錐形帶輪注塑工藝與模具設(shè)計

錐形帶輪注塑工藝與模具設(shè)計,錐形帶輪注塑工藝與模具設(shè)計,錐形,注塑,工藝,模具設(shè)計 2 Table of Contents Page 1. INTRODUCTION.3 2. PART DESIGN .4 3. MOLD DESIGN .6 4. HOT RUNNER SYSTEMS.17 5. OVERMOLDING.20 6. MACHINE SELECTION.21 7. MATERIAL HANDLING AND PREPARATION.22 8. PROCESSING CONDITIONS.24 9. TROUBLESHOOTING.28 3 INTRODUCTION Objective This document provides guidelines for part design, mold design and processing of styrenic block copolymer (SBC) TPEs. The GLS product families that include styrenic TPEs are Kraton compounds, Dynaflex TPE compounds and Versaflex TPE alloys. SBC Rheology One major characteristic of SBCs is that they are shear dependent. A material is shear dependent when its viscosity is higher at low shear rates (such as extrusion) and lower at high shear rates (as in injection molding). Therefore, SBC compounds will flow more easily into thin areas of the mold at high shear rates. The shear thinning behavior of SBCs should be considered when designing injection molds and also when setting mold conditions during processing. Figure 1.The effect of shear rate on the viscosity of GLS styrenic TPE compounds (measured at 390F (200C). To obtain information regarding the viscosity of an individual grade, refer to the Product Technical Data Sheet, available at or contact your GLS representative. 4 PART DESIGN General Part Design Concepts When designing a TPE part, there are a few general rules to follow: The part wall thickness should be as uniform as possible. Transitions from thick to thin areas should be gradual to prevent flow problems, back fills, and gas traps. Thick sections should be cored out to minimize shrinkage and reduce part weight (and cycle time). Radius / fillet all sharp corners to promote flow and minimize no-fill areas. Deep unventable blind pockets or ribs should be avoided. Avoid thin walls that cannot be blown off the cores by air-assist ejection. Long draws with minimum draft may affect ease of ejection. Flow Length and Wall Thickness The maximum achievable flow length is dependent on the specific material selected, the thickness of the part, and processing conditions. Generally, GLS compounds will flow much further in thinner walls than other types of TPEs. The flow to thickness ratio should be 200 maximum, however this is dependent on the material and the part design. High flow GLS TPE compounds (such as Versalloy) have been used successfully to fill flow ratios up to 400. The measurement of spiral flow offers a comparative analysis of a materials ability to fill a part. The spiral flow test is performed by injecting a material into a spiral mold (similar to a ribbon formed into a spiral). The distance the material flows is measured in inches. In this case, the spiral flow test was conducted using two different injection speeds (3 in/sec and 5 in/sec). The typical spiral flow lengths for the various GLS product families are summarized in Table 1. With specific compounds, flow lengths of up to 40 inches (at 5 in/sec injection speed) are possible. Table 1. Typical Spiral Flow Lengths for GLS Compounds* Flow length, in Series 3 in/sec 5 in/sec Dynaflex D 13 15 18 20 Dynaflex G 12 22 18 30 Versaflex 9 16 13 26 *Spiral flow tests performed using 0.0625 in thickness and 0.375 in width channel at 400 F. For spiral flow information about a specific grade or additional details about the spiral flow test procedure, please refer to the GLS Corporation TPE Tips Sheet #7, available at or by contacting your GLS representative. 5 Undercuts The flexibility and elastic nature of TPEs allows for the incorporation of undercuts into the part design. Because of their excellent recovery characteristics, GLS compounds are capable of being stretched and deformed, allowing them to be pulled from deep undercuts (Figure 2). If both internal and external undercuts are present on the same part, slides or core splits may be necessary. Parts with internal undercuts (e.g. bulb- shaped parts) may be air ejected from the core by use of a poppet valve in the c ore. Minor permanent elongation (3% - 8%) due to deformation may occur during ejection. Figure 2. An example of TPE parts with large undercuts. Gate and Knit Line Locations The product engineer should indicate the areas of the part that are cosmetic and those that are functional and include this information on the drawing. This will help the mold designer to determine the allowable gate and knit line locations. Anisotropy Thermoplastic materials that have different properties in the flow direction versus the cross-flow direction (90 perpendicular to the flow direction) are characterized as “anisotropic” materials. Properties that may be affected are shrinkage and tensile properties. Anisotropy is caused when the polymer chains orient in the direction of flow, which leads to higher physical properties in the flow direction. Wall thickness, injection speed, melt temperature and mold temperature are a few variables that affect anisotropy. Depending on the processing conditions and mold design, most GLS styrenic TPE compounds exhibit a degree of anisotropy. Shrinkage Due to their anisotropic nature, GLS styrenic TPE compounds shrink more in the flow direction than in the c ross-flow direction. Generally, SEBS compounds have higher shrinkage and are more anisotropic than SBS compounds. Typical shrinkage values for SEBS-based compounds are 1.3% - 2.5%, whereas those for SBS based compounds are 0.3% - 0.5 %. Softer SEBS compounds (below 30 Shore A) will shrink more than harder 6 materials. Some grades, such as Dynaflex G7700, G7800, and G7900 Series contain filler, which reduces their shrinkage. The shrinkage values reported by GLS are determined using a 0.125” thick plaque. It should be noted that shrinkage is not an exact number, but a range value. This range can be affected by the part wall thickness, melt temperature, mold temperature, injection speed, hold/pack pressures and also the time between molding and measuring. As a result, prototyping is strongly recommended for parts with close tolerances to better quantify the realistic shrinkage of a specific grade of material in a specific application. For shrinkage values for specific grades, please refer to the product T echnical Data Sheet, available at or by contacting your GLS representative. MOLD DESIGN Types of Molds GLS SBC compounds can be molded in two- and three-plate molds. Both conventional and hot runner tool designs have been used with GLS compounds. Self-insulating hot runner tool designs are not recommended due to the potential for material degradation in the stagnation zones. Two-shot molds and insert molds can also be used. If a family mold is required, the cavity volumes should be similar, otherwise overpacking and flashing of the smaller cavity may occur. Steel Selection GLS styrenic TPEs are generally non-abrasive and non-corrosive. The selection of tool steel will depend on the quantity and quality of parts to be produced. For high volume production, the initial expense of quality tooling is a sound investment. A wide variety of tool steels are available for injection mold construction. Table 2 lists the properties of common tool steels and the typical mold components for which they are used. Soft metals, such as aluminum and beryllium copper, can be used for prototype parts or short production runs up to 10,000 parts. 7 Table 2. Typical Tool Steel for Injection Mold Construction Steel Type Steel Properties Mold Component P-20 Pre-hardened, machines well, high carbon, general-purpose steel. Disadvantage: May rust if improperly stored. Mold bases, ejector plates, and some cavities (if nickel or chrome plated to prevent rust). H-13 Good general purpose tool steel. Can be polished or heat-treated. Better corrosion resistance. Cavity plates and core plates. S-7 Good high hardness, improved toughness, general-purpose tool steel. Machines well, shock resistant, polishes well. Disadvantage: Higher cost. Cavity plates, core plates and laminates, as well as thin wall sections. A-2 Good high toughness tool steel. Heat-treats and polishes well. Ejector pins, ejector sleeves, and ejector blades. D-2 Very hard, high wear characteristics, high vanadium content, somewhat brittle. Disadvantage: Difficult to machine. Gate blocks, gib plates to prevent galling, gate blocks to prevent wear. 420 SS Tough corrosion resistant material. Heat-treats and polishes well. Disadvantage: High cost. Cavity blocks, ejector pins, sleeves, etc. Some part designs may benefit from the use of higher thermal conductivity materials such as beryllium copper. This material is less durable than steel and may hob or wear faster than steel if used at the parting-line. Beryllium copper can be used for inserts, slides or cores to increase heat transfer rates and reduce cycle times. In cases where there is a long draw core, a fountain-type bubbler may be beneficial. Mold Surface Treatment, Finishing and Texturing Most GLS materials replicate the mold surface fairly well. To produce a glossy surface, a polished mold is required and an unfilled grade should be used. A highly polished tool and a transparent material are required to produce a part with good clarity. If a matte finish similar to that of a thermoset rubber is required, a rougher mold texture should be used (or a GLS product such as GLS Versalloy TPV alloys, which naturally produce a matte surface). In general, an EDM surface will produce a good texture and may improve release from the tool during part ejection. Matte surfaces can also help to hide any flow marks or other surface defects. Vapor honing, sand or bead blasting and chemical etching are also used to produce textured surfaces with varying degrees of gloss and appearance. To aid in release, the cavity or core may be coated with a release coating such as PTFE impregnated nickel after it has been given a sandblast or EDM finish. 8 Sprue and Sprue Puller Design The sprue should have sufficient draft, from 1 to 3 to minimize drag and sprue sticking. Longer sprues may require more taper (3 - 5 ), as shown in Figure 3. Typically, the sprue diameter should be slightly larger than the nozzle diameter. An EDM finish is acceptable for most styrenic TPE materials. Permanent surface lubricant treatments have also been used successfully. Sprue puller designs vary with the hardness of the material. The different sprue designs possible and their relative dimensions are shown in Figures 4 through 7. In addition, Table 3 shows the typical h ardness range for which a particular sprue design is applicable. Table 3. Typical Sprue Designs for Various Hardness Values Typical TPE Hardness Range Most Common Sprue Puller Types Figure 50 Shore A Tapered, Pin, Z-Type 3, 4 and 6 40-70 Shore A Undercut 5 5-40 Shore A Pine Tree 7 Hot sprue bushings and extended nozzles may also be used with GLS compounds. In many molds, the sprue is the thickest wall section in the mold and will control the minimum cooling time. The use of a hot sprue, which may be viewed as an extension of the machine nozzle, can sometimes reduce cycle time. Extended machine nozzles may also be used to reduce sprue length and size. When hot sprues are used, the machine nozzle tip should be a free-flow nozzle rather than a reverse tip. Figure 3. Tapered Sprue Puller Figure 4. Z-Pin Sprue Puller 9 Figure 5. Undercut Sprue Puller Figure 6. Sucker Pin Sprue Puller 10 Figure 7. Pine Tree Sprue Puller Conventional Runner Configuration and Design A balanced runner configuration is critical to achieve uniform part quality from cavity to cavity. In a balanced runner system, the melt flows into each cavity at equal times and pressure. The runner balance can be designed by using computer mold-flow analysis programs and verified by performing short-shot studies. An unbalanced runner may result in inconsistent part weights and dimensional variability. The cavity closest to the sprue may be overpacked and flashing may occur. As a result of overpacking, parts may also develop high molded-in stresses, which lead to warpage. Examples of balanced runner systems are shown in Figures 8 and 9. 11 Figure 8. Example of Balanced Spider Runner Figure 9. Example of Balanced Cross-Runner Figure 10 shows different runner cross-sections and their associated efficiency. Full- round runners have the least resistance to flow and surface area, allowing the material to stay molten longer. The second most efficient runner cross-section is the modified trapezoid. This runner geometry most closely simulates a full round runner but only requires machining in only one plate. Figure 11 shows typical ball cutter dimensions and the corresponding modified trapezoid runner sizes. Figure 12 illustrates typical runner dimensions. Figure 10. Typical Runner Cross-Sections W = 1.25D 1/8 MODIFIED TRAPEZOID RUNNER 3/16 MODIFIED TRAPEZOID RUNNER 1/4 MODIFIED TRAPEZOID RUNNER 3/8 MODIFIED TRAPEZOID RUNNER 1/2 MODIFIED TRAPEZOID RUNNER 12 Figure 11. Modified Trapezoid Runner Sizes Figure 12. Runner Design and Dimensions Cold slug wells should be used at each runner transition (turn). Cold slug wells serve to remove the leading edge of the melt. The slug well associated with the sprue should be large enough to trap the cold material formed in the machine nozzle during the mold-open cycle. Typical slug well dimensions are approximately 1.5 to 2.0 times the diameter or width of the feed runner. 13 Runner Keepers Runner keepers or sucker pins provide undercuts to keep the runner on the desired plate but should not restrict material flow through the runner. Figures 8 and 9 show typical locations for runner keepers and sucker pins. Figure 13 illustrates an example design of a runner keeper. Figure 13. Runner Keeper design Gate Design and Location Most conventional gating types are suitable for processing GLS styrenic TPE compounds. The type of gate and the location, relative to the part, may affect the following: Part packing Gate removal or vestige Part cosmetic appearance Part dimensions (including warpage) The type of gate selected is dependent on both part and tool design. The gate location is equally important. To prevent the chances of jetting, locate the gate entrance in an area where the flow will impinge on a cavity wall. For automatically degating tools, the highly elastic nature of softer TPEs makes submarine gate designs or three plate tools with self- degating drops more difficult. Higher hardness and filled grades usually have lower ultimate elongation and therefore are more easily degated. To assure the gates will break at a specific location, they should have a short land length to create a high stress concentration. Tab/Edge Gates Tab or edge gates (Figure 14) most commonly utilize a conventional sprue and cold runner system. They are located along the tool parting line. A small undercut can be placed where the gate meets the part to minimize gate vestige caused by degating. Advantages of edge gates are ease of fabrication, modification and maintenance. The 14 gate depth (D) should be 15% - 30% of the wall thickness at the gate entrance. Common practice is to start “steel safe”. A good starting point for the gate width should be 1.0 - 1.5 times the gate depth. The gate land should be equal to or slightly longer than gate depth. The gate size may also depend on the part volume. The gate area may be inserted to facilitate gate maintenance or modification. Figure 14. Tab or edge gate Figure 15. Submarine Gate Submarine or tunnel gates are self-degating. During part ejection, the tool steel separates the part and the runner. Figure 15 shows a typical design of a submarine gate. Cashew type submarine gates should not be used for medium to soft hardness compounds due to their high coefficient of friction and high elongation. Fan Gates A fan gate is a streamlined variation of a tab gate (Figure 16). The fan gate distributes material into the cavity more evenly; thus it is normally used in parts that require a high degree of flatness and absence of flow lines. It also minimizes the possibility of gate pucker or part warpage. Figure 16. Fan gate 15 Sprue or Direct Gate The sprue or direct gate is often used on prototype parts because it is inexpensive. This type of gating is not recommended for GLS styrenic compounds because of their high elongation. In addition, the sprue will need to be trimmed thus appearance quality of the part is usually poor. If sprue gating is selected, care should be taken to keep both the sprue length and diameter as short and small as possible. Diaphragm Gate The diaphragm gate is used to maintain the concentricity of round parts. It allows even flow into the cavity and minimizes the potential for knit lines. Due to anisotropic shrinkage, flat round parts using center or diaphragm gating may not lay flat. A ring gate may also be used on the outside of a circular part. Table 4 compares the advantages and disadvantages of the various gate types discussed in this section. Table 4. Advantages and Disadvantages of Various Gate Types Gate Type Advantage Disadvantage Edge/Tab/Fan Gate Appropriate for flat parts Easy to modify Post-mold gate/runner removal is difficult Poor gate vestige Submarine Gate Automatic gate removal Minimal gate vestige More difficult to machine Diaphragm Gate Concentricity Appropriate for round parts No knit lines Scrap Post-molding gate removal Pin gate (3-plate) Automatic gate removal Minimal gate vestige Localized cooling Requires floater plate More scrap Higher tool cost Valve gate (Hot runner systems) Minimal gate vestige Positive shut-off Minimizes post pack Higher tool cost Higher maintenance Only for hot runner systems Gate Location Styrenic TPE compounds are anisotropic, thus they have different physical properties in the flow direction versus the cross-flow direction. Depending on the products intended usage, these property differences could be critical to the performance of the final part. As a result, the anisotropic nature of the styrenic TPE needs to be taken into consideration when determining the gate location on the part. The material flow may be estimated by eye or by using flow analysis programs. For higher shrinkage grades, the part may shrink near the gate, which causes “gate pucker” if there is a high molded-in stress at the gate. Parts shaped like a handle grip may warp toward the gate side of the part. Locating the gate at the top of the part minimizes this problem. Using two gates on opposite sides of the part can also address the issue, but it will result in two knit lines. If filling problems exist in thin walled parts, adding flow channels or minor changes in wall thickness can alter the flow. In some cases, it may be necessary to add a second gate to properly fill the parts. 16 The gate should be placed so that the flow path is as short as possible. Locating the gate at the heaviest cross section of the part can improve packing and minimize voids or sinks. If possible, the gate should be positioned so as to avoid obstructions (flowing around cores or pins) in the flow path. The flow path of the material should minimize the possibility of formation of knit lines and flow marks. Upon injection, the material should impinge off the cavity wall to reduce the possibility of jetting. To minimize the effect of molded-in stress (at the gate) on part performance, the gate should be located in noncritical areas of the part. Also, the gate location should allow for easy manual or automatic degating. Mold Venting Mold venting is critical to the quality and consistency of the finished part. Venting is required to allow the air in the sprue, runner and cavity to leave the tool as the melt flows into the cavity. Inadequate venting may cause short-shots, poor surface appearance, or weak weld-lines. Potential air traps in the part design can be predicted by flow simulation software. Once the tool has been built, short-shot studies can be used to find the critical venting areas. Vents should be placed at the last place to fill and in areas where weld l