1、原文三INJECTION MOLD DESIGN AND GUIDEMold constructionA standard injection mold is made of a stationary or injection side containing one or more cavities and a moving or ejection side. Relevant details are shown in the figure below.Impression of a standard injection mold.High quality molds are expensiv
2、e because labor and numerous high- precision machining operations are time-consuming. Product development and manufacturing costs often can be significantly reduced if sufficient attention is paid to product and mold design.The way in which the mold is constructed is determined by: shape of the part
3、 number of cavities position and system of gating material viscosity mold venting A simple mold with a single parting line is shown in the figure above. More complex molds for parts with undercuts or side cores may use several parting lines or sliding cores. These cores may be operated manually, mec
4、hanically, hydraulically, pneumatically or electro-mechanically.The figure below shows an example of a sliding cam. The cam pins that operate the cams are mounted under a maximum angle of 20 - 25 in the injection side. The angle is limited because of the enormous force that is exerted on these pins
5、during mould opening and closing. Cammed mold for part with undercut cams move in vertical direction when mold is opened.Multi-cavity moldsThe number of cavities and mold construction depend both on economical and technical factors. Important is the number of parts to be molded, the required time, a
6、nd price in relation to mould manufacturing costs. The figure below shows the relation between the total part costs and the number of cavities. Total part costs in relation to number of cavities.The gating system and gate location can limit the design freedom for multi-cavity molds. Dimensional accu
7、racy and quality requirements should be accounted for. The runner layout of multiple-cavity molds should be designed for simultaneous and even cavity filling. The maximum number of cavities in a mold depends on the total cavity volume including runners in relation to the maximum barrel capacity and
8、clamping force of the injection molding machine.Number of cavitiesA given molding machine has a maximum barrel capacity of 254 cm3, a plasticizing capacity of 25 g/s, 45 mm screw and a clamping force of 1300 kN. A PC part of 30 cm3, (shot weight 36 g) and a projected area of 20 cm2 including runners
9、 requires about 0.5/tons/ cm2 (5 kN/cm2 ) clamping force. The maximum number of cavities based on the clamping force would be 12. It is advisable to use only 80% of the barrel capacity, thus the number of cavities in this example is limited to 6. When very short cycle times are expected the total nu
10、mber of cavities may be further reduced. A 6-cavity mold in this example requires a shot weight of 216 g. The cooling time must be at least 8.7 seconds.Gate locationAppearance Whenever possible locate gates on non-visual surfaces thus eliminating problems with residual gate vestiges after the gate h
11、as 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 pa
12、rt. This will also help prevent sink marks and voids forming.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
13、 to the weld line location this should help maintain a high melt temperature that is beneficial to a strong weld line.WarpageAn incorrectly dimensioned or located gate may also result in undesirable flow patterns in the cavity. This can lead to moldings with visible weld line (see figure below). Inf
14、luence of gate location on flow behavior of the melt.Undesirable flow patterns in the cavity can also lead to deformation by warping or bending (see figure below).Warpage due to unfavorable gate location.Gate typeAs important as selecting the optimal gate size and location is the choice of the type
15、of gate. Gate types can be divided between manually and automatically trimmed gates. 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 t
16、o 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 flow distribution across a wide front to achieve specific orientation of fibers of molecules often precludes a
