* Polyactetal(POM) Mold Deposits

 

1. Outline

Mold deposits(hereinafter refered to as M/D) are mainly white precipitates that occur when decomposition

factors during long-term plastic injection molding are attached to the product in mold. Polyacetal resin in

particular can readily form M/D, and occasionally, attached M/D cannot be removed easily.

M/D can cause injection issues, poor gloss, weight loss, and surface issues of injection parts, in addition to ejection.

Volatile gases from polyacetal resin can be easily deposited on the surface of the mold if the mold temperature is

too low.

 

2. Root causes of M/D

  (1) The polyacetal resin (pellets absorbed moisture)

  (2) Migrated additives from polyacetal resin are left behind on the surface of the injection part 

  (3) Paraformaldehyde[HO(CH₂O)_nH] re-polymerized on the surface of the mold caused by a small amount of

          remaining formaldehyde(HCHO) in the polyacetal resin.

  Of the above causes, paraformaldehyde is the most difficult to remove as the binding force between it and

  the mold surface is very strong.

 

3. The effect of injection parameters for M/D

  (1) Resin temperature : The lower the resin temperature is, the fewer M/D occurrences it has.

       1) Cylinder temperature : 180 ~ 210 ℃

  (2) Injection speed : The faster the injection speed is, the more M/D occurs.

  A too-fast injection speed can stimulate the occurrence of M/D due to adiabatic air compression resulting from

   poor air emissions in the cavity.

 

4. Precaution from mold

5. Mold gas vents

6. Precaution for removing M/D and mold keeping

 

* Please refer to attached file below.