The TPE adhesive material has a soft and comfortable touch that is close to the human skin, which is the essence of TPE's application in adhesive packaging. By adjusting and optimizing the process parameters during the adhesive coating process, we believe that more and more TPE functional products will be added to enhance and enhance our lives.

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

1、 Possible factors affecting the adhesion between TPE material and substrate during secondary injection molding:

1. Matrix type - additive (with glass fiber, mineral filling, heat stabilized treatment, and added lubricating oil)

2. Type of color masterbatch carrier

3. Moisture content in the secondary injection layer and 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 molds

2、 Precautions for TPE molding and processing:

1. Machine selection (injection molding), with a locking capacity of 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, and the injection amount should be 25-75% of the machine chamber capacity. Typically, the longest dwell time at processing temperature should be 8-10 minutes. During injection molding, a small sharp hole should be used for the injection nozzle to aid in shear heating. The recommended injection nozzle diameter is 1/16 (0.0625 "or 1.59 mm) to 3/16 (0.1875" or 4.76 mm)

2. Mainstream channel. Generally, a standard sprue design with a 3C die angle can be used. Soft SBS and SEBS materials cannot be used with "Z" type pull pins. Reverse buckle fingers, reverse conical cold wells, and narrow groove rod pull pins are typical designs.

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

4. Drying. TPE generally does not have hygroscopicity and does not need to be dried. However, excessive moisture in the secondary injection materials or matrix can have a negative impact on the bonding force. Therefore, the E3 and E4 series of secondary injection products of Hong Kong Hengsu International Holdings Limited need to be dried. For varieties that require drying, the moisture content is required to be below 0.1%. In order to maximize drying efficiency and produce high-quality workpieces, it is strongly recommended to use a dehumidifier or vacuum dryer. The typical drying condition is to dry for 2-3 hours at a temperature of 70-80 degrees Celsius.

5. Coloring. The color masterbatch carrier should be compatible with the selected TPE material, and for SBS substrate TPE, it is more appropriate to use PS or EVA as the base color; For the TPE of SEBS substrate, it is more appropriate to use PE or PP as the base color, while EVA as the base color has poor thermal stability. Therefore, PVC as the base color cannot be used for TPE. In order to facilitate the dispersion process, the color masterbatch should have a lower viscosity (higher melt flow index) than the base TPE composite material. For many types of secondary injection molding materials, the use of polyethylene (PE) carriers may have an adverse impact on the bonding force. If the wax content in the color masterbatch is high, it may also reduce the bonding force.

6. Recycled materials. For many applications that use two materials, adding recycled materials is not advisable. Clean TPE recycled materials can be utilized, with a maximum proportion of up to 20%. Black materials can accommodate a high proportion of recycled materials. Natural products, light colored or transparent composite materials may exhibit contamination or discoloration unless properly controlled. When the residence time is too long or the mixing ratio of recycled materials is high, yellow, red, blue, and green organic pigments are more likely to be burned or faded. If possible, the proportion of recycled materials should always be consistent.

7. Clean the machine. SBS has good thermal stability, and it is not necessary to clean the barrel after every interruption of raw materials. PS can be used to clean the barrel. SEBS shells have excellent thermal stability, and even if left at processing temperature for two hours, the material inside the barrel will not degrade. PP or HDPE with a low melt flow index can be used to clean the barrel. If different colors are changed during the production process, it is recommended to use PP with a lower melt flow index to clean the barrel.

8. Pressure and rate of fire. Usually, the required or achievable injection pressure is 200-600 psi. To take advantage of the benefits of shear thinning, the injection molding speed should be adjusted to fill the mold within 1-3 seconds. Injecting SBS materials usually uses a medium injection speed, but injecting SEBS materials requires a higher injection speed to avoid the appearance of the melt during the filling process due to cooling. For secondary injection materials, higher pressure and injection speed can achieve better adhesion.

9. Temperature. The TPE soft segment of SBS substrate has unsaturated bonds, which are prone to oxidation. High temperature or high shear rate processing has a high possibility of material degradation. To avoid material degradation, the processing temperature must be maintained at 150-200 degrees Celsius, and the melt adhesive should not stay in the barrel for too long. If the processing temperature exceeds 200C, the viscosity of the melt adhesive will increase (SBS molecules may have bridging phenomenon), thus reducing productivity. The soft segment molecules of SEBS projectile are 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, and high shear rate processing can be used. If the material degrades due to high temperature or high shear rate processing, the viscosity of the melt will decrease due to molecular chain breakage. For some special specifications of TPE, temperatures of 250 or even higher will be used for production. For secondary injection molding, in order to achieve the best adhesive strength, it is often required to use a melt temperature higher than normal. In some critical applications, this temperature can be close to the upper limit of the processing temperature of the TPE. In order to shorten the residence time at high temperatures, the temperature of the rear section of the injection molding unit should be reduced as much as possible, and only the last section and injection nozzle should be kept at higher processing temperature conditions. Please refer to the physical properties table of Hong Kong Hengsu International Holdings Limited for the specific processing temperature of the brand. For secondary injection molding, this can be achieved by preheating the substrate before embedding it into the mold. Preheating can also reduce any moisture on the substrate surface, thereby improving adhesion. For two injection molding, the time interval between the first and second injection molding should be minimized as much as possible to achieve optimal adhesion. However, this time interval must be long enough to enable the matrix to develop the ability to resist injection pressure, and not to melt or deform again due to the second injection molding. 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. To prevent moisture from condensing inside the mold and bringing impurities into the mold cavity, the mold temperature in the molding area should be set above the dew point temperature. If the workpiece has very long or thin parts and it is impossible to achieve filling by changing other molding parameters, then it may be necessary to increase the mold temperature. The mold temperature of SBS substrate is generally 10-40 degrees Celsius, while the mold temperature of SEBS substrate is 35-65S degrees Celsius. Higher mold temperatures can promote the flow of sol in the mold, and low-pressure injection can also fill the mold. Appropriate mold temperatures can achieve better surface effects.

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 and are easier to demould than softer varieties. Due to the poor thermal conductivity of the plastic matrix, TPE can only be cooled from one side, so the workpiece for secondary injection molding needs a longer time for cooling. Compared to the thickness of every 0.100 "second injection layer, the cooling time required for the second injection of the workpiece is generally 20-40 seconds.

The secondary injection molding process of TPE encapsulation requires attention to the following parameter conditions:

1. The compatibility between TPE and hard adhesive structural components needs to be matched, and the molecular solubility is close to each other in order to achieve better molecular compatibility;

2. In the design, it is necessary to avoid sharp corners as much as possible to ensure excellent contact between TPE and hard adhesive parts and improve the bonding effect;

3. Avoid gas retention in the mold cavity through appropriate exhaust;

4. Balance the thickness of TPE with the expected tactile sensation;

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

6. TPE material encapsulation requires drying and reprocessing to reduce surface water marks and achieve uniform surface color;

7. The selected color masterbatch has a carrier resin that is compatible with TPE and structural component materials;

8. For smooth surfaces, special treatment is required to increase the bonding contact surface between soft and hard adhesives and enhance the bonding effect;

9. TPE should have good flowability, as the thickness to size ratio of TPE adhesive layer is very small, TPE usually needs to flow through a longer path and thin-walled area to fill the mold.

10. The flow length/product thickness ratio of TPE is less than 150:1;

11. Use good adhesive.