#1 Shrinkage Rate
Factors affecting thermoplastic molding shrinkage are as follows:
- Plastic Varieties During the molding process of thermoplastic plastics, there are still volume changes caused by crystallization, strong internal stress, large residual stress frozen in plastic parts, and strong molecular orientation. Therefore, compared with thermosetting plastics, the shrinkage rate is lower. Large, wide shrinkage range, obvious directionality, in addition, the shrinkage after molding, annealing or humidity conditioning treatment is generally larger than that of thermosetting plastics.
- Characteristics of plastic parts When the molten material is in contact with the surface of the cavity, the outer layer is immediately cooled to form a low-density solid shell. Due to the poor thermal conductivity of plastic, the inner layer of the plastic part is cooled slowly to form a high-density solid layer with large shrinkage. Therefore, those with thick walls, slow cooling, and thick high-density layers will shrink greatly. 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, so the characteristics of plastic parts have a greater impact on shrinkage and directionality.
- The form, size and distribution of the feed inlet directly affect the direction of material flow, density distribution, pressure holding and feeding and molding time. The direct feed port and the feed port with large section (especially thicker section) will have small shrinkage but large directionality, and the wide and short length of the feed port will have small directionality. The ones that are close to the feed inlet or parallel to the flow direction will shrink greatly.
- Molding conditions The mold temperature is high, the molten material cools slowly, the density is high, and the shrinkage is large, especially for the crystalline material because of the high crystallinity and the large volume change, so the shrinkage is greater. The mold temperature distribution is also related to the internal and external cooling and density uniformity of the plastic part, which directly affects the shrinkage and direction of each part. In addition, maintaining pressure and time also have a greater impact on shrinkage, and those with high pressure and long time will have small shrinkage but large directionality. The injection pressure is high, the viscosity difference of the molten material is small, the shear stress between layers is small, and the elastic rebound is large after demoulding, so the shrinkage can also be reduced appropriately. The high temperature of the material causes large shrinkage, but the directionality is small. Therefore, adjusting various factors such as mold temperature, pressure, injection speed and cooling time during molding can also appropriately 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 feed inlet, the shrinkage rate of each part of the plastic part is determined according to experience, and then the cavity size is calculated. For high-precision plastic parts and when it is difficult to grasp the shrinkage rate, it is generally suitable to design the mold by the following method:
- Test the mold to determine the form, size and molding conditions of the pouring system.
- The plastic parts to be post-processed shall be post-processed to determine the dimensional change (measurement must be made after 24 hours after demoulding).
- Correct the mold according to the actual shrinkage.
- Try the mold again and change the process conditions appropriately to slightly correct the shrinkage value to meet the requirements of the plastic part.
#2 Mobility
The molecular weight is small, the molecular weight distribution is wide, the molecular structure is poor, the melt index is high, the spiral flow length is long, the apparent viscosity is small, and the flow ratio is large, the fluidity is good. For the plastic with the same product name, it is necessary to check its manual to judge whether its fluidity is applicable. for plastic injection molding.
According to the 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.
High material temperature increases fluidity, but different plastics also have differences, PS (especially impact-resistant and high MFR value), PP, PA, PMMA, modified polystyrene (such as ABS, AS), The fluidity of PC, CA and other plastics changes greatly with temperature. For PE and POM, the temperature increase or decrease has little effect on its fluidity. Therefore, the former should adjust the temperature to control the fluidity during molding.
As the pressure injection molding pressure increases, the molten material is subject to greater shearing action and fluidity also increases, especially PE and POM are more sensitive, so it is advisable to adjust the injection molding pressure to control fluidity during molding.
The form, size, layout, cooling system design, and molten material flow resistance of the mold structure gating system (such as surface finish, channel section thickness, cavity shape, exhaust system) and other factors directly affect the molten material in the cavity. The actual fluidity of the material, the fluidity will decrease if the temperature of the molten material is lowered and the fluidity resistance is increased. 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 properly adjust the filling situation to meet the molding needs.
#3 Crystallinity
Thermoplastics can be divided into two categories: crystalline plastics and non-crystalline (also known as amorphous) plastics according to their lack of crystallization during condensation. The so-called crystallization phenomenon is that when the plastic is from the molten state to the condensation, the molecules move independently and are completely in a disordered state, and then the molecules stop free movement, according to a slightly fixed position, and have a tendency to make the molecular arrangement into a regular model. phenomenon.
The appearance standard for distinguishing these two types of plastics depends on the transparency of 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, such as poly(4) methylpentene is a crystalline plastic but has high transparency, and ABS is an amorphous material but is not transparent. When designing molds and selecting injection molding machines, you should pay attention to the following requirements and precautions for crystalline plastics: the heat required to raise the material temperature to the molding temperature is large, and equipment with large plasticizing capacity should be used.
#4 5 Key Points Of Injection Molding Process For Spray-Free Materials
1.Drying control – moisture content below 5%
No-spray materials, like most materials, absorb moisture from the air. It must be dried before molding, otherwise it may cause silver threads and water splashes on the surface of the product and affect the surface gloss of the product.
It is usually required that the water content of the spray-free material be controlled below 0.05 after drying. For example, for ABS-based spray-free materials, the drying temperature is usually 70°C-85°C, and the drying time is 3-4 hours.
If the temperature is too high, the performance of the material will be reduced; if the temperature is too low, the drying time will not be enough, and the moisture content of the resin will be too high, which will easily cause air marks on the surface of the product.
2.Injection temperature – medium and low temperature injection molding
Due to the addition of metal pigments in the spray-free resin, under the high shear action of the screw, it is easy to cause discoloration of the metal pigments. Therefore, the principle of setting the molding temperature of spray-free materials is to use medium and low temperature injection molding as much as possible to prevent material degradation under the premise of ensuring that the materials are fully plasticized.
For example, the processing temperature of spray-free materials based on ABS is 210°C-230°C. If the temperature is too high, the material will degrade and air marks will appear on the surface of the product.
3.Injection speed—medium-speed step-by-step control
When using spray-free materials for injection molding, the selection of injection speed mainly considers the appearance of the product, the exhaust of the mold and the resistance of the resin flow in the cavity of the injection molding machine.
Faster injection speed will generally lengthen the melt process, suitable for filling thin-walled products, and form better surface finish and surface effect. However, too fast injection speed will easily cause the melt to be strongly sheared, resulting in damage to the shape and particle size of the pearlescent toner and reducing the special color effect of the product. In addition, problems such as ejection marks and poor exhaust are prone to occur near the gate.
Therefore, on the premise of ensuring the surface effect and quality of the product, it is recommended to choose a medium-speed step-by-step speed control to ensure smooth filling and product appearance.
4.Injection pressure – lower pressure
The injection pressure is to overcome the resistance of the melt during the flow process, to give the melt a certain filling speed, and to compact and feed the melt to ensure the smooth progress of the filling process. Actual injection pressure is related to many variables, such as: melt temperature, mold temperature, part geometry, wall thickness, flow length, and other mold and equipment conditions.
In general, it is best to choose a lower pressure that satisfies performance, appearance and injection molding cycles. At the same time, pay attention to the difference between the actual injection peak pressure and the set value. In general, the proper holding pressure is 60-80% of the injection pressure.
5.Mold temperature – higher mold temperature
The mold temperature directly affects the surface brightness, weld line and strength of the final product. Using a high mold temperature can increase the fluidity of the material, obtain a higher bonding line strength, and reduce the internal stress of the molded product, making it better in heat resistance and chemical resistance, and at the same time improving the reproducibility of the melt on the mold surface , Improve product gloss and special color effects.
In order to achieve the ideal surface quality effect, when using spray-free materials, a higher mold temperature should be used as much as possible. For spray-free materials with ABS as the base material, the mold temperature is usually 70-90°C.
In short, only reasonable mold design, appropriate drying temperature, injection temperature, injection speed, and reasonable mold temperature are essential conditions for making beautiful-looking products from spray-free materials.