7 Factors that Affect the Injection Molding Process

7 factors affecting the injection molding process

1. Shrinkage rate

The factors that affect thermoplastic molding shrinkage are as follows:

1.1 Plastic varieties

In the plastics molding process of thermoplastic, there are the volume change caused by crystallization, strong internal stress, large residual stress frozen in the plastic part, and strong molecular orientation. Therefore, compared with thermosetting plastics, it has a larger shrinkage rate, a wide range of shrinkage rate, and obvious directionality.

1.2 The characteristics of the plastic part

When the molten material is in contact with the wall of the cavity, the outer layer is immediately cooled to form a low-density solid shell. Due to the poor thermal conductivity of the plastic, the inner layer of the plastic part is slowly cooled to form a high-density solid layer with large shrinkage. Therefore, the plastic part with a thick wall, slow cooling, and high-density layer thickness will shrink more.

In addition, the presence or absence of inserts and the layout and quantity of inserts directly affect the direction of material flow, density distribution and shrinkage resistance. Therefore, the characteristics of plastic parts have a greater impact on shrinkage and directionality.

1.3 The form, size and distribution of the mold gate

The form, size and distribution of the mold gate can directly affect the melt plastic flow direction, density distribution, pressure maintaining and shrinking effect and molding time. Direct feed gate and gate with large cross-sections (especially thicker cross-sections) have less shrinkage but greater directionality. While the shorter gate are with shorter width and length have less directionality. Those that are close to the gate or parallel to the direction of the material flow will shrink more.

1.4 Molding conditions

The high mold temperature, the slowly cooling molten plastic can the high density lead to a large shrinkage, especially for the crystalline material, the shrinkage is greater due to high crystallinity and large volume changes. The mold temperature distribution is also related to the internal and external cooling and density uniformity of the plastic part. And that directly affects the size and directionality of the shrinkage of each part.

In addition, holding pressure and holding time also have a greater impact on contraction. The contraction is smaller but the directionality is larger when the pressure is high and the time is long. The injection pressure is high and the viscosity difference of the molten material is small. Besides, the interlayer shear stress is small, and the elastic rebound after demolding is large. So the shrinkage can also be reduced by an appropriate amount. The material temperature is high, the shrinkage is large, but the directionality is small. Therefore, adjusting the mold temperature, injection pressure, injection speed and cooling time during molding can also appropriately change the shrinkage of the plastic part.

Design 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 gate form. Estimate the shrinkage rate of each part of the plastic part according to experience, and then calculate the cavity size. For high-precision plastic parts and when it is difficult to confirm the shrinkage rate, the following methods are generally used:

Mold Design:

Take a smaller shrinkage rate for the outer diameter of the plastic part, and a larger shrinkage rate for the inner diameter, so as to leave room for correction after the test mold.

The mold trial determines the form, size and molding conditions of the gate/runner system.

The post-processed of the plastic product shall be done to determine the size change (measurement must be 24 hours after demolding).

Correct the mold according to the actual shrinkage.

Retry the mold and appropriately change the process conditions to slightly modify the shrinkage value to meet the requirements of the plastic part.

2. Fluidity

2.1 The fluidity of thermoplastics can generally be analyzed from a series of indexes such as molecular weight, melt index, Archimedes spiral flow length, apparent viscosity and flow ratio (process length/plastic part wall thickness). The plastic is of small molecular weight, wide molecular distribution, poor molecular structure regularity, high melt index, long spiral flow length, low apparent viscosity, high flow ratio. Then, its fluidity is good. We must check the instructions of plastics resin with the same product name to determine whether their fluidity is applicable for injection molding. According to mold design requirements, the fluidity of commonly used plastics can be roughly divided into three categories:

Good fluidity: PA, PE, PS, PP, CA, poly(4) methylpentene

Medium fluidity: Polystyrene series resin (such as ABS, AS), PMMA, POM, polyphenylene ether

Poor fluidity: PC, hard PVC, polyphenylene ether, polysulfone, polyarylsulfone, fluoroplastics

2.2 The fluidity of various plastics also changes due to various molding factors. The main influencing factors are as follows:

Higher material temperature increases fluidity, but different plastics have their own differences. such as PS (especially those with high impact resistance and higher MFR value), PP, PA, PMMA, modified polystyrene (such as ABS, AS) , PC, CA and other plastics. Their fluidity varies greatly with temperature.

For PE and POM, the temperature increase or decrease has little effect on their fluidity. Therefore, the former should adjust the temperature during molding to control fluidity.

As the injection pressure of injection molding increases, the molten material is subject to greater shear and fluidity, especially PE and POM. They are more sensitive. So the injection pressure should be adjusted to control fluidity during molding.

The form, size, layout, cooling system design of the mold structure, the flow resistance of the molten plastic (such as the wall finish, the thickness of the runner, the shape of the cavity, the air-exhaust system) and other factors directly affect the the actual fluidity of molten plastic inside the mold. If the molten material is promoted to lower the temperature and increase the fluidity resistance, the fluidity of the melt plastic will decrease.

When designing the mold, a reasonable structure should be selected according to the fluidity of the plastic used. During molding, the material temperature, mold temperature, injection pressure, injection speed and other factors can also be controlled to appropriately adjust the filling condition to meet the molding needs.

3. Crystallinity

Thermoplastics can be divided into crystalline plastics and non-crystalline (also known as amorphous) plastics according to their absence of crystallization during condensation.

The so-called crystallization phenomenon is: When the plastic changes from a molten state to a condensation state, the molecules move independently and are completely in a disordered state. But then the molecules stop moving freely, in a slightly fixed position, and have a tendency to arrange themselves to a regular model.

The appearance criteria for judging these two types of plastics can be determined by the transparency of the thick-walled plastic parts. Generally, crystalline materials are opaque or translucent (such as POM, etc.), and amorphous materials are transparent (such as PMMA, etc.). But there are exceptions. For example, poly methylpentene is a crystalline plastic but has high transparency. While ABS is an amorphous material but not transparent.

Requirements for crystalline plastics when designing molds and selecting injection molding machines

Requirements and precautions:

A lot of heat is required for the material temperature to rise to the molding temperature. So in this situation, we need a equipment with large plasticizing ability.

It emits a large amount of heat during cooling back and needs to be cooled sufficiently.

The specific gravity difference between the molten state and the solid state is large. And the molding shrinkage is large. Then, the shrinkage and pores are prone to occur.

Fast cooling, low crystallinity, small shrinkage and high transparency. The crystallinity is related to the wall thickness of the plastic part. If the wall thickness is slow to cool, the crystallinity will be high, the shrinkage will be large and the physical properties will be good. Therefore, the mold temperature of the crystalline material must be controlled as required.

Significant anisotropy and large internal stress. Molecules that are not crystallized after demolding have a tendency to continue to crystallize, are in an energy imbalance state, and are prone to deformation and warpage.

The crystallization temperature range is narrow, and the unmelted material is prone to be injected into the mold or block the mold gate or runner.

4. Heat-sensitive plastics and easily hydrolyzed plastics

4.1   Heat sensitivity refers to the tendency of certain plastics to be sensitive to heat. The heating time is longer at high temperature or the cross section of the mold gate is too small. Besides, the shearing effect is large. Then, the material temperature increases and it is prone to discoloration, degradation, and decomposition. The plastic has the above characteristics is called heat-sensitive plastic. Such as hard PVC, polyvinylidene chloride, vinyl acetate copolymer, POM, polychlorotrifluoroethylene, etc.

Heat-sensitive plastics produce monomers, gases, solids and other by-products during decomposition. In particular, some decomposition gases have irritating, corrosive or toxic effects on the human body, equipment, and molds. Therefore, we should pay attention to mold design, injection molding machine selection and molding. Screw injection molding machine should be used. The section of the injection system should be large. The mold and barrel should be chrome-plated. Add stabilizer to weaken its thermal sensitivity.

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

5. Stress cracking and melt fracture

5.1  Some plastics are sensitive to stress. It is easy to produce internal stress during molding and is brittle and easy to crack. The plastic part will crack under the action of external force or solvent. For this reason, in addition to adding additives to the resin to improve crack resistance, we should pay attention to drying of the plastic resin. And select the molding conditions reasonably to reduce internal stress and increase crack resistance.

What’s more, we should choose a reasonable shape of plastic parts, it is not appropriate to install inserts and other measures in order to minimize stress concentration. When designing the mold, increase the demolding angle, and select a reasonable feed inlet and ejection mechanism. The material temperature, mold temperature, injection pressure and cooling time should be adjusted appropriately during molding. And try to avoid demolding when the plastic part is too cold and brittle. After molding, we should post-treat the plastic partsto improve crack resistance, eliminate internal stress and prohibit it contact with solvents.

5.2  A polymer melt with a certain melt flow rate passes through the nozzle hole at a constant temperature and its flow rate exceeds a certain value. Then, some obvious lateral cracks appear on the melt surface and we called it melt fracture. And it will damage the appearance and physical properties of the plastic part. Therefore, when selecting polymers with high melt flow rate, the cross-section of the nozzle, runner, and feed opening should be increased to reduce the injection speed and increase the material temperature.

6. Thermal performance and cooling rate

6.1  Various plastics have different specific heat, thermal conductivity, heat distortion temperature and other thermal properties. Plasticizing with a high specific heat requires a large amount of heat, and an injection molding machine with a large plasticizing capacity should be used. The cooling time of the plastic with high heat distortion temperature can be short and the demoulding is early, but we should prevent the cooling deformation after demolding. Plastics with low thermal conductivity have a slow cooling rate (such as ionic polymers, etc.). Therefore, they must be sufficiently cooled to enhance the cooling effect of the mold. Hot runner molds are suitable for plastics with low specific heat and high thermal conductivity. Plastics with large specific heat, low thermal conductivity, low thermal deformation temperature, and slow cooling rate are not conducive to high-speed molding. Appropriate injection molding machines and enhanced mold cooling system must be selected.

6.2  Various plastics are required to maintain an appropriate cooling rate according to their types, characteristics and shapes of plastic parts. Therefore, the mold must be equipped with heating and cooling systems according to the molding requirements to maintain a certain mold temperature.

When the material temperature increases the mold temperature, it should be cooled down. So that it can prevent the plastic part from deforming after demolding, shorten the production cycle, and reduce the crystallinity.

When the plastic waste heat is not enough to keep the mold at a certain temperature, the mold should be equipped with a heating system. The heating system can keep the mold at a certain temperature to control the cooling rate, ensure fluidity, improve filling conditions or control the slow cooling of the plastic parts. Prevent uneven cooling inside and outside of thick-walled plastic parts and increase crystallinity.

For those with good fluidity, large molding area, and uneven material temperature, depending on the molding conditions of plastic parts, sometimes it is necessary to heat or cool alternately, or use both local heating and cooling. Therefore, the mold should be equipped with a corresponding cooling or heating system.

7. Hygroscopicity

Due to various additives, the plastics have different degrees of affinity for moisture. And so plastics can be roughly divided into two types: moisture absorption, moisture adhesion, and non-absorption and non-stick moisture. The water content in the plastic resin must be controlled within the allowable range. Otherwise, the moisture will become gas or hydrolyze under high temperature and high pressure. And it will cause the resin to foam, decrease the fluidity, and have poor appearance and mechanical properties. Therefore, hygroscopic plastics must be preheated with appropriate heating methods and specifications as required to prevent re-absorption of moisture during use.

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