Ribs and Undercuts

Case in point

The line of Liberty Ships from the era of World War II have often made up for an interesting case study for ship builders worldwide. As history has it, it took the loss of both stock and life to put design into perspective. An immediate inspection surmised cold cracking and hydrogen induced cracking (HIC) for the fatal accidents, but experts knew otherwise. Experts today know for a fact that the welded hull construction chosen over the good old design with rivets and the grade of steel, both transformed ductile ships into frangible ones. Often dubbed as “Kaiser’s Coffins”, it makes up for an interesting case study even in this century.

Like many major accidents, this incident too highlighted structural errors embraced by manufacturers. But such errors are not just limited to structures but are often a part of design as well.

Changing times call for changing methods

Things are much different now than they were then, modern ship industry has followed the lead of other major sectors and have slowly started reaping the benefits of plastic in ship building. Not only does it cut down on manufacturing cost, it also involves reduced hours of ship building, flexible parts and minimal waste (if done right). Ships these days make use of plastic precast sets for its skeleton. Injected styrene, cast resin model and Fiber Reinforces Plastic (FRP) are some of the commonly used terms in the shipping industry of this age.

While one would consider this transition from metal to plastic as a break from structural faults, it’s challenging nonetheless. These challenges are not exclusive to the shipping industry but are inherent to each such sector which makes use of plastic. Tooling cost, inaccurate cost estimate and design restrictions are commonly faced challenges in plastic dependent industries. Thanks to the advancement in technology, parts today can be analyzed (alerting one of the anticipated challenges) well before it is constituted in structures such as bridges, ships or even bottles, for that matter. One can plan their design or propose an alternate design based on these analysis. Design for Manufacturing software not only analyze designed parts but also serve as design rules filter, thereby ensuring an optimized design cycle with minimum rework.

The apt tool

An effective Design for Manufacturing software which specializes in injection molding is a handy tool for a plastic part designer. Plastic molded parts often offer challenges like Screw engagement failures, Silver streaking, voids, sink marks, cracking, crazing and the like. Many of these challenges are either faced due to the unavailability of the apt injection-molding machine, injection speed, injection pressure, material or design errors. Often resulting in added cost and wasted time, a significant number of challenges can always be anticipated beforehand, making the required changes right at the stage of design.

Undercuts

For instance, Plastic parts with undercuts are often deemed difficult to manufacture and therefore, scare use of them is often stressed upon by both designers and manufacturers. These critical parts not only add to the manufacturing cost but also entail long design cycles.

For parts which cannot be removed from the mold in the direction of its opening and where an undercut is an absolute must from the functional aspect. In such a case, certain design rules must always be taken into consideration-   

  • The obtruding depth of the undercut should be two-third (or, less) of the wall thickness.
  • Parts with external undercuts are always difficult to manufacture without additional tooling. Tugging at a part from its open mold can pose to be challenging at the time of manufacture. Therefore, external undercuts should always be paired with small degrees of internal undercut.

While deciding the place where an undercut would be placed, the property of the plastic, ejection temperature, the required tolerance for the dimension, all these should also be taken into consideration.

For example, Plastics like TPEs (thermoplastic elastomers) can have large undercuts, whereas rigid plastics should always have smaller undercuts.

  • In case of special tools, the undercut shouldn’t be solely decided according to the pliant nature of the material used. If the manufacturing process includes lifters, collapsible cores and the like, then undercut rules are subject to change.

It’s almost physically impossible to consistently adhere to this trial and error approach. Taking every minute detail into consideration and then designing a part can wear the designer and consequently delay the design cycle. Given this, DFMPro takes all these into consideration, thereby, suggesting design alternates. The manufacturing process, special tools at manufacturer’s disposal, the fastest and efficient ways in which a plastic product can be manufactured, these are just the early benefits which a design for manufacturability software user tends to reap.

Going the extra mile

Ribs and undercutsDFMPro also understands that only undercuts do not summarize an injection molded part.  Ribs, for instance, are commonly used in injection molding to boost the stiffness of a plastic part. Some of the commonly established rib rules are as follows-

  • Rib Thickness

While ribs in a part design tend to add to the overall strength of a part, inclusion of it in the wrong area can reverse its benefits. Ribs should be oriented and placed to support the loading conditions. Including ribs which are 50%-80% of the nominal wall thickness would ensure strong parts which can be easily manufactured.

  • Rib Height

As a rule of thumb, three times the nominal wall thickness of a part should always be greater than the rib height. In case the rib height exceeds three times the adjacent wall thickness then manufacturer are often known to face issues at the time of ejection.

  • Multiple ribs with sufficient spacing between corresponding ribs

While multiple ribs instead of one often ensure a strong part, insufficient space between these ribs can prove to be challenging as well. Taking the manufacturing difficulties into consideration which if dodged would compromise on the quality of the product, spacing between two adjacent ribs is always recommended to be twice that of the nominal wall.

  • Rapidly transitioning ribs must be avoided

While differential ribs are commonly used in plastic design, avoid using ribs which swiftly runs from thin to thick. This designing error often leads to obstructive flow, thereby, stalling at places or freezing altogether. A weakened part with a questionable product life is the last thing a designer aspires to achieve.

Given the challenging nature of plastic part design and manufacture, a designer would need a utilitarian Design for Manufacturability software which knows the best practices of plastic design by heart. Further, it should be able to adapt to both the designer’s and manufacturer’s needs, providing the former with customizable solutions, as a result of which rework can be eliminated, cost can be cut and parts are manufactured in minimal time. DFMPro prides in understanding the needs of an organization starting from scratch- the designing phase!

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