Vertical injection molding machine molded article shrinkage (6)

Date：2015-10-18 20:54:59

Shrinkage 2.2 center gate injection molding gear
        By shrinking the actual vertical injection molding machine injection molding products for analysis, we can further clarify the law contraction injection molded article. Injection molding gear is typical precision injection molded products, computer disc drives and printers, copiers, cameras and other precision mechanical transmission mechanism pinion large number of applications engineering plastics are manufactured in these complexes have a size requirement, plastic products shape dimensional accuracy must be very high, for injection molding gear selection as an example analysis.
        2.2.1 theoretical derivation
        Decided involute shape is a major factor in the modulus of teeth, pressure angle. In m, a, referral indicates modulus plastic tooth vertical injection molding machine, pressure angle, base radius indicates modulo tooth cavity, pressure angle, base radius,: Indicates the number of teeth ,, represents the average contraction of plastic rate. The literature suggests that the plastic tooth curves obtained positive displacement curve tooth cavity; literature that deal with tooth cavity analog to digital correction and pressure angle correction, amendment modulus of m = m (1 + 2) , pressure angle correction views are inconsistent. Base radius remain unchanged. To determine these design methods are correct and that the tooth cavity with plastic toothed curve curve what is the relationship, we must first determine when the plastic shrink is shrunk along what direction.
        Literature that plastic gear contraction along the direction of the radius of curvature (ie, the surface normal) in. According to this contraction rule, when the plastic surface is not smooth surfaces (ie tangent plane plastic surface is not continuous), the place of the plastic surface normal direction mutated, can not make a point on the cavity surface and plastic parts The point on the table - one correspondence.
        For involute, if the direction along the curvature of the plastic parts shrink to involute curve ABC represents plastic tooth, involute A'B'C 'represents the cavity tooth curve, due to the straight line AA' is both a point of law A line, but also the point A 'normals, so the two on the same involute base circle. Let the base center of the circle is O, base circle radius r. Straight line AA 'and the base circle tangent to the point P, linear BB' and the base circle tangent to the point Q, according to the contraction direction curvature direction, indicating the amount of shrinkage and basic dimensions PA ', QB' has nothing to do, which is inconsistent with the actual situation .
        2.2.2 Experimental Verification
        To test the above derivation result, the object being analyzed were vertical injection molding machine injection experiments. Vertical injection molding machine die as the standard involute tooth spur gear. Cavity using CNC wire cutting, error 0.olmm. Single cavity mold structure, center feed, six rams uniform, the top position in the convergence of the board. Molding material is polyoxymethylene. Plastic parts obtained from the same injection pressure and temperature conditions, optionally three, plastic tooth curve point by point measurements with the projector, 50 times magnification, precision 1pm.
        Tooth cavity with a universal tool microscope curve point by point measurements, 50 times magnification, precision 0.2 co m. Mountains of the measured data were plotted curve and cavity plastic tooth tooth curve.
        Order data processing is:
        1. The coordinates of each point measured by the addendum circle, tooth root circle, Solver addendum circle's center and radius, center of the circle and the tooth root radius.
        2. By ra = r + h. 'M = r + m, r / = r a (ha' + c ') m =, a 1.35m, too few 1.35ra + 2.35. Using this formula, according to the above fitting addendum circle radius ra, the tooth root radius' determined pitch circle radius: value.
        3. For each angle involute, according to the coordinates of each point on the vertical injection molding machine to solve the planning group circle center (x., Yo), base radius referral, involute starting position with the x-axis Arts c to $ O
        4. by one tooth left and right angle between two involute base circle radius starting position was determined based arc tooth thickness sre.
        5. According to a one arccos (re / :), by the:, r, determined pitch circle pressure angle a value.
        6. by the base circle center (x., Yo), base radius r. Involute starting location and the angle between the X-axis length of the pitch circle Ca dagger pressure angle a, find a certain involute coordinate values ​​at the pitch circle. According to one tooth of the left and right two involute coordinate values ​​at the pitch circle, pitch circle chord determined tooth thickness.
        Experimental results show:
        Radial shrinkage 1. involute, from the tooth root circle, base circle, pitch circle to the addendum circle decreases. Analysis of the reasons that low because of the addendum enclosed cavity being located, and the cooling circuit located just outside the ring cavity, so than at the tooth root solidified melt faster, larger cooling rate freeze more non-equilibrium molecular free volume, resulting in a large solid hematocrit, the result is tooth tip of the radial contraction rate is less than the tooth root. To reduce the difference between the two, we need to adjust the temperature distribution of the mold, for example, the outer ring of the tooth cavity oil heating, or cooling pipe placed inside the cavity of the ring gear.
        2. Since the ratio cosa and c0s mouth very close to 1, can be considered a pressure angle and cavity plastic parts involute involute pressure angle a 'are equal.
        3. According to the resulting cavity involute positional relationship between the experimentally measured positional relationship between the line and the two plastic parts involute consistent. According to the principle of vertical injection molding machine direction shrinkage along the curvature of the plotted cavity with plastic parts involute involute positional relationship is obviously inconsistent with the experimental results.
        4. In calculating the shrinkage base arc tooth thickness and the pitch circle of chord tooth thickness, modulus experiments because gear is very small, so that the tooth thickness dimension is small, so even a few microns in a measurement error will cause data processing results diverge.
        But certainly, shrink vertical injection molding machine tooth thickness direction is there, the magnitude of shrinkage values ​​can be compared with the radial shrinkage, it is often too small leaving its basic dimensions overshadowed negligible shrinkage. Through theoretical analysis and experimental research, that the center gate injection molding gear is to shrink the center of the gate, the relationship between tooth cavities between teeth and plastic parts approximation. So say "approximately" because the distribution is not the same from the top to the root of the tooth shrinkage, with variations with the cavity surface temperature uniformity improvement and narrowing.
        If the temperature of the cavity surface can be the same everywhere, it can be "approximately" word removed, this time by a plastic cavity curve curve by the same shrinkage zoom to get, but this is the ideal situation can only wish, in fact, difficult to achieve. Engineering applications of plastic gears basically in the center of the gear has the hole, it is impossible to use the central gate, plus you do not want to stay clear of the gate marks on plastic products, it is generally taken more points runner feed method evenly distributed, so that each point on the plastic gear to gear center not contracted, but along the respective flow path contraction.
        Since the vertical injection molding machine lost its central symmetry structure, even if the cavity surface temperature reaches a uniform distribution, shrinkage plastic gears can not be evenly distributed, the distance between each tooth and a gate within a gate control area Different shrinkage, two plastic gears some parts may be hampered by the cavity. When the vertical injection molding machine plastic gear contraction throughout the difference, if the mold is involute toothed, plastic tooth could not be strictly involute.
        To get involute tooth vertical plastic injection molding machine, the mold should not be processed into a tooth involute shape, the best way is to get the points difference in the degree of shrinkage on tooth curve by numerical simulation, accordingly mold tooth curve calculated coordinates of each point, let WEDM a given coordinate point processing mold tooth.

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