Plastic injection molding product dimension control

There are different control technologies and methods for different types of molds and products. Today I will talk about my experience in the control of the molding dimensions of injection molded products. When talking about injection molded products, we must talk about injection molds. Generally, I start from the following aspects.

1. Control of mold design

1. First of all, we must fully understand the technical requirements of users in many aspects, such as mold structure, material, hardness, precision, etc., including whether the shrinkage rate of the molded plastic material is correct, whether the 3D size of the product is complete, and reasonably process and analyze.

2. Fully consider all places that affect the appearance of the injection molded product, such as shrinkage holes, flow marks, draft angles, weld lines and cracks.

3. Under the premise of not hindering the function and pattern shaping of the injection molded product, simplify the processing method of the mold as much as possible.

4. Whether the selection of the parting surface is appropriate, the mold processing, molding appearance and deburring of the molded parts must be carefully selected.

5. Whether the ejection method is appropriate, whether to use ejector rods, stripping plates, ejector sleeves or other methods, and whether the positions of ejector rods and stripping plates are appropriate.

6. Whether the side core pulling mechanism is suitable, flexible and reliable, and there should be no jamming.

7. Which method of temperature control is more suitable for plastic products, which structural circulation system is used for temperature control oil, temperature control water, coolant, etc., and whether the size, quantity, and position of the coolant hole are appropriate.

8. Whether the gate form, the size of the material channel and the feed port, the gate position and size are appropriate.

9. Whether the influence of heat treatment deformation of various modules and mold cores and the selection of standard parts are appropriate.

10. Whether the injection volume, injection pressure and clamping force of the injection molding machine are sufficient, and whether the nozzle R, gate sleeve aperture, etc. are matched appropriately.

Comprehensive analysis and preparation are carried out from these aspects, and strict control should be imposed from the initial stage of the product.

2. Control of process manufacturing

Although comprehensive and sufficient consideration and arrangement have been made in the design stage, there will still be many problems and difficulties in actual production. We should try our best to meet the original intention of the design in production and find more effective and economical process means in actual processing.

1. Choose economically adaptable machine tools and equipment to make 2D and 3D processing plans.

2. You can also consider appropriate fixtures for auxiliary preparation work in production, the rational use of tools, to prevent deformation of product parts, prevent fluctuations in product shrinkage, prevent product parts from being deformed during demolding, improve mold manufacturing accuracy, reduce errors, prevent changes in mold accuracy, etc., a series of production process requirements and solutions.

3. Here is a list of the causes of mold size errors and their proportions from the British Plastics Federation (BPF):

A: mold manufacturing error is about 1/3, B error caused by mold wear is 1/6 C error caused by uneven shrinkage of molded parts is about 1/3, D error caused by inconsistency between the expected shrinkage and the actual shrinkage 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 mold size tolerance, otherwise the mold will find it difficult to guarantee the geometric dimensions of the molded parts.

3. General production control

Fluctuation of geometric dimensions after plastic parts are formed is a common problem and a phenomenon that often occurs:

1. Control of material temperature and mold temperature

Different plastics have different temperature requirements. Plastics with poor fluidity and the use of more than two kinds of mixed materials will have different situations. Plastics should be controlled within the optimal flow value range. These are usually easy to do, but the control of mold temperature is more complicated. Different molded parts have different geometric shapes, sizes, and wall thickness ratios. There are certain requirements for the cooling system. The mold temperature controls the cooling time to a large extent;

Therefore, try to keep the mold at an allowable low temperature to shorten the injection cycle and improve production efficiency. If the mold temperature changes, the shrinkage rate will also change. If the mold temperature remains stable, the dimensional accuracy will also be stable, thereby preventing defects such as deformation, poor gloss, and cooling spots of the molded parts, so that the physical properties of the plastic are in the best state. Of course, there is also a debugging process, especially for multi-cavity molded parts. More complicated.
2. Adjustment and control of pressure and exhaust

The appropriateness of injection pressure and the matching of clamping force should be determined when debugging the mold. The air in the gap formed by the mold cavity and the core and the gas generated by the plastic must be discharged from the exhaust groove outside the mold. If the exhaust is not smooth, insufficient filling, welding marks or burns will occur. These three forming defects sometimes appear in the same part from time to time;

When there are thick walls around the thin-walled part of the molded part, insufficient filling will occur when the mold temperature is too low, and burning will occur when the mold temperature is too high. Usually, welding marks will appear at the burned part at the same time. The exhaust groove is often ignored and is generally in a small state. Therefore, under normal circumstances, as long as there is no burr, the depth of the exhaust shoulder is as deep as possible, and a larger ventilation groove is opened at the rear of the shoulder so that the gas behind the shoulder can be quickly discharged from the mold. If there is a special need, the exhaust groove is opened on the ejector. The reason is the same. First, there is no flash, and second, the exhaust is fast and can play a good effect.

3. Supplementary shaping control of injection molded parts size

Some plastic parts will deform and warp in different situations due to the changes in temperature and pressure loss after demolding due to different shapes and sizes. At this time, some auxiliary tooling and fixtures can be used for adjustment. Remedial measures can be taken promptly after the molded parts are ejected from the mold. After they are naturally cooled and shaped, better correction and adjustment effects can be achieved. If strict management is guaranteed throughout the injection molding process, the size of the injection molded parts can be controlled very well.