There’s an old adage in horse racing — “Don’t change a winning horse’s feed” — which is honestly good advice unless that feed is going to be discontinued or is constantly out of stock. In plastics engineering, we face a similar dilemma if a “winning” material’s supply status changes and replacement resins need to be sourced for an established part or component. When considering material replacement, mechanical performance is one of the most important factors, and the motivations behind switching typically boil down to cost reduction or supply chain issues.
The gold standard for material replacement is a “drop-in” resin, which is a direct replacement requiring minimal or no design or tooling changes. A genuine drop-in must offer geometric compatibility (shrinkage, dimensional stability), processing compatibility (processing window, rheological profile) and mechanical equivalence.
The first in a series on drop-in resins, the goal of this article is to highlight common mechanical performance pitfalls that aren’t always considered when selecting a replacement material. If you take away one lesson, it is this: Look beyond the datasheet.
Drilling Into Datasheets
First, let’s start with what properties we can compare on a datasheet. With any drop-in resin, it would be best to find a material that is the same exact polymer as the one to be replaced. Ideally it would have consistent reinforcements (glass fiber, carbon fiber, etc.), fillers (talc, mica, calcium carbonate, etc.), and additives (UV stabilizer, impact modifier, etc.). A good filter to find a resin with similar mechanical properties is to search for resins with equivalent melt flow rates or melt flow indices (MFR/MFI). For fiber-reinforced materials, sometimes intrinsic viscosity (IV) is listed instead.
Figure 1. Comparison tensile stress/strain curves for Samples 1-4. (Source: The Madison Group)
These tests directly correlate to the molecular weight of the polymer, which corresponds to the mechanical properties. You should try to match MFR as closely as possible, keeping drift to no more than ±20%, which is typical for a material’s lot-to-lot variation. For IV, a tighter variation is warranted around ±7%. Be cautious though as MFR values cannot be directly compared to one another if the material is not run at the same conditions. Furthermore, both MFR and IV comparisons are not helpful for polymers that are not identical (e.g., nylon 6 to nylon 66).
Next, you will want to match key mechanical properties of the resin: stiffness (modulus), strength, impact resistance and elongation or strain at break. Most datasheets will have these values listed, which is helpful. But sometimes different conditions can be used to perform a test to make a material appear higher performing than it truly is. Tensile and flexural modulus as well as tensile strength values are typically more consistent depending on the strain rate used. Elongation at break and strain at yield (if reported) are more reliant on the testing rate (0.2 in./min., 2.0 in./min., 20 in./min.). So, strain values should be taken with a grain of salt.
Finally, impact resistance may be the most difficult to compare based on datasheets alone, especially if low-temperature impact is important to the application. Comparisons can only be made within the same test method and specimen type (Izod, Charpy, notched, unnotched, falling dart, etc.), temperature of testing, specimen geometry and specimen thickness. Ensure all datasheet parameters are consistent before comparing impact strength values. Otherwise, the resins must be tested under identical conditions to ensure a valid comparison.
Single-Point Data Deficiencies
Once we have a few material matches based on datasheet properties, it’s time to look beyond the datasheet. One of the properties that cannot be reported with single-point data is toughness. Toughness gives an indication of energy absorbed by a material before it fails. Toughness can help differentiate the tradeoffs between stiffness/strength and elongation at break. It can also indicate how the materials will perform in impact testing at that temperature.
Table 1. Tensile testing results on potential replacement resins. (Source: The Madison Group)
One way to measure toughness is by calculating the area under the curve for a tensile test. Table 1 and Figure 1 show examples of three similar materials (Samples 2-4) that were proposed by a material supplier as potential replacements for Sample 1. Based on the datasheets, the modulus, tensile strength and elongation at breaks were all similar, giving the initial impression that there were multiple options for a replacement resin. However, by running tensile testing on these materials and calculating the area under the curve to determine toughness, Samples 2 and 4 could easily be eliminated as inadequate replacements.
Table 2. Mechanical properties of PP copolymer and impact modified resins. (Source: “Dynamic mechanical properties and morphology of polypropylene block copolymers and polypropylene/elastomer blends,” Polym Eng Sci, 27: 254-262. Karger-Kocsis, J. and Kiss, L.)
All in the (Material) Family
This example leads to another common pitfall for replacement materials: Not all polymers within a material family are the same. In the previous example, all polymers were from the same family, but only Sample 3 was an adequate replacement. A common assumption is that all nylon (polyamide) polymers will behave similarly enough to be quasi drop-in resins.
Figure 2. Tensile strength vs. moisture content for various nylon resins. (Source: Nylon Plastics Handbook, Kohan, Melvin I.)
While all nylons’ properties change with moisture absorption, there are varying degrees of changes depending on the specific polymer. As observed in Figure 2, tensile strengths of PA 6 and PA 66 are similar when dry as molded (DAM), but at 2% moisture content, PA 6 tensile strength drops to half of DAM, while PA 66 still maintains about 75% of DAM tensile strength. Similarly, dimensions of polyamides change at different rates depending on the relative humidity (Figure 3). Therefore, simply finding other nylons for a part that will be exposed to moisture is not selective enough.
Figure 3. Dimensional change vs. relative humidity for various nylon resins. (Source: Nylon Plastics Handbook, Kohan, Melvin I.)
Another example of resins that are treated similarly could be polypropylene (PP) copolymers as well as impact-modified PP. Table 2 shows mechanical properties of a PP block copolymer as compared to a couple of impact-modified PP resins. The PP block copolymer and the PP/EPDM/TPO blend show roughly equivalent properties at the temperatures listed. The PP/elastomer blend is stiffer and stronger across the board at room temperature, but all three are equivalent at -20°C. This shows a different rate of change for these properties due to the difference in composition. If the room temperature properties were all closer together, there could be lower strength than expected at sub-ambient temperatures for the PP elastomer blend. For these resins, the level of impact modifier is typically not listed on the datasheet either.
Table 3 – Glass transition temperature and storage modulus vs. temperature from DMA testing of PP homopolymer with various levels of TPO. (Source: “Dynamic mechanical properties and morphology of polypropylene block copolymers and polypropylene/elastomer blends,” Polym Eng Sci, 27: 254-262. Karger-Kocsis, J. and Kiss, L.)
Table 3 shows the effects of different levels of TPO on the glass transition (Tg) and storage modulus (stiffness) of a PP homopolymer resin at various temperatures as measured with dynamic mechanical analysis (DMA). The lowered Tg and reduced stiffness with increasing TPO could affect long-term properties such as creep resistance. Multipoint data from DMA makes these differences more clear than single values from a datasheet and could easily differentiate different replacement resins. On this same note, a significant limitation of standard material datasheets is the absence of long-term performance metrics — specifically creep and fatigue resistance. Most datasheet values are derived from short-term mechanical tests that illustrate how a resin responds to instantaneous overload or impact.
Datasheets frequently omit critical data regarding sub-ambient and elevated temperature performance, as well as general time-dependent degradation. For critical components subjected to sustained static loads or cyclical stresses, relying on room temperature specs is risky. Direct creep and fatigue testing are essential to accurately benchmark how materials will truly compete in the field. We will examine these variables further in detail in the second installment of this series.
Reinforced and Filled Resins
Finally, let’s tackle reinforced and filled resins. Finding replacement materials for fiber-reinforced resins can be especially tricky, and molders should use the utmost caution when comparing datasheet properties. First, the mechanical properties experienced in a molded part are typically reduced as compared to those reported on a datasheet for fiber-reinforced materials. This is due to the high alignment of the fibers when molding into a tensile bar.
Table 4 – Tensile strength of a 30% glass fiber reinforced PP – datasheet vs. plaque (Source: The RTP Company)
In the data shown in Table 4, a 30% glass-fiber-reinforced PP was molded into a plaque, and tensile bars were machined from the flow and cross-flow directions. The tensile strength in the flow direction was reduced by 16% as compared to the datasheet. However, in the cross-flow direction, there was a 34% reduction.
Table 5 – Retention of tensile strength at weldline of various fiber reinforced resins. (Source: Runner and Gating Design Handbook, Beaumont, J.P.)
Datasheets also do not consider the significant reduction in strength at weld lines in fiber-reinforced materials. Table 5 shows the tensile strength determined at the weld line in various resins with fiber-reinforced materials. While there is already a reduction in strength without fibers, many fiber-reinforced resins show more than a 50% reduction in strength at a weld line. Not taking this into account could result in reduced part performance and possibly failure.
There are other considerations for fiber-reinforced materials, including fiber breakage in the final part. Furthermore, uncoupled fibers do not create as strong a bond to the resin matrix as coupled fibers do. A material with uncoupled fibers might be more cost effective, but it will create a part that’s mechanically inferior in the field. Overall, if you are sourcing a replacement fiber-reinforced thermoplastic, it’s critical to perform mechanical testing to ensure that it will be equivalent to the incumbent material.
Figure 4. Tensile testing of HDPE with 10% wollastonite and mica fillers. (Source: Lubomír Lapcík, David Manas, Barbora Lapcíková, et. al., Effect of filler particle shape on plastic-elastic mechanical behavior of high density poly(ethylene)/mica and poly(ethylene)/wollastonite composites, Composites Part B: Engineering, Volume 141, 2018.)
There are similar considerations for filled materials. Different types of fillers, their particle sizes and their coatings can play a large role in the final mechanical properties. Figure 4 exhibits the stress-versus-strain behavior of neat high-density polyethylene (HDPE), HDPE with 10% mica and HDPE with 10% wollastonite filler. Mica significantly increased the modulus and tensile strength of the HDPE, but due to the shape of mica, the elongation at break and toughness were significantly reduced. Wollastonite filler, on the other hand, showed more moderate increases in stiffness and strength but retained the toughness of the HDPE matrix. Thus, if elongation at break was ignored on a datasheet, there could be unintended consequences in the final part.
In conclusion, finding drop in resins is not quite as straightforward as you might initially expect. Datasheets are helpful tools to quickly identify potential replacement candidates, but without performing comparative testing yourself, there could be large mechanical performance differences between materials that otherwise seem equivalent on paper. It is always important to look beyond the datasheet and perform further material testing or product testing to qualify drop-in resins.
This article was originally published in the June 2026 edition of Plastics Technology.
