Dimensional control of injection molded parts from mold design to product end

With the rapid development of the injection mold industry in recent years, at the same time the application of new technologies, and new processes has expanded and progressed, from the traditional accumulation of experience to the application of software development has been a qualitative change.

The wide application of CAD, CAM, and CAE has opened up an upside for our geometric dimension control technology for molds and mold products.

Since the market demand for injection molds and mold products varies greatly, there are many different kinds of molds and mold products, and they are highly variable and demanding in shape, size, material, and structure, which makes us encounter many problems and difficulties in the product manufacturing process of molds and products.

How to effectively control the geometry of molds and products is also very intuitively placed in front of us. For different types of molds and products, there are different control techniques and methods.

Today I am here to talk about the injection molding product forming size control experience, talking about injection molding products must be talked about injection mold, in general, I started from the following aspects.

The mold design control  

1. First of all, we should fully understand the technical requirements of users in many aspects such as mold structure, material, hardness, and precision, including whether the shrinkage rate of molding plastic material is correct, whether the 3D size of the product is complete, and whether the processing analysis is reasonable.  

2. To fully consider the appearance of the injection molded products such as shrinkage, flow marks, mold slope, fusion line, and cracks.  

3. Simplify the processing method of the mold as much as possible without hindering the function and pattern-making of the injection molded products.  

4. Whether the selection of parting surface is appropriate or not, careful selection should be made for mold processing, molding appearance, and deburring of injection molded parts.  

5. Whether the pushing and topping method is appropriate, using the push rod, unloading plate, pushing and topping casing, or other ways, whether the position of the push rod and unloading plate is appropriate. 

6. Whether the use of the side core extraction mechanism is appropriate, flexible, and reliable action, there should be no jamming phenomenon. 

7. What kind of temperature control method is more suitable for plastic products, which structure circulation system is used for temperature control oil, temperature control water, and coolant, and whether the size, quantity, and position of coolant holes are appropriate?  

8. The form of the sprue, the size of the feed channel and inlet, and whether the location and size of the sprue are appropriate. 

9. The effect of heat treatment deformation of various modules and mold cores and the appropriateness of the choice of standard parts.  

10. Is the injection volume, injection pressure, and clamping force of the injection molding machine adequate, the nozzle R, the gate sleeve aperture, etc. matched with the appropriate.  

From these aspects of the comprehensive analysis of the preparation, the initial stage of the product parts should be strictly controlled.

The control of process manufacturing  

Although in the design stage for a comprehensive and full consideration and arrangements, in actual production there will be a lot of problems and difficulties, we should try to comply with the original intention of the design in production, to find the actual processing more effective, more economical and reasonable means of technology.  

1. Choose the economic adaptation of the machine tool equipment, for 2D and 3D machining programs.  

2. Can also consider the appropriate tooling fixtures for the production of auxiliary preparations, the reasonable use of tools, to prevent the product pieces from deformation, prevent product pieces shrinkage fluctuations, to prevent product pieces deformation, to improve the accuracy of injection mold manufacturing, reduce errors, to prevent changes in mold accuracy, etc., a series of production process requirements and solutions. 

3. Here mention the British Plastics Council (BPF) of the molding part size error generated by the causes and its proportion of the distribution of the situation.  

A: mold manufacturing error of about 1/3, B by the mold wear error 1/6 C uneven shrinkage of molded parts generated by the error of about 1/3, D scheduled shrinkage rate and the actual shrinkage rate inconsistent error of about 1/6  

Total error = A + B + C + D, so it can be seen that the mold manufacturing tolerance should be below 1/3 of the size tolerance of the forming part, otherwise it is difficult for the mold to ensure the geometric size of the forming part. 

The usual production control  

Geometry fluctuation after forming plastic parts is a common problem and is an often occurring phenomenon:

1. The material temperature, mold temperature control

Different grades of plastics require different temperature requirements, poor plastic material fluidity and the use of more than two kinds of mixed injection molding materials will have different situations, the plastic material flow should be controlled in the best flow value range, these are usually easy to do.

However, the control of mold temperature is a bit more complicated, different forming part geometry, size, and uniform wall thickness ratio of the different cooling system have certain requirements, and mold temperature to a large extent control the cooling time.

Therefore, try to keep the mold at a permissible low temperature to help shorten the injection molding cycle and improve production efficiency.

Mold temperature changes, then the shrinkage rate will also change, mold temperature remains stable, and the dimensional accuracy is also stable, thus preventing deformation of injection molded parts, bad gloss, cooling spot, and other defects, so that the physical properties of plastic in the best condition, of course, there is a debugging process, especially multi-cavity molded parts more complex.  

2. Adjustment and control of pressure and exhaust

The appropriate injection pressure and clamping force match should be determined when debugging the mold, the air in the mold cavity and core formed by the gap and the gas generated by the molten plastic must be discharged from the exhaust slot outside the injection mold, such as poor exhaust will be underfilled, resulting in fusion marks or burns, these three molding defects sometimes appear intermittently in the same part.

When there is a thick wall around the thin-walled part of the forming part, the mold temperature is too low, there will be underfilling, and the mold temperature is too high, then there will be burns, usually in the burn area and at the same time there will be fusion marks, the exhaust slot is often ignored, generally in a small state.

Therefore, in general, as long as no burrs are produced, the depth of the exhaust shoulder should be as deep as possible, and a larger venting slot should be opened at the back of the shoulder so that the gas can be quickly discharged from the mold after passing through the shoulder.

If there is a special need to open a venting slot on the top bar, the reason is the same, one is not to appear flying edge, the second is the gas out quickly can play a good effect on the line.  

3. Injection molding size of the supplementary plastic control

Some plastic parts due to the shape and size of the different, after demoulding with the temperature and loss of pressure changes, will occur in different cases of deformation warpage, etc.

when some auxiliary tooling fixtures can be adjusted in the plastic injection molding parts out of the mold in timely and rapid remedial measures, to be naturally cooled and deformed to achieve a better correction and adjustment effect.

If we ensure strict management in the whole injection molding process, the size of the injection molded parts will be very ideal control.