Injection molding is a very technical field that involves years of gathering experience, an aptitude for invention or improvisation, and a knack for the whole injection molding process. It takes a lot of trial and error, mistakes, and happy accidents to properly learn the ins and outs of injection molding. But given the fact that we at Creative Mechanisms are adept at the processes, we can help you stay on the right track with an injection molding project – without having to suffer the pitfalls of inexperience.
Here are four major design choices you need to make if you are planning on producing injection molded parts.
Mold design parameters: Designing the mold for injection molding is the most important part of a molding project and also very time consuming because a lot of thought goes into it. Getting things right the first time is critical to the success of the injection molding venture. There are many design parameters that need to be considered in order to achieve the best possible injection molded product and some of the most important are draft and cavity bends.
More complex molds have a lot of intricate cavities that the plastic must flow through in order to completely fill the cavities. These turns, especially when they are very acute, will result in molded-in stresses from the atomic structural level and also from the part cooling differently from the outside in. Designing the mold to have smoother and gentler turns will avoid sudden changes in flow direction, preventing these stresses from building up in the part.
Draft (or, more accurately, draft angle) is a tapering of the part which serves the purpose of making it easier to eject the part when the mold opens. Draft sometimes is not compatible with the part design, either from an aesthetic point of view or a functional one, but the smallest draft is preferable to no draft at all. The draft required also varies with the surface finish of the tooling or the texture requirement of the part. A smoother tooling will require less draft than a regular one.
Part design parameters: Designing a part for injection molding requires a mixture of manufacturability, utility, and elegance. Compromises are made between factors such as reproducibility, cost, strength, functionality, and aesthetics. A balance will be sought that ensures none of these factors fall below the minimum requirements. From the point of view of injection molding, design parameters such as the uniformity of the part wall thickness and the part wall thickness itself are the most crucial elements. A uniform part thickness will ensure the minimization of such defects as warpage and twists or cracks due to uneven wall thickness, which occurs as a result of the part cooling at different rates. The part thickness is a consideration mostly born out of cost, strength of the part, and the speed of production. Thicker parts cost more because they consume more plastic, whereas thinner parts cool faster and can be ejected earlier, saving crucial time in the production process. Despite the extra cost, however, thicker parts are generally stronger.
Gate design and location: There are generally two types of gates in injection molding – manually trimmed gates and automatically trimmed gates. As the names imply, the parts are removed either manually or automatically from the mold, and each type is used for certain reasons. Gates can be manually sheared if they are too cumbersome to use an automatically incorporated de-gating shearer. Automatically trimmed gates are used to reduce downtime from manual gate removal, which achieves an even production rate.
Gate locations are generally close to the portions of the part that are thickest and from where the melt flows to the thinner areas of the mold. This ensures a minimization of part defects such as sink marks, voids, and insufficient part packing. In some cases, it may be necessary to incorporate a secondary gate to ensure the proper filling of the cavities. Tapering the wall thickness so that it is thickest at the gate location and then thinner down along the cavity will ensure a good flow.
Molding conditions and parameters: These are readily adjustable variables such as mold and melt temperature, cooling time, injection pressure, injection speed, holding pressure and time, etc. that can affect the outcome of the finished product. All these factors have effects on the part and sometimes these effects are adverse ones that need to be minimized. However, adjusting one variable in order to limit a particular defect might make the part susceptible to another defect; therefore, a balancing act ensues with the correct tradeoffs to achieve the necessary requirements and for the part to fulfill its intended function satisfactorily.