As a rule, an injection mold should be made for only one type of product.
The main reason for this is that the different shapes may have different molding characteristics and the slower running shape will control the molding cycle and production output. However, products that require the same quantities of parts and are made out of the same color to make up one finished product and the quantities are relatively low, may be produced in a family mold. The initial injection mold costs are very high but the payback on the injection molding process is usually short. Making one mold to produce four different parts for a product is a lot cheaper than making four different molds.
The injection molding process is used to produce parts as fast as possible. The cycle time is usually dictated by part thickness and determines the length of the cooling time. In a four cavity mold, if one of the parts is thicker than the other three, then the cooling time is longer then what is needed for those three parts. One of the critical parts of injection molding is controlling the part shrinkage. Extra time and work is needed for adjustments to achieve the correct part dimensions in a family mold.
Toth Mold/Die Inc. has made numerous family molds and has learned through a great deal of experience what features need special attention in a family mold. We have built a series of family molds for projects consisting of up to twenty-two parts to make up one finished product.
Designing the cavity layout to achieve the optimum runner and gating design is done by having years of experience in the injection molding industry. Listed below are some of the advantages and disadvantages of family molds used in injection molding:
- Cost savings on one mold versus multiple molds.
- Parts are produced in the same cycle and will have the same shrink.
- Part price is cheaper than running multiple molds.
- Increased scrap rate – if one part is scrapped out in a four cavity mold then the other three are scrap because of the uneven total for the production run.
- Cycle time increased due to different part thicknesses.
- Extra design cost.
- Increased part price due to increased scrap rate.
My final take on using family molds for injection molding is, if you are going to have one made, have it made by a shop that has the experience designing and building them.