The wide range of applications of CAD, CAM and CAE has opened up room for the geometric size control technology of our molds and mold products. Due to the large differences in market demand for molds and mold products, there are a wide variety of shapes, dimensions, materials, structures, etc. The large changes and high requirements in various aspects have caused us to encounter many problems and difficulties in the production of molds and products. Among them, how to effectively control the geometric dimensions of molds and products is very intuitively placed in front of us. Under normal circumstances, I start from the following aspects.
control of mold design
1. Firstly, we must fully understand the user's technical requirements in many aspects such as mold structure, material, hardness, precision, etc., including whether the shrinkage rate of the formed plastic material is correct, whether the product 3D size modeling is complete, and reasonable processing analysis.
2. Full consideration should be given to the various places that affect the appearance of injection products, such as shrinkage holes, flow marks, draft angles, weld lines, and cracks.
3. Simplify the processing method of the mold as much as possible without hindering the product function and patterning of the injection molded part.
4. Whether the selection of the parting surface is appropriate, the mold processing, the appearance of the forming and the deburring of the formed parts must be carefully selected.
5. Whether the ejection method is appropriate, whether the push rod, discharge plate, ejection sleeve, etc. are used or other methods, and whether the position of the push rod and the discharge plate is appropriate.
6. Whether the side core pulling mechanism is suitable, the action is flexible and reliable, and there should be no jamming.
7. Which method of temperature control is more suitable for plastic products? What kind of structure circulation system is used for temperature control oil, temperature control water, coolant, etc.? Whether the size, quantity, and location of the coolant holes are appropriate.
8. The gate type, the size of the material channel and the inlet, and whether the gate position and size are appropriate.
9. The heat treatment deformation of various modules and mold cores and whether the selection of standard parts is appropriate.
10. Whether the injection volume of the injection molding machine, the injection pressure and the clamping force are sufficient, whether the nozzle R, the aperture of the sprue bushing, etc. match properly.
, etc. These aspects are comprehensively analyzed and prepared, and the product parts should be strictly controlled from the initial stage.
Process and manufacturing control
1. Choose economically suitable machine tools and make 2D and 3D processing plans.
2. Appropriate tooling and fixtures can also be considered for auxiliary preparations in production. The reasonable use of tools can prevent product deformation, prevent product shrinkage fluctuations, prevent product demoulding deformation, improve mold manufacturing accuracy, and reduce Small errors, prevent changes in mold accuracy, etc., a series of production process requirements and solutions.
3. Here are the reasons for the dimensional errors of the shaped parts of the British Plastics Association (BPF) and the distribution of the proportions:
A: The mold manufacturing error is about 1/3, B the error caused by mold wear is 1/6, the error caused by the unbalanced shrinkage rate of the molded part is about 1/3, and the error caused by the inconsistency between the predetermined shrinkage rate and the actual shrinkage rate is about 1/6
Total error=A+B+C+D, so it can be seen that the mold manufacturing tolerance should be less than 1/3 of the dimensional tolerance of the formed part, otherwise it is difficult for the mold to guarantee the geometric size of the formed part.
General production control
The fluctuation of the geometric dimensions of the plastic part after forming is a common problem, and it is a common phenomenon:
1. Control of material temperature and mold temperature
Different grades of plastics must have different temperature requirements. Plastic materials with poor fluidity and the use of two or more mixtures will have different situations. Plastic materials should be controlled within the optimal flow value range. These are usually easy to achieve. However, the control of the mold temperature is more complicated. Different forming parts geometry, size, and wall thickness ratio have certain requirements on the cooling system, and the mold temperature controls the cooling time to a large extent;
Therefore, try to keep the mold at an allowable low temperature to help shorten the injection cycle and improve production efficiency. When the mold temperature changes, the shrinkage rate will also change, the mold temperature remains stable, and the dimensional accuracy is stable, thereby preventing forming parts Defects such as deformation, poor gloss, cooling spots, etc., make the physical properties of the plastic in the best state. Of course, there is still a debugging process, especially the multi-cavity mold forming part is more complicated.
2. Adjustment and control of pressure and exhaust:
The injection pressure is appropriate and the matching of the clamping force should be determined when debugging the mold. The air in the gap formed by the mold cavity and core and the gas generated by the plastic must be discharged from the exhaust groove outside the mold, such as exhaust Poor air will cause insufficient filling, resulting in weld marks or burns. These three forming defects sometimes appear at the same place occasionally;
When there are thick walls around the thin wall of the molded part, insufficient filling will occur when the mold temperature is too low, and burns will occur when the mold temperature is too high. Usually, weld marks will appear at the burned part at the same time, and the exhaust groove It is often overlooked and is generally in a small state. Therefore, as long as there is no burr, the depth of the exhaust shoulder should be as deep as possible, and a larger vent groove is opened at the back of the shoulder to pass the gas behind the shoulder. It can be quickly discharged from the mold. If there is a special need to open a vent slot on the ejector, the reason is the same. One is that there is no waste edge, and the other is that the vent is fast and the effect is good.
3. Supplementary shaping control for the size of injection molded parts
For some plastic parts due to different shapes and sizes, after demolding, with temperature and loss of pressure, different deformations and warpages will occur. At this time, some auxiliary tooling and fixtures can be used to adjust, and the molded parts can be quickly and quickly after being ejected from the mold. Remedial measures are taken, and a better correction and adjustment effect can be obtained after it is naturally cooled and shaped. If strict management is ensured in the entire injection molding process, the size of the injection molded parts will be ideally controlled.