【正文】 ype of gate. There are two main considerations when designing a runner layout. The runner length should always be kept to a minimum to reduce pressure losses, and the runner system should be balanced. (i) The crosssectional area of the runner must be sufficient to permit the freezes and for packing pressure to be applied for shrinkage pensation if required, Because of this, runners below 2 mm (3/32 in) diameter are seldom used and even this diameter is normally limited to branch runners under 25mm (1 in) in length. (Runner balancing means that the distance the plastic material travels from the sprue 1o the gate should be the same for each molding This system ensures that all the impressions will fill uniformly and without interruption providing the gate lands and the gate areas are identical, Figure shows example~ of molds all based on the balanced runner principle. It is not always practicable, however, to have a balanced runner system and this particularly applies to molds which incorporate a large number of differently shaped impressions (Figure ). In these cases balanced filling of the impression can be achieved ~y varying the gate dimensions. That is by balanced gating (Section ). SingleImpression MOLDS Singleimpression molds are usually fed by a direct sprue feed into the impression (Figure ) and hence no runner system is required. However, it may be desirable 。 The Injection Mold The design of Runner basic The runner is a channel machined into the mod plate to connect the sprue with the entrance(gate) to the impression. In the basic twoplate mod the runner is positioned on the surface while on the more plex designs the runner may be positioned below the parting surface The wall of the runner channel must be smooth to prevent any restriction to flow. Also, as the runner has to be removed with the molding, there must be no machine marks left, which would tend to retain the runner in the mod plate. To ensure that these points are met, it39。s cutters available and (v) the plastics material to be used. (i) The crosssectional area of the runner must be sufficient to permit the freezes and for packing pressure to be applied for shrinkage pensation if required, Because of this, runners below 2 mm (3/32 in) diameter are seldom used and even this diameter is normally limited to branch runners under 25mm (1 in) in length. (ii) The further the plastic melt has to travel alone the runner the greater is the resistance to flow. Hence the distance the impression is from the sprue has a direct bearing on the choice of crosssectional size of the runner. For example, whereas a 5mm (3/16 in) (iii) The crosssectional area of the runner should not be such that it controls the injection cycle, although this is sometimes unavoidable for very light moldings The larger he crosssec ~tion area of the runner the greater is the bulk of material it contains and the longer the period it takes to cool sufficiently to enable the mod to be opened and the moldings and runner ejected. For this reason it is undesirable to make the runner larger than 10 mm (] in) diameter for most materials. However, the rigid PVCs and the acrylics are exceptions due to their high viscosity and diameters up to 13 mm (1/2 in) are used. (iv) The size chosen for the runner should be in a range consistent with the mouldmakers39。為了確保這些要點得到滿足，要求模具設(shè)計者在圖紙中注明這條流道需要沿取出方向拋光?？梢姀倪@種角度考慮圓形和方形的流道是兩種最滿意的設(shè)計。當(dāng)然，假如要求相似的橫截面積，則 D 值（圖 ）必須相應(yīng)地增加。（圖 ） 整圓流道的主要問題是這種流道是由分別加工在兩塊模板上的兩個半圓合成的。這時，總是采用基本的梯形流道設(shè)計，其流道開在注射半模上，當(dāng)開模時流道廢料被拉出。因此型腔和主流道之間的距離，直接和流道橫截面尺寸的選擇有關(guān)。對于硬PVCs 和聚丙烯，計算得到的直徑值增加 25%。 整圓流道的主要問題是這種流道是由分別加工在兩塊模板上的兩個半 圓合成的。這時，總是 采用基本的梯形流道設(shè)計，其流道開在注射半模上，當(dāng)開模時流道廢料被拉出。假如澆口的長度和截面積相同，系統(tǒng)要求確保所有的型腔將均勻地充滿 而沒有阻斷現(xiàn)象。這里要注意的是采用這種方法澆注單一型腔時，型腔自身必須要有一個偏置。 要達到這種兩端澆口的目的，必須采用其他的流道布置設(shè)計方案，其中既可以用圖 延伸到型腔一邊的 T 形流道再以短的分流道連接到澆口 的方案；也可以用圖 中 S 形，延伸到澆口，不需分流道的方案。例如，含有四腔、五腔、六腔和八腔模具的平衡式流道如圖 所示。