We are talking about a thick part, made of nylon with glass fiber. Once injected, in the surface it seems to be well compacted but in the interior there are "bubbles".

At first sight I tought they were air traps, but reading injection manuals and courses, now, I think they are CONTRACTION VOIDS.

If anyone know how to improve this problem, it would be very good to me. It's a hot runner injection part. It's injected with 6 gates in the core (I mean the thickest part of the piece), but I'm not able to eliminate these voids.

To eliminate voids in any hydroscopic material:-

Make sure the granules are dry.

Increase effective pack time and pressure. This is achieved by increased hold time and a larger gate. It is the minimum dimension of the gate that is critical as that is what controls freeze off time. There is no point to holding up once any point in the feed system is frozen off.

If you are using nylon 6.6, nylon 6 may provide a better moulding with regards to voids.

Harry is correct re aiming the stream from the gate into a wall so it does not jet or stream from the gate.

A very hot mould will produce sinks rather than voids if it is under packed as the surface stays soft longer, thus allowing it to suck in as a result of contraction.

We need details of runner, nozzle and gates and heat system to the hot runner to form an informed opinion.

Are there also voids in the runners.

45mm sounds unbelievably thick and would indicate you need a gate with a minimum of at least 15mm thickness on the smallest dimension.

Sometimes an extrusion grade base resin helps with voids as you can pack harder without flash.

Plenty of clamp pressure (at least 3 tonnes per square inch of projected area is required.

A good ring check valve that holds a cushion is required. When moulding GF nylons, these require regular replacement.

The barrel bore must also be in good condition in the ring check seal area for the ring check to seal.

If the hot runner extends to a heated gate, the heat must be sufficient to avoid freezing before the part is fully packed.

At 45mm section thickness, I would expect a hold time of several minutes is required.

You can check effective hold time by starting low, and progressively increasing time and weighing parts until it is seen that the part weight does not increase with increased hold time.

The hold pressure should be such that it is just short of flashing the mould or is just showing the slightest signs of flash.

Blowing agents can also remove contraction voids but replace them with thousands of small well dispersed bubbles. This foam will be stronger than contraction voids. The aesthetics of foam vs void is subjective.

45mm really is unbelievable. Can you post a dimensioned drawing or sketch with gate and runner and nozzle positions and sizes shown.To eliminate voids in any hydroscopic material:-

Make sure the granules are dry.

Increase effective pack time and pressure. This is achieved by increased hold time and a larger gate. It is the minimum dimension of the gate that is critical as that is what controls freeze off time. There is no point to holding up once any point in the feed system is frozen off.

If you are using nylon 6.6, nylon 6 may provide a better moulding with regards to voids.

Harry is correct re aiming the stream from the gate into a wall so it does not jet or stream from the gate.

A very hot mould will produce sinks rather than voids if it is under packed as the surface stays soft longer, thus allowing it to suck in as a result of contraction.

We need details of runner, nozzle and gates and heat system to the hot runner to form an informed opinion.

Are there also voids in the runners.

45mm sounds unbelievably thick and would indicate you need a gate with a minimum of at least 15mm thickness on the smallest dimension.

Sometimes an extrusion grade base resin helps with voids as you can pack harder without flash.

Plenty of clamp pressure (at least 3 tonnes per square inch of projected area is required.

A good ring check valve that holds a cushion is required. When moulding GF nylons, these require regular replacement.

The barrel bore must also be in good condition in the ring check seal area for the ring check to seal.

If the hot runner extends to a heated gate, the heat must be sufficient to avoid freezing before the part is fully packed.

At 45mm section thickness, I would expect a hold time of several minutes is required.

You can check effective hold time by starting low, and progressively increasing time and weighing parts until it is seen that the part weight does not increase with increased hold time.

The hold pressure should be such that it is just short of flashing the mould or is just showing the slightest signs of flash.

Blowing agents can also remove contraction voids but replace them with thousands of small well dispersed bubbles. This foam will be stronger than contraction voids. The aesthetics of foam vs void is subjective.

45mm really is unbelievable. Can you post a dimensioned drawing or sketch with gate and runner and nozzle positions and sizes shown.

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