Venting design of molds: The hidden causes of air bubbles and material shortages

Bubbles and material shortages (trapped gas) account for a significant proportion of defective silicone components. Tracing back to the source, most of it is not a parameter issue, but rather a failure to properly exhaust the mold. Exhaust is an inconspicuous part of mold design - without it, the mold can still be opened and the product can still be produced, but the defect rate cannot be reduced. This article explains the issue of exhaust thoroughly.

Why do silicone molds require special exhaust

The flow of silicone adhesive (especially liquid silicone) in the mold cavity has a characteristic of low viscosity and fast filling. The rubber material in the mold cavity is "pushed by air", and the air is pushed to the final filling position. If it cannot be discharged, it will be trapped in the mold cavity - forming bubbles, material shortage, and burning (the air is compressed and rapidly heated).

Solid silicone compression molding also has an additional source: the air between the rubber material and the mold cavity, as well as the gases that may be generated during vulcanization reactions, all need to be discharged through exhaust channels.

Where to turn on the exhaust

The exhaust groove is located at the last position where the rubber material reaches. Where is the final destination? There are two ways to determine:

Moldflow and other simulation software for filling liquid silicone gel can predict the fusion line and trapped gas position, and products with high accuracy are worth making.

Trial mold verification - The trapped gas position, material shortage position, and fusion line position on the trial mold are the places where the exhaust should be turned on. The first version of the mold leaves room for repairing the exhaust groove (leaving processing capacity on the parting surface), and after trial molding, it is a common practice in the industry to make up for the actual trapped gas position.

The size of the exhaust groove is important

The exhaust groove is too shallow - the air cannot be completely discharged, and the problem persists. The exhaust groove is too deep - the rubber material enters the exhaust groove, forming burrs and blocking the passage. The exhaust groove needs to be cleaned regularly.

The depth of the exhaust groove of liquid silicone molds is usually in the range of 0.005-0.02mm - thinner than a strand of hair, and if it is deeper, it can be drilled with glue. The exhaust of solid-state molding can be slightly deeper. The width and number of exhaust slots are designed according to the area of the trapped gas zone, and the edges are connected to the outside of the mold cavity.

Vacuum exhaust

For products with high requirements (transparent parts, appearance parts, precision seals), relying solely on exhaust slots is not enough. Vacuum molds are used: after closing the mold, the air in the mold cavity is first evacuated, and then the rubber material is injected. When the vacuum degree reaches a negative pressure state, the poor trapped gas is basically eliminated.

The cost of vacuum molds: the mold needs to have a sealed structure and the machine needs to be equipped with a vacuum system, which increases the mold cost by 20-30%. Whether it's worth it or not depends on the product's defect losses - the 30% gas trapped scrap rate for transparent parts and the additional cost of two to three percent for molds - it's clear at a glance.

Three common misconceptions related to exhaust

Treat poor exhaust as a parameter issue. When encountering bubbles on site, the first reaction is to adjust the temperature and pressure - the exhaust groove is blocked or not fully opened, and no matter how the parameters are adjusted, it is only a relief. First check the exhaust, then adjust the parameters, reversing the order is a waste of time.

The exhaust duct is not cleaned. The exhaust groove is a consumable function: the burrs and debris that the rubber material drills into accumulate inside, and the more it gets clogged, the more it becomes dead. Cleaning the exhaust duct during daily maintenance (as discussed in the previous section on mold maintenance) is a necessary task.

Repairing the exhaust groove has been a last resort for repairing the mold. Some factories feel that opening the exhaust groove requires moving the parting surface, fearing that it will affect the accuracy of the mold. They can drag it if possible, and use parameter selection and strengthened quality inspection to handle it. The cost of having a high defect rate by one or two points far exceeds the cost of repairing the exhaust system in the long run.

Give a hint to one party

You don't need to know where the exhaust groove is located, but you should be able to ask during quotation and factory inspection: How to handle the exhaust of this product's mold? Do you want to evacuate? Experienced factories will directly tell you: where is the structure prone to gas entrapment, how do you plan to exhaust, and whether the first version of the mold has the amount of exhaust left for repair. For those who cannot answer, or those who pat their chest and say 'no problem, definitely no bubbles', their experience points will be discounted.

The bad accounts of bubbles are ultimately calculated in terms of yield and delivery time. The exhaust investment during the mold stage is the most direct return among all mold investments.