10,000-shot continuous molding experiment What causes mold fouling and what can be done about it?

Introduction.

In injection molding, continuous molding is essential for stable mass production of products. However, ‘mold surface contamination’, which occurs gradually over a series of consecutive shots, is an important problem that affects production quality.

In this issue, based on the results of 10,000 shots of continuous molding, the main factors and mechanisms of mold contamination and the countermeasures that can actually be taken on site are introduced.

Outline of the experiment

• Type of machine used: 150-tonne class general-purpose injection moulding machine

  
  

  Moulding conditions

  
  

  Resin: PA6 (standard grade)

  Injection temperature: 250°C

  Mould temperature: 80°C

  Cycle time: 30 sec.

  Mould release agent used: none

  
  

  Evaluated at: parting line, cavity surface, contamination around gas vents.

  

  
  

1. main causes of mold fouling

(1) Gas generation and its adhesion.

Thermoplastic resins release minute quantities of gases (outgassing) at high temperatures and pressures during molding. resins with many additives, such as PA6, tend to decompose and volatilise additive components, which do not escape through the vent and parting surface and adhere to the mold surface, resulting in brown to black staining. .

2) Deposition of additives (bleed-out)

PA6 resin contains various additives such as plasticisers, lubricants and flame retardants. Over time, some of these additives seep out onto the resin surface and are pressed against the mold side by the pressure of injection, resulting in the accumulation of a faint oil film-like stain.

(3) Inadequate mold vent design.

Insufficient or blocked venting (air venting) allows gases to remain trapped in the mold, causing scorching and contamination. The vent width, depth and arrangement must be optimised for each resin.

(4) Harsh molding conditions.

If the temperature setting is too high or the residence time of the resin is too long, the resin will partially thermally decompose. As a result, carbides (charring) and fines are generated, which adhere to the cavity surface.

In the case of PA6, drying is recommended before molding, but if the drying temperature is too high, the material turns brown, as if burnt, so it can be said to be a difficult material to mold. ➡Here.

2. gas burn marks

At the end of the 10,000 shots, dirt was particularly concentrated around the vent and parting line. As you can see in the image above, you can see blackened burn marks in the gas venting path. If these burn marks accumulate during continuous molding, the gas vent will no longer function and gas burns will eventually appear on the molded product. The need for regular mold cleaning was strongly felt.

3. measures to prevent mold contamination

Conclusion.

Mould surface contamination causes a variety of adverse effects, such as reduced molding quality, deteriorated mold release and increased maintenance man-hours. However, it can be adequately controlled and prevented by understanding the characteristics of the resin, optimising injection conditions and carrying out regular mold management.

The 10,000-shot continuous molding experiment this time allowed us to visually check when, where and what kind of contamination occurs, and we felt that this would be useful for establishing maintenance standards at the site.

Please try to accumulate data on trends and countermeasures against contamination at your own molding site, and aim for **"trouble-preventive production ’**!