The 5 Critical Factors to produce a Successful Injection Molded Product - 7/1/2012

Successful molding is defined as when the molded part meets all the quality requirements and satisfies the needs of the end user. However, for the molder the definition also includes two more requirements.
These are the consistency and repeatability of the molding process. To successfully mold parts there are five critical factors that need to be carefully selected as mentioned below. (See figure).

  1. The Part Design
  2. The Material Selection
  3. The Mold Design and Construction
  4. The Molding Machine
  5. The Molding Process

Successful applications of scientific principles and techniques to each of these factors will lead to the overall success of the project. Each of these with some of their requirements are described below.
1. Part Design: The part must be designed for molding. Rules for plastic part design are considerably different form metal part design because of the inherent nature of the plastic. For example, thick sections cannot be present in a plastic part to avoid sink, or all corners must have a radius to avoid stress concentration and premature failure. Parts must also be designed for assembly. With the need for plastics recycling, the concept and requirement of Design for Disassembly is getting more common.
2. Material Selection: Based on the part design and the part performance requirements, the plastic material must be selected. Material selection must be also selected based on the tolerance requirement of the part. Tight tolerance parts must be molded with low shrink materials unless they are micromolded products or parts that are dimensionally small (around half inch in their maximum dimension). If a thick section must be present, a filled material may need to be selected or if there is a sliding surface then a additive to reduce the coefficient of friction needs to be added to the plastic.
Material selection should be done when the basic part design is done and the requirements are detailed. The material manufacturers are the best source of information.
3. Mold Design and Construction: The mold must be designed and constructed such that the mold is robust enough to withstand the molding process and the plastic material. For example, during the molding process, the mold can be subjected to high mechanical stresses especially during the plastic injection and the packing. All these material specific factors such as shrinkages must be considered. The required amount of parts from the mold is another factor that will dictate the actual material of construction since the wear on the components must be considered.
4. Machine Selection: Selecting the right machine for the mold should be done once the mold design is complete or should be done concurrently during the mold construction stage. The machine plays a very important role in the stability of the molding process. For example, machines with large shot sizes must not be used to mold small shots since the part quality consistency suffers. Vice versa, using a large percentage of the shot size can give rise to problems with melt homogeneity and therefore issues with fill and dimensions.
5. Molding Process: Process optimization is the last step and is often not done correctly or is even completely ignored. Injecting plastic into a mold without any scientific basis leads to scrap, inconsistent quality in the parts and inefficiencies in the production process. If the above four factors and activities are not properly selected or performed, process optimization can be a challenge.
Since there are various departments involved in the production of the molded part and therefore regular meetings between the different departments must be held. Each department will have specific knowledge of the selection process and can then not just contribute to the process but more importantly predict issues when the mold comes over to their department.


Figure 1.