Factors To Be Considered In Injection Molding Process Setting

First,

The form and calculation of the forming shrinkage of thermoplastic plastics, as mentioned above, affect the molding shrinkage of thermoplastic plastics as follows:

1.1 plastic molding process, due to the existence of crystalline form of volume changes, internal stress, freezing in plastic parts of residual stress, strong molecular orientation and other factors, therefore, compared with thermosetting plastics, shrinkage rate is large, shrinkage range is wide, square direction is obvious, and after molding shrinkage, annealing or humidity treatment shrinkage rate is generally larger than thermosetting plastics.

1.2 plastic parts are molded when molten materials contact with the surface of the cavity and the outer layer is cooled immediately to form a low density solid shell.

Due to the poor thermal conductivity of plastics, the inner layer of plastic parts is cooled slowly to form a high density solid layer with large shrinkage.

Therefore, the wall thickness, slow cooling and high density thickness shrink greatly.

In addition, the layout and quantity of inserts and inserts directly affect the direction of flow, density distribution and shrinkage resistance. Therefore, the characteristics of plastic parts have a great influence on the size and direction of shrinkage.

1.3 the form, size and distribution of feed inlet directly affect the direction of flow, density distribution, pressure maintaining and feeding time and molding time.

The section of the direct feed port and the feed port is large (especially the thick section), but the shrinkage is small, but the direction is large.

Large shrinkage occurs near the inlet or parallel to the material flow direction.

1.4 molding conditions, mold temperature is high, melting material is slow, high density, large shrinkage, especially for crystalline materials because of high crystallinity and large volume changes, so shrinkage is greater.

The mold temperature distribution is also related to the internal and external cooling and density uniformity of the plastic parts, which directly affects the shrinkage and direction of each part.

In addition, keeping pressure and time also has a greater effect on contraction. When the pressure is large and the time is long, the contraction is small but the direction is large.

The injection pressure is high, the viscosity difference of the melted material is small, the interlaminar shear stress is small, and the elastic rebound is large after demoulding, so the shrinkage can also be reduced to a proper degree. The material temperature is high and the shrinkage is large, but the direction is small.

So molding temperature, pressure, injection speed and cooling time can also change the shrinkage of plastic parts properly.

According to the shrinkage range of various plastics, the thickness and shape of plastic parts, the size and distribution of feed ports, the shrinkage rate of each part of plastic parts is determined according to experience, and then the cavity size is calculated.

For high precision plastic parts and difficult to grasp shrinkage, it is generally advisable to design moulds with the following methods:

(1) the smaller shrinkage rate of the outer diameter of the plastic part, and the larger shrinkage of the inner diameter, so that there is room for correction after the trial die.

Second, determine the form, size and molding conditions of gating system.

(3) after processing, the size of the plastic parts must be determined after processing (the measurement must be 24 hours after the mold release).

Fourth, correct the mold according to the actual shrinkage condition.

Retry the die and appropriately modify the technological conditions to slightly modify the shrinkage value to meet the requirements of plastic parts.

Two. Liquidity

2.1 the mobility of thermoplastic plastic can be analyzed from a series of indices, such as molecular weight, melt index, Archimedes spiral flow length, apparent viscosity and flow ratio (process length / plastic wall thickness).

Low molecular weight, wide molecular weight distribution, poor molecular structure, high melting index, long flow of screw, low viscosity, and high flow rate. The same name must be checked to determine whether its fluidity is suitable for injection molding.

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

1. Fluidity is good: PA, PE, PS, PP, CA, poly (4) methylene.

(II) mobile medium polystyrene series resins (such as ABS, AS), PMMA, POM and polyphenylene oxide.

3. Poor mobility PC, hard PVC, polyphenylene oxide, polysulfone, polysulfone and fluoroplastics.

2.2 the mobility of various plastics has also changed due to various molding factors.

(1) when the temperature and material temperature were high, the fluidity increased, but different plastics also varied. PS (especially impact resistant and MFR high), PP, PA, PMMA, modified polystyrene (such as ABS, AS), PC, CA and other plastics had greater fluidity with temperature.

For PE and POM, temperature increase and decrease have little effect on their fluidity.

Therefore, the former should adjust the temperature to control the fluidity.

2. When pressure injection pressure increases, the melt is shear and the flow is increased. Especially PE and POM are sensitive. Therefore, injection pressure should be adjusted to control fluidity.

3. The form, size, layout, cooling system design, melt flow resistance (such as surface finish, cross section thickness, cavity shape, exhaust system) and other factors directly affect the actual fluidity of molten material in the mold cavity, and the fluidity of the molten material decreases with increasing temperature and increasing resistance.

Mold design should be based on the liquidity of the plastic used, and choose a reasonable structure.

During molding, material temperature, mold temperature and injection pressure, injection speed and other factors can be controlled to properly adjust filling conditions to meet the molding needs.

Three. Crystallinity

Thermoplastic plastics do not crystallize according to their condensation. They can be divided into two categories: crystalline plastic and non crystalline (also amorphous) plastics.

The so-called crystallization phenomenon is that when molten plastic is fused to condensate, molecules move independently, completely in an inordered state, and become molecules that stop moving freely, in a slightly fixed position, and have a tendency to make molecules arranged as regular models.

As a criterion for judging the appearance of these two kinds of plastics, the pparency of the thick plastic parts of the visible plastics is determined. The crystalline materials are opaque or plucent (such as POM), and amorphous materials are pparent (such as PMMA).

But there are exceptions, for example, poly (4) methylene is crystalline plastic but has high pparency. ABS is amorphous but not pparent.

In mold design and selection of injection molding machines, attention should be paid to the following requirements and precautions for crystalline plastics.

(1) the amount of heat needed to increase the material temperature to the molding temperature is large, and the equipment with large plasticizing capacity is needed.

2. When the cooling is discharged, the heat will be released.

3. The specific gravity difference between the molten state and the solid state is large, the shrinkage is large, and shrinkage pores and pores are easy to occur.

Fast cooling, low crystallinity, small shrinkage and high pparency.

The degree of crystallinity is related to the wall thickness of the plastic parts. The thickness of the wall is slow, the crystallinity is high, the shrinkage is large and the physical properties are good.

Therefore, the mold temperature must be controlled according to the requirements.

The anisotropy is significant and the internal stress is large.

The molecules that are not crystallized after demoulding have the tendency to crystallize and are in the state of energy imbalance. They are prone to deformation and warpage.

The crystallization temperature range is narrow, and it is easy to injecting mold into the unmelted end or clogging the feed port.