Injection Molded Part Design Part 4: Warp

Christian Bourgeois . March 18, 2020

We have been talking about different challenges and flaws that can come about in injection molded parts that can cause cosmetic defects, internal stresses in these parts and cause delays to market, and how to design for injection molded parts to prevent these problems.  One area we have mentioned but not addressed in injection molded part warp after the injection cycle.  This is caused by differential cooling of the plastic in the freshly molded part, and can be due to varying thickness of material, internal features or wall junctions, part aspect ratios, and the injection molding process. 

The same conditions that drive sink marks can cause whole walls to warp out of plane.

Injection molded plastic part warpage is really its own challenge and can be a bit of a surprise.  Although mold flow analysis using 3D design CAD files can help predict this there are several parameters affecting warp, not just part geometry, so predictions are not exact.  Often, small amounts of predicted warp can end up being more severe as the internal stresses in the plastic part, and cold flow of the plastic material after part creation, can cause more bending which will not go unnoticed when the product is assembled.  Although many fit, form and function aspects of a new plastic part design can be confirmed during the prototype stages, using machined or printed pieces, warp will not necessarily manifest itself here. 

It is for these reasons it is key to follow several general guidelines to maintain relatively consistent plastic part wall thicknesses with gradual transitions, and follow recommendations for rib and boss sizes, which we discussed recently.  However, this is not always enough.  Some product designs due to their required functions or styles can still have significant challenges with regards to preventing warp in the injection molded parts.  Sharp corners, very large flat sides, and non-symmetric or open part shapes can all have issues with maintaining their intended form and fit up.   

A classic warp flaw is a 5 sided enclosure piece with its side walls deformed inwards.  Although the part has relatively consistent wall thickness on all 5 sides of the enclosure the corners can represent thicker sections that can cool and shrink more slowly, pulling on the inside of the part walls more than the outer sides.


Care needs to be taken to be careful in these corners with regards to thickness or add coring reliefs.  One option is to increase the corner radii on the outside of the part and maintain a common wall thickness to the inside fillet’s radii. 

Alternately, if sharper edged look is needed on the outside of the product, cored inside corners can locally thin the thickness by the sharp inside corners. 

As noted this coring approach applies to the intersection of side walls and the floor wall as well.  

Products designed with large flat planes have a defined clean, modern look for companies that often want to re -brand their image and set themselves apart from competitors.  But plastic parts like this have their own challenge in maintaining flatness across larger surfaces. 

With a shiny finish any sag, dips or misalignment’s are highlighted and very noticeable.  Fish eyeing of reflected light can make surfaces look weird.  The parts for the Protein Simple line of lab equipment, WES model shown, did not even have draft to optimize the orthogonal look that was desired.  Even though these parts were molded using structural foam plastic, this process only alleviates some of the concerns for sinks and warp.  The key to getting perfect parts was designing them with minimal abrupt wall thickness changes, and working closely with the molding team to communicate the required flatness and help devise post injection fixtures to prevent warp during cool down. 

Since the parts were produced in lower quantities the fixtures were a viable option but had to be designed to achieve the desired results.  In many cases a molder will design their own fixtures to meet the tolerances and form callouts in the engineering drawings.  These fixtures were jointly developed by the product design team at StudioRed and the mold house.    

For the challenging molded door part, a custom fixture was designed for post injection cooling to ensure the perfectly planar look that the new product language sought.

The extra effort in engineering and the use of fixtures achieved the desired results for Protein Simple’s new product look, and at lower cost than additional hand finishing warped parts.  The new look was not only recognized for a design award but brought the intended branded message to market, and sales responded with a 6 fold increase in the first month. 

We have just looked at plastic molding challenges where flatness and alignment are crucial, and how thick sections can distort the intended shape.  There are also situations where an injection molded plastic part may be designed to have large openings or missing regions driven by assembly, part break up or other functional requirements like venting.  These asymmetric shapes and thin spans of plastic can lead to warp.  This is often the challenge with bezel designs for rack mounted servers, switches and storage products. 

The bezel can be key to providing branding for the rack mounted product and lift it above a generic metal chassis.  As well as provide for some security and disguise OEM vendor hardware platforms underneath. 

If the injection molded portion of the bezel becomes too spindly or has to bridge very large openings to meet venting open area, access and other part fit up needs, then the part can easily warp across the bridging sections. 

The part warping may not even reveal itself until second article plastic parts are shot as the mold process is further dialed in.  But fit up issues to the chassis of the computing product can telegraph sags and bends very visibly. 

2U bezel part on the right is an early shot and shows signs of warp along edge with ruler. This warping both side to side and front to rear. This was corrected in later molding runs like the injection molded part on the left.

Many times additional structural support from other pieces, like metal perf screens, cannot be counted on as they may be removable or they may not be formed very accurately due to material and cost targets. 

These challenges with injection molded bezel pieces with large open regions through the parts or eccentric cross sections were resolved by close review of material thicknesses and transitions throughout the part, working closely with the molding team, adjusting process and cool down times, and adding post injection fixtures to hold the plastic parts shape after molding in the more extreme cases. 

The results were high quality components that fit well to various chassis and provided brand definitions for companies wanting to differentiate themselves. 

So although warp can be a challenge for some part designs, even a surprise during first mold shots, following basic design guidelines on part thicknesses and working with the molding team can rapidly resolve issues to yield high end product designs that look as intended and function as needed. 

See More Like This: Undercuts, Sink Marks, Bosses