Designing for Injection Molding




Injection molding machines perform a wide range of mechanical movements with differing characteristics. Mold opening is a low-force high-speed movement, and mold closing a high-force low-speed movement. Plasticizing involves high torque and low rotational speed, while injection requires high force and medium speed. A source of motive power is needed to drive these movements. The modern injection molding machine is virtually always a self-contained unit incorporating its own power source. Early machine frequency ran from a centralized source serving an entire shop or factory. In this respect, injection molding machines have undergone the same metamorphosis as machine tools.





Oil hydraulics has become firmly established as the drive system for the vast majority of injection molding machines and until recently was almost unchallenged as the power source. Put at its simplest, the injection molding machine contains a reservoir of hydraulic oil which is pumped by an electrically-driven pump at high pressure, typically at up to 2000 psi, to actuating cylinders and motors. High and low pressure linear movements are performed by hydraulic cylinders, and rotary movements for screw drive and other purposes are achieved by hydraulic motors. Hybrid machines, in which the screw is driven by electric motor while the linear movements remain hydraulically powered, are not uncommon.






In recent years, the supremacy of the hydraulic machine has been challenged by all-electric machines. These use new brushless servo motor technology to power the various machine movements. The capital cost of all-electric machines is higher than that of conventional machines but the energy consumption in production is much lower. This is because the electric motors run only on demand, and there are no losses due to energy conversion, pipelines, or throttling. The elimination of hydraulic oil makes the all-electric machine inherently cleaner, so these machines are attractive for sterile or clean room use. There is also evidence that all-electric machine movements can be resolved with a higher degree of precision and repeatability than hydraulic systems.






The Process


The process may involve either a thermoplast or a duroplast as the polymeric binder. With a thermoplast, solidification of the melt occurs on cooling; with a duroplast, a hardener is added to the feed mixture and solidification results from a binder-hardener reaction that occurs at elevated temperature. Figure 1 shows the viscosity-temperature relation for each type of binder. The reversibility of a thermoplast in terms of solidification makes recycling the reject a possibility but can lead to deformation of the compact during the subsequent burnout stage (see Sect. 3). For the duroplast process, solidification is irreversible and no deformation can occur during reheating, but the time for hardening is relatively long and the mold temperature is relatively high, and the reject cannot, of course, be recycled. The thermoplast process is the most widely used for ceramics and so this is discussed here.






Injection molding,such as commodity mold, is a prevalent manufacturing process utilized across a variety of applications, from full-scale productions of consumer products to smaller volume production of large components like car body panels.






The process involves a tool or mold, typically constructed from hardened steel or aluminum. The mold is precision machined to form the features of the desired auto part, and thermoplastic material is fed into a heated barrel, mixed and forced into the metal mold cavity where it cools and hardens.






With precise tooling and high-quality results, thin wall injection parts molding produces parts reliably and cost-effectively at large volumes.


Stratasys Direct has decades of experience in all phases of tooling, including part design, tool design, material sciences, post-processing and project management. Capabilities include injection molding, pad printing, silk screening, painting, EMI/RFI shielding and light assembly.






For streamlined operations, we offer Fast Track tooling, an operation that delivers parts in as little as ten days at volumes of 25 to 1,000 units.






Whatever the project, industrial designers, engineers and product designers may face some challenges when designing for plastic injection parts molding. The following details three mistakes designers should avoid for successful injection auto molded parts.






Non-Uniform Walls


On average, the minimum wall thickness of an injection molded part ranges from 2mm to 4mm (.080 inch to .160 inch). Parts with uniform walls thickness allow the mold cavity to fill more precisely since the molten plastic does not have to be forced through varying restrictions as it fills.






If the walls are not uniform, the thinner sections cool first. As the thicker sections cool and shrink, stresses occur between the boundaries of the thin and thick walls. The thin section doesn’t yield to the stress because the thin section has already hardened. As the thick sections yields, warping and twisting of the part occurs, which can cause cracks.






If design limitations make it impossible to have uniform wall thicknesses, the change in thickness should be as gradual as possible. Coring is a helpful method where plastic is removed from the thick area, which helps to keep wall sections uniform. Gussets support structures can also be designed into the part to reduce the possibility of warping.






Not Utilizing Draft


Mold drafts facilitate part removal from the metal thin wall mold. The draft must be in an offset angle that is parallel to the mold opening and closing. The ideal draft angle for a given part depends on the depth of the part in the mold and its required end-use function.






Allowing for as much draft as possible will permit parts to release from the mold easily. Typically, one to two degrees of drafts with an additional 1.5 degrees per 0.25mm depth of texture is sufficient.The mold part line will need to be located in a way that splits the draft in order to minimize it.






Sharp Corners


Sharp corners greatly increase stress concentration, which, when high enough, can lead to part failure. Sharp corners often come about in non-obvious places, such as a boss attached to a surface, or a strengthening rib, and the medical parts.






The radii of sharp corners needs to be watched closely because stress concentration varies with radius for a given thickness. The stress concentration factor is high for R/T values, less than 0.5, but for R/T values over 0.5 the concentration lowers. It is recommended that an inside radius be a minimum of 1 times the thickness.






In addition to reducing stresses, the fillet radius provides a streamlined flow path for the molten plastic, resulting in an easier fill of the mold, such as fiberglass mold. At corners, the suggested inside radius is 0.5 times the material thickness and the outside radius is 1.5 times the material thickness. A bigger radius should be used if part design allows.






Working with customers across a variety of industries, Stratasys Direct has developed thorough methods to provide solutions for fast tooling in order to serve your versatile needs.