| A new molding technique, gas-assisted injection molding (GAIM)which is capable of producing parts having both thick and thinsections, with structured rigidity and not sacrificing surfacequality, has received extensive attention in recent years. In thisprocess, molten plastics are injected into a mold followed by aninjection of pressurized nitrogen gas into the core of the melt toproduce hollow parts. Despite some pitfalls in the practicalapplication of GAIM process, it offers a number of advantages overconventional injection molding (CIM). â‘ GAIM can substantiallyreduce cycle time. The melt injection time and cool time are decreased,the pressure packing time is eliminated. It is propitious to reduceparts cost. â‘¡GAIM can reduce the clamp tonnage greatly. During theCIM, clamp tonnage must large enough to meet the needs of packingstage, especially to solve the problem of sink marks. However, thelow gas pressure is needed for packing in GAIM, thus, the clamptonnage is reduced. â‘¢GAIM can save material. The first stage of thisprocess is shot short, so the material is saved. â‘£GAIM can eliminatethe pitfalls of parts greatly. Part quality can be improved byreducing residual the stress, warpage, sink marks and shrinkage thatare normally encountered in CIM.Focused on the formation mechanism of gas bubble blemish insidethe gas channel wall for a GAIM part, the main study results aresummarized as follows:1. Study of formation mechanism of gas bubble blemish inside the gaschannel wall for a gas-assisted injection molding partWhen the fluid temperature and pressure reach their critical pointsor above, the fluid will be in a state called the supercritical fluid(SCF). In this state, the fluid is dissolvable. In other words, itmay dissolve in all melted polymers. During the stage of gas-holdingin the GAIM process, the gas pressure in the channel is usually over10 MPa, and the temperature of the gas close to the melt is also veryhigh (over room temperature highly). As a result, both gas pressureand temperature are over their critical points. Obviously, the N2inside the gas channel cavity has been the SCF during the stage ofgas holding.Let us take a thick and rod-like molding part as an analyticalexample. When the process condition is not controlled well, thesupercritical N2 and the polymer melt may diffuse into each otherduring the stage of gas holding. After the gas holding is over, thedissolved N2 in the polymer achieves a supercritical state, and theN2 close to the wall of gas channel diffuses out. In this process,the N2 dissolved in the melt deeply come together and gradually growup until the cellular bubbles are formed in the parts.2. Study of mechanism of the supercritical N2 diffusing into thepolymer melt for GAIMThe stage of gas holding for GAIM is a stage that the supercriticalN2 may diffuse into the polymer melt continually. Analysising thetheory of diffusing will help to understand the formation mechanismof gas bubble blemish inside the gas channel wall for a GAIM fully.As the first of all,we analysis the mechanism of the supercriticalN2 diffusing into the polymer melt and then establish the equationfor gas diffusion at the different coordinate based on the Fick'sdiffusion law. Secondly, we settle the equation. Following the resultof diffusion eqution, the primary causation of the forming dept aboutgas bubble blemish inside the gas channel wall for GAIM is holdingpressure, temperature of melted polymers and holding time.3. Study of matter of forming about gas bubble blemish inside thegas channel wall for GAIMBased on the result of diffusion eqution, We can get to know thematter of the relt about gas bubble blemish inside the gas channelwall for GAIM, that is holding pressure, temperature of meltedpolymers and holding time. With the melt temperature going down, thediffusing dept will reduce. On the other hand, the diffusing deptwill augment. The diffusion time is an second important factor ofaffecting the diffusion depth. With increasing gas-holding time, thedepth for the supercritical N2 into the polymer will increase, andvice versa. At lase, the holding pressure is an important factor ofaffecting the gas bubble blemish inside the gas channel wall for GAIM.Adding the holding pressure will add the thickness of gas bubbleblemish, debasing the holding pressure will reducing the thicknessof gas bubble blemish.
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