Dimensional control analysis from molding design to product end

Time:2020-05-13 08:44:54 / Popularity: / Source:

molding design 
With rapid development of mold industry in recent years, application range of new technologies and 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 molded products. Due to wide market demand for molds and molded products, there are many kinds of changes such as shape, size, material, structure and requirements are high, which makes us encounter many problems and difficulties in the production process of molds and products. How to effectively control geometry of molds and products is very intuitive to 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 molding products. When it comes to injection molding products, we must talk about injection molding. In general, I started from following aspects.

First, control of molding design

1 First of all, we must fully understand technical requirements of mould structure, material, hardness, precision, etc., including shrinkage rate of formed plastic material, whether 3D size of product is complete, and reasonable processing analysis.
2 Full consideration is given to various places that have an influence on appearance of shrinkage holes, flow marks, draft angles, weld lines and cracks of injection molding products.
3 Mould making process method is simplified as much as possible without impeding function of injection molding products and shape of pattern.
4 Whether selection of parting surface is appropriate or not, mould making process, forming appearance and deburring of forming part should be carefully selected.
5 Whether push-up method is appropriate, whether push rod, discharge plate, push-up sleeve or others is used, or whether position of push rod and discharge 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 Which kind of method of temperature control is more suitable for plastic products. Which structure is used for temperature control oil, temperature control water, coolant, etc. If size, quantity and position of coolant holes are appropriate.
8 Whether gate form, size of feed and feed ports, proper location and size of gate are appropriate.
9 Whether various modules, core heat treatment deformation effects and selection of standard parts are appropriate.
10 Whether injection volume of injection molding machine, injection pressure and clamping force are sufficient. Whether nozzle R, nozzle sleeve diameter, etc. match properly.
These aspects are prepared for comprehensive analysis and should be strictly controlled from initial stage of product.
molded products 

Second, control of process manufacturing

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, find out the more effective and economical / reasonable process in actual mould making process.
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 in shrinkage of product parts, and demoulding deformation of product parts, to improve precision of mold manufacturing, reduce errors, prevent mold accuracy changes, and other 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:
Mold manufacturing error is about 1/3, error caused by wear of mold is 1/6. Error caused by unevenness of shrinkage of formed part is about 1/3, and 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.
Mould making process 

Third, usual production control

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

1 Material temperature, mold temperature control

Different grades of plastics must have different temperature requirements. Poor fluidity of plastics and use of two or more kinds of mixes can be different. It is usually easy to control plastics within optimum flow range. However, control of mold temperature is more complicated. Different geometric shapes, sizes and wall thickness ratios of formed parts have certain requirements for cooling system. Mold temperature largely controls 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 changes. Mold temperature remains stable and dimensional accuracy is stable, thereby preventing deformation of molded products, poor gloss, cooling spots and other defects, so that physical properties of plastic are in an optimal state. Of course, there is a debugging process, especially multi-cavity moldings are more complicated.

2 Pressure and exhaust adjustment control

Proper injection pressure, matching of clamping force should be determined when mold is adjusted. Air in gap formed by mold cavity and core, gas generated by plastic must be discharged from exhaust groove outside mold. If gas is not smooth, there will be insufficient filling, resulting in weld lines or burns. These three forming defects sometimes appear in 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, and burnt phenomenon will occur when mold temperature is too high. Usually, weld spots will appear at the same time in burned part. It is often overlooked and generally in a small state. Therefore, as long as no burrs are generated, depth of exhaust shoulder is as deep as possible, and 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 ejector, reason is same. First, there is no flash, and second is that gas exhaust can be very good.

3 Supplementary shaping control of injection molding products size

Due to difference in shape and size of some plastic parts, deformation and warping may occur in different situations with change of temperature and loss of pressure after demolding. At this time, some auxiliary fixtures can be adjusted to make timely adjustments after forming parts are released. Remedial measures can be taken, after it is naturally cooled and shaped, better correction and adjustment effects can be obtained. If strict management is ensured throughout injection molding process, size of injection molded part is very well controlled.

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