With rapid development of mold industry in recent years, application range of new technologies and new processes has been continuously expanded and improved, application from traditional experience accumulation to software development has undergone qualitative changes.
Wide range of applications for CAD, CAM and CAE opens up space for geometry control technology of our molds and mold products. Due to wide market demand for molds and mold products, there are many kinds of changes, such as shape, size, material, structure and other aspects, requirements are high, which makes us encounter many problems and difficulties in the production process of molds and products. How to effectively control geometric dimensions of molds and products is very intuitively placed in front of us.
There are different control techniques and methods for different types of molds and products. Today, I am here to talk about molding size control of injection moulding products. When it comes to injection moulding products, we must talk about injection moulding. In general, I started from following aspects.

 

1. First of all, we must fully understand technical requirements of mold structure, material, hardness, precision, etc., including whether shrinkage rate of formed plastic material is correct, whether 3D size of product is complete, and reasonable processing analysis.
2. Full consideration should be given to various places that have an influence on the appearance of shrinkage, flow marks, draft inclination, weld lines and cracks of injection moulding products.
3. Simplify processing method of mold as much as possible without impeding function of injection moulding products and shape of pattern.
4. Whether selection of parting surface is appropriate, mold processing, forming appearance and deburring of forming parts should be carefully selected.
5. Whether pushing method is appropriate, whether push rod, unloading plate, push-up sleeve or other solution is used, or whether position of push rod and unloading plate is appropriate.
6. Whether side core pulling mechanism is suitable for use, action is flexible and reliable, there should be no stuck phenomenon.
7. What method is suitable for temperature control? For temperature control oil, temperature control water, coolant, etc., which structure is used to cycle system? Whether size, quantity and position of coolant holes are appropriate.
8. Whether form of gate, size of material channel and feed port, position and size of gate are appropriate.
9, Whether impact of various types of modules, core heat treatment deformation and selection of standard parts is appropriate.
10. Whether injection volume, injection pressure and clamping force of injection molding machine are sufficient, whether nozzle R, nozzle sleeve diameter match properly.
These aspects are prepared for comprehensive analysis and should be strictly controlled from initial stage of product.

Although comprehensive consideration and arrangement have been carried out in design stage, there are still many problems and difficulties in actual production. We should try our best to meet original intention of design in production and find out the most effective, economical and reasonable process in actual processing.
1. Select economical machine tools for 2D and 3D processing.
2. Can also consider appropriate fixtures for auxiliary preparation in production, rational use of tool, to prevent deformation of product parts, fluctuations of shrinkage of product parts, demoulding deformation of product parts, improve precision of mold manufacturing, reduce errors, prevent mold accuracy from changing, etc., a series of production process requirements and solutions.
3. Here is a summary of reasons for size error of British Plastics Association (BPF) and proportion of its proportion:
A: Mold manufacturing error is about 1/3, B: Error caused by mold wear is 1/6, C: Error caused by unevenness of shrinkage of formed part is about 1/3, D: Error caused by difference between predetermined shrinkage rate and actual shrinkage rate is about 1/6.
Total error = A + B + C + D, so it can be seen that mold manufacturing tolerance should be less than 1/3 of dimensional tolerance of formed part, otherwise mold is difficult to guarantee geometrical dimensions of formed part.

 

Occurrence of geometrical fluctuations after forming of plastic parts is a common problem and is a phenomenon that often occurs:

Different grades of plastics must have different temperature requirements. Poor fluidity of plastic materials and use of two or more kinds of mixture will have different conditions. It is usually easy to control plastic material within optimal flow value range, but control of mold temperature is more complicated. Different geometry, size and wall thickness ratio of forming parts have certain requirements for cooling system, mold temperature largely controls the cooling time;
Therefore, mold should be kept at an allowable low temperature state as much as possible, so as to shorten injection cycle, improve production efficiency. When mold temperature changes, shrinkage rate also change. When mold temperature remains stable, dimensional accuracy be stable, thereby preventing defects such as deformation, poor gloss, and cooling spots of molded part, so physical properties of plastics are optimally performed. Of course, there is a debugging process, especially multi-cavity moldings are more complicated.

Proper injection pressure, matching of clamping force should be determined when mold is adjusted. Air in cavity formed by mold cavity and core, gas generated by plastic must be discharged from exhaust groove outside mold. If exhaust is not smooth, filling may be insufficient filling, resulting in weld lines or burns. These three forming defects sometimes appear in the same place.
When there is a thick wall around thin-walled part of formed part, filling will be insufficient when mold temperature is too low, burnt phenomenon will occur if mold temperature is too high. Usually, weld line will appear at the same time in burned part. Exhaust groove is often neglected, and it is generally in a small state. Therefore, as long as no burrs are generated, depth of exhaust shoulder is as deep as possible, a large vent groove is formed at the rear of shoulder so that gas passing through shoulder can be quickly discharged outside mold. If there is a special need to open exhaust slot on the ejector, reason is same: first, there is no flash, and the other is that venting can be very effective.

Due to difference in shape and size of some injection molded parts, deformation and warping may occur in different situations with change of temperature and loss of pressure after demolding. At this time, some auxiliary tooling fixtures can be used for adjustment, remedial measures can be quickly taken after forming parts are demoulding, a good correction and adjustment effect can be obtained after natural cooling and shaping. If strict management is ensured throughout injection molding process, size of injection molded parts is very well controlled.