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What are the issues to pay attention to in TPE secondary injection molding?

Hits:Updated:2020-05-26【Print】

TPE encapsulation material has a soft and comfortable touch close to human skin, which is the main point of TPE application in encapsulation. By adjusting and optimizing the process conditions parameters in the encapsulation process, it is believed that there will be more and more functional products made of TPE material to enhance and decorate our lives.

What should be paid attention to in TPE secondary injection molding?

First Possible factors affecting the adhesion between the overmolded TPE material and the substrate:

1.Matrix type-Additives (add glass fiber, mineral filled, heat stabilized, lubricated)

2. Masterbatch carrier type

3. Moisture content in the overmolded layer and substrate

4. Recycled material quality and addition ratio

5. Cleanliness, preparation and preheating of the substrate

6.Processing conditions (melt temperature, pressure, injection speed, cooling time)

7.Design of workpiece and mold

Second, TPE molding processing matters needing attention:

1. Machine selection (injection molding), the clamping capacity should be 1.5-3 tons per square inch of projected molding area, and a universal screw with a compression ratio of 2: 1 to 3: 1 can be used, and the injection volume should be the bore capacity 25% to 75%, usually the longest residence time at the processing temperature should be 8-10 minutes. The injection nozzle should use a small sharp hole during injection to help generate shear heating. Recommended injection nozzle diameter is 1/16 (0.0625 "or 1.59 mm) to 3/16 (0.1875" or 4.76 mm)

2. Mainstream Road. Generally, a standard mainstream design with a 3C die angle can be used. Soft SBS and SEBS materials cannot use "Z" type pull pins. Inverted fingers, reverse tapered cold wells and slot rod pull pins are all typical designs.

3. Shrinkage. The shrinkage of TPE materials varies according to the formula. In the range of (0.5-2%), the shrinkage of the melt flowing into the mold in the reverse direction will be higher. Generally speaking, the shrinkage of SBS is smaller, while the shrinkage of SEBS is smaller. It will be slightly larger. The shrinkage rate of higher density materials is smaller than that of lower density materials. In addition, processing conditions such as mold temperature, melt temperature, injection speed and product thickness will affect product shrinkage.

4. Dry. TPE generally does not have hygroscopicity and does not need to be dried. However, too much moisture in the secondary injection material or matrix will adversely affect the adhesion. Hong Kong Hengsu International Holding co.,Limited E3 series and E4 series secondary injection products need to be carried out. dry. For varieties that need to be dried, the moisture content is required to be 0.1% or less. In order to maximize the drying efficiency and produce high-quality workpieces, it is strongly recommended to use a dehumidifying dryer or a vacuum dryer. Typical drying conditions are drying at a temperature of 70-80 degrees Celsius for 2 to 3 hours.

5. Coloring. The color masterbatch carrier should be compatible with the selected TPE material. For SBS substrate TPE, it is more appropriate to use PS or EVA as the base color; and for SEBS substrate, TPE is to use PE or PP as the base color. Appropriately, EVA-based color species have poor thermal stability. TPE cannot use PVC-based color species. In order to make the dispersion process easier, the color concentrate should have a lower viscosity (higher than the base TPE composite material Melt flow index). For many secondary injection materials, if a polyethylene (PE) carrier is used, it may adversely affect the adhesion. If the wax content in the masterbatch is high, the adhesion may also be reduced.

6. Recycled material. For many applications where two materials are used, the addition of recycled materials is not desirable. Can use clean TPE recycled materials, the proportion of up to 20%. Black materials can accommodate a higher percentage of recycled materials. Natural products, light-colored, or transparent composites may appear contaminated or discolored unless properly controlled. When the residence time is too long or the proportion of recycled materials is high, yellow, red, blue and green organic pigments are more likely to be burned or discolored. If possible, the proportion of recycled material should always be the same.

7. Clean up. SBS has good thermal stability. It is not necessary to clean the barrel after each raw material interruption. PS can be used to clean the barrel. The thermal stability of the SEBS projectile is very good. Even if it is parked for two hours at the processing temperature, The materials in it will not degrade. PP or HDPE with lower melt flow index will be used to clean the barrel. If during production, it is recommended to use a PP with a lower melt flow index to clean the barrel when changing different colors.

8. Pressure and rate of fire. Generally, the required or achieved injection pressure is 200-600 psi. In order to take advantage of shear thinning, the injection speed should be adjusted to fill the mold in 1-3 seconds. SBS materials are usually injected at a medium injection speed, but SEBS materials must be injected at a higher injection speed to avoid the melt from affecting the appearance of the melt halfway due to cooling during mold filling. For secondary injection materials, higher pressure and The rate of fire allows better adhesion.

9. Temperature. The SPE substrate TPE soft segment has unsaturated bonds, which are easily oxidized. The possibility of material degradation is high due to high temperature or high shear rate processing. To avoid material degradation, the processing temperature must be maintained at 150-200 degrees Celsius. The melt glue should not stay at If the inside of the barrel is too long, if the processing temperature exceeds 200C, the melt viscosity will increase (the SBS molecules may be bridging), thus reducing productivity. The soft segment of the SEBS projectile is saturated, and will not be oxidized. High temperature or high shear rate processing does not affect stability. Generally, the processing temperature is between 190-230 degrees Celsius. High shear rate processing is possible. If the material is subjected to high temperature or high shear, Degradation at processing rate, melt viscosity will be reduced due to molecular chain breaks, and for some special specifications of TPE can be used to produce 250 or higher temperatures. For secondary injection molding, in order to achieve the best bond strength, higher than normal melt temperatures are often required. In some critical applications, this temperature can approach 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 specific brand processing temperature, please refer to the physical property table of Hong Kong Hengsu International Holding co.,Limited
 Company. For overmolding, this can be achieved by preheating the substrate before it is inserted into the mold. Preheating can also reduce any moisture on the surface of the substrate, thereby improving adhesion. For two injection moldings, the time interval between the first and second injection should be as short as possible to achieve the best adhesion. However, this time interval must be long enough to enable the base body to resist the injection pressure, and not to melt or deform again due to the second injection. The total cycle time depends on the cooling of the thickest part of the wall thickness and the time required to rotate the mold and load the insert.

10. Mold temperature. To prevent moisture from condensing in the mold and bringing impurities from the water into the cavity, the mold temperature in the molding area should be set above the dew point temperature. If the workpiece has long or thin sections and it is not possible to achieve filling by changing other molding parameters, then the mold temperature may have to be increased. The mold temperature of the SBS substrate is generally 10 to 40 degrees Celsius, and the mold temperature of the SEBS substrate is 35 to 65S degrees Celsius. A higher mold temperature can promote the flow of the sol in the mold. Low-pressure injection can also fill the mold. An appropriate mold temperature can be obtained. 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 set faster than soft varieties and are easier to demold. Due to the poor thermal conductivity of the plastic matrix, TPE can only be cooled from one side, so the over-injected workpiece requires a longer time to cool. Relative to the thickness of the 0.100 "overmolded layer, the cooling time required for the overmolded workpiece is generally 20-40 seconds.

Pay attention to the following parameters and conditions for TPE rubber overmolding:

1. The compatibility of TPE and hard rubber structural parts needs to be matched, and the molecular solubility is close, so the compatibility of molecules is better;

2. Try to avoid sharp corners in the design to ensure that the TPE is in good contact with the hard rubber parts and improve the bonding effect;

3, through appropriate exhaust to avoid gas in the mold cavity;

4. Balance the thickness of TPE with the expected touch;

5. Maintain the temperature of the TPE melt to ensure the bonding effect;


6. The TPE material needs to be reprocessed for baking molding to reduce the water ripple on the surface of the product and obtain the uniform surface color.

7. The carrier resin of the selected masterbatch is compatible with TPE and structural materials;

8. The smooth surface should be specially treated, the purpose is to increase the contact surface between soft rubber and hard rubber to enhance the bonding effect;

9. TPE should have good fluidity. Because the thickness and size ratio of the TPE coating layer is small, TPE usually needs to flow through a longer path and thin wall to fill the mold.

10, TPE flow length / product thickness ratio is less than 150: 1;

11. Use good adhesive.
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