At Creative Mechanisms the process of prototype injection molding has many steps. It typically starts with the product design. Once the product is designed we then move to the engineering for production phase. The next phase, mold design, is the phase where the mold is drawn and all of the mold details are determined. The prototype injection mold is then built and then the parts are shot in an injection molding machine
Most of the products that we prototype here at Creative Mechanisms get produced by Plastic Injection Molding. Designing parts for injection molding is in our DNA. We do it naturally. There are numerous details to keep in mind when engineering a part for injection molding. During the initial design phase you don't want to get hung up on the details of part design. The goal in this phase is to quickly establishdesign feasibility and get to a proof of function model. There is a fine line here though. You certainly don't want to be developing a product with parts that are so far from being producible that the essence of the design will have to change when you do get to the engineering for production phase. You have to maintain the right balance of speed and production feasibility. This ability comes with experience for the most part. Knowing if the part as designed will be able to be engineered for production becomes almost a sixth sense or an art form to a seasoned engineer.
Once the product design is complete and the function has been proved successfully, it is time to engineer all of the parts of the product for production. The parts have to be engineered for production so that they can be produced in a steel tool. The parts need to be drafted, or tapered, so that they can be easily ejected from the tool. The parts need to have a relatively even wall thickness with no undercuts which would prevent the steel halves from opening or cause the part to be trapped in one-halfof the tool. The gate, the location where the plastic is injected, needs to be determined and the flow of the material into the part analyzed. An experienced engineer will adjust the wall thicknesses of the part to allow the material to flow from the gate location evenly throughout the part without trapping any air. This is another area where a seasoned engineer can call on his experience to look at a part and visualize how the material will flow through it and make adjustments to improve what he sees. Our engineers will also collaborate with our mold design experts and our tool builder at this stage so that everyone is in agreement as to the best part design for moldability.
Science lends a hand here too with Mold Flow Analysis software that can simulate the flow of specific materials through the mold, help determine the gate locations, and show if there will be any gas traps or areas that won't fill properly. Here is a sample of a Mold Flow Analysis of a part:
Now that the part has been designed and engineered, it is time to design and engineer the mold that will create the part. Mold designers have a special ability to see the negative of the part, the cavity that the part creates, in a block of steel. The mold design determines how the tool is built to create the part. There are plates that need to be created with pins that will push the part out of one-half of
the mold. Water lines need to be run in order to cool the part quickly once the cavities have been filled with plastic. If the part has complicated geometry there may need to be additional parts added to the tool, beside the two halves of the steel which will allow an undercut to be molded. These actions are called cams or slides. Each of the numerous parts that comprise the mold is drawn and detailed in the mold design.
The tool build is the process of creating the mold per the mold design. This consists of fabricating every part per the design drawing. Most injection molds are made of steel, but for injection molded prototype molds we often use a very hard 7075 aluminum. Detailed parts with fine texture or complicated surfaces may require an EDM process. This involves building a copper electrode that is used to create aspark that burns the steel into the shape of the electrode. If there is a texture on the surface the part may have to be sent out to a company that applies that texture by way of a chemical etch. A mask is put on the tool that blocks the chemical in certain areas and the chemical is applied, etching the texture into the surface of the steel or aluminum. The final process is polishing all of the non textured surfaces to get the part to have a polished smooth finish. When the mold is complete, it is put into an injection molding machine and shot for the first time. The mold rarely works perfectly the first time it is shot. There are typically adjustments that have to be made to the tool in order to get everything operating properly. These are the small things that you can't see without testing the mold and seeing how it runs. Once the mold is operating properly, adjustments need to be made to the tool to get the part to perform exactly the way you want it to. This phase is called the tool debug phase. People often do not allow enough time or forget to include this phase in the schedule. If the tool maker gives you an 8 week delivery on the tool that does not necessarily mean that you will have usable parts in 8 weeks. The tool maker should always keep to the "steel safe" side of the tolerances of the dimensions. That means you always have enough room to remove a little more steel to make the plastic fit just the way you want it to. It is very difficult to add steel to a tool so you want to intentionally make sure that you can groom the tool by removing a few thousandths of steel to make the part's dimensions fit just the way you want them to with the mating parts that you are producing. When this phase is complete you can run your parts and assemble your product.