Designing for Injection Molding

Designing for Injection Molding
Posted by bnui7uibnui7ui on 2021/09/06
Designing for Injection Molding

    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.


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