產品肉厚的考量 製造生產上的影響 產品性能的影響 補強與防止變形的方法 Lesson 8. 產品肉厚的設計 產品肉厚的考量 製造生產上的影響 產品性能的影響 補強與防止變形的方法

產品肉厚的選擇考量 結構需求 成本 外觀及觸感 模具容不容易開 產品凹陷、翹曲變形等控制 成型週期

品質及成本考量 成型週期時間隨著肉厚增加而增加 較厚的產品容易翹曲 較薄且均一肉厚的產品品質較佳 一般利用減少肉厚的方法來控制 Thick Part Warped Part Uniform Thickness

對製造生產的影響 一般而言，減少肉厚需要增加壓力來填滿模穴 有時候，材料的剪切熱反而會使塑料更容易流動，降低射出壓力 Higher Pressure Thin Part Thick Part Lower Figure 2-2 in the Design Guide covers a number of different influencing factors on injection pressure. We will divide this list into the four key areas of influence. In the category of part design, the most dominant factor that influences injection pressure is wall thickness. Wall thickness and pressure are inversely related. As wall thickness decreases, the required injection pressure increases. As you increase wall thickness, injection pressure decreases. What are some problems you see if wall thicknesses are too thin? (flash, short shots, burning, etc.)

肉厚與流長的關係 GE Plastics/PC Lexan 101 Melt temperature: 624(F) Pressure (psi) 25 20 15 10 5 22050 17985 13923 9863 Max. Flow Length (in) First, let’s take a look at the part thickness vs. maximum flow length for this resin. This plot is available in C-MOLD to help you select a resin and set process conditions before running a simulation. For a given melt temperature, 624 F in this case, we see that the plastic will flow approximately 7 inches at the lowest pressure shown. However, if we apply the highest pressure we can push the plastic as far as 17 inches. Since our flow length is only 5-6 inches, we should require the lower pressure to fill the part. 0.020 0.040 0.060 0.080 0.100 0.120 Part Thickness (in)

肉厚的改變 肉厚的突然改變會改變充填模式 塑膠會往較厚的地方充填 可能會發生許多問題，包括了短射，不均勻的體積收縮，以及翹曲變形 Another part design influence on injection pressure is surface area. Because the figure on the left touches more steel, there is more cooling and also more drag force on the plastic as it flows. This causes the required pressure to fill this part to be higher than the part at right which has less surface contact area. What might be an application where you would see this type of pressure influence? (any type of speaker grill pattern, vent type effect, or a surface grid pattern)

其他肉厚變化問題 當接觸模具面的塑膠面積增加時，射出壓力也隨之增加 表面缺陷的問題可能會隨之發生 Higher Pressure More Cooling & Drag Force Less Cooling Lower Another part design influence on injection pressure is surface area. Because the figure on the left touches more steel, there is more cooling and also more drag force on the plastic as it flows. This causes the required pressure to fill this part to be higher than the part at right which has less surface contact area. What might be an application where you would see this type of pressure influence? (any type of speaker grill pattern, vent type effect, or a surface grid pattern)

Injection Pressure (psi) 肉厚與成型視窗的關係 650 640 630 溢料，毛邊 成型區 Melt Temperature (F) 620 Let me take a moment now to define what I mean by a “process window”. This is basically the range of injection pressures and melt temperatures that you can mold the part within. You want to have as large a process window as possible so that you don’t run into many problems molding the part. Just outside the left side of the window where the pressure is low what problem might you expect to see? (short shots) What problem might you expect outside the right side of the window where the pressure is very high? (flash) What about if the temperature is too low? (short shots) Too high? (flash, burning) 610 短射 600 590 5000 10000 20000 Injection Pressure (psi)

Injection Pressure (psi) Injection Pressure (psi) 肉厚與成型視窗的關係 0.100” thickness 0.075” thickness 650 640 630 620 610 600 590 650 640 630 620 610 600 590 Melt Temperature (F) Melt Temperature (F) Look at the figures again in Figure 6-2 through 6-4. What advantages/disadvantages do you get by changing your gating from that shown in Figure 6-2 to the configuration shown in Figure 6-3? (no gate mark, less expensive 2 plate mold, weld line) Would you expect the pressure to be different? (probably not since the flow length and thickness have not changed significantly). What about if we use the configuration shown in Figure 6-4; what would change? (no weld line, no gate mark, less expensive mold, very high pressure and clamp force) The pressure would be much higher because the flow length has basically doubled. This figure shows what you might see in terms of the process window for Figure 6-2 and 6-3 (on the left) and Figure 6-4 (on the right). So you see that flow length greatly influences your ability to fill the part consistently. It’s much easier to fall outside the small process window during production and start to see problems. 5000 10000 15000 20000 5000 10000 15000 20000 Injection Pressure (psi) Injection Pressure (psi)

肉厚與保壓的關係 薄殼成型會限制保壓壓力 流長越長，保壓壓力越小 在填滿小特徵區域時，必須避免過保壓

肉厚與冷卻時間的關係 Thermal Diffusivity of the Melt Cooling Time ~ (Heaviest Wall Thickness)2 Thermal Diffusivity of the Melt Allow me to quickly touch on the significance of wall thickness in determining cooling time. The basic equation for calculating cooling time for part walls is shown here. More detail is shown on pg. 83 of the Design Guide. This is Figure 7-16 and a discussion on design rules are also found in the Design Guide. The point to note here is that cooling time is proportional to the square of the thickest wall section. It really does not matter how wide or long the wall section is, the thicker it is between core and cavity mold surfaces, the longer it will take to cool. What happens if you have isolated areas of thick regions in your part and you try to eject the part too soon? (sinks or non-uniform shrinkage causing warpage) Original Design Better Design

肉厚與翹曲變形的關係 Shrinkage Wall Thickness Well, that was pretty technical information. You cannot really do much about the characteristics of the plastic resin except for changing to a different resin. However, there are a few things that you can control that have an impact on part shrinkage. These are shown in Figure 9-2. Among these are the melt and mold temperature, which reduce shrinkage as you reduce temperature. The injection rate has some affect, though not as dominant.

Low Crystallization Level High Crystallization Level 肉厚對翹曲變形的影響 High Shrinkage High Cooling Rate Low Crystallization Level Low Cooling Rate High Crystallization Level Warped Part Figure 9-5 discusses another cause of warpage, which is non-uniform volume shrinkage. In this case, the cooling from core to cavity may be even. However, the thickness of the part varies causing different shrinkage rates between sections of the part. This difference in shrinkage causes stress differences between the sections and can lead to part warpage. This is especially true for semi-crystalline resins where more crystallization can occur in the thicker sections where the plastic stays hot longer. The thin sections cool more rapidly and do not have time to crystallize significantly. This occurrence increases the shrinkage difference between the two sections more than it would be had an amorphous resin been used to mold the part. 不均勻的產品肉厚設計

補強與防止變形 肋件設計 面與面相接的圓角 變化肉厚和改變形狀

肋件設計 補強肋（Rib）可增加產品的強度和剛性， 防止殘留應力造成的變形，也可改善塑膠的 流動性 肋與產品的肉厚有適當的比例 肋與成型品相接處應採用圓弧 避免採用大斷面的肋設計

面與面相接的圓角 面與面相接處為應力集中處，尤其是尖銳 的交角更容易造成集中應力。若在面相接 處以一定的圓弧相接，即能減少應力集中 的現象，同時還可使塑膠在成型時有良好 的流動性，並有助於產品脫模

變化肉厚和改變形狀 變化肉厚或改變成型品形狀，可防止 側壁或邊緣變形，以及增加成型品的 強度及剛性。有時還可以改善塑膠的 流動性