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附件2：外文原文
Gas-Assisted Injection Molding
Injection molding is a very popular operation for production of commercial plastic parts with its sophisticated control and superior surface details. However, it has limitations, such as long cycle time for parts with thick sections due to slow cooling. Also packing of thick sections can produce sink marks on the part surface. Large thin parts can have warpage because the residual stress and strain induced during filling and packing. Thus traditional injection molding can be modified to solve these kinds of problems, also to improve the quality of the part and lower the cost of production.
Currently, gas-assisted injection molding is in use and being developed worldwide. In the US, the process is known as Gas-Assisted Injection Molding (GAIM); it is also called Gas Injection Technique (GIT) in Europe (see Fig.4.3.1). This process is developed for the production of hollow plastic parts with separate internal channels. It is unique because it combines the advantages of conventional injection molding and blow molding while differing from both. GAIM offers a cost effective means of producing large, smooth surfaced and rigid parts using lower clamping pressure with little or no finishing. By introducing the gas before complete filling, numerous problems such as warpage, sink marks, and high filling pressure are mostly overcome. Moreover, the process gives great benefits in terms of higher stiffness-to-weight ratio than the solid parts with the same overall dimensions due to the elimination of material placed inefficiently near the neutral axis of the cross section, thus increasing the freedom of part design.
In comparison with conventional injection molding, the gas-assisted process is more critical in terms of process control, especially for multi-cavity applications. The quality of the part is determined by both tool and process variables such as degree of under-fill, gas injection conditions, and mold temperature, thus indicating the importance of process control. The process is attracting many molders due to the demand for highly automated production of gas-assisted injection molded parts.
The gas-assisted injection molding process is the most rapidly growing field
with considerable work going on in the field of controls and the process development. Research interest is drawn towards the development of new gas injection units, the study of the process variable, the efficiency of the production process, and advantages offered by the new process. Many different companies are offering gas injection-molding units with the various options, which are mainly pressure controlled or volume controlled processes.
In gas injection molding, the mold is partially filled with molten thermoplastic, and an inert gas, usually nitrogen, is injected into the plastic. Gas is injected into the molten thermoplastic material using either of two procedures. In one method, a measured volume of gas is pressurized in a container. A valve is opened to allow the gas to flow into the polymer, and a piston is activated to force all gas from the container into the mold. As the gas expands in the mold, its pressure drops. A second method holds gas pressure, rather than gas volume, constant. The gas rapidly travels down the thickest-and therefore the hottest-section of the part, advancing the melt front and filling and packing the mold. Additional plastic volume may be displaced by the pressurized gas as the material shrinks. After the plastic cools, the gas is allowed to escape, leaving a molded plastic part containing internal voids.
The standard GAIM process can be divided into four partial steps. The first step is a stage of melts injection [Fig.4.3.2 (a)]. The cavity is partially filled with a defined amount of melt. The required volume is empirically determined by performing filling studies in order to avoid blowing the gas through at the flow front and to ensure an ideal blowhole volume. Typically the polymer fills the
cavity between 75%~95% before the melt and gas transition.
The gas inlet phase is the second stage, which is shown in Fig. 4. 3. 2(b). Gas may be added at any point in time either during or shortly after melts injection. The gas can enter only if the gas pressure exceeds the melt pressured. In the interior of the molded part, the gas expels the melt from the plastic nucleus until the remainder of the cavity is completely filled. Gas injection pressures range from 0.5~30Mpa (70~4500psi).
At the gas holding pressure phase, [Fig. 4.3.2(c)] the gas continues to push the polymer melt into the extremities of the cavity of the molded article acts as a holding pressure to compensate for path of least resistance as it pushes through the polymer.
The final stage is a gas return for recycling or a gas release to atmosphere [Fig. 4. 3. 2 (d)]. After the gas holding phase, the gas pressure in the molded article is released to the outside by suitable gas return and/ or by pressure release.
A. Advantages of the GAIM process
Gas injection provides a solution to a number of problems
that occurs in conventional injection molding.
(1) Reducing stress and warpage
With gas, the pressure is equal everywhere throughout the continuous network of hollow channels. When designed properly, these provide an internal runner system within the part, enabling the applied pressure, and therefore the internal stress gradients, to be reduced markedly. This reduces a part’s tendency to warp.
(2) Elimination of sink marks
Sink marks resulting from ribs or bosses on the backside of a part have long been a problem. These surface marks result from the volume contraction of the melt during cooling. Sink marks can be minimized or eliminated if a hollow gas channel can be directed between the front surface of the part and the backside detail. With gas injection, the base of the rib made somewhat thicker to help direct the gas channel. With a gas channel at the base of a rib, material shrinks are away from the inside surface of the channel as the molded part cools because the material is the hottest at the center. Therefore, no sink mark occurs on the outside surface as the part shrinks during cooling.
(3) Smooth surface
Unlike structural foam, gas injection permits lighter weight and saves material in a structurally rigid part. With gas holding, a good surface quality can be achieved.
(4) Reduced clamp tonnage
In conventional injection, the highest pressure occurs during the packing phase. The maximum injection pressure is significantly lower in GAIM and a controlled gas pressure through a network of hollow channels is used to fill out the mold. This means that clamp tonnage requirements can be reduced by as much as 90%.
(5) Elimination of external runners
One of the best features of gas injection is that flow runners can be built right into the part. Frequently, all external runners (both hot and cold) can be eliminated, even on a larger and complex part. These benefits include the reduced tooling costs, the lower quantities of regrind from runners, and the improvement of temperature control over the plastic melt. Often the internal runners can improve the flow pattern in the mold and eliminate or control knit-line location resulting from multiple injections from multiple injection gates. In addition to serving as flow channels, the ribs and thick sections can provide structural rigidity when required.
(6) Permitting different wall thickness
A constant wall thickness is maintained in the plastic parts. With gas injection, this design rule is flexible. Different wall thicknesses are possible if gas channels are designed into the part at the transition points. This permits uniform material flows in the mold and avoids the high stresses and warpage that normally result from this sort of geometry.
(7) Cycle time Reduction
Compared with structural foam, gas-injection parts do not have the same inherent insulating characteristics, so that cycle times are faster-reportedly even faster than would be conventional injection of the same part with no hollow sections.
(8) Resin saving
Gas assist plays a direct role in part-weight saving in the conversion of current tools. The main factor in reducing weight is that the part cavity is never completely filled. Another major contributor to resin saving is scrap reduction. With proper tool design, gas assisted allows scrap-free startups and production runs.
B. Disadvantages of the GAIM process
All processes have their disadvantages, but those of GAIM and GAIMIC (Gas-assisted injection molding with internal-water cooling) appear relatively minor compared with their significant advantages.
(1) Large hollow sections
GIAM is not well suited for thin-walled hollow parts such as bottles or tanks. However, the thin-wall part has also tried out for some specific applications.
(2) Vent hole
The gas must be vented prior to opening the mold, leaving a hole somewhere on the part. Normally this can be placed in a non-visible location, but if appearance or function is affected or secondary operations are required, it may be necessary to seal the hole.
(3) Mold temperature control
Since wall thickness along the gas flow channel is a function of cooling rate, consistent wall thickness requires precise mold temperature control.
(4) Surface blush
The gas channel may leave surface blush, which arises from differences in surface gloss leaves. The tendency for blush is a function of processing conditions and types of plastics.
(5) Unique design
The unique part design and mold design required in most cases to fully utilize that GAIM might be considered by some to be a disadvantage. The gas part design takes a relatively longer time than with the conventional injection molding process.
(6) Extra cost of controller
In order to control the gas injection, the process requires extra equipment. Gas-assisted injection molding with internal cooling requires a system for controlling the gas and the water, an expense not required with traditional injection molding.
C. Types of process defects in the GAIM
Fingering, gas bubbles, hesitation lines, burning of resin, witness line cold slug, and gas blowout are typical defects normally encountered in GAIM.
Fingering, or gas permeation, is a common problem encountered in GAIM. In fingering, gas escapes from the gas channel and migrates into undesired areas of the part. Severe gas fingering can result in significant reduction n in part stiffness, impact strength and reliabitity of the final molded part. During the gas holding phase, the transitional region between the gas channel and the flat area is possible for fingers to form within the flat area. In this case, the main cause of the fingering effect is the higher its shrinkage potential, and hence the greater danger of the fingering effect. In order to largely exclude the fingering effect through design, it is necessary to implement the following criteria: a basic wall thickness of 4mm or greater should be avoided for flat areas, a material with favorable solidification behavior should be selected, and the lowest possible gas pressure should be applied.
Gas bubbles are caused by fingering. When fingering occurs, gas sometimes gets trapped in the thin-wall sections of the part where the gas is unable to fully vent. These trapped gases can cause bubbles that will still be in the gas core after the mold is opened.
Hesitation lines appear on the surface of a part produced by GAIM when the short shot of resin stops in the cavity, then starts moving again as the gas completes the fill.
Burning of the resin can appear on either the outer surface of the part or within the gas channel itself. Burning of the part surface can be caused by gas pressure that is too high or by insufficient venting of the mold. Burning, the resin within the hollow sections of the part is also possible. Burning within the gas channel can cause gas injection pins to become plugged.
On thin-walled parts molded in certain resins, a witness line, or gloss-level change, can occur over the gas channel. Excessive gas pressure can also cause witness lines over gas channels.
When gas is injected through the molding machine nozzle, cold slugs of resin may occur on the part surface. A cold slug is caused when a small amount of unmelted resin is injected into the part.
Gas blowout occurs when there is not enough resin in the cavity to hold the gas inside the part. If the part is short, gas will migrate to the non-filled area of the cavity and blow through. When blowout occurs, the part will sometimes look like a short shot.
Most cases of defects are produced by the interface of the gas and the melt. These problems can be overcome by internal water-cooling between the interface of the gas and the melt.
氣輔注射成型
注射成型是一種很普通的生產方法，用于加工那種生產時難以控制和有復雜表面的商業(yè)塑件。但是這種成型方法也有一些局限性，如因為壁厚太厚，而要很長的冷卻時間，使工件的生產周期變長。還有在局部厚壁處成型時，其表面會產生凹陷現象。因為在保壓和補縮時會產生殘余應力和應變，所以大的薄壁件會發(fā)生翹曲變形。因此可通過改良傳統(tǒng)的注射成型工藝來解決這些問題，提高產品的性能和降低產品的成本。
當前氣輔成型工藝已投入使用，并在全球迅速發(fā)展。在美國，這種工藝被稱為氣體輔助成型（GAIM）；在歐洲這種工藝過程又被稱為氣體注射技術（GIT），[見圖4.3.1]。這種工藝的發(fā)展是為了生產那種有內部通道的中空塑料產品。這是一種非常獨特的工藝，因為它集合了傳統(tǒng)的注塑成型和中空成型的優(yōu)點，但跟兩者又不一樣。GAIM是一種高效率的生產方式，因為它能在比較小的鎖模力，很小的或不需要保壓力的情況下生產出表面光滑且堅硬的大型塑件。在塑料熔體完全充滿型腔前，注入氣體能解決許多問題，例如翹曲變形、表面凹陷和注塑壓力的高需求量。這種工藝過程提供的許多便利是依據剛度-重量比比模具與制件尺寸比高（模具與制件尺寸比為一），因為這會減少塑件在橫截面中心軸附近的物料，因此能擴大結構設計的自由空間。
與傳統(tǒng)的注塑成型工藝相比，氣輔成型在成型的控制方面有比較多的優(yōu)點，特別是多型腔的應用。制品的質量由工具和生產過程中的工藝變量決定，這些工藝變量包括注塑溫度，注塑氣體的狀態(tài)和模具溫度，即過程控制是非常重要的。這種工藝過程吸引了許多模具設計師投入其中，因為社會需要高自動化的氣體輔助注塑模具。
隨著控制過程和工藝方面的發(fā)展，在復雜工藝發(fā)展過程中，氣輔成型工藝發(fā)展最快。這方面的研究方向是新型氣體注射裝置的發(fā)展、工藝變量的確定、產品的生產效率，新工藝過程的一些優(yōu)勢。多數公司能出售一系列可供選擇的氣體輔助模具的標準模架，這些模架一般是由壓力控制或體積控制工藝過程。
氣輔注射成型時，先用熱塑性塑料部分充填模具形腔，然后再注入一種惰性氣體，通常是氮氣?？捎脙煞N工序中的一種將氣體注入到熱固性塑料內部。一種方法是將等體積的氣體注入到容器里。將閥門打開使氣體流入聚合物中，并且用活塞來推動所有氣體從容器流入到模具型腔中。當氣體在模具中膨脹時，其壓力將下降。第二種方法是保持氣體的壓力（不是體積）為恒量。氣體迅速沿著最厚也是最熱的部位向料流前鋒推進，然后充滿模具型腔并保壓。另外在塑件收縮時，壓力氣體可以充填收縮的那部分塑料所占的空間。塑件冷卻后，將氣體排出塑料件，留下內部中空的且與模具形狀相同的塑件。
GAIM的標準工作過程分四步。第一步是熔體注射，[見圖4. 3.（2）]。在型腔中注入定量的熔體（沒有充滿型腔）。所需的體積量是通過模擬實驗來計量和決定的，以防止氣體流穿塑件并且能保證理想的充氣空間。通常在塑料熔融和注入氣體前注入75%~95%型腔體積的聚合物。
第二步氣體注射，[見圖4. 3. 2（b）]。可在熔體注射時或其后短暫一段時間進行氣體注射。只有在氣體壓力超過熔體壓力時氣體才能流進熔體。在模具內部，氣體迫使塑料熔體從聚集物團狀流到其完全充滿行型腔。氣體注射壓力的范圍為0.5~30Mpa（70~4500pis）。
第三步氣體保壓，[見圖4. 3. 2（c）]。氣體繼續(xù)使聚合物熔體完全充滿型腔。在這個階段，氣體推著聚合物流動時，它總是沿著抵抗力最小的路徑前進。
第四步氣體泄壓，[見圖4. 3. 2（ d）]。在氣體保壓后，塑件中的氣壓可通過適當的氣體轉換或卸壓方式來釋放。
A. GAIM工藝的優(yōu)勢
氣體輔助注射成型能解決許多發(fā)生在傳統(tǒng)注射成型階段的問題。
（1）減少應力和翹曲變形
因為氣體通過連通著的各個通道，使塑件各處的壓力相等。經過很合理的設計，塑料制品內部可以提供合理的氣流通道，以保證塑件的使用壓力，因此塑件內部應力能平穩(wěn)的下降。這樣就可以減少塑件翹曲變形的趨勢。
（2）消除收痕現象
塑件背面的肋和凸臺所引起的收痕現象會導致塑件長期的使用問題。這些表面缺陷是塑件在冷卻時因體積收縮而引起的。如果在塑件的前、后表面間設計合理的氣體通道，收痕現象將會減少或消除。氣體輔助注射時，設計比較厚的肋骨，可以方便地在塑件內部形成氣體通道。因為塑件內部溫度最高，所以在肋處設有氣體通道，這樣塑件冷卻時材料的收縮變形將遠離塑件內部的氣體通道。因此當塑件冷卻時其表面就不會因收縮而發(fā)生收痕現象。
（3）增加表面光澤度
不像泡末成型，氣體注射成型不但能節(jié)省材料，而且能生產出結構堅固且輕質的塑件。在保壓狀態(tài)下，塑件能自動完善其表面光澤度。
（4）減小鎖模力
在傳統(tǒng)注射成型過程中，保壓階段需要很高的壓力。在GAIM生產過程中的最高注射壓力顯然低于因氣體流經氣體通道而需要的控制壓力。也就是GAIM所需的鎖模力將減少到原來的90%。
（5）消除表面熔接痕
氣體輔助注射成型的一個最大特點是在塑件中設計合理的氣流通道。所以塑件表面的熔接痕（冷的和熱的）經?？梢韵?，即使在大而復雜的塑件上。這有許多優(yōu)點，包括模具設計、制造費用降低，研磨熔接痕的次數減少和塑料熔融時的溫度控制的發(fā)展。通常在模具中設計合理的氣流通道可以改變熔體的流動方向，和通過開幾個注射通口來減少或控制熔接痕。另外為了很好地保證氣流通道的順暢，在需要時可在肋和厚壁處進行結構的加強。
（6）滿足不同的壁厚要求
在用一般注塑機注射時，塑件的壁厚必須等尺寸。氣體輔助注射時，壁厚的設計彈性很大。如果在塑件上的交接處設有氣流通道，那就可以生產出不同壁厚的塑件。因為這種方法能保證塑料在模具中均勻流動，所以避免了大的應力和翹曲變形的發(fā)生，這些現象通常發(fā)生在有復雜的幾何形狀的塑件上。
（7）縮短成型周期
與泡末成型件相比，氣輔成型件也沒有絕緣性，因此這種方法生產出的塑件的成型周期比較短。相對于用傳統(tǒng)注射方法生產等尺寸的塑件，這種方法生產的塑件沒有凹陷缺陷。
（8）節(jié)省樹脂
在用傳統(tǒng)工具來減輕塑件的重量上，氣輔成型起著很直接的作用。減輕塑件重量的主要因素是塑件型腔沒有被完全充滿。另一種節(jié)省樹脂的方法是減少廢料。合理的模具設計和氣體輔助成型能使廢料減少和熔體流動順暢。
B GAIM工藝過程的局限性
所有的工藝過程都有它的局限性，但是GAIM和GAIMIC的局限性相對于自身的優(yōu)點就少多了。
（1）大的凹陷缺陷
GAIM這種工藝過程不適宜于生產那種薄壁且有凹陷的塑件，如瓶子和箱子。而且這種薄壁塑件也不適宜使用于一些特殊場合。
（2）氣孔
注射的氣體必須在開模前排出，這會在塑件的表面留下排氣口。通常將此口布置在隱蔽處，但是，如果塑件有表面質量的要求，或排氣口會影響塑件的使用，或下一步加工的需要，也可將此洞口封閉。
（3）模具溫度的控制
因為氣體流道附近的塑件壁厚可影響冷卻速率，所以與之相同壁厚的塑件其他地方就需要精確的模具控制溫度。
（4）表面缺陷
氣體在流道中流動時會在塑件表面留下缺陷，這種缺陷隨著亮度的不同而變化。這種趨勢是由工作條件和塑料的種類來影響的。
（5）特殊的設計
絕大多數的情況下，需要對模具和塑件進行單獨的設計，這被一些人認為是GAIM的不足之處。在設計時，氣輔注射模具比傳統(tǒng)的需要更長的時間。
（6）控制的額外費用
為了控制氣體注射，就需要額外的設備。帶有內冷卻系統(tǒng)的氣輔注射模具需要氣體和水的控制系統(tǒng)，這方面的費用是傳統(tǒng)模具不需要的。
C. 在GAIM工作過程中的一些工藝缺陷
指紋效應、氣泡、遲滯線、樹脂熱分解、亮度變化線、冷料和氣體吹穿現象都是在GAIM工作過程中經常碰到的典型缺陷。
指紋效應或氣體穿透現象是GAIM工藝過程中常碰到的問題。在有指紋現象時，氣體從氣體流道中流出，侵入到塑件中沒有設計流道的部分。嚴重的指紋效應將導致塑件堅硬處和有強烈沖擊處發(fā)生明顯的變形，也會影響塑件的使用性能。在氣體保壓過程中，在有氣流通道和沒有的過渡處，將可能在沒有氣流通道處發(fā)生指紋效應。在這種情況下，主要影響指紋效應的因素是塑件沒有氣流通道處的壁厚。壁厚越厚，指紋收縮現象就越有可能，指紋效應的危害就越大。為了通過設計來減少指紋效應，就需要遵循下列原則：在沒有氣流通道處必須避免大于等于4mm的基本壁厚，需要選擇易于凝固的材料，應用盡可能低的氣體壓力。
氣泡是由指紋效應引起的。當發(fā)生指紋效應時，氣體將在塑件的薄壁處被困住，此處的氣體將不能被完全排出。這些留在塑件中的氣體就引起氣泡現象，在模具開啟后，這些氣泡將一直留在塑件中。
欠料注射的樹脂在型腔中先停頓一下，再重新開始流動直到完成注滿型腔，在此過程中塑件表面就會出現遲滯線。
在塑件的外表面或氣體通道內可能會發(fā)生樹脂的熱分解。塑件表面的熱分解是由注射氣體的壓力太高，或模具內部沒有充分的出氣通道。在塑件的中空部分發(fā)生樹脂熱分解也是有可能的。在氣體通道內發(fā)生樹脂熱分解會使氣體注射機的氣針堵塞。
在用定量樹脂成型薄壁塑件時，在有氣流通道的塑件表面會發(fā)生光澤變化，或亮度變化線。過大的氣體壓力也會引起有氣流通道的塑件表面出現亮度變化線。
如果氣體通過模具注射機的噴嘴注入型腔，塑件表面就會產生冷料現象。當有少量沒有熔化的樹脂注入型腔，也會產生冷料現象。
如果模具型腔中沒有足夠的樹脂來包住壓力氣體，就會在塑件上產生氣體吹穿現象。如果欠料注射，氣體就會侵入型腔中未被充滿的地方，并且會沖破塑件。如果發(fā)生氣體吹破現象，塑件看起來就像中了子彈。
GAIM工藝的絕大多數缺點是由氣流通道和熔體引起的。可以在氣腔壁和熔體表面間開冷水通道來解決這些問題。