, down
to 60-90 seconds, depending on part size and resin selection. Frequently, existing tools,
without modification of wall thickness, can utilize Gas Assist by eliminating all gates
except one central sprue, and by adding properly designed gas channels to orient the
resin and gas to the desired locations. A uniform-wall part of any nominal wall thickness
and without channels would not be acceptable for the Gas Assist process, but a part with
the differentials resulting from flow/gas channels and intersecting ribs will usually be a
candidate for this type of conversion.
There are no inherent disadvantages to the Gas Assist process. There are, however, a
few circumstances where it would not be recommended. For example, resins with very
high melt flow indexes should not be used, as they displace too easily in the tool to allow
the gas to be confined to the thicker sections/channels in the cavity. This is due to
reduced surface tension of the melt in the channel areas. Gas could permeate the
thinner sections, resulting in weakened parts. The potential penetration into the nominal
wall sections would also be variable, resulting in loss of control of the process entirely.
This is usually overcome if the base resin is available with a lower melt index, which is
usually the case. As previously noted, the tool will fill much easier with gas injection,
and, a very high melt flow resin was probably originally selected only due to anticipated
difficulty in filling of the cavity.
As mentioned previously, gas must not be injected through a hot runner. An existing hot-
runner tool, however, may be converted to Gas Assist by utilizing gas pins, and if
necessary, adding gate valves to the tool to prevent resin and/or gas from the cavity from
backing into the runner system.
Gas injection should not be used where there are relatively thin or uniform wall thickness
without gas channels. Again, the gas has to be directed where to go. In a part without
the differential of nominal wall thickness to flow/gas channel thickness, it will be
impossible to control direction of the gas flow.
III. Processing Methods
A. Gas Assist Systems and Equipment
1. Control Systems and Accessories
The GAIN Gas Assist injection molding process is made possible through finite control,
regulation, and injection of high pressure, (up to 10,000 PSI) nitrogen gas. This involves
a means to intensify gas pressure, control of the high-pressure gas, and delivery of the
gas to the desired areas within the molded product. GAIN’s Senior Gas Kits receive
purified nitrogen at a minimum pressure of 500 PSI (2,000 to 2,500 is ideal and
recommended), intensify and store the gas at 10,000 PSI, and with PLC controllers,
regulates and delivers the gas with precise control of pressure and time. Junior Gas Kits
and Satellite I systems receive the high-pressure gas from a Senior Gas Kit or central
high-pressure system, and operate in exactly the same manner.
The GAIN Gas Kit uses a unique phased pressure control system, providing up to six
phases of time and pressure following the gas injection delay period. (Gas injection
delay may be from 0 seconds to any delay desired. Usually, the delay will be less than
one second.) The Gas Kit system requires minimal interface with the molding machine;
only two signals are mandatory. 1) A signal that the mold clamp is closed, and 2) a
signal indicating the injection ram has completed its forward stroke. “Ram forward” is the
set-point to begin the gas injection sequence. The gas delay timer begins, and the
subsequent time and pressure phases follow. All control is through a “touch screen”
Allen Bradley PLC controller; the process is programmed by the injection-molding
technician at the time of molding machine and tooling set-up. System setup is a simple
process, requiring only connection of the signal inputs to indicate clamp closed and ram
forward position, and inputting of the gas delay and pressure/time phases. After setup,
the process is automatic, controlled by the PLC and valves in the GAIN Gas Kit.
2. Injection through Sprue
The most common and simplest method of gas injection is through the sprue. With this
method, the part is gated directly into the tool cavity (such as with center gating) or via a
cold runner or runners, which feed to the flow/gas channels of the tool cavity. The tool
may have multiple gates to a single cavity, as long as there is careful attention to balance
of the fill of the cavity. An out-of-balance resin fill will cause a condition of out-of-balance
gas distribution. This method is also the most cost effective method, as tool build costs
are minimized, and potential problems such as plugging of gas pins are avoided. This
method may be utilized on single or multiple cavity tools; on multiple cavity tooling, it is
imperative to carefully balance the cavities. Although this is true in any multiple cavity
tool, as the Gas Assist process is a “short-shot” process, it is impossible to compensate
for unbalanced filling by “packing-out”.
It must be noted that with gas injection through the sprue, a hot runner system may not
be utilized, as the nitrogen will either mix with the melt and create foaming: cool the resin
causing defects in the product; or contaminate the melt characteristics of the resin in a
manner that will be detrimental to the appearance and mechanical attributes of the final
product. The success of the process is based on displacement of resin in channels and
intentionally designed heavy sections; not mixing with the resin or creating hollow areas
in a product.
3. Injection through gas pins
Gas injection through a pin in-article or in-runner is the second most common method of
injecting nitrogen into a part. The same features are accomplished, however the high
pressure gas is introduced through precise injection and venting through one or more
gas pins into the tool cavity. In many cases, this is the preferred method, such as in the
case of a hot runner multiple cavity tool being converted to Gas Assist. It does add
some complexity to tool design, and adds cost for acquiring, assembly and maintenance
of the pins. This is said not to discourage the practice, however, as with gas injection
through pins in the cavity, the process is enhanced far beyond the minimal cost
differentials involved. Done properly, this is a sophisticated method that will furnish the
The decision of whether to inject nitrogen “through the sprue” or via gas pins should be
based solely on the design of the part to be molded. As an example, a product may be
designed where it is desirable to have a “nominal wall thickness” at the same point where
it is advisable to edge-gate the part. Gas should never be injected into or through a
nominal wall of the molded product, but into the flow/gas channel, thereby immediately
displacing the resin from the channel into the nominal wall. If gas is injected into the
nominal wall, control of the flow-path of the gas is lost, and so is control of the entire
process. Distribution of the gas will vary, and product integrity will be compromised, as
the remaining resin in the nominal wall will be inadequate for the product design.
B. Nitrogen Supply
Gas Assist injection molding requires a reliable supply of clean nitrogen gas. This is
accomplished by supplying the Gas Kit with nitrogen from pressurized bottles, a GAIN
Nitrogen Generator, or from a central nitrogen system. The primary requirements are a
clean supply of nitrogen (as contaminated gas can cause problems with the gas delivery
system), and a minimum nitrogen input pressure of 500 PSI. The GAIN Senior Gas Kit
intensifies the pressure to 10,000 PSI internally from the gas source. Maximum input
pressure is 2,500 PSI. Obviously, the higher the input pressure, the less the pressure
has to be intensified internally. GAIN’s Nitrogen Generator delivers a continuous supply
of nitrogen at 2,500 PSI, at up to 99% purity, and can supply nitrogen to up to 3 GAIN
Gas Kits. When a molder operates numerous Gas Kits, they may find it economically
advantageous to convert to a central nitrogen system.
C. Molding Machine Requirements
Gas Assist injection molding does not require any special adaptation or modifications to
a molding machine. GAIN Technologies develops and markets “stand-alone” Gas Kits
as well as “modular” systems, which molding machine manufacturers integrate with their
machines. The only molding machine requirement is control shot size, as the volume of
the gas channels and hollow areas in the product is controlled by the short shot, not by
the gas pressure or as sometimes implied, volume control of the gas.
D. Tooling Considerations
1. New Tooling
New tooling considerations involve only a few items that are not part of a conventional
tooling design. Quality input is imperative to determine location and shape of gas
channels and for design advice regarding ribs, bosses and heavy sections of the molded
product. Most often, tool design is simpler than with conventional injection molding, as
the constraints of part design are fewer. What we see most often is the need for quality
input to what new options are available in part design, and to assist the molder or tool
builder in understanding the concepts of flow and gas channels in the process and part
When injecting through the nozzle, gate size and location is critical, and simplicity is the
key. A single gate is preferred in the majority of parts that are conducive to that
configuration. Flow channel/gas channel feed to ribs, bosses and heavy sections require
input from experienced Gas Assist engineers or molders. Please note that a hot runner
must be avoided when injecting nitrogen through the sprue. This does, however, allow
for reduced tooling costs. As a part of our service function, GAIN Technologies offers
consultation on any developmental or current molded part. GAIN provides design input
and recommendations on molded parts, part prints, and tool prints for existing products.
Once the concept of the displacement of resin in the flow/gas channels is realized, the
concept becomes simple. A molder new to the Gas Assist process should consult with
GAIN Technologies to acquire the input necessary for successful startup of the Gas
Assist process. GAIN regularly performs tryouts and demonstrations at prospective
customers plants to demonstrate the system and prove-out the process to the
application. It should be noted that there are no mechanical additions to an injection
mold in order to facilitate gas injection, unless injecting through pins in the tool.
2. Converting Existing Tooling
Existing tooling can easily be converted to Gas Assist injection, provided the basic part
design is conducive to the process. As mentioned previously, nitrogen cannot be
injected through a hot runner. A tool with a hot runner has two options: 1) Inject gas
through pins in the tool, or 2) modify the tool to eliminate the hot runner and revise the
gating method. Gas channels may need to be cut to facilitate the flow/gas channels, and
to follow to the areas where gas is desired. The channels would lead to areas of
intersecting ribs, under heavy bosses, and to any additional areas that would benefit from
a gas channel.
Spillovers are pockets cut into an injection mold, outside the area of the cavity itself,
which are used to receive “spilled over” resin from the cavity during gas injection. The
spillover is used for one of the following basic functions:
i. The spillover receives resin displaced from flow channels and sections
of the molded product when the part design prevents resin in those
areas from being displaced to areas further down-stream during gas
injection. In certain product design, this condition is unavoidable due to
part design considerations. This condition results in from an areaof the
cavity, which would usually be last to fill, filling early in the plastic
injection phase. When gas injection takes place, there is no area for
the resin in those areas to be displaced to, therefore the spillover is cut
into the area adjacent to the cavity, and during gas injection, the resin is
displaced from that area into the spillover
ii. A spillover can be used to create a complete, continuous gas channel,
with the channels actually meeting, joining, and extending into the
spillover. This is the method used to join (2) separate channels in a
iii. A spillover may be used to eliminate weld lines or hesitation lines, and
undesirable surface appearances in a product by displacing the resin,
which includes those characteristics into the spillover, rather than
remaining in the molded product.
IV. Product Considerations
A. Resin Selection
1. Basic Resin Applications
There have been no problems with any base resins, other than those with high melt flow
indexes. It should be noted that reinforced materials are not detrimental to the process,
but actually facilitate the flow of the injected gas through the channels and heavy
2. Melt Flow Indexes
High melt flow indexes should be avoided or changed, as noted above. High melt flow
materials are generally selected when filling a cavity is a problem, however, with gas
injection and proper channel design, cavity fill is not a problem, and resins can be
switched to those with lower melt flow indexes.
3. Reinforced Materials
Gas Assist injection molding has been successfully applied to parts molded in
engineering grade resins up to 50% reinforcement, by weight. These materials have
been fiberglass, calcium, talc, glass bead, and combinations of the same.
B. Part Design
The Gas Assist process does not encumber part design. Many of the constraints for rib,
boss, and heavy design sections are reduced or eliminated. The key to successful Gas
Assist molding is uniformity of the short shot, control of the gas injection pressure and
time, and careful design and location of the thickness differentials for proper gas flow.
Generally speaking, a gas channel leading from the gate in two different directions are
acceptable, as long as they phase out distant from each other, and are balanced with
respect to fill of the cavity. The flow distance attainable is dependent on the nominal wall
thickness and gas channel size and location. Thick tubular sections do not have to be
avoided. Parts with geometry such as shovel handles and baseball bats have been
successfully molded with GAIN Gas Assist.
C. Gating Methods
Gating methods should be kept simple. Frequently, tools with 5 to 10 gates, such as in
many television cabinets, have been reduced to one or two gates, each which will lead to
a gas channel. Excessive gating locations can lead to opposing gas channels, and
should be avoided. Too many gates and/or channels may lead to opposing channels
and pressures within the tool and will inhibit optimization of the process. The gas flow, in
the same manner as resin flow, is linear and should not encounter opposing flows in the
D. Gas Channel Design
The ideal gas channel design is spherical, although this is often impractical. Most
channels, if designed properly, will become hemispherical. The gas channel section
should usually be two to three times the nominal wall thickness. The eventual voided
section in these areas will be approximately 50% of the total thickness in the overall
wall/channel. Smaller channels will not allow the resin flow channel (which the gas
channel is, initially) to provide the necessary reservoir for adequate resin flow and
eventual evacuation. Significantly larger flow/gas channels are also not recommended.
Oversized channels will result in excessive resin being retained, causing unbalanced
filling in those channels and nominal wall sections, increased part weight, and extended
cooling times. In either of the examples of improper channel design, optimum efficiency
of the process will not be achieved.
A. Molding Hollow Plastic Parts
Numerous technical publications and articles have been written implying or stating
directly that with Gas Assist methods, it is possible to produce large, structurally sound,
“hollow plastic parts”. This is, at best, a serious mis-statement. Production of hollow
parts is best left to blow molding. The only case in which production of a hollow part is
possible is with tubular products, such as thick handles or any product with a significantly
or entirely tubular geometry.
The avoided area in a gas-injected product is the by-product of the Gas Assist process,
and only occasionally an intentional “hollowing” of the part or an isolated area of the part.
It should be remembered that Gas Assist is a process enhancement that allows the
molder to reduce or eliminate all of the undesirable characteristics of conventional
injection molding, while affording the cost and product benefits explained above. The
allowance of a significantly hollow area in a part may be possible, but is usually never the
sole and primary intent. We make particular note of this issue, as we frequently see
design concepts and even tool drawings where a product intended for Gas Assist is a
substantially hollow product, and clarification of the abilities of the process are only
realized after considerable cost has been put into the product and process design.
B. Injection Through the Nozzle vs. Through Gas Pins
Many custom injection molders and original equipment manufacturers believe that the
decision whether to inject gas through the nozzle or via gas pins depends on the vendor
of the Gas Assist injection system. This is a common mistake that causes a molder to
base decisions on considerations other than those which are meaningful to the
production program. The decision whether to inject through the sprue or through gas
pins should be made solely on product design, and, to attain the most reliable and
repeatable processing method.
As noted previously, injection of gas through the sprue is the simplest and most uniformly
reliable method. Injection through gas pins should be implemented when product design,
existing tooling design, cavity configuration, or desired gas channel layout does not
appear to be applicable to injecting through the sprue.
C. Gas Injection Into Uniformly Thick Wall Products
Although not as widespread as the items noted above, GAIN occasionally sees
applications where gas is injected directly into thick-wall products, or into tools originally
intended to be structural foam. Although the gas will displace resin following the partial
filling of the cavity, the practice will not allow for efficient processing. When practiced in
this manner, there is no constraint on the travel of the gas within the tool and resulting
product. The gas will travel to the most fluid area in the melt within the cavity, which will
be where the melt is the most fluid.
When gas is injected into a partially filled uniform wall, variations of the tool cavity
environment may result in the remaining wall thickness on each side of the cavity to
change and flow from side-to-side of the cavity may vary. This will usually result in areas
much thicker than desired and/or potentially very thin wall sections opposing a heavy wall
section. Cycle times will be increased, product weights will be heavier than desired or
possible and product integrity will be greatly diminished. The costs involved to properly
convert a tool to the recommended channel configuration will be insignificant when the
benefits of proper configuration are quantified.
D. Difficulty at New Tool Startup
Many published articles state that a molder should allow for additional time beyond that
usually required for startup of a new production tool. This is only true when first applying
Gas Assist technology without proper application and design support. In reality, tooling
startup may take less time, as injection molds with gas injection capabilities may be far
simpler than when hot runners and additional action in the tool is required.
VI. Summary Points
A. Three criteria of utmost importance for successful implementation of the
Gas Assist Process are:
1. Control of the size of the “short-shot”
2. A highly controllable gas injection system
3. Qualified input on the method of injecting the gas, and location and size of
the gate (s) and flow/gas channels
B. The practice of Gas Assist is a process enhancement to conventional
injection molding which will:
1. Decrease cycle times
2. Reduce resin usage and possibly base resin cost
3. Improve product appearance
4. Reduce tooling costs
5. Reduce tooling maintenance costs
6. Increase product design possibilities
7. Enable tools to be run in smaller molding machines
8. Facilitate significantly greater resin flow lengths
C. The gas injection method, whether to inject via nozzle or gas pins, should
be based on product and tooling considerations alone
D. Common problems encountered when processing with Gas Assist most
usually are caused by injection molding techniques or application of the
process to the product, not from gas injection control variations.
E. Gas Assist processing is not universally applicable to every injection
molded product. The decision to implement Gas Assist should be based
1. A need to increase output/productivity
2. A need to improve quality based on problems with
i. Sink marks
iii. Weld lines
iv. Burnt areas in the part due to high injection pressures and shear
3. The need to run a tool in a smaller molding machine
4. A desire to reduce costs through shorter cycle times and decreased resin
5. A desire to contain and reduce tooling maintenance costs
Commercial Plastics Company Inc. believes the information and recommendations within
this Technical Guide to be accurate and reliable. However, since this book is a compilation
of data from various industry sources provided without charge, and inasmuch as Commercial
Plastics Company Inc. has not control over the use to which others may put this material, it
does not guarantee that the same results described herein will be obtained.
No warranty, express or implied, is given. Further, nothing herein shall be construed as a
recommendation to use any product in conflict with patents covering any material, process,
application or its use.
800 E. Allanson Rd., Mundelein, Illinois, 60060 (847) 566-1700 (Fax) (847) 566-4737