Gas injection moulding: Gas Assist Injection Moulding is the most effective method of applying pressure to an injection-moulded part during cooling.
Gas assist injection moulding is a low-pressure process, reducing the clamp tonnage required for moulding.
Nitrogen gas is applied to the part internally, either directly into thicker sections of the part or via a network of gas channels that are added to the part.
Through proper design, this lower gas pressure is applied evenly throughout the part and can be sustained during the entire cooling phase. High-pressure differential within the mould cavity is avoided, thereby eliminating the associated defects.
The processing in gas injection moulding takes follows:
1. Closing the mould.
2. Injecting the plastic.
3. Gas injection in the plastic melt.
4. Maintence of the gas pressure during solidification.
5. Reduction of gas pressure.
6. Opening the mould.
7. Removal of part from mould.
Gas assisted injection:
1. Injection through the machine nozzle.
2. Injection directly into the mould.
Involvement of gaim for large complex parts: GAS-assist injection moulding, a process pioneered morn than a decade ago, injects high pressure gas through hollow part channels, forcing the molten plastic against the inside of the mould. Conventional injection molding makes solid parts, but the gas assist process makes a hollow in the part. The advantages are two fold it reduces cycle time, it reduces the amount of the plastic needed to make a part, uses smaller press size and gives an excellent surface finish. The results are faster production at lower cost. The large complex parts are moulded by GAS assisted injection moulding because the following problem aries in injection moulding.
1. Cycle time is more
2. More plastic raw material is used
3. Cooling time is more
4. Sink mark
5. Cost of large complex parts is increased
6. Decrease in strength of large complex part
In GAIM we overcome from these problems.
Introduction and principle of gaim: GAS- assisted injection moulding process as the name implies, comprises a partial injection of polymer melt into the mould cavity, followed by an injection of compressed inert gas into core of the polymer melt through the runner or any designed gas channel into the mould cavity. Hence a lower injection/packing process and clamp tonnage are required than the conventional injection moulding of thick walled parts, producing significant weight saving, avoiding part warpage and sink mark & due to reduced filling pressure and residual stress and resulting in shorter cooling time by coring out thick part of the part. The trick of using injected gas is not entirely now the first mention of the process was recorded 20 years ago, when Ernst Mohrbach registered a patent for the production or heels for ladies shoes. Due to problem faces during the injection moulding of large complex part and thin wall parts, GAIM is comes into use. This tech. involves the injection of an inert gas, usually nitrogen gas is used. This is not confused with structural foam moulding as in the case no foams core is introduced instead the gas forms a series of inter connecting hollow channel in the thicker section of the part. The gas pressure is maintained through out the cooling cycle. Due to which gas packs the plastic into the mould without a second stage high pressure packing in the cycle as used in injection moulding which requires high tonnage to mould high parts.
Material used in gaim process: The experience to date of raw material and machinery suppliers as well as plastics processors who work with gas pressure has found that as a general rule all injectable polymers material can be used with the internal gas-pressure method. Very soft, unfilled materials such as thermoplastic elastomers are considered problematic. PVC processing can also be critical if the gas is injected via the m/c nozzle, which may be result in thermal damage to the material. For certain material e.g polyamide, Polybutylene terephthalate, some specialized manufactures offer modified system specifically for this process. PA6 and PA66 as well as poly-propylene often in a glass fiber-reinforced form account for the same lion's share of application using gas pressure today. It has been found that not only thermoplastic but also thermosets are suitable for internal gas pressure technology. Various materials have been tested such as PF, unsaturated polyester resin, melamine and epoxy resin. It has been found that weight saving of between 30 and 40 percent can be achieved depending on the product being tested. The key material characteristics for the process are melt viscosity and elasticity. Polyolefins are widely used in gas assist, have higher degree of shrinkage and there for more secondary gas penetration.
Equipment used in gaim process: The main equipment which determines the GAS-assist injection moulding process is given below:
Type of nozzle: Nozzle design is an area where advance are being made in terms of reliability, case of use and resistance to plugging.
The nozzle used for injection of gas when the gas is injected directly into the tool is same as that used in injection moulding m/c. But when the gas is injected through sprue, runner or gate a typical nozzle is used in that case called directional nozzle that ensures nitrogen gas flow into the mould in the same direction as resin flow. The nozzle has a tip angled at 90o of the body. According to Roger Harwood for RG mold & engineering, Grand Rapids Mich. a tool maker experienced in building gas assist molds, every job is different. The fist thing you look for is how to get gas into the desired area. Designing the gas channel is depended on the part design as well as the particular gas assist process used. The part has to be gates in such a way that the areas last to fill with resin are at the ends of the gas channel in the thick section. After the part is designed correctly he adds gas entry timing and pressure critically influence control of gas penetration. Gas channel layout is particularly critical when gas is injected through the nozzle and has to travel some distance to the desired place in the mould. If the channels are too big, you have a racetrack effect where the melt stays in the channels and does crazy things to your filling process. If the gas channels are too small, the gas goes all over the place and does not stay in the channel. It is important to consider the resins prop.and flow characteristics to help size the gas channels prop.
Mould for gaim: C-mould gas assisted injection moulding simulates the initial filling of a cavity with polymer, followed by the injection of pressurized gas through the nozzle runner or directly into the tool, at a specified time under gas pressure or gas volume control.
C-stands for Cordnel University which is the manufacture of mould used for as assisted injection moulding. Depending on the m/c and using of gas injection system. Gas is injected at a regulated pressure profile with gas volume control; gas initially is metered into a compression cylinder and injected under compression with movement of plunger. C-mould now provides full control simulation over gas value gate closing and opening us can put gas anywhere, at any time in any part.
Clamping system: Clamping system in gas-assist injection moulding process is same as used in injection moulding. Both systems hydraulic and toggle can be adopted. GAIM permits lower clamping force.
Actuation system: Various hydraulic cylinders are provided which perform different factions by the indication of the limit switches (sensor switch) provided in the m/c. Functions such as injection unit forward, injection forward then injection unit back ward.
Gas unit: Gas-assisted injection molding process as the name implies, comprises a partial injection of polymer melt into the mold cavity, followed by an injection of compressed inert gas into the core. The process utilizes compressed gas as the packing medium one or more gas cylinder are provided in the system for the aforesaid process. The compressor/controller increases the gas pressure from the 70 psi produced by N2 generator to the 3,000 needed for gas as molding.
Supply of N2:
1. Nitrogen supply can be arranged as desired with.
2. Nitrogen bottles or banks of bottles
3. Liquid nitrogen
4. Nitrogen generation from atmosphere.
5. The decision for the types of N2 supplies rest soon the volume used and other factors.
Some processors replaced its cylinder banks with a membrane nitrogen production system, which produces N2 from compressed air at costs below cylinder or bulk purchases. The system includes a compressor and storage cylinder bank to ensure that the gas-assist injection moulding control unit recent a consistent, un-interrupted N2 supply. A range of high pressure nitrogen gas generation equipment has been developed for gas assisted moulding processes. The lower range units, the MP-120 and MP-220 can supply upto five m/c. Higher N2 output units the MP-400 and MP1000 have capacities upto 1000/minute, which is sufficient for between eight and 12 m/c operation depending on molding conditions. The compressed air feeded to the generator is filtered and dried. N2 supply upto 350 bar is standard, but can be intensified upto 700 bars if required. Pricing start at US$ 45,000 for the generator and compressor system.
2. Gas injection time: Duration of gas injection which includes gas injection during the filling stage and the subsequent "Gas Holding" stage. The gas injection time begins when the timer for core or gas injection triggers injection of the gas and ends at the movement injection of the gas and ends at the movement the gas pressure is released or when the cools sufficiently. So that gas is no longer penetrating.
3. Recycling of N2: Many users recycle the N2 for use in subsequent cycles although this is not common practice with lower gas cost.
Recovery rates in the 70-90% range are readily attainable, but gas recovery is not possible with some resin. N2 is recovered through the in-mold gas injection nozzles after each cycle, filtered and then fed to a fixed volume receiver. Recovery rated of up to 95% are possible and the shortfall is topped off by 99.5% pure N2 generated by the PSA unit. The gas can then be fed to a high pressure intensifier. GE plastics mike coropreso cautions that molders should watch the purity of the N2 when using membrane system which is very rate-dependent 99% N2 purity is good for just about any plastics. If the purity is too low, O2 can cause burning and oxidation in the mould.
Advantages of n2 gas: N2 is the preferred gas for gas-assist injection because it is non-flammable and does not discolor the part as oxygen containing compressed air does. But finding a cost effective production supply of N2 with the required purity, flow volume and pressure had been a problem for processors.
Gaim process: GAIM process is a low pressure process it facilitates thick section in parts, which act as conduct for resin flow. This permits lower injection pressure and clamping forces. The gas assisted injection moulding is different from blow moulding or injection blow moulding in which a much large volume of air is used to inflate the plastic melt parison to a finished shape inside the mould. In GAIM process the inner gas (usually N2) is used to hollow out a thick section of plastic product the same difficulties are arises in GAIM of materials having high shrinkage and temp. Sensitive material as in injection moulding.
GAIM the ADVANTAGE:
1) Less Cost: - Less material, improved quality, Fewer rejects.
2) Reduced tool costs: - Shorter Cycle Time, Cycle time reduced 10% or more, Mass is reduced in thick sections
3) More Strength: - Eliminates undercuts, Permits part consolidation
4). Less Weight: - Heavier ribs and bosses are possible, Allows structural tubular sections
5). Quality Improvement: - Eliminates knit lines, No stresses, and Removes sink marks
6). More Design Options: - Realize from 10% to 50% weight reduction.
The GAIM process: can create large or intricate parts with no "moulded-in" stress or sink marks.
The process begins when a "short shot" of thermoplastic is injected into a mould, partially filling it with a predetermined amount of resin.
A resin/gas shut off valve is used to isolate the resin in the mould from the barrel.
Using either a nozzle or gas pins, nitrogen is injected into the center of the hot resin, following the path of least resistance it forms hollow channels and directs the resin to the last area of the mould to fill.
As the gas expands in the cavity, forcing the plastic in front of it, all of the surfaces receive an equal pressure creating an even "pack out" of the part. This brings the following benefits: shortened cooling cycle, elimination of sink marks and improved dimensional stability
The nitrogen gas is vented either through a sprue break, if gas was injected through the nozzle, or back through the self-venting pins if injected by pins.
Procedure of gaim: The Procedure of GAIM is some what similar to injection moulding but in this process the pressurized N2 gas is injected to penetrate the part via network of thicker sectioned channels.
1. The process starts with a standard injection moulding process, partially filling the cavity with melt.
2. At a pre-determined time, high pressure gas is introduced into the melt.
3. The gas pressure causes the melt to continue to fill the cavity.
4. At the end of fill the gas pressure is maintained the polymer shrinks as it cools and the gas pocket expands.
5. Gas pressure in the pocket acts like a holding pressure material ahead of the gas pocket acts as a melt reserved.
Because the gas has a very low viscosity, the gas pressure is the same at all location with in the gas stream. That is why the gas/holding pressure is more uniform then in possible with conventional injection moulding where modification needed. After the cooling time product is ejected out from the mould.
1. Process cycle for gaim:
1. FILLING STAGE
2. Cycle begins
3. Resin injection stage
4. Resin injection ENDS
5. Delay time
6. GAS injection begins
7. GAS injection during filling
8. Cavity filling completed.
Post filling stage:
1. Gas pressure hold
2. Gas pressure released (Gas injection in post filling)
3. Mould opens
4. Mould opening stage
Advantages/disadvantages:
1. It offers a way to mould large parts with only 10 to 50% of clamp tonnage that would be necessary in conventional injection m/c.
2. Cycle time is reduced in this case
3. The thick but hollow sections provide excellent rigidity.
4. In case of rod shape moulding upto 50% weight reduction is there.
5. Upto 50% reduction in cooling time.
6. It does not create sink marks or warpage problem.
7. The finished parts have excellent surface finish with least distortion.
8. Scrap rate is reduced from 15% to 20%
Dis-advantages:
1. Gas channel with sharp banks many introduce structural weakness due to the tendency of gas to travel close to the inner radius or even to jump across in this section.
2. With C-mold gas assisted moulding may potential problem such as poor gas penetration undesired weld line location and air traps can be detected prior to making the mould.
3. 100% pure N2 is required. If purity is too low; O2 can cause burring and oxidation in the mould.
4. Recycling of N2 is very costly. Prices of membrane separation systems are coming down but typical out-lays range from $ 10,000 to $ 70,000 depending on flow rate required.
Process techniques:
1. Nitrogen gas is used in the process.
2. A simulations injection of plastic and gas is not possible as the gas would immediately brake through the melt front.
3. The timing of gas injection and the gas pressure are of great importance to produce useful moulding.
4. Pressure versus time plot’s with and without brake through.
5. As long as the gas pressure is great that the melt pressure in the unfilled mould the gas will advances and eventually break through the melt.
6. The gas can be injected only when the gas pressure is a high as the melt temp.
7. A typical pressure diagram of mould filling.
Rim (Read ion injection moulding): When the mix head removes back two or more liquid reaction flow at high pressure (1500 to 3000 PSI) into the mix head to polymerize as they flow into the mould.
Rim processes consist of:
1. Supply track (used to store and blend component) also maintain the level in conditioning tracks.
2. Conditioning tanks controls temperature and degree of dispersion of the reactants by low pressure recirculation.
3. High pressure metering of the reactants to the mix head at sufficient flow rate of good mixing and at the proper ratio for complete polymeration.
4. The reaction mix flows into the mould and filling in less than five seconds where the polymer cures and solidifies.
Application includes rigid use than foam for automatic bumpers furniture parts etc.
Advantage of RIM:
1. Large part size.
2. Low energy consumption.
3. Complex part compatibility.
4. Paintable surfaces.
5. Light weight parts.
6. Corrosion resistance.
7. Variable chemistry.
8. Lower temperature.
9. Lower moulding pressures are employed.
Disadvantage of RIM:
1. High wastage.
2. Rim processing equipments are very costing.
Application of RIM:
1. Bumper and fascians.
2. Cushions.
3. Shoe soles ferames.
4. Fender, pandas, cunrests steering wheels.
5. Ben key ends.
Gas assisted injection moulding/water assisted injection moulding:
1. Developments in water-assisted injection moulding tech. are now leading to their first commercial application.
2. The first molder to go public with a WIT product is Sulo, in Herfort Germany.
3. Sulo originally planned to use GIT but switched to WIT when it found that cycle tine could be cut 70% because of extra cooling power of the water. As a result productivity at Sulo will be 3.5 tunes higher.
4. The trolleys made of filled poly-propylene weigh 17 kg which is less than half that of a metal trolled yet the are just as strong.
5. Nylon, polyolefins, acetyl and stridencies have all been tested at the 1 kv.
6. WIT enables large hollow part to be molded to a better standard of a quality and for more economically that with GIT because water is incompressible, it not only produce better shipped hollow sections with smoother surface but also permits better process control and taster cut.
7. Despite the generally lower working pressure required for water upto 240 bar compared with 400 bars for N2 his possible to mold hollow part with cavity diameter of 20 mm and more.
8. WIT scores over GIT for cooling of parts with walls of 20 mm thickness or greater.
9. WIT is also a better process for parts with many bonds and curves or with wide variation in wall thickness.
TOP TEN GAS ASSISTED INJECTION MOULDING TIPS
Below is an outline of design and processing tips for gas assist injection moulding.
Following these tips will allow manufacturing the best product utilizing GAIM equipment.
Always design gas channels to direct gas to the last area to be filled.
Never use gas with a hot runner system, unless valve gate shutoffs are installed.
When spillovers are used, keep the thickness dimensions low. Gas takes the path of least resistance. If it is easy for gas to enter the spillover, it will occupy space that was intended for displaced resin.
4.) Gas channel dimensions should be 2 to 2 1/2 times nominal wall thickness. If desired results cannot be obtained with the smaller dimension, it is easier to remove more steel than it is to add more steel. (Remember, this dimension includes nominal wall thickness: i.e., .100 wall,
gas channel .200 to .250 including nominal wall).
5.) Ensure alignment of the barrel is present. A nozzle tip which does not seat properly can allow gas leakage when injecting gas through the nozzle
6.) Make sure seat of nozzle tip is true. A damaged nozzle tip can allow for gas leakage.
7.) Always use the least amount of gas pressure, gas time, and resin as possible.
8.) Before shutdowns, manually inject gas to assure the gas valve is clear.
9.) Assure clean, dry air is used in all cases of air supply to gas machines and generators.
Contaminated air supplies will cause premature wear on components of the gas equipment.
10.) Use the largest allowable orifice in a nozzle tip. This will allow maximum gas flow through the tip.
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