What is the root cause of the burr issue after liquid silicone molding

After the formation of liquid silicone products, there is often a thin edge (also known as a flash or cape) on the edge of the product. Almost everyone who makes silicone products has encountered this problem - slight burrs are acceptable, while severe burrs directly affect the appearance and assembly.

The burrs are not caused by "careless operation", their root lies in mold design and process parameters. Only by understanding the reasons can we solve the problem accordingly.

How are burrs produced?

Liquid silicone gel is a liquid that has good fluidity after injection into the mold before vulcanization. If the parting surface of the mold (the contact surface between the upper and lower molds) is not completely sealed, the rubber material will be squeezed out from the gap, forming burrs.

So the essence of burrs is that the rubber material runs from the mold cavity to the wrong place.

Five main sources

Root cause one: Insufficient fitting accuracy of the parting surface

The upper and lower halves of the mold are joined together, and the contact surface is called the parting surface. If the machining accuracy of the parting surface is not sufficient - poor flatness, micro unevenness on the surface - there will be small gaps after the mold is closed, and the adhesive material will drill in.

The fluidity of liquid silicone is much higher than that of solid silicone, and the requirements for the accuracy of the parting surface are also higher. The precision of the parting surface of solid silicone molds is slightly poor, and the burrs may not be obvious; Using liquid silicone with the same precision may result in severe burrs.

Solution: Improve the machining accuracy of the parting surface, and ensure that the flatness and surface roughness meet the standards of liquid silicone molds. After using the mold for a period of time, the parting surface may wear out and require regular maintenance.

Root cause 2: Insufficient clamping force

When injecting glue, there is pressure inside the mold. If the clamping force of the injection molding machine is not strong enough, the mold will be stretched open by internal pressure, and the rubber material will be squeezed out.

The calculation of clamping force is related to the projected area of the mold cavity and the injection pressure. The larger the mold cavity and the higher the injection pressure, the greater the required clamping force. When selecting equipment, it is necessary to leave a margin and not just enough.

Root cause three: Excessive injection pressure and speed

If the injection pressure is too high and the speed is too fast, the impact force of the rubber material in the mold will be strong, making it easier to rush out from the parting surface.

The faster the liquid silicone injection, the better. The appropriate injection speed should allow the rubber material to smoothly fill the mold cavity, rather than "pouring forcefully". For molds that are prone to burrs, reducing the injection pressure and speed appropriately can usually improve the situation.

Root cause four: Unreasonable exhaust design

As mentioned earlier, the air inside the mold needs to be expelled. If the exhaust groove is opened too deep or too wide, the rubber material will also run out along the exhaust groove, forming burrs.

The depth of the exhaust groove is important - it should allow air to be expelled, but not allow the rubber material to enter. The depth of the exhaust groove for liquid silicone molds is generally between 0.005-0.02mm, depending on the viscosity of the silicone and the injection pressure.

Root cause five: The viscosity of the adhesive material is too low

Liquid silicone has different viscosity grades. Adhesive materials with low viscosity have good fluidity and strong filling ability, but they are also more prone to leakage from gaps.

When choosing rubber materials, the higher the fluidity, the better. If the product structure is complex and the wall is thin, low viscosity adhesive is needed to fill the mold; If the product structure is simple but the burrs are severe, changing to a slightly higher viscosity adhesive may improve the situation.

How to determine where the burr problem lies?

To troubleshoot the issue of burrs, you can follow this order:

First, let's take a look at the location of the burrs - is it all around or only in certain areas? The whole circle has it, mostly due to mold clamping force or parting surface issues; Locally present, possibly due to exhaust slots or local accuracy issues Check the glue injection parameters - try reducing the glue injection pressure and speed. If the burrs are reduced, it indicates that the parameters are too high Check the parting surface of the mold - use a feeler gauge or blue oil to check the fit of the mating surface and see if there are any gaps Check the exhaust groove - measure the depth of the exhaust groove to confirm if it is within the design range If all of the above are normal, consider changing to a higher viscosity adhesive material to try Post processing of burrs

If burrs cannot be completely avoided (which is often the case), post-processing is needed to remove them:

Manual trimming: suitable for small batches, manually trimmed with scissors or blades. Low efficiency and poor consistency. Freezing to remove burrs: suitable for large quantities, cool the product to the temperature where silicone becomes brittle, spray with frozen pellets (dry ice or liquid nitrogen), and the burrs will be shot down due to low temperature brittleness, without damaging the product body. High efficiency and good consistency. Mechanical cutting: Suitable for products with regular parting surfaces, use a cutting die to remove burrs.

The choice of method depends on the production volume, product shape, and appearance requirements. Large scale production usually involves freezing to remove burrs, which strikes a good balance between effectiveness and efficiency.

In summary, burrs are not isolated problems, but reflect the comprehensive results of mold accuracy, equipment capability, process parameters, and material selection. Solving from the source is more cost-effective than spending money on post-processing.