Low pressure injection molding process
This low pressure injection process is very similar to that of thermoplastic injection molding.The granular hot melt adhesive is heated to melt for further processing in the liquid state, as shown in figure 1.Different from the traditional injection molding technology, this kind of single-component hot-melt adhesive only needs 2 to 40 bar low pressure in the specially designed mold to complete the process of packaging electronic components.This low pressure range is possible because the hot melt adhesive has a low viscosity in the molten state, only between 1000 and 8000 mpa.s.In addition, the temperature range of injection molding ranges from 180 to 240 degrees Celsius. In this way, the precise and sensitive electronic components such as wire bundles, connectors, microswitches, sensors and circuit boards can be meekly encapsulated without causing any harm.Figure 2 is an encapsulated part encased in a low pressure injection material of amber or dark color.After the hot melt is injected into the mold, it begins to cool and solidify. The curing time varies according to the amount of glue, about 10 to 50 seconds.In addition to protecting the components from the surrounding environment, the low pressure injection material can also play the role of shock resistance and buffer stress.In addition, the material can also be used as electric edge material.The picture on the front page shows an electronic component packaged in an amber hot melt adhesive provided by Siemens VDO.
Low pressure injection material
The chemical used in this technique is polyamide hot melt adhesive based on dimer fatty acids.The fatty acids come from renewable sources, such as soybeans, rapeseed and sunflower seeds, which are then condensed into dimers.In the process of condensation, the dimer fatty acid reacts with diamine, releases water, and is born with polyamide hot melt adhesive.The obvious characteristic of this kind of products is the wide range of temperature resistance, that is to say, the products have low temperature flexibility, and at the same time, they also have high temperature creep resistance.
These products have properties similar to plastics because they are stronger and stronger than other hot melt materials.During injection, these adhesives do need to function as plastics - in other words, adhesives are not just a layer of film between two substrate surfaces, but an integral part of a three-dimensional exterior structure.Thermoplastic shells can be completely replaced by these adhesives.
In addition to its mechanical superiority, another important characteristic of such products is its stickiness.It can be a perfect waterproof system by firmly binding the layers of the package together (such as the wire end material, the shell material, and the circuit board).
The diversification of a material can only be achieved by blending different materials.Due to such fusion, the polyamide material does not have a definite melting point, but has a wider softening range.The same is true for the vitrification temperature, or, more accurately, the range of temperatures.These changes can be illustrated by DSC thermal difference scanning (DSC) graph, as shown in table 1.This is - 120 ℃ to 250 ℃ between the second round of the recorded data.The melting peak on the right represents the melting point at which a solid changes into a liquid.On the left is the vitrification range, which depicts the softening process from the glassy state to the elastomer state from left to right.The vitrification temperature is defined as the middle point of the vitrification range  and .The softening point of ASTM E 28 in table 2 depicts the transformation temperature from solid to liquid.This value is important for the process because the injection temperature must exceed it.This softening point at the end of the DSC melting peak has little to do with the working temperature range of the polyamide material, since the polyamide is soft enough before reaching this softening point.
Unlike polyamide materials such as PA 6, polyamide based on dimer fatty acids is mainly amorphous in structure, because its crystal composition is very small.Figures 3 and 4 show different molecular structures.The structure of PA 6 is very uniform, so it can form highly crystalline lens with very compact structure, while the molecular structure of polyamide hot melt adhesive is extremely complex and very uneven.The strength and temperature creep of common polyamide materials are stronger than that of polyamide hot-melt adhesive based on dimer fatty acids, while the polyamide hot-melt adhesive shows stronger flexibility and cold flexure.
Due to the large molecular weight, the viscosity of fused common polyamide materials is much higher than that of polyamide hot-melt adhesive, so it can only be processed by traditional injection molding machine.The low - viscosity polyamide hot - melt adhesive can be constructed by low - pressure hot - melt adhesive injection molding machine.
Due to the sequence of fatty acids, polyamide based on dimer fatty acids has non-polar parts, but its overall structure is mainly polar and can absorb water.Because of its fatty acid content, it is usually less absorbent than ordinary polyamide materials.Prior to processing, the polyamide thermoplastic material must be preserved in moisture-proof conditions to prevent bubbles from forming during melting.
In addition to the characteristics of mechanical properties and vitrifying temperature, the characteristics related to application are also important factors, such as fire resistance and electric exhaustion.Thermal expansion is also important for temperature cycles.And besides that, there must be chemical corrosion resistance that must be considered in the automobile industry.
Table 2 shows the operating temperature range of some polyamide hot-melt adhesive products.However, the choice of the product depends on the specific application, the characteristics of the material itself, the compatibility with the base material and the expansion coefficient are all decisive factors.For these reasons, it is recommended to test the encapsulated and sealed parts according to the corresponding temperature requirements.The expansion coefficient of polyamide hot melt material is by TMA measurement standards to determine, in - 45 ℃ to 95 ℃ between about 300 PPM/K.Due to good cold flexure and high ductility, the polyamide hot melt material also performs well under severe temperature impact tests.In application, the material is usually not subjected to heavy mechanical loads, with the exception of temperature changes.However, if for some special applications, high mechanical loads may occur at different temperatures, the specific thermal properties of the material must be considered.This material will gradually soften as the temperature rises.
The most obvious characteristic of polyamide hot melt adhesive is that VOC value is very low after temperature rise.Test for 30 minutes under 100 ℃, VOC value is less than 30 PPM.According to DIN 75201 standard, test 16 hours under 100 ℃, atomized value less than 0.1 mg.
There are many ways to measure flammability or fire resistance.For car manufacturers, the official catalog for Underwriters Laboratories is often decisive.The test according to FMVSS 302 is specific to the automobile manufacturing industry, especially the internal components of the automobile.The products listed in table 2 meet UL 94 v-0 certification and pass FMVSS 302 test.In the hot-wire ignition test conducted according to ASTM D3874 standard, the products reached the grades of 3 to 4 respectively.In addition, the product also meets the requirements of DIN en 60695-2-12 hot wire test. The flammability index (GWFI) of hot wire is 960/3.0.
Table 2 lists some data on the electrical properties of the polyamide thermoplastic, such as volume resistivity, dielectric strength, other limiting properties, and comparative tracking index.Since we are talking about polyamide here, we must also consider the effect of moisture on it.
Injection materials used in automobile manufacturing must be corrosion resistant to many chemicals.In most cases, however, contact is brief and sporadic.Corrosion resistance is determined by applying a small amount of liquid to the surface or by rapidly dipping the test body into the liquid.Then, the test body is usually kept at the appropriate working temperature.The polyamide materials discussed in this paper are resistant to chemicals commonly encountered in automobile industry.Surface decomposition occurs only when the battery is exposed to acid.