Mold and product design differences

Compared with thermoplastic materials, LSR materials have many differences in mold and product design. Many of the guidelines for thermoplastic materials are not applicable to LSR doors.

This is particularly important in the application of LSR secondary forming. In general, the secondary forming is to form LSR on thermoplastic substrate, or one grade of LSR on another LSR. These products usually require many small adjustments to allow for more user-friendly mold design and molding processes. In the design process, it is necessary to ensure that there is proper shut-off valve, proper ventilation, proper gate position, and most importantly, proper matching base plate and secondary molding material. The thermoplastic base resin is required to withstand half the mold temperature of LSR. If the thermoplastic starts to deform below the LSR curing temperature, the project will fail. Another issue that needs to be discussed early is the bonding method of two materials. Are we using self bonding LSR or mechanical keys or both? These items will affect the design of the mold. Once the product has passed all approvals, it is difficult to change, so these factors need to be discussed as early as possible.

Cold deck and hot runner

Both thermoplastic and LSR can be designed by runner free die, but there are significant differences in design. Thermoplastics require a hot runner to maintain the temperature and viscosity of the material between shots. LSR, on the other hand, requires a cooling channel to prevent the material from solidifying before it reaches the heated cavity. (once the two reactive components are mixed, the LSR can start curing at room temperature.) From the point of view of keeping the heat separation of material conveying system and die body, the engineering concepts of the two systems are similar.

When using the LSR, the barrel and the cooling table (runner system) on the molding machine use a water mixture to keep cool. The typical temperature range for these components is 15 to 38 degrees Celsius. The most critical area is the contact point between the cold plate nozzle and the mold, where a needle valve gate is usually used. If the area is not designed properly, molding defects can easily occur, such as the appearance of "wet" Gates due to the area being too cold, or expansion gates due to the area being too hot. With the increase of die size and the number of cavities, the temperature sensitivity and exhaust characteristics of LSR make the challenge more difficult. That's why most LSR mold makers and mold makers don't like more than 64 cavities.

The economic motivation of using LSR mold without runner is even greater than that of using thermoplastic, for several reasons.

First, because solidified LSR waste cannot be reused.

Second, because LSR is often expensive material.

Third, the curing cycle of LSR is usually longer than that of thermoplastic. When forming small LSR parts, the runner may be thicker than the part, so after the part is ready for demoulding, the mold worker must wait for the runner to solidify. In addition, hot runner LSR systems are more difficult to automate because they require the removal of formed parts and runner scrap. The waste must also be collected and eventually removed from the forming area.