Typically, plastic injection molding can be controlled to within 0.005 inches of accuracy, making it one of the most reliable manufacturing process for producing parts.

Injection molding involves injection molds, injection materials, and injection machines which all have a direct impact on the accuracy of injection molding.

In this article, we will analyze the impact of plastic injection moulding accuracy from these 3 aspects in detail.

Factors affecting the dimensional accuracy of injected products

1. Injection mold precision

2. Shrinkage rate of injection molding plastic material

3. Injection molding machine equipment and injection moulding process

Accuracy of injection mold

In order to make the injection mold processing to achieve high processing accuracy and reduce the error of the products, it is not only required to meet the requirements of the injection mold processing accuracy, but also to set a reasonable margin in the processing process, to ensure the smooth completion of the injection mold processing premise, as far as possible to improve the accuracy of the injection mold processing.

About the accuracy requirements of mold processing, including the processing accuracy of injection mold processing dimensions, verticality, coaxiality, parallelism, etc.

a. Generally, the processing accuracy of injection mold should be within 0.005-0.02mm, while the verticality should be within 0.01-0.02mm, the coaxiality should be within 0.01-0.03mm, and the parallelism of the upper and lower planes of the movable and fixed mold parting surfaces should be within 0.01-0.03mm.

b. At the same time, the clearance between the parting surfaces after the mold is closed should be less than the overflow value of the molded plastic; while the parallelism of the rest of the mating surfaces of the template is required to be within 0.01-0.02mm; the mating accuracy of the fixed part of the injection mold processing is generally within 0.01-0.02mm.

c. Injection mold processing with the sliding part of the accuracy of the general choice of H7/e6, H7/f7, H7/g6 three, it should be noted that the mirror surface such as the inserts made to hang on the steps, then the requirements with can not be too tight, otherwise in the inserts from the front back knocking tool used to knock easy to touch the mirror surface, without affecting the size of the product, can take bilateral 0.01 ~ 0.02mm Gap fit.

d. Another thing to note is the heat treatment processing of the workpiece, requiring its heat treatment before the size of the preparation plus 0.25mm grinding machine margin on one side; and for the mold kernel, inserts need CNC machining of the part, unilateral reserve margin, to know a reasonable margin will also affect the accuracy of the workpiece.

How to improve the precision of injection mould?

Injection mold mold accuracy mainly depends on the mould cavity size accuracy, accurate cavity positioning or parting surface accuracy to meet the requirements. Generally, the dimensional tolerance of precision injection mold should be controlled below 1/3 of the product dimensional tolerance.

Machinability and rigidity, in the mold structure design, the number of cavities should not be too many, and the base plate, support plate, cavity wall should be thicker, in order to avoid parts in high temperature, high pressure under the action of violent elastic deformation.

Product release, the mold should try to take fewer cavities, fewer and shorter runners and a higher degree of finish than ordinary molds, which is conducive to mold release.

The material of mold, choose the alloy steel with high mechanical strength. The materials for making cavities and runners should undergo strict heat treatment, and choose materials with high hardness (molding parts should reach about HRC52), good wear resistance and strong corrosion resistance.

Molding Shrinkage

The molten plastic will be injected to the molds using extreme pressure. Once the molten plastic takes the shape of the molds and harden, it will be ejected out of the machine. Molding shrinkage refers to the percentage of shrinkage of the original uncooled size of a molded part at room temperature after it has been taken out of the mold and cooled to room temperature.

Since shrinkage is not only the thermal expansion and contraction of the resin itself, but also related to various plastic molding factors, the shrinkage of the molded part after plastics molding should be called the molding shrinkage.

When designing injection molds, it is often necessary to determine the shrinkage rate of each part of the plastic parts according to the shrinkage range of various plastics, the wall thickness and shape of the plastic part, the form size and distribution of the feed opening, and then calculate the cavity size according to experience.

How to consider the shrinkage rate in designing injection mold?

a. Take the smaller shrinkage rate for the outer diameter and the larger shrinkage rate for the inner diameter to allow room for correction after the mold trial.

b. Determine the form, size and molding conditions of the pouring system after the mold trial.

c. Determine the dimensional change of the molded part after post-processing (note: the measurement must be done after 24 hours of demolding).

d. The injection mold is corrected according to the actual shrinkage.

e. When re-testing the mold, the molding process conditions can be changed appropriately to slightly correct the shrinkage value to meet the requirements of the plastic part.

Shrinkage affecting the accuracy of injection molding

There are four factors affecting shrinkage: thermal shrinkage, phase change shrinkage, orientation shrinkage and compression shrinkage.

a. Thermal shrinkage is a thermophysical property inherent to the molding material and the mold material.

The mold temperature is high, the temperature of the product is also high, and the actual shrinkage rate will increase, so the mold temperature for precision molding should not be too high.

b. Phase change shrinkage is called phase change shrinkage due to the crystallization of macromolecules in the orientation process, which is accompanied by the reduction of specific volume.

High mold temperature, high crystallinity, large shrinkage; but on the other hand, the increase in crystallinity will increase the density of the product, reduce the coefficient of linear expansion, shrinkage rate is reduced. Therefore, the actual shrinkage rate is determined by the combined effect of both.

c. Orientation shrinkage is due to the forced stretching of molecular chains in the flow direction, so that the macromolecules in the cooling tend to re-curl and recover, which will produce shrinkage in the orientation direction.

The degree of molecular orientation is related to injection pressure, injection speed, resin temperature and mold temperature. But the main one is the injection speed.

d. Compression shrinkage and elastic restoration, general plastics are compressive in nature. That is, the specific volume changes significantly under high pressure.

In general temperature, increase the pressure molding products specific volume will decrease, density will increase, expansion coefficient will decrease, shrinkage will be significantly reduced.

Corresponding to the compressibility, the molding material has an elastic reset effect, so that the product shrinkage is reduced. The factors affecting the shrinkage of the product molding are related to the molding conditions and operating conditions.

Principles for the selection of injection molding materials.

High mechanical strength, good dimensional stability, good creep resistance, and wide range of environmental adaptation.

There are four kinds of engineering plastics commonly used

1. POM and carbon fiber reinforced ( CF) or glass reinforced (GF). This material is characterized by good creep resistance, fatigue resistance, weatherability, good dielectric properties, flame retardant, easy to release by adding lubricant.

2. PA and glass-reinforced PA66, its characteristics: strong impact resistance and wear resistance, good flowability, can be molded 0.4mm wall thickness of the product.

Glass fiber reinforced PA66 has heat resistance (melting point of 250 ℃), its disadvantage is that it has moisture absorption, generally after molding are to be processed by moisture conditioning.

3. PBT reinforced polyester, molding time is short. Forming time comparison is as follows: PBT ≤ POM ≈ PA66 ≤ PA6.

4.PC and GFPC. characteristics: good wear resistance, enhanced rigidity, good dimensional stability, weather resistance, flame retardant and good molding processability.

Injection molding machine equipment and injection molding process

The five elements of injection molding process include: pressure, flow, temperature, time, position, a good injection molding machine in these five elements to reflect more accurate and sensitive.

1. Temperature

Temperature is the first step of injection molding, temperature can be divided into: drying temperature, material temperature, mold temperature, these are some vital factors.

Drying temperature is to ensure the molding quality and prior drying of polymer to ensure that the polymer dry and wet are in a certain amount; material temperature is to ensure that the polymer plasticization is normal, and smooth mold filling, molding.

2. Speed

Speed is divided into: injection speed, melt temperature, loosening and retreating speed, opening and closing the mold speed, ejector forward and backward speed. Injection speed will affect the increase in mold pressure, increase the injection speed can make the flow length increase, product quality uniformity to ensure product quality, high speed injection for long flow products, low speed control product size.

Melt speed affects the plasticizing ability, plastic mold processing is an important parameter of plasticizing quality, the higher the speed, the higher the temperature of the melt plasticizing ability is stronger.

Loosening retreat speed is to reduce the melt specific volume, so that the melt cavity The speed of opening and closing the mold is one of the important conditions to ensure the production operation of the machine.

The speed of forward and backward of the ejector is to ensure the smooth release of the product does not make the product deformation, top crack.

3. The pressure

The pressureis divided into: injection pressure, holding pressure, back pressure, clamping pressure, mold protection pressure, ejection pressure. Plastic shell injection molding processing injection pressure is used to overcome the melt from the nozzle – runner – gate – cavity pressure loss to ensure that the cavity is filled.

Pressure is to reduce the injection pressure of the parts due to excessive injection pressure and lead to sticky mold burst or bending; be pressed to avoid foreign objects in the mold at high speed and high pressure state and lead to mold crushing; ejector pressure is to prevent the product from leaving the mold surface caused by bad.

4. Position

Positioncan be divided into: mold opening position, ejecting back stroke, metering stroke, loose leg volume, injection stroke, buffer volume. The opening position is to ensure that the mold can be taken off the normal operation of the parts; ejected backward stroke on the part of the spring to reset the role of the top limit protection; metering stroke is to ensure that there is enough plastic to fill the mold to ensure product quality.

5. Time

Time is divided into: cooling time, melting time, drying time, cooling time, melt time, drying time, cycle, injection time, holding time.

Plastic shell spraying products need a certain period of molding; injection time to determine the size of the product; holding time is to prevent the melt backflow, the role of cooling shrinkage; cooling time to ensure that the product curing, to prevent deformation; melt time is to ensure that the melt is sufficient; drying time to ensure that the product does not become elongated, bad and other factors.

Summary

In order to get precision injection molding, we should consider the structure of the product, the choice of raw materials, the precision of the mold, the precision injection moulding equipment and the experience of the injection molding factory from the beginning of the product design, in order to get satisfactory products in the end.