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原文三 INJECTION MOLD DESIGN AND GUIDE Mold construction A standard injection mold is made of a stationary or injection side containing one or more cavities and a moving or ejection side. Relevant details are shown in the figure below. Impression of a standard injection mold. High quality molds are expensive because labor and numerous high- precision machining operations are time-consuming. Product development and manufacturing costs often can be significantly reduced if sufficient attention is paid to product and mold design. The way in which the mold is constructed is determined by: · shape of the part · number of cavities · position and system of gating · material viscosity · mold venting A simple mold with a single parting line is shown in the figure above. ?More complex molds for parts with undercuts or side cores may use several parting lines or sliding cores. These cores may be operated manually, mechanically, hydraulically, pneumatically or electro-mechanically. The figure below shows an example of a sliding cam. The cam pins that operate the cams are mounted under a maximum angle of 20° - 25° in the injection side. ?The angle is limited because of the enormous force that is exerted on these pins during mould opening and closing. Cammed mold for part with undercut cams move in vertical direction when mold is opened. Multi-cavity molds The number of cavities and mold construction depend both on economical and technical factors. Important is the number of parts to be molded, the required time, and price in relation to mould manufacturing costs. The figure below shows the relation between the total part costs and the number of cavities. Total part costs in relation to number of cavities. The gating system and gate location can limit the design freedom for multi-cavity molds. Dimensional accuracy and quality requirements should be accounted for. The runner layout of multiple-cavity molds should be designed for simultaneous and even cavity filling. The maximum number of cavities in a mold depends on the total cavity volume including runners in relation to the maximum barrel capacity and clamping force of the injection molding machine. Number of cavities A given molding machine has a maximum barrel capacity of 254 cm3, a plasticizing capacity of 25 g/s, 45 mm screw and a clamping force of 1300 kN. A PC part of 30 cm3, (shot weight 36 g) and a projected area of 20 cm2 including runners requires about 0.5/tons/ cm2 ?(5 kN/cm2 ) clamping force. The maximum number of cavities based on the clamping force would be 12. It is advisable to use only 80% of the barrel capacity, thus the number of cavities in this example is limited to 6. ? When very short cycle times are expected the total number of cavities may be further reduced. ?A 6-cavity mold in this example requires a shot weight of 216 g. The cooling time must be at least 8.7 seconds. Gate location Appearance Whenever possible locate gates on non-visual surfaces thus eliminating problems with residual gate vestiges after the gate has been removed. Stress Avoid areas exposed to high external stress (mechanical or impact). The gate area has high residual stresses and also rough surfaces left by the gate act as stress concentrators. Pressure Locate the gate in the thickest section to ensure adequate pressure for packing out the part. This will also help prevent sink marks and voids forming. Weld lines Place gates to minimize the number and length of weld lines or to direct weld lines to positions that are not objectionable to the function or appearance of the part. When weld lines are unavoidable try to locate the gates close to the weld line location this should help maintain a high melt temperature that is beneficial to a strong weld line. Warpage An incorrectly dimensioned or located gate may also result in undesirable flow patterns in the cavity. This can lead to moldings with visible weld line (see figure below). Influence of gate location on flow behavior of the melt. Undesirable flow patterns in the cavity can also lead to deformation by warping or bending (see figure below). Warpage due to unfavorable gate location. Gate type As important as selecting the optimal gate size and location is the choice of the type of gate. Gate types can be divided between manually and automatically trimmed gates. Manually trimmed gates Manually trimmed gates are those that require an operator to separate parts from runners during a secondary operation. The reasons for using manually trimmed gates are: · The gate is too bulky to be sheared from the part as the tool is opened. · Some shear-sensitive materials (e.g., PVC) should not be exposed to the high shear rates inherent to the design of automatically trimmed gates. ? · Simultaneous flow distribution across a wide front to achieve specific orientation of fibers of molecules often precludes automatic gate trimming. Gate types trimmed from the cavity manually include: · Sprue gate · Edge gate · Tab gate · Overlap gate · Fan gate · Film gate · Diaphragm gate · External ring · Spoke or multipoint gate Automatically trimmed gates Automatically trimmed gates incorporate features in the tool to break or shear the gate as the molding tool is opened to eject the part. Automatically trimmed gates should be used to: · Avoid gate removal as a secondary operation. · Maintain consistent cycle times for all shots. · Minimize gate scars. Gate types trimmed from the cavity automatically include: · Pin gate · Submarine (tunnel) gates · Hot runner gates Sprue gate Recommended for single cavity molds or for parts requiring symmetrical filling. This type of gate is suitable for thick sections because holding pressure is more effective. A short sprue is favored, enabling rapid mold filling and low-pressure losses. A cold slug well should be included opposite the gate. The disadvantage of using this type of gate is the gate mark left on the part surface after the runner (or sprue) is trimmed off. Freeze-off is controlled by the part thickness rather than determined the gate thickness. Typically, the part shrinkage near the sprue gate will be low; shrinkage in the sprue gate will be high. This results in high tensile stresses near the gate. Di mensions The starting sprue diameter is controlled by the machine nozzle. The sprue diameter here must be about 0.5 mm larger than the nozzle exit diameter. Standard sprue bushings have a taper of 2.4 degrees, opening toward the part. Therefore, the sprue length will control the diameter of the gate where it meets the part; the diameter should be at least 1.5 mm larger than or approximately twice the thickness of the part at that point. The junction of sprue and part should be radiused to prevent stress cracking · A smaller taper angle (a minimum of one degree) risks not releasing the sprue from the sprue bushing on ejection. · A larger taper wastes material and extends cooling time. · Non-standard sprue tapers will be more expensive, with little gain. Edge gate The edge or side gate is suitable for medium and thick sections and can be used on multicavity two plate tools. The gate is located on the parting line and the part fills from the side, top or bottom. Dimensions The typical gate size is 80% to 100% of the part thickness up to 3.5 mm and 1.0 to 12 mm wide. The gate land should be no more than 1.0 mm in length, with 0.5 mm being the optimum. Tab gate A tab gate is typically employed for flat and thin parts, to reduce the shear stress in the cavity. The high shear stress generated around the gate is confined to the auxiliary tab, which is trimmed off after molding. A tab gate is often used for molding P. Dimensions The minimum tab width is 6 mm. The minimum tab thickness is 75% of the depth of the cavity. Runner layout There are 3 basic layout systems used for multi-cavity systems. These can be catorigized as follows: · Standard (herringbone) runner system · "H" bridge (branching) runner system · Radial (star) runner system Unbalanced runner systems lead to unequal filling, post-filling and cooling of individual cavities that may cause failures like: · Incomplete filling · Differences in product properties · Shrinkage differences/warpage · Sink marks · Flash · Poor mold release · Inconsistency Example of unbalanced feed systems. Although the herringbone is naturally unbalanced, it can accommodate more cavities than its naturally balanced counterparts, with minimum runner volume and less tooling cost. With computer aided flow simulation it is possible to adjust primary and secondary runner dimensions to obtain equal filling patterns. Keep in mind that non-standard runner diameters will increase manufacturing and maintenance costs. Adjusting runner dimensions to achieve equal filling may not be sufficient in critical parts to prevent potential failures. Special attention is required for: · Very small components · Parts with thin sections · Parts that permit no sink marks · Parts with a primary runner length much larger than secondary runner length. It is preferred to design naturally balanced runners as shown in the figure below. Naturally balanced feed systems. The "H" (branching) and radial (star) systems are considered to be naturally balanced. The naturally balanced runner provides equal distance and runner size from the sprue to all the cavities, so that each cavity fills under the same conditions. When high quality and tight tolerances are required the cavities must be uniform. Family moulds are not considered suitable. Nevertheless, it might be necessary for economical reasons to mold different parts in one mold. The cavity with the largest component should be placed nearest to the sprue. Runners for multi-cavity molds require special attention. Runners for family molds, molds producing different parts of an assembly in the same shot, should be designed so that all parts finish filling at the same time. This reduces over- packing and/or flash formation in the cavities that fill first, leading to less shrinkage variation and fewer part-quality problems. Consider computerized mold- filling analysis to adjust gate locations and/or runner section lengths and diameters to achieve balanced flow to each cavity. The same computer techniques balance flow within multi-gated parts. Molds producing multiples of the same part should also provide balanced flow to the ends of each cavity. Naturally balanced runners provide an equal flow distance from the press nozzle to the gate on each cavity. Spoked runner designs work well for tight clusters of small cavities. However they become less efficient as cavity spacing increases because of cavity number or size. Ejection systems The method of ejection has to be adapted to the shape of the molding to prevent damage. In general, mould release is hindered by shrinkage of the part on the mould cores. Large ejection areas uniformly distributed over the molding are advised to avoid deformations. Several ejector systems can be used: · Ejector pin or sleeve · Blades · Air valve · Stripper plate When no special ejection problems are expected, the standard ejector pin will perform well. In case of cylindrical parts like bosses a sleeve ejector is used to provide uniform ejection around the core pin. Blades are poor ejectors for a number of reasons: they often damage parts; they are prone to damage and require a lot of maintenance. Blade ejectors are most commonly used with ribbed parts. Blade ejectors. A central valve ejector is frequently used in combination with air ejection on cup or bucket shaped parts where vacuum might exist. The air valve is thus only a secondary ejection device. A high-gloss surface can have an adverse effect on mould release because a vacuum may arise between cavity wall and the molding. Release can be improved by breaking the vacuum with an ejection mechanism. A stripper plate or ring is used when ejector pins or valves would not operate effectively. The stripper plate is often operated by means of a draw bar or chain. Three-plate molds, as shown in the figure below, have two parting lines that are used in multi-cavity molds or multiple gated parts. During the first opening stage automatic degating takes place when the parts are pulled away from the runners. Three plate mold with two stripper plates for ejection. Runners Unlike sprees, which deliver material depth wise through the center of the mold plates, runners typically transport material through channels machined into the parting line. Runner design influences part quality and molding efficiency. Overly thick runners can lengthen cycle time needlessly and increase costs associated with regrind. Conversely, thin runners can cause excessive filling pressures and related processing problems. The optimum runner design requires a balance between ease of filling, mold design feasibility, and runner volume. Material passing through the runner during mold filling forms a frozen wall layer as the mold steel draws heat from the melt. This layer restricts the flow channel and increases the pressure drop through the runner. Round cross-section runners minimize contact with the mold surface and generate the smallest percentage of frozen layer cross-sectional area. As runner designs deviate from round, they become less efficient (see figure 7-20). Round runners require machining in both halves of the mold, increasing the potential for mismatch and flow restriction. A good alternative, the “round-bottomed” trapezoid, requires machining in just one mold half. Essentially a round cross section with sides tapered by five degrees for ejection, this design is nearly as efficient as the full-round design. The runner system often accounts for more than 40% of the pressure required to fill the mold. Because much of this pressure drop can be attributed to runner length, optimize the route to each gate to minimize runner length. For example, replace cornered paths with diagonals or reorient the cavity to shorten the runner. Runners for Multi-cavity Molds Figure 7-23 Family Molds The runner diameter feeding the smaller part was reduced to balance filling. Runners for multi-cavity molds require special attention. Runners for family molds, molds producing different parts of an assembly in the same shot, should be designed so that all parts finish filling at the same time. This reduces over- packing and/or flash formation in the cavities that fill first, leading to less shrinkage variation and fewer part-quality problems. Consider computerized mold- filling analysis to adjust gate locations and/or runner section lengths and diameters to achieve balanced flow to each cavity (see figure 7-23). The same computer techniques balance flow within multi-gated parts. Molds producing multiples of the same part should also provide balanced flow to the ends of each cavity. Naturally balanced runners provide an equal flow distance from the press nozzle to the gate on each cavity. Spoked runner designs (see figure 7-24) work well for tight clusters of small cavities. However they become less efficient as cavity spacing increases because of cavity number or size. Other Gate Designs Pinpoint gates feed directly into part surfaces lying parallel to the mold parting plane. On the ends of three-plate runner drops, multiple pinpoint gates can help reduce flow length on large parts and allow gating into areas that are inaccessible from the part perimeter. For clean degating, the gate design must provide a positive break-off point (see figure 7-40) to minimize gate vestige Set in recesses or hidden under labels, properly designed and maintained pinpoint gates seldom require trimming. Because gate size must also be kept small, typically less than a 0.080-inch diameter, pinpoint gates may not provide sufficient packing for parts with thick wall sections. Parts with holes in the center such as filter bowls, gears, and fans often use the “filter-bowl” gate design to provide symmetrical filling without knit lines. Typically, the gate extends directly from a sprue and feeds the cavity through a continuous gate into the edge of the hole (see figure 7-41). Degating involves trimming away the sprue and conical gate section flush with the outer surface. Another design variation, the diaphragm gate, feeds the inside edge of the hole from a circumferential edge gate extending from a center disk (see figure 7-42). Degating usually involves punching or drilling through the hole. 譯文三 注塑模具設(shè)計與指導 模具結(jié)構(gòu) 一個標準的注塑模具是由定模和動模部分組成，由一個或多個型腔，以及一個可以移動的脫模部分組成。 有關(guān)的細節(jié)在下面的圖片中被顯示出。 標準注塑模具的圖片： 圖1 高質(zhì)量高精度模具的價格是非常貴的。因為大量的勞動力和很多的高精密機器操作都被運用到生產(chǎn)制造中，經(jīng)濟消耗巨大。 產(chǎn)品發(fā)展和制造費用如果想要被極大地減少，我們就需要充分地重視和關(guān)注產(chǎn)品模具的設(shè)計。 如下方面是我們在制造中應(yīng)該解決的： 制件的形狀 型腔的數(shù)量 澆口的位置與澆口形式的選擇 材料的流動性 模具的排氣系統(tǒng) 帶有1個分型面的簡單注塑模具如下圖所示。 一個比較復雜的，用來成型結(jié)構(gòu)復雜和帶有側(cè)抽芯結(jié)構(gòu)的模具通常需要使用多次分型和側(cè)抽芯結(jié)構(gòu)。這種側(cè)抽芯結(jié)構(gòu)通常是通過人工，機械，液壓以及電子機械裝置來完成的。 下面的圖片是一個簡單的滑動凸輪結(jié)構(gòu)?？刂仆馆喌匿N被放置在定模一側(cè)，其角度在20° - 25°之間。 這個角度也是有限制的，這是由于它要在開模和合模中受到一個很大的力。當開模時這種模具是豎直方面移動的。 圖2 多型腔模具 型腔的數(shù)目與它的結(jié)構(gòu)主要取決于經(jīng)濟和技術(shù)這兩個方面。 最重要的是，按照制件的數(shù)量來決定型腔的數(shù)量， 花費的時間，金錢以及與其相關(guān)的模具制造費用也要被考慮進去。下面的這個圖，顯示出模具型腔數(shù)量與制造全部費用的關(guān)系。 圖3 澆注系統(tǒng)與澆口位置限制了多型腔模具設(shè)計的隨意性。同時，模具的尺寸精度和質(zhì)量要求也需要被考慮。 流道結(jié)構(gòu)在設(shè)計時，應(yīng)該將其設(shè)計為可以同時充滿型腔的形式。 一個模具中型腔的最大數(shù)量應(yīng)該取決于型腔的全部容量，包括流道和相關(guān)的最大注塑量，還需要考慮到注塑壓力的大小。 型腔數(shù)量 一個給定的注塑機，他的最大注塑量是254立方厘米, 塑化能力是25 g/s, 45mm螺桿和1300 kN的鎖模力。 一個體積是30立方厘米的PC制件(注射重 36 g), 他的投影面積是20平方厘米，包括流道，大約在0.5噸/CM2（0.5KN/CM2）的注塑壓力。 以注塑壓力為設(shè)計基準的模具，他的型腔數(shù)量最大是12個。建議型腔數(shù)量的容積是注塑機注塑量的80%。因此，通常情況下型腔數(shù)目被限制在6個以內(nèi)。 當循環(huán)周期很短時，型腔的數(shù)量建議被大打的減少。以這個為例，一個有6個型腔的模具，他的冷卻時間必須不能低于8.7秒。 澆口位置 外形 澆口無論被設(shè)計在什么位置，我們都不可能從外表面看到他的位置所在。因此在澆注系統(tǒng)被除去后，清除殘余的廢料是一個要被解決的問題。 應(yīng)力 避免表面直接受到高壓，避免澆口上存有殘余應(yīng)力和粗糙的表面，可以通過設(shè)置一個壓力集合器來解決該問題。 壓力 把澆口設(shè)計在制件壁最厚的地方來確保有適當?shù)膲毫沓尚椭萍?。這樣也可以用來減少尺寸收縮。 熔接痕 塑件表面的一種線狀痕跡，是由注射或擠出中若干股流料在模具中分流匯合，熔料在界面處未完全熔合，彼此不能熔接為一體，造成的熔合印跡，影響塑件的外觀質(zhì)量以及力學性能。 翹曲 不正確地形狀尺寸或一個已經(jīng)確定的澆口可能會造成一個難以預料的結(jié)果。這可能導致成型時的熔接痕會出現(xiàn)在明顯的位置.(見下面的圖片) 融料流動對澆口位置的影響 圖4 不理想的流動形式可能會產(chǎn)生翹渠與彎曲。（見下圖） 翹曲對產(chǎn)生澆口位置的不利影響 。 圖5 澆口類型 一個最為理想的澆口尺寸和澆口位置是非常重要的。 澆口類型通常被被分為人工和自動脫出兩種。 手動脫澆口系統(tǒng)是需要一個操作者在第2次分型時將制件與流道用手將他們分開。使用手動脫澆注系統(tǒng)的使用地方: 澆口面積過大不能夠在開模時被自動剪開。 一些修剪敏感的材料 (舉例來說, pvc) 接觸在一起，這樣使其黏附在模具上面，不能自動脫澆。。 同時的流動分別穿過一個寬的澆口，前面的部分無法通過自動脫澆注結(jié)構(gòu)取出。 澆口包括: 直澆口 邊緣澆口 扇行澆口 爪形澆口 平縫澆口 點澆口 輪輻澆口 圓環(huán)澆口 多點澆口 自動脫澆口 自動脫澆口是用工具將澆口自動的打破并剪掉，在模具打開取件時，其經(jīng)常被使用的地方有如下： 兩次分型時避開澆口清除。 成型時間很短。 將澆口痕跡減小至最小。 澆口類型包括: 針點澆口 潛伏式澆口 熱流道 直流道 推薦單型腔注塑模具與那些需要對稱布置來成型的塑件來使用這種結(jié)構(gòu)。 這種類型的澆道適合那些有較大壁厚的塑件，因為這樣可以使保壓變得更為有效。 一個短小的主流道是有利的, 這樣可以促使熔料迅速填充滿模具而且還可以減少壓力損失，需要在澆口的對面設(shè)置一個冷料井。 使用這種類型的澆口的缺點就是留在制件和側(cè)流道上的廢料很難被清除掉。凝固是受塑件壁厚影響的，但是他并不能決定塑件的壁厚。很明顯，主流道口附近的塑件收縮較低;而主流道口的收縮較明顯。這樣就造成了在主流道口處出現(xiàn)高的應(yīng)力拉深。 尺寸 在臥式注塑機用模具中，主流道一般垂直于分型面，而角式注塑機用模具的主流道則開設(shè)在分型面上，前者便于流道凝料的拔出，通常設(shè)計成2-4度的錐角，內(nèi)壁的粗糙度在Ra=0.4um左右。主流道小端直徑應(yīng)該比注塑機的噴嘴孔徑大0.5-1mm,通常取4-8mm，具體視制件大小及補料要求決定。大端直徑應(yīng)該比分流道深大約1.5mm左右。 一個小角度的流道廢料很難從主流道中取出。 大的角度則會造成材料的浪費而且還會增加冷卻時間。 圖6 邊緣澆口： 這種澆口相對于分流道來說斷面尺寸較小，屬于小澆口的一種。邊緣澆口一般開設(shè)在分型面上，從制件的邊緣進料。邊緣澆口斷面形狀一般是矩形或者接近矩形。 尺寸： 典型的澆口尺寸是流道尺寸得80%-100%其數(shù)值一般為3.5 毫米，1.0-12 毫米寬。這種澆口的定位長度方向不應(yīng)該多出1.0mm，0.5mm左右最為適宜。 圖7 定位澆口： 定位澆口一般用于平、薄的制件，以便減小在型腔中剪切力。出現(xiàn)在澆口附近的高剪切力被限制在輔助片中，注塑完成后這個片就會被除掉。 尺寸： 最小量定位片寬度是 6 毫米， 最小量定位片厚度是洞的深度的 75%左右。 圖8 流道布局 對于多型腔的模具有3種使用情況。 如下: 標準的流道結(jié)構(gòu) " H" 形 (分枝式) 流道結(jié)構(gòu) 星形流道結(jié)構(gòu) 非平衡流道也常使用，但是會產(chǎn)生一些不利影響: 不能完全被充滿 產(chǎn)品的特性不同 產(chǎn)品的收縮不同 洗滌槽痕跡明顯 光澤度不好 降低了模具的使用壽命 各個制件的形狀不一致 非平衡流道的例子。 圖9 盡管人字型的流道是非平衡的，但是與平衡式相比它可以容納更多的型腔，有更小的流道容量和更低得加工費用，可以使用電腦輔助軟件來模仿塑料的流動以及對主流道和分流道的尺寸來保證設(shè)計的合理。記住非標準流道會增加使用制造費用。 調(diào)整流道的直徑來完成澆注，可能不能滿足某些要求高的制件，會產(chǎn)生一些缺點，特別需要注意的是: 小的制件 帶有薄壁的制件 制件表面不允許有沉積痕跡 制件的主流道長度要比分流道的長度要長的多。 平衡式流道： 圖10 " H"(分岐型) 和光線式的 (星型) 系統(tǒng)通常被被當做平衡式流道。 平衡式流道有著相同的流道長度和直徑尺寸，所以每個型腔的情況大概相同。 當要求質(zhì)量和公差較好時，型腔就要統(tǒng)一。不過，對于在一個模具中成型不同制件來說，經(jīng)濟因素非常重要的，帶有很大組件的型腔要放在主流道最近處。 在設(shè)計多型腔模具的分流道時應(yīng)特別注意。在一次填充成型一個組合件的不同制