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外文原文
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 appearance of the finished part. 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 dimensions or position
- Balance flow paths to ensure uniform filling and packing
- Prevent weld lines or direct to less critical sections
- Minimize entrapped air to eliminate burn marks
- Avoid areas subject to impact or mechanical 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 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 shaped products or gears, because it will assure more uniform distribution of material, temperatures, and packing, and better orientation effects it gives very predictable results. On the other hand, linear flow and cross flow properties often differ. In flat parts, this can induce additional stress and results in warpage or uneven shrinkage.
Because of their simplicity and ease of manufacture, 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.
Figure 63 Influence of gating on glass fiber orientation and shrink- age of the product.
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 expense.
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 gate 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 controls the packing time.
A larger gate will reduce frictional heating, permit lower velocities, and allow the application 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 advantageous.
A minimum size of 0.8 mm is recommended for unreinforced materials. Smaller gates may induce high shear and thus thermal degradation. Reinforced thermoplastics require slightly larger gates > 1 mm. As a rule it should not exceed the runner or sprue diameter. The maximal land length should be 1 mm.
Gate location. Location items to consider include:
Appearance
Whenever possible locate gates on nonvisual 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.
Orientation
Molecular orientation becomes more pronounced in thin sections, the molecules usually align themselves in the ?ow direction. High degrees of orientation result in parts having 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.
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 bene?cial to a strong weld line
Figure 64 Influence of gate location on flow behavior of the melt.
Glass ?ber reinforced materials
Fiber-?lled materials require larger gates to minimize breakage of the ?bers when they pass through the gate. Using small gates such as submarine, tunnel, or pin gates can damage the ?llers in ?lled materials. Gates that deliver a uniform ?lling pattern (such as an edge gate) and thus, a uniform ?ber orientation distribution are preferable to point-type gates. Fiber orientation will normally be the determining 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 ?ber orientation in thinner wall sections, e.g. less than 2 mm and as injection speed increases. A high injection speed is required to obtain a smooth surface. The direction of orientation is in?uenced 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 ?ow patterns in the cavity. This can lead to moldings with visible weld line (see Figure 64). Undesirable ?ow 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 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 ?ow distribution across a wide front to achieve speci?c orientation of ?bers of molecules often precludes automatic gate trimming.
Gate types trimmed from the cavity manually include sprue gates, edge gates, tab gates, overlap gates, fan gates, film gates, diaphragm gates, external rings, and spoke or multipoint gates.
Sprue Gate
Recommended for single cavity molds or for parts requiring symmetrical ?lling. This type of gate is suitable for thick sections because holding pressure is more effective. A short sprue is favored, enabling rapid mold ?lling 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.
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 ?lls from the side, top or bottom. 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 ?at and thin parts, to reduce the shear stress in the cavity. The high shear stress generated around the gate is con?ned 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.
Overlap Gate
An overlap gate is similar to an 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 gate is a wide edge gate with variable thickness. This type is often used for thicksectioned moldings and enables slow injection 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 thickness, to maintain a constant cross sectional area. This will ensure that:
- The melt velocity will be constant.
- The entire width is being used for the ?ow.
- The pressure is the same across the entire width.
As with other manually trimmed gates, the maximum 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.
Film or Flash Gate
A ?lm or ?ash gate consists of a straight runner and a gate land across either the entire length or a portion of the cavity. It is used for long ?at thin walled parts and provides even ?lling. Shrinkage will be more uniform which is important especially for ?ber reinforced thermoplastics and where warpage must be kept to a minimum. The gate size is small, typically 0.25mm to 0.5mm thick. The land area (gate length) must also be kept small, approximately 0.5 to 1.0 mm long.
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 diameter. 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 external 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.
Spoke Gate or Multipoint Gate
This 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 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 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 without 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 ?lling or where long ?ow paths must be reduced to assure packing to all areas of the part. Gate diameters for unreinforced thermoplastics range from 0.8 up to 6 mm. Smaller gates may induce high shear and thus thermal degradation. Reinforced thermoplastics require slightly larger gates > 1 mm. The maximal land length should be 1 mm. Advised gate dimensions can be found in Figure 74.
Submarine (tunnel) Gates
A submarine gate is used in two-plate mold construction. An angled, tapered tunnel is machined from the end of the runner to the cavity, just below the parting line. As the 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 ?xed half. A sub-gate is often located into the side of an ejector pin on the non-visible side of the part when appearance is important. To degate, the tunnel requires 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 sprueless gating. The nozzle of a runnerless mold is extended forward to the part and the material is injected through a pinpoint gate. The face of the nozzle is part of the cavity surface; this can cause appearance problems (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 freezing of the nozzle.
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 controls the packing cycle, better control of the packing cycle is maintained with more consistent quality.
Mold Cooling
Mold cooling serves to dissipate the heat of the molding quickly and uniformly. Fast cooling is necessary to obtain economical production and uniform cooling is required for product quality. Adequate mold temperature control is essential for consistent molding. The layout of the cooling circuit warrants close attention especially if you consider cooling typically accounts for two thirds of a products cycle time.
Optimal properties of engineering plastics can be achieved only when the right mold temperature is set and maintained during processing. The mold temperature has a substantial effect on:
- Mechanical properties
- Shrinkage behavior
- 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 thicknesses are likely to deform because of local differences in the degree of crystallization. Additionally the required cooling time increases rapidly with wall thickness. This calculation is shown in Cooling system equations.
Cooling channel con?guration. In general, the cooling system will be roughly drilled or milled. Rough inner surfaces enhance turbulent ?ow of coolant, thus providing better heat exchange. Turbulent ?ow achieves 3 to 5 times as much heat transfer as does non turbulent 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 considered when designing the cooling system.
Some thermoplastics may require mold temperatures of 100°C (212?F) or higher for optimal processing and properties. Effective mold insulation is advised to minimize heat loss between the mold and the machine mounting platens. Insulation boards with low thermal conductivity and relatively high compressive strength are commercially available.
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 Figure 81). Seals or O-rings should be resistant to elevated temperatures and oils.
Guidelines for optimal mold temperature control include:
- Independent symmetrical cooling circuits around the mold cavities.
- Cores need effective cooling (see baf?es, bubblers & thermal pins).
- Short cooling channels to ensure temperature differences between in and outlet do not exceed 5?C (41?F).
- Parallel circuits are preferred over serial cooling as shown in Figure 82.
- Avoid dead spots and/or air bubbles in cooling circuits.
- Heat exchange between mold and machine should be minimized.
- Differences in ?ow resistance of cooling channels, caused by diameter changes, should be avoided.
Mold parts that are excessively heated, like sprue bushings and areas near the gates, must be cooled intensively. Rapid and even cooling is enhanced by the use of highly conductive metals, such as beryllium-copper. These metals are used to full advantage in places where it is impossible to place suf?cient cooling channels. Beryllium copper transfers twice as much heat as carbon steel and four times as much heat as stainless steel. This does not mean beryllium copper molds will run 4 times faster tan a stainless steel mold but they will run some thin walled parts signi?cantly faster. Beryllium copper is not recommended for materials that require high mold temperatures as they allow so much heat transfer that it is dif?cult to maintain adequate heat economically.
Alternative cooling devices. For areas in the mold where it is not possible to use normal drilled cooling channels, alternative methods must be employed to ensure these areas are uniformly cooled with the rest of the part. The methods employed usually include baffles, bubblers, or thermal pins.
Baf?es
Baffles and bubblers are sections of cooling lines that divert the coolant flow into areas that would normally lack cooling, e.g. cores. A baffle is actually a cooling channel drilled perpendicular to a main cooling line, with a blade that separates one cooling passage into two semi-circular channels. The coolant flows in one side of the blade from the main cooling line, turns around the tip to the other side of the baffle, and then flows back to the main cooling line.
This method provides maximum cross sections for the coolant, but it is difficult to mount the divider exactly in the center. The cooling effect and with it the temperature distribution on one side of the core may differ from that on the other side and this is the main disadvantage. The use of a helix baffle will solve the problem by conveying the coolant to the tip and back in the form of a helix. It is useful for diameters of 12 to 50 mm, and makes for a very homogeneous temperature distribution. Another logical development of baffles are single or double-flight spiral cores (see Figure 84).
Bubblers
A bubbler is similar to a baf?e except that the blade is replaced with a small tube. The coolant ?ows into the bottom of the tube and "bubbles" out of the top, like a fountain. The coolant then ?ows down around the outside of the tube to continue its ?ow through the cooling channels. For slender cores this is the most effective form of cooling. The inner and outer diameters must be adjusted so that the flow resistance in both cross sections is equal. The condition for this is: Inner Diameter / Outer Diameter = 0.7.
Thermal Pins
A thermal pin is an alternative to baf?es and bubblers. It is a sealed cylinder ?lled with a ?uid. The ?uid vaporizes as it draws heat from the tool steel and condenses as it releases the heat to the coolant. The heat transfer ef?ciency of a thermal pin is almost ten times as greater than a copper tube. For good heat conduction, avoid an air gap between the thermal pin and the, or ?ll it with a highly conductive sealant.
Cooling of large cores
For large core diameters (40 mm and larger), a positive transpor