Cavity

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The term cav­i­ty is de­rived from the Latin cavum (cav­i­ty). In mold and die mak­ing, cav­i­ty refers to the free or hol­low space be­tween the low­er and up­per part of the form­ing mold in­to which the liq­uid or flow­able ma­te­r­i­al (melt) flows or is pressed. The cav­i­ty is al­so re­ferred to as a mold cav­i­ty. To en­sure that di­men­sion­al­ly ac­cu­rate mold­ed parts can be pro­duced, the ma­te­r­i­al shrink­age fac­tor must be in­clud­ed in the de­sign of the cav­i­ty. The cav­i­ty di­men­sion must be used when cal­cu­lat­ing warpage as part of the in­jec­tion mold­ing simulation.

Number of cavities

The num­ber of cav­i­ties in the mold can vary.

• One cav­i­ty = 1‑cavity mold
• Two cav­i­ties = 2‑cavity mold
• Four cav­i­ties = 4‑cavity mold
• Eight cav­i­ties = 8‑cavity mold

A dis­tinc­tion must be made as to whether the cav­i­ties are al­ways the same or have dif­fer­ent shapes.

Positioning of the cavities in the mold

In a 1‑cavity mold, the cav­i­ty is usu­al­ly planned in the cen­ter of the mold. The sprue is placed di­rect­ly on the mold­ed part. Oth­er place­ments are pos­si­ble, but re­sult in a larg­er mold.

From a 2‑cavity mold, it is im­por­tant when po­si­tion­ing the cav­i­ties that the melt has the same path from the sprue to all cav­i­ties. Oth­er­wise, the cav­i­ties will fill with the melt in an off­set man­ner. This can re­sult in over­mold­ing of the cav­i­ties that are filled too ear­ly, or the cav­i­ties that are filled too late are not com­plete­ly filled.

Why are more cavities planned in a mold?

The more cav­i­ties a mold has, the more in­di­vid­ual parts can be pro­duced in one in­jec­tion mold­ing cy­cle. This en­ables a rapid re­duc­tion in the cost per mold­ed part. On the oth­er hand, there is the greater ef­fort to de­sign the mold and is al­so lim­it­ed by the ma­chine size. 

Shrinkage is always

Ma­te­r­i­al shrink­age must be con­sid­ered and planned for when cre­at­ing the cav­i­ty. In or­der to in­sert the plas­tic in­to the cav­i­ty with an ex­act fit, it is melt­ed. This al­lows the melt to fit eas­i­ly in­to any mold. As it cools in the tar­get lo­ca­tion, the part re­tracts min­i­mal­ly, mak­ing it small­er. This process is called shrink­age and is dif­fer­ent for each material.

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