翻譯.pdf

1、大連交通大學2017屆本科生畢業(yè)設計（論文）外文翻譯 Figure 63 Influence of gating on glass fiber orientation and shrink- age of the product. Gate Design Gates are a transition zone between the runner system and the cavity. The location of gates is of great importance for the properties and appear- ance of the finished part.

2、 The melt should fill the entire cavity quickly and evenly. For gate design the following points should be considered: Locate the gate at the thickest section Note gate marks for aesthetic reasons Avoid jetting by modifying gate dimen- sions or position Balance flow paths to ensure uniform filling a

3、nd packing Prevent weld lines or direct to less critical sections Minimize entrapped air to eliminate burn marks Avoid areas subject to impact or mechan- ical stress Place for ease of degating Single vs. multiple gates. Unless the length of the melt flow exceeds practical limits a single gate is the

4、 preferred option. Multiple gates always create weld lines where the flows from the separate gates meet. A distinction can be made between center and edge gating of a part. Center gated parts show a radial flow of the melt. This type of gate is particularly good for symmetrical parts, such as cup sh

5、aped products or gears, because it will assure more uniform distribu- tion of material, temperatures, and packing, and better orientation effects it gives very pre- dictable results. On the other hand, linear flow and cross flow properties often differ. In flat parts, this can induce additional stre

6、ss and results in warpage or uneven shrinkage. Because of their simplicity and ease of manu- facture, edge gates are the most commonly used. These work well for a wide variety of parts that are injection molded. Long narrow parts typically use edge gates at or near one end in order to reduce warpage

7、. 外文原文 大連交通大學2017屆本科生畢業(yè)設計（論文）外文翻譯 But it is very difficult to mold round parts using this type gate, as they tend to warp into an oval shape. While a single gate into the body of the part might incur a higher initial tool cost, lower scrap rates and higher part quality will quickly justify this expe

8、nse. Figure 64 Influence of gate location on flow behavior of the melt. Gate dimensions. The cross section of the gate is typically smaller than that of the part runner and the part, so that the part can easily be de-gated (separated from the runner) without leaving a visible scar on the part. The g

9、ate thickness is typically between one half and two-thirds the part thickness. Since the end of packing can be identified as the time when the material in the gate drops below the freeze temperature, the gate thickness con- trols the packing time. Pressure Locate the gate in the thickest section to

10、ensure adequate pressure for packing out the part. This will also help prevent sink marks and voids forming. Orientation Molecular orientation becomes more pro- nounced in thin sections, the molecules usually align themselves in the flow direction. High degrees of orientation result in parts having

11、unaixal strength, resistance to loading only in one direction. To minimize molecular orientation the gate should be located so that as the melt enters the cavity it is diverted by an obstruction such as the cavity wall or an ejection pin. A larger gate will reduce frictional heating, permit lower ve

12、locities, and allow the applica- tion of higher packing pressure for a longer period of time. If appearance, low residual stress, and better dimensional stability were required then a larger gate would be advan- tageous. A minimum size of 0.8 mm is recommended for unreinforced materials. Smaller gat

13、es may induce high shear and thus thermal degrada- tion. Reinforced thermoplastics require slight- ly larger gates 1 mm. As a rule it should not exceed the runner or sprue diameter. The maximal land length should be 1 mm. Weld lines Place gates to minimize the number and length of weld lines or to d

14、irect 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. Gate location. Locat

15、ion items to include: consider 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

16、and also rough surfaces left by the gate act as stress concentrators. 大連交通大學2017屆本科生畢業(yè)設計（論文）外文翻譯 Figure 65 Warpage due to unfavorable gate location. Glass fiber reinforced materials Fiber-filled materials require larger gates to minimize breakage of the fibers when they pass through the gate. Using

17、small gates such as submarine, tunnel, or pin gates can damage the fillers in filled materials. Gates that deliver a uniform filling pattern (such as an edge gate) and thus, a uniform fiber orien- tation distribution are preferable to point-type gates. Fiber orientation will normally be the determin

18、ing factor for warpage problems with this type of material and the gate location and choice of gate type are 2 of the primary factors in controlling the orientation. In general, there will be a higher glass fiber orientation in thinner wall sections, e.g. less than 2 mm and as injection speed increa

19、ses. A high injection speed is required to obtain a smooth surface. The direction of orientation is influenced by gate type and location and, of course, by the shape of the product (see Figure 63). Warpage An incorrectly dimensioned or located gate may also result in undesirable flow patterns in the

20、 cavity. This can lead to moldings with visible weld line (see Figure 64). Undesirable flow patterns in the cavity can also lead to deformation by warping or bending (see Figure 65). Manually trimmed gates. Manually trimmed gates are those that require an operator to separate parts from runners duri

21、ng a sec- ondary operation. The reasons for using man- ually 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 automati- cally trimmed gates. S

22、imultaneous flow distribution across a wide front to achieve specific orientation of fibers of molecules often precludes auto- matic gate trimming. 大連交通大學2017屆本科生畢業(yè)設計（論文）外文翻譯 Gate types trimmed from the cavity manually include sprue gates, edge gates, tab gates, overlap gates, fan gates, film gates,

23、 diaphragm gates, external rings, and spoke or multipoint gates. Figure 66 Sprue gate. 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, ena

24、bling 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 determ

25、ined 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. The starting sprue diameter is controlled by the machine nozzle. The sprue diameter here must be about 0.5 mm larger

26、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 thick- ness of the par

27、t 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

28、more expensive, with little gain. 大連交通大學2017屆本科生畢業(yè)設計（論文）外文翻譯 Figure 67 Edge gate. Edge Gate The edge or side gate is suitable for medium and thick sections and can be used on multi- cavity two plate tools. The gate is located on the parting line and the part fills from the side, top or bottom. The t

29、ypical 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

30、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. The minimum tab width is 6 mm. The minimum tab thickness is 75% of the depth of the cavity. Figure 68 Tab gate. Overlap Gate An overlap gate is similar to an

31、 edge gate, except the gate overlaps the wall or surfaces. This type of gate is typically used to eliminate jetting. The typical gate size is 10% to 80% of the part thickness 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. Fan Gate A fan g

32、ate is a wide edge gate with variable thickness. This type is often used for thick- sectioned moldings and enables slow injec- tion without freeze-off, which is favored for low stress moldings or where warpage and dimensional stability are main concerns. The gate should taper in both width and thick

33、ness, to maintain a constant cross sectional area. This will ensure that: Figure 69 Overlap gate. The melt velocity will be constant. The entire width is being used for the flow. The pressure is the same across the entire width. 大連交通大學2017屆本科生畢業(yè)設計（論文）外文翻譯 As with other manually trimmed gates, the ma

34、ximum thickness should be no more than 80% of the part thickness. The gate width varies typically from 6 mm up to 25% of the cavity length. Figure 70 Film or flash gate. Film or Flash Gate A film or flash gate consists of a straight runner and a gate land across either the entire length or a portion

35、 of the cavity. It is used for long flat thin walled parts and provides even filling. Shrinkage will be more uniform which is important especially for fiber reinforced ther- moplastics and where warpage must be kept to a minimum. The gate size is small, typical- ly 0.25mm to 0.5mm thick. The land ar

36、ea (gate length) must also be kept small, approx- imately 0.5 to 1.0 mm long. Figure 71 Internal ring gate. Diaphragm Gate A diaphragm gate is often used for gating cylindrical or round parts that have an open inside diameter. It is used for single cavity molds that have a small to medium internal d

37、iameter. It is used when concentricity is important and the presence of a weld line is not acceptable. Typical gate thickness is 0.25 to 1.5 mm. External Ring Gate This gate is used for cylindrical or round parts in a multicavity mold or when a diaphragm gate is not practical. Material enters the ex

38、ter- nal ring from one side forming a weld line on the opposite side of the runner this weld line is not typically transferred to the part. Typical gate thickness is 0.25 to 1.5 mm. Figure 72 External ring gate. 大連交通大學2017屆本科生畢業(yè)設計（論文）外文翻譯 Figure 73 Multi-point gate. Spoke Gate or Multipoint Gate Thi

39、s kind of gate is used for cylindrical parts and offers easy de-gating and material savings. Disadvantages are the possibility of weld lines and the fact that perfect roundness is unlikely. Typical gate size ranges from 0.8 to 5 mm diameter. Automatically trimmed gates. Automatically trimmed gates i

40、ncorporate 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. Figure 74 Pin gates. Gate types

41、trimmed from the cavity automati- cally include pin gates, submarine (tunnel) gates, hot runner gates, valve gates. Pin Gates Pin gates are only feasible with a 3-plate tool because it must be ejected separately from the part in the opposite direction The gate must be weak enough to break off withou

42、t damaging the part. This type of gate is most suitable for use with thin sections. The design is particularly useful when multiple gates per part are needed to assure symmetric filling or where long flow paths must be reduced to assure packing to all areas of the part. Gate diameters for unreinforc

43、ed thermoplastics range from 0.8 up to 6 mm. Smaller gates may induce high shear and thus thermal degrada- tion. Reinforced thermoplastics require slight- ly larger gates 1 mm. The maximal land length should be 1 mm. Advised gate dimen- sions can be found in Figure 74. Figure 75 Dimensions of gates

44、(* wall thickness larger than 5 mm should be avoided). 大連交通大學2017屆本科生畢業(yè)設計（論文）外文翻譯 Submarine (tunnel) Gates A submarine gate is used in two-plate Figure 76 Tunnel gate. mold construction. An angled, tapered tunnel is machined from the end of the runner to the cavity, just below the parting line. As t

45、he parts and runners are ejected, the gate is sheared at the part. The tunnel can be located either in the moving mold half or in the fixed half. A sub-gate is often located into the side of an ejector pin on the non-visible side of the part when appear- ance is important. To degate, the tunnel requ

46、ires a good taper and must be free to bend. Typical gate sizes 0.8 mm to 1.5 mm, for glass reinforced materials sizes could be larger. Hot Runner Gates Hot runner gates are also known as sprue- less gating. The nozzle of a runnerless mold is extended forward to the part and the mate- rial is injecte

47、d through a pinpoint gate. The face of the nozzle is part of the cavity surface; this can cause appearance prob- lems (matt appearance and rippled surface). The nozzle diameter should therefore be kept as small as possible. Most suitable for thin walled parts with short cycle times, this avoid freez

48、ing of the nozzle. Figure 77 Hot runner gates. Valve Gates The valve gate adds a valve rod to the hot runner gate. The valve can be activated to close the gate just before the material near the gate freezes. This allows a larger gate diameter and smoothes over the gate scar. Since the valve rod cont

49、rols the packing cycle, better control of the packing cycle is maintained with more consistent quality. Figure 78 Valve gate. 大連交通大學2017屆本科生畢業(yè)設計（論文）外文翻譯 Figure 79 Basic principle of cooling channels. Mold Cooling Mold cooling serves to dissipate the heat of the molding quickly and uniformly. Fast co

50、oling is necessary to obtain economical production and uniform cooling is required for product quality. Adequate mold temperature control is essential for consistent The layout of the cooling circuit close attention especially if you molding. warrants consider cooling typically accounts for two thir

51、ds of a products cycle time. Optimal properties of engineering plastics can be achieved only when the right mold temperature is set and maintained during pro- cessing. The mold temperature has a sub- stantial effect on: Figure 80 Position of cooling channels. Mechanical properties Shrinkage behavior

52、 Warpage Surface quality Cycle time Flow length in thin walled parts In particular semi-crystalline thermoplastics need to cool down at optimal crystallization rate. Parts with widely varying wall thickness- es are likely to deform because of local differ- ences in the degree of crystallization. Add

53、itionally the required cooling time increas- es rapidly with wall thickness. This calculation is shown in Cooling system equations. Cooling channel configuration. In general, the cooling system will be roughly drilled or milled. Rough inner surfaces enhance turbu- lent flow of coolant, thus providin

54、g better heat exchange. Turbulent flow achieves 3 to 5 times as much heat transfer as does non tur- bulent flow. Cooling channels should be placed close to the mold cavity surface with equal center distances in between (see Figures 79 and 80). The mechanical strength of the mold steel should be cons

55、idered when designing the cooling system. 大連交通大學2017屆本科生畢業(yè)設計（論文）外文翻譯 Some thermoplastics may require mold temper- atures of 100C (212F) or higher for optimal processing and properties. Effective mold insulation is advised to minimize heat loss between the mold and the machine mounting platens. Insul

56、ation boards with low thermal con- ductivity and relatively high compressive strength are commercially available. Figure 81 Sealing and cooling channel lay-out. Care is required in the correct placing of seals; they may be damaged by the sharp edges of the pocket when the mold insert is mounted (see

57、 Figure 81). Seals or O-rings should be resistant to elevated temperatures and oils. Guidelines for optimal mold temperature control include: Figure 82 Cooling of the mold. Independent symmetrical cooling circuits around the mold cavities. Cores need effective cooling (see baffles, bubblers they are

58、 prone to damage and require a lot of maintenance. Blade ejectors are most commonly used with ribbed parts. Figure 91 Three plate mold with two stripper plates for ejection. 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 mold release because a vacuum may arise between cavity wall and the molding. Release can be improved by breaking the vacuum with an ejection mechanism. Figure 92 Eject