For previous series, Please refer to Mold design experience (serial one).
31. After mold has been used for a period of time, it needs to be repaired. During process of repairing mold core, try not to use oil stone, because repeated use of oil stone will deform mold; it is best to use sharpened cork or soft bamboo chopsticks.
32. In molds with sliders, four support legs should be added between #102 and #103 plates.
33. When forming parts with a mold core inside and a mold core inside, a secondary core-pulling mechanism should be considered to avoid difficulty in demoulding and damage to part; if inlet is on fixed side or on slider, always pull insert in first; if inlet is on movable side and is close to fixed side, counterbore of inlet can be made deeper. When ejecting, part should be ejected first, then inlet can be ejected. If it is not broken, it should be taken off first, and corresponding mold structure should be changed.

34. If breaking surface between fixed side and movable side is a plane that is not perpendicular to mold opening direction, it should be designed as a slope to reduce possibility of flash due to wear. At the same time, it will also form a pre-pressure when break occurs and strengthen fit between two surfaces. Length direction should be designed to have a positive tolerance of +0.02 during design. However, it should be noted that when there is a draft angle between fixed side and movable side, careful consideration should be given to fact that draft angle between fixed side and movable side is in opposite direction, joints that are inconsistent with original design drawing of part will be formed on broken slope. If not considered carefully, burrs or breaks will be formed that are difficult to eliminate.

35. When fixed side needs to be embossed, outer dimensions of fixed side should be based on degree of embossing, with one side smaller than 0.03-0.05mm during design.
36. Electrodes are generally polished with 1000 grit sandpaper, but appearance electrodes need to be polished with more than 1200 grit sandpaper; mold core is polished with 1500 grit, but if a mirror surface is required, 3000 grit sandpaper is used, finally diamond paste and absorbent cotton are used for fine polishing. When fitting inserts, first use 400-grit sandpaper and then 800-grit sandpaper. However, inserts in Japanese molds seem to have been polished with 1000-1200 grit sandpaper.
37. After plastic gear is formed, main gear parameters to be measured are tooth tip circle and span-tooth thickness. If two gears are too close or too loose, transmission performance will be affected; there are special measuring instruments for measurement of span-tooth thickness.
38. In mold design, if thickness of parts is uneven and gates of parts are evenly distributed, uneven pouring will easily occur.
39. Although gears made of PC+30GF have better forming dimensions and can generally be molded in four pieces, their rigidity and wear resistance are not as good as those of PBT+GF30. Therefore, although size of PBT is difficult to control in terms of molding and can only be molded in two pieces, manufacturers that pay attention to quality still choose quality in the face of quality and cost.
40. In mold design, in order not to affect use of part, it is often necessary to make a recess on the surface of part so that gate shear residue is lower than the surface of part. Depth of recess is as shallow as possible on the premise that gate residue is lower than surface of part. Generally it is 0.3-0.5mm. If it is too deep, it will affect size during molding.
41. In order to improve filling performance of parts far away from gate, escape grooves can be set up in these parts to increase inlets. This should be especially considered before designing. When determining structure, there should be such a concept: try to keep pressure and temperature of each part constant when fluid flows in mold cavity.
42. For molds with thin parts and difficult forming, forming performance can be improved by enlarging gate, but bigger is not always better. If it is too large, gate will be torn off part when cut, form a pit. At the same time, orientation effect of parts will increase and it will be easy to deform. Therefore, point gate size is ¢0.5-1.2mm.
43. In EDM machining, discharge gap is directly related to machining accuracy (generally considered to be 3:1).
44. Slope of pressing block for large mold core is 1°, 3°, 5°.
45. In order to facilitate ejection of oblique pin, oblique pin should be designed to be 0.1-0.3mm shorter than normal, that is, thickness in this part is 0.1-0.3mm thicker than normal.
46. When designing a mold, you should first consider processing technology of parts, try to avoid using electric discharge and wire cutting, try to consider using milling machines and grinders, because in terms of processing cost, processing accuracy and processing time, the former is not as good as the latter, although accuracy of slow wire cutting is good.
47. When designing, we should avoid designs that have a simple shape but require a large area of plane discharge, which is time-consuming, difficult to ensure accuracy, and increases workload of fitter.
48. When designing, we should try to avoid ladder-shaped upper and lower mold core designs that require surfaces to fit together. This is often difficult to process.
49. Disadvantage of ultrasonic grinding is that it is easy to distort surface shape of mold due to inaccurate control of feel.
50. When mass production requirement of mold is 10,000-15,000/month, mold core material is NAK55.

 

51. A good injection molding machine can perform more than 5 segments of injection by adjusting parameters. For example, the first segment can be set to fill flow channel; the second segment fill one-third of part; the third segment filled with one-half of parts...and so on. Therefore, problems existing in injection molding can be solved by analyzing filling conditions of parts in these situations.
52. For molds that are difficult to mold, or have surface requirements, or accuracy of some parts cannot reach required size in previous mold trials, consider using multi-stage injection molding during mold trials.
53. Among injection molding machines, both Japanese and Taiwanese machines can perform multi-stage injection molding, but generally speaking, Taiwanese machines can not only change injection speed and injection speed. In addition, injection pressure can be changed.
54. Factors that determine cavity number of mold include: molding cost of a single part, average mold production cost of each part, accuracy requirements of part, difficulty of mold production, etc.
55. When molding corrosive resin, mold material must be corrosion-resistant, or anti-corrosion treatment should be performed on mold surface. When molding resin containing high-strength filling materials such as glass fiber, mold parts must have corresponding hardness.
56. Distance between water pipe and mold core should be greater than 4mm.
57. If it is estimated that part will be difficult to form and molding pressure needs to be increased, strength of mold should be considered during design, strength of mold core should be increased, support columns should be added, tolerances between fitting surfaces should be paid attention to.
58. Forced demoulding mechanism should not be considered in design of precision molds, otherwise it will have a great impact on mass production of mold, accuracy of parts, and even surface of parts.
59. In mold design, from perspective of cost and manufacturing, try to avoid sliders and inclined tip mechanisms.
60. If there are only 15-20 mold cores left after processing by milling machine, two to four pieces per mold are used, then even electrodes with sharp corners can generally be made one rough and one fine.