[Verification Experiment] Measurement of Gas Volume within Moulds Using the Water Displacement Method

Introduction

In recent years, there has been a trend towards increasing the use of market-recovered recycled materials due to environmental considerations.

However, recycled materials tend to release more decomposition gases than virgin materials during molding, due to their thermal history and the effects of impurities. Consequently, molding recycled materials using conventional mold designs makes them more prone to molding defects such as gas burn and short shots.

This experiment verified whether differences could be identified by capturing gases using the water displacement method to determine the extent of gas generation during molding with virgin PP and recycled PP materials respectively.

[Verification Method]

・Molded parts

Basic wall thickness 2mm, flat plate shape. Some sections have a reduced wall thickness of 0.5mm.

Volume including gate: 10.6cm³ (10.6cc)

・Materials

　Nippon Polypropylene Co., Ltd. BC03B (PP) : Referred to as virgin PP in this article

　Recycled PP from household appliances + vehicles : Referred to as market-recycled PP02 in this article

　Recycled PP from vehicles : Referred to as market-recycled PP04 in this article

・Moulding conditions

Conditions were established to prevent molding defects with virgin PP, determining parameters such as injection speed and holding pressure.

・Mold

Three gas vent inserts (SG-WIND) are installed on the cavity side.

These three locations each connect to exhaust ports outside the mold.

The exhaust ports outside the mold were connected to hoses, allowing the gas emitted from the hoses to be collected within the cylinder.

Gas from ten consecutive shots was collected and their volumes compared.

【Measurement Results】

Results of gas volume comparison

The highest volume was observed in market-recycled PP02, followed by market-recycled PP04.

While the composition of the generated gases is currently under investigation, market-recycled PP tends to produce more gas than virgin PP.

The standard volume should have been 10.6 cc × 10 shots = 106 cc,

but both virgin PP and market-recycled PP yielded volumes below this standard.

[Gas Generation Process]

The video below shows the actual gas collection process. Gas generation is linked to the timing of the molding cycle (injection).

【Considerations】

・The collected gas volume is smaller than the standard volume (106cc)

Gas may have escaped due to stagnation within the hose, the mold parting line (PL), sensor holes, etc. However, as all three materials were molded on the same day under identical conditions, the amount of gas escape is considered to be the same for each.

・Can similar results be obtained with materials other than PP?

➡This experiment used PP. The effect on materials prone to gas burn, such as virgin PA6 or PPS, will be verified in a subsequent test using different materials.

・Is there a difference in gas capture volume between nested and non-nested configurations?

➡SG-WIND features slit-like holes, providing excellent gas venting performance. However, in the case of porous venting structures, reduced venting performance may result in lower gas capture compared to SG-WIND. We will verify this by changing the nested configuration in the next experiment.