How should TPE package be adjusted if its gluing effect is not good
Time：2019-12-26Hits：1755
What should be paid special attention to in TPE plastic-coated injection molding?
Thermoplastic elastomer TPE is a material with the characteristics of high elasticity, high strength, high resilience of rubber and injection molding. The invention has the advantages of environmental protection, innocuity, safety, wide hardness range, excellent coloring property, soft touch feeling, weather resistance, fatigue resistance and temperature resistance, excellent processing performance, no need of vulcanization, recycling, cost reduction, secondary injection molding, coating and bonding with PP, PE, PC, PS, ABS and other matrix materials, and can also be formed separately.
The following points should be paid attention to during TPE molding process:
1. Machine Selection (Injection Molding)
The clamping capacity should be 1.5-3 tons per square inch of projected molding area. A universal screw with a compression ratio of 2: 1-3: 1 can be used. The injection volume should be 25%-75% of the machine chamber capacity. Generally, the longest residence time at the processing temperature should be 8-10 minutes. During injection molding, the injection nozzle should use a small sharp hole to help generate shear heating. The recommended injection nozzle diameter is 1/16(0.0625 "or 1.59mm) to 3/16(0.1875" or 4.76mm).
2. Main Roads
Generally, the standard main runner design with 3C die exit angle can be adopted. soft SBS and SEBS materials cannot use "z" type drawing needle. the reverse buckle finger, reverse taper cold well and slot rod drawing needle are all typical designs.
3. Contraction rate
The shrinkage rate of TPE materials varies according to the formula. In the range of 0.5-2%, the reverse shrinkage rate of melt glue flowing into the mold will be higher. Generally speaking, the shrinkage rate of SBS will be smaller, while the shrinkage rate of SEBS will be slightly larger. The shrinkage rate of materials with higher density will be smaller than that of materials with lower density. In addition, processing conditions such as mold temperature, melt glue temperature, injection speed and product thickness will affect the shrinkage rate of products.
4. Drying
TPE generally has no hygroscopicity and does not need to be dried. However, excessive moisture in the secondary injection molding material or matrix will adversely affect the adhesive force. The secondary injection molding products of E3 series and E4 series of Lihong Company need to be dried. For varieties that need to be dried, the moisture content is required to be less than 0.1%. In order to improve the drying efficiency and produce high-quality workpieces as much as possible, it is strongly recommended to use dehumidification dryer or vacuum dryer. Typical drying conditions are drying at a temperature of 70-80 degrees Celsius for 2-3 hours.
5. Coloring
The color masterbatch carrier should be compatible with the selected TPE material. For SBS substrate TPE, the color based on PS or EVA is more appropriate. However, the TPE of SEBS substrate is more suitable with PE or PP as the base color species, and EVA as the base color species has poor thermal stability. TPE cannot use PVC as the base color species. In order to make the dispersion process easier, the color masterbatch should have lower viscosity (higher melt flow index) than the base TPE composite material. For many kinds of secondary injection molding materials, if polyethylene (PE) carrier is used, the adhesive force may be adversely affected, and if the wax content in color masterbatch is high, the adhesive force may also be reduced.
6. Recycled materials
For many applications using two materials, adding recycled materials is not desirable. Clean TPE recycled materials can be used, with a proportion of up to 20%. Black material can accommodate a higher proportion of recycled materials. Natural products, light-colored or transparent composites may exhibit contamination or discoloration unless properly controlled. If the residence time is too long or the blending ratio of recycled materials is high, the organic pigments of yellow, red, blue and green are more likely to burn or fade. If possible, the proportion of recycled materials should always be consistent.
7. Cleaning machine
SBS has good thermal stability. It is not necessary to clean the barrel after each interruption of raw materials. The barrel can be cleaned by PS. SEBS projectile has very good thermal stability. Even if it is parked for two hours at processing temperature, the materials in the barrel will not degrade. PP or HDPE with low melt flow index will clean the barrel. If PP with lower melt flow index is recommended to clean the barrel when changing different colors during production.
8. Pressure and firing rate
Generally, the required or achieved injection pressure is 200-600 psi. In order to take advantage of the benefits of shear thinning, the injection speed should be adjusted to fill the mold in 1-3 seconds. The medium injection speed is usually used for injecting SBS material, but a higher injection speed must be used for injecting SEBS material in order to prevent the appearance of molten glue from being affected by cooling halfway during mold filling. For secondary injection materials, higher pressure and injection speed can obtain better adhesion.
9 temperature
SBS base material TPE soft segment molecular segment has unsaturated bond, which is easy to be oxidized. High temperature or high shear rate processing produces high possibility of material degradation. To avoid material degradation, processing temperature must be kept at 150-200 degrees Celsius. Melt glue should not stay in gun barrel for too long. If processing temperature exceeds 200 degrees Celsius, melt glue viscosity will increase (SBS molecules may have bridging phenomenon), thus reducing productivity. SEBS elastomer soft segment molecules are saturated and will not be oxidized. High temperature or high shear rate processing does not affect stability. Generally, the processing temperature is 190-230 degrees Celsius, and high shear rate processing is available. If the material degrades due to high temperature or high shear rate processing, the melt viscosity will be reduced due to molecular chain fracture, while for some special specifications of TPE, it can be produced at temperatures of 250 or higher. For secondary injection molding, in order to achieve the best bonding strength, it is often required to use melt temperature higher than normal. In some key applications, this temperature may be close to the upper limit of the TPE processing temperature. In order to shorten the residence time at high temperature, the temperature of the rear section of the injection unit should be reduced as much as possible, and only the last section and the injection nozzle should be kept at a higher processing temperature. For the processing temperature of specific brands, please refer to the physical property table of lihong company. For secondary injection molding, the substrate can be preheated before being embedded into the mold to achieve this. Preheating can also reduce any moisture on the surface of the substrate, thus improving adhesion. For two injection molding, the time interval between the first injection and the second injection should be shortened as much as possible to achieve the best adhesion. However, this time interval must be long enough to enable the matrix to generate the ability to resist injection pressure and not melt or deform again due to the second injection. The total cycle time depends on the cooling of the thickest part of the wall and the time required to rotate the mold and load the insert.
10. Mold temperature
In order to prevent moisture from condensing in the mold and bringing impurities in the water into the mold cavity, the mold temperature in the molding area should be set above a certain temperature. If the workpiece has a very long or thin part and it is impossible to fill it by changing other molding parameters, then the mold temperature may have to be increased. The mold temperature of SBS substrate is generally 10 ~ 40℃ and that of SEBS substrate is 35 ~ 65s℃. Higher mold temperature can promote the flow of sol in the mold, and low pressure injection can also fill the mold. Proper mold temperature can obtain better surface effect.
11. Cooling time
The required cooling time depends on the melt temperature, the wall thickness of the workpiece and the melt index of the material. Harder varieties solidify faster than softer varieties and are easier to demould. Because the thermal conductivity of the plastic matrix is very poor, TPE can only be cooled from one side, so it takes a long time to cool the workpiece after secondary injection molding. With respect to the thickness of each 0.100 "secondary injection layer, the cooling time required for the secondary injection workpiece is generally 20-40 seconds.
12. The factors that affect the adhesive force between TPE material and substrate (PP, ABS, PC, PA) for secondary injection molding may include:
1) matrix type-additive (glass fiber, mineral filling, thermal stabilization, lubricating oil added);
2) type of color masterbatch carrier;
3) moisture content in the secondary injection layer and the matrix;
4) quality and addition ratio of recycled materials;
5) cleanliness, preparation and preheating of the substrate;
6) Processing conditions (melt temperature, pressure, injection speed, cooling time);
7) Design of workpieces and dies (especially cut-off design)
德创化工 (edit：dechuang)
