Injection moulding process considerations

Table of Contents

Injection molding is a popular manufacturing process that can be used to produce a variety of parts and products. However, several factors must be considered when selecting this process.

First, the type of material to be molded must be considered. Some common injection molding materials are better suited for injection molding than others, and certain types of materials may require special handling or processing.

Second, the size and complexity of the desired part must be taken into account. Injection molding is well suited for producing large numbers of relatively simple parts, but more complex parts may be better suited for other manufacturing processes.

Finally, the cost of injection molding types of equipment and tooling must be considered. In many cases, the initial investment in injection molding equipment can be significant, but the cost per part is usually lower than for other types of manufacturing processes.

Injection molding is a manufacturing process involving the injection of molten material into a mold cavity. The material cools and hardens to take the shape of the mold cavity. Injection molding is used in a wide variety of industries, from automotive to consumer products.

When designing an injection molded parts, several considerations must be taken into account.

First, the wall thicknesses of the parts must be uniform.

Thick walls can lead to warpage and excessive shrinkage, while thin walls can cause parts to fracture or break.

Second, the material must be compatible with the injection molding process. Some materials, such as glass or metal, cannot be injected into the mold cavity.

Third, the dimensions of the part must be within the tolerance of the injection molding machine. If the size is too large or too small, the part may not eject properly from the mold or may not meet the customer’s specifications.

Finally, the injection mold must be designed for proper cooling and ventilation to prevent defects in the finished part. By considering all of these factors, the designer can produce a high-quality injection molded part that meets the customer’s requirements.

The performance of plastic products is determined by the interaction of material properties and molding process parameters. The choice of material has a significant impact on the product properties, as different plastics have different physical and chemical properties.

The molding process also plays an important role, as different parameters can lead to significant variations in the final product. To achieve the desired properties, materials and molding processes must be carefully selected. By doing so, it is possible to produce high-quality plastic products that meet the specific needs of the application.

Plastic product properties are influenced by material properties and molding process parameters, and different plastics require process parameters tailored to their properties to obtain the best physical properties.

The key points of injection molding are as follows:

Shrinkage of Plastic Materials

Thermoplastic molding shrinkage form and calculation as described earlier, the following factors affect the thermoplastic molding shrinkage.

a. Plastic species thermoplastic molding process because there is also the crystallization of the volume of the shape of the change, internal stress, frozen in the plastic parts of the residual stress, molecular orientation and other factors, so compared with thermosetting plastics are larger shrinkage, shrinkage rate range, directional obvious.

In addition to the shrinkage after molding, annealing, or moisture conditioning treatment shrinkage is generally larger than in thermosetting plastics.

Sequence NumberPlastic MaterialsShrinkage Rate Range
1PA661%-2%
2PA61%-1.5%
3PA120.5%-2%
4PBT1.5%-2.8%
5PC0.1%-0.2%
6POM2%-3.5%
7PP1.8%-2.5%
8PS0.4%-0.7%
9PVC0.2%-0.6%
10ABS0.4%-0.5%

b. Characteristics of plastic parts When molding, the molten material, and cavity surface contact the outer layer and immediately cool to form a low-density solid shell.

Due to the poor thermal conductivity of the plastic, the inner layer of the plastic part cools slowly and forms a high-density solid layer with large shrinkage. Therefore, the proper wall thickness, slow cooling, and high-density layer are thick shrinkage.

In addition, the presence or absence of inserts and the layout and number of inserts have a direct impact on the direction of material flow, density distribution, and shrinkage resistance size, so the characteristics of the plastic parts on the size of shrinkage, directional impact.

c. Inlet form, size, and distribution of these factors directly affect the direction of material flow, density distribution, pressure-holding and shrinkage effect, and molding time.

Direct inlet, inlet cross-section large (especially thicker cross-section) is small shrinkage but directional, the inlet wide and short length is small directional. Those close to the inlet or parallel to the direction of material flow will have large shrinkage.

d. Molding conditions mold temperature is high, the molten material cooling slow, high density, shrinkage, especially for crystalline materials due to high crystallinity, volume change, so shrinkage is greater.

Mold temperature distribution and cooling inside and outside the plastic parts and density uniformity is also related, directly affecting the size and direction of the shrinkage of each part.

In addition, the holding pressure and time also have a greater impact on the shrinkage, the pressure is large, and the time is long shrinkage is small but directional.

High injection pressure, the molten material viscosity difference is small, the interlayer shear stress is small, the elasticity after the mold jump, so the shrinkage can also be moderately reduced, high material temperature, shrinkage, but the direction of small.

Therefore, adjusting the mold temperature, pressure, injection speed, and cooling time during molding can also change the shrinkage of plastic parts.

When designing the mold, according to the shrinkage range of various plastics, the wall thickness, and shape of the plastic part, the size and distribution of the inlet, and the shrinkage rate of each part of the plastic part are determined empirically, and then the cavity size is calculated.

For high-precision plastic parts and difficult grasp the shrinkage rate, it is generally appropriate to use the following methods to design the mold.

1. Take a smaller shrinkage rate for the outer diameter of the plastic part and a larger shrinkage rate for the inner diameter to leave room for correction after the trial mold.

2. Test mold to determine the form, size, and molding conditions of the pouring system.

3. To, post-process the plastic parts by post-processing to determine the size change (measurement must be after 24 hours after demolding). 

4. Correct the mold according to the actual shrinkage

5. Try the mold again and slightly correct the shrinkage value by changing the design process conditions appropriately to meet the requirements of the plastic part.

The factors affecting the shrinkage of thermoplastic molding are as follows:

1. In Different varieties of plastic, the shrinkage rate of the material is different. Crystalline materials shrink more, amorphous materials shrink and s, and the higher the filling, the smaller the shrinkage of the material.

2. The size and structure of the plastic molding mold. If the uniform wall thickness of the molded part is too large or the cooling system is not good will affect the shrinkage rate. In addition, the presence or absence of inserts and the layout and number of inserts directly affect the direction of material flow, density distribution, and shrinkage resistance size.

3. The form, size, and distribution of the material mouth. These factors directly affect the direction of material flow, density distribution, pressure-holding and shrinkage effect, and molding time.

4. Mold temperature and injection pressure. High mold temperature and high melt density during molding will result in high plastic shrinkage, especially for plastics with high crystallinity. The temperature distribution and density uniformity of plastic parts also directly affect the size and direction of shrinkage.

Holding pressure and holding time also have an impact on shrinkage. If the pressure is high and the time is long, the shrinkage is small but the directionality is large. Therefore, adjusting the mold temperature, pressure, injection speed, and cooling time during molding can also change the shrinkage of plastic parts.

When designing the mold, according to the shrinkage range of various plastics, the wall thickness, and shape of the plastic part, the size and distribution of the inlet, and the shrinkage rate of each part of the plastic part are determined empirically, and then the cavity size is calculated.

For high-precision plastic parts and difficult grasp the shrinkage rate, it is generally appropriate to use the following methods to design the mold.

a) Take a small lower shrinkage rate for the outer diameter of plastic parts and a larger shrinkage rate for the inner diameter, to leave room for correction after the mold trial.

b) Test mold to determine the form, size, and molding conditions of the pouring system.

c) The plastic parts to be post-treated will be post-treated to determine the dimensional change (measurement must be done after 24 hours after demolding).

d) Correct the mold according to the actual shrinkage.

e) The mold is tested again and the shrinkage value can be slightly corrected to meet the requirements of the molded part by changing the process conditions as appropriate.

Plastic material fluidity

a. Thermoplastic fluidity size, generally from the molecular weight size, melt index, Archimedes spiral line flow length, performance viscosity, and flow ratio (process length / plastic wall thickness), and a series of indices to analyze.

Small molecular weight, wide molecular weight distribution, poor molecular structure regularity, high melt index, long spiral flow length, small performance viscosity, flow ratio is good, the same name of the plastic must check its instructions to determine its liquidity is suitable for injection molding.

According to the requirements of mold design, the fluidity of commonly used plastics can be roughly divided into three categories.

1. Good fluidity PA, PE, PS, PP, CA, poly (4) methyl garlicene.

2. medium fluidity polystyrene series resins (such as ABS, AS), PMMA, POM, polyphenylene ether.

3. poor fluidity PC, hard PVC, polyphenylene ether, polysulfone, polyaryl sulfone, fluoroplastics.

b. The fluidity of various plastics also changes due to various molding factors, the main factors affecting the following.

1. Temperature material temperature increases the fluidity, but different plastics also vary, PS (especially impact-resistant and MFR value higher), PP, PE, PMMA, modified polystyrene (such as ABS, AS), PC, CA, and other plastics fluidity with temperature changes. For PE and POM, the temperature increase or decrease has less effect on its liquidity. So the former in the molding of the temperature should be adjusted to control the fluidity.

2. Pressure injection pressure increases, the molten material is subject to shear, liquidity also increases, especially PE, POM is more sensitive, so it is appropriate to adjust the injection pressure to control the liquidity when molding.

3. Mold structure pouring system form, size, layout, cooling system design, molten material flow resistance (such as surface finish, channel cross-section thickness, cavity shape, exhaust system), and other factors directly affect the actual liquidity of the molten material in the cavity, where the molten material to reduce the temperature and increase the flow resistance of the liquidity will be reduced.

Mold design should be based on the fluidity of the plastic used, and choose a reasonable structure. When molding, we can also control the material temperature, mold temperature and injection pressure, injection speed, and other factors to properly adjust the filling situation to meet the molding needs.

The crystallinity of plastic materials

Thermoplastics can be divided into two categories: crystalline and non-crystalline (also known as amorphous) plastics, according to whether the crystallization phenomenon occurs when condensing.

The so-called crystallization phenomenon is the plastic from the molten state to condensation, molecules from independent movement, completely in a state of no order, into molecules to stop free movement, according to a slightly fixed position, and a tendency to make the molecular arrangement into a regular model of a phenomenon.

As a criterion to distinguish the appearance of these two types of plastics depending on the transparency of the thick-walled plastic parts, the generally crystalline material is opaque or translucent (such as POM, etc.), and amorphous material is transparent (such as PMMA, etc.).

However, there are exceptions, such as poly (4) methyl garoulein is crystalline plastic but has high transparency, and ABS is amorphous material but is not transparent.

In the mold design and selection of injection molding machines, the following requirements and considerations should be noted for crystalline plastics.

1. More heat is needed to raise the material temperature to the molding temperature, so use equipment with a large plasticizing capacity.

2. The heat released during cooling and tempering is large and should be fully cooled.

3. The specific gravity difference between molten state and solid-state is large, molding shrinkage is large, easy to occur shrinkage, porosity.

4. Fast cooling, low crystallinity, small shrinkage, and high transparency. The crystallinity is related to the wall thickness of plastic parts, the wall thickness is slow cooling, high crystallinity, high shrinkage, and good physical properties. So the crystalline material should be required to control the mold temperature.

5. Significant anisotropy and high internal stress. The uncrystallized molecules tend to continue to crystallize after demolding and are in a state of energy imbalance, prone to deformation and warpage.

6. The crystallization temperature range is narrow, and it is easy to inject the unmelted material into the injection mold or block the inlet.

Heat-sensitive plastic and easy hydrolysis plastic

a. Heat-sensitive refers to some plastics that are more sensitive to heat, heat at high temperatures for a long time or feed port cross-section is too small, the shear effect is large, the material temperature increases susceptible to discoloration, degradation, decomposition tendency, with this characteristic of plastics called heat-sensitive plastics. Such as hard PVC, polyvinyl chloride, vinyl acetate copolymer, POM, polyvinyl trifluoride, etc.

Thermosensitive plastics produce monomers, gases, solids, and other by-products during decomposition, especially some decomposition gases that are irritating, corrosive, or toxic to the human body, equipment, and molds.

Therefore, the mold design, selection of injection molding machine and molding should pay attention to, should use screwmolding injection molding machine, pouring system cross-section should be large, mold and barrel should be chromium-plated, there should be no * corner stagnant material, must strictly control the molding temperature, plastic to add stabilizers to weaken its heat-sensitive performance.

b. Some plastics (such as PC) will decompose under high temperature and pressure even if they contain a small amount of water, this property is called easy hydrolysis, which must be heated and dried in advance.

Stress cracking and melt rupture

a. Some plastics are stress-sensitive, easy to produce internal stress, brittle, and easy to crack, plastic parts under the action of external forces or in the role of the solvent that cracking phenomenon.

For this reason, in addition to adding additives to the raw materials to improve the crack resistance, the raw materials should pay attention to dry, reasonable choice of molding conditions to reduce internal stress and increase the crack resistance. And should choose a reasonable shape of the plastic parts, and should not set the inserts and other measures to minimize the stress concentration.

Mold design should increase the slope of the mold release, choose a reasonable feed port and ejector mechanism, molding should be appropriate to adjust the material temperature, mold temperature, injection pressure, and cooling time, and try to avoid plastic parts too cold and brittle when the mold release, molding plastic parts should also be post-treatment to improve anti-cracking, eliminate internal stress and prohibit contact with solvents.

b. When a certain melt flow rate of the polymer melt, at a constant temperature through the nozzle hole when its flow rate exceeds a certain value, the melt surface occurs in lateral cracks called melt rupture, the appearance and physical properties of plastic parts. Therefore, in the selection of high melt flow rate of polymer, etc., should increase the nozzle, sprue, inlet cross-section, reduce the injection speed, and increase the material temperature.

Thermal performance and cooling speed

a. Various plastics have different thermal properties such as specific heat, thermal conductivity, and heat deflection temperature. High specific heat requires a lot of heat when plasticizing, so you should use an injection molding machine with a large plasticizing capacity.

The cooling time of plastics with high heat deformation temperature can be short and the mold can be released early, but the cooling deformation should be prevented after the mold is released.

The cooling speed of plastics with low thermal conductivity is slow (such as ionic polymers, etc. cooling speed is extremely slow), so it must be fully cooled, and the cooling effect of the mold should be strengthened.

Hot sprue molds are suitable for plastics with low specific heat and high thermal conductivity. Plastics with high specific heat, low thermal conductivity, low heat deflection temperature, and slow cooling rate are not suitable for high-speed molding, so an appropriate injection molding machine must be used and the cooling of the mold should be strengthened.

b. According to the characteristics of various types of plastics and the shape of plastic parts, it is necessary to maintain the appropriate cooling speed. Therefore, the mold must be set up with a heating and cooling system according to the molding requirements to maintain a certain mold temperature.

When the material temperature makes the mold temperature rise, it should be cooled to prevent the deformation of plastic parts after demolding, shorten the injection molding cycle and reduce the crystallinity.

When the residual heat of plastic is not enough to keep the mold at a certain temperature, the mold should be equipped with a heating system to keep the mold at a certain temperature to control the cooling rate, ensure fluidity, improve filling conditions or control the plastic parts to make them cool slowly, prevent uneven cooling inside and outside the thick-walled plastic parts and improve crystallinity, etc.

For good fluidity, large molding area, and uneven material temperature, it is sometimes necessary to use alternate heating or cooling or partial heating and cooling according to the molding situation of plastic parts. For this reason, the mold should be equipped with the corresponding cooling or heating system.

Moisture absorption

There are various additives in plastics, so they have different degrees of affinity to water, so plastics can be broadly divided into hygroscopic, water-adhesive, and non-absorbent, and are not easy to adhere to water.

The common plastics with strong moisture absorption are PMMA, PA, PC, ABS, and so on. And poor moisture absorption is PE, PP, PS, fluorine plastic, etc.

The water content in the material must be controlled within the allowable range, otherwise, the water will become gas or hydrolysis under high temperature and pressure, making the plastic resin blistering, liquidity decline, and appearance and mechanical properties bad.

Therefore, moisture-absorbing plastics must be preheated according to the requirements of appropriate heating methods and specifications to prevent re-absorption of moisture when in use.

Conclusion

The injection molding process involves the injection molding machine equipment, the design of the injection molding product, the design and production of the injection mold, the information related to the injection molding material, and the debugging of the injection molding production process, etc. Each link needs to be considered thoroughly to ensure the final product is of high quality.

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