In the previous article we covered the top 5 easiest plastics to injection mold. That was mainly targeted at those looking for less challenging materials for injection molding projects that perhaps don’t require such high performance plastics or they are beginner injection molders. In this article we are discussing plastics that pose more challenges for injection molding than most other plastics. Perhaps you are inclined to use one of these plastics and you’d like to know what challenges you are likely to face or you are in the process of material selection and you’d like some heads up on what plastics to avoid if you are really not up for anything too challenging to deal with beyond standard processing requirements.
The injection molding processing condition is particularly challenging compared to other plastic processing techniques because it exerts high shear stress on the material and occurs at relatively high pressures compared to other processes. This is important to achieve the level of precision, yield and efficiency that are characteristics of injection molding. Therefore the material selection process must make careful consideration that the material should be as fit to go through the process as it is fit for product performance.
In injection molding the easiest materials to work with will melt at a moderate temperature, flow easily into molds without risk of cavitation, hotspots or other issues, and allow a broad processing window such that slight change in temperature or pressure would not disturb part uniformity or lead to any defect. More challenging materials tend to present significant technical challenges that require more specialized tooling and peripheral equipment, precise process control over parameters, and extensive expertise beyond standard injection molding process.
In this article we explore five of the most challenging plastics for injection molding. It begins with discussing the criteria used for selection. The rest of the article then discusses each plastic selected. Each section explains why the specific plastic poses a challenge in the injection molding process, discusses common defects, and highlights some strategies for addressing the challenges in order to achieve successful production.
Key Factors Considered
In making this selection we considered the parameters that are most significant in the injection molding process. These are the main parameters that determine the yield and efficiency of the process. How much energy and time goes into producing each part using the particular type of plastic.
These factors include melt viscosity, thermal stability, moisture sensitivity, crystallization behavior, shrinkage characteristics, and the material’s susceptibility to degradation. These parameters determine the occurrence of defects such as warpage, sink marks, flash, burn marks, voids, incomplete filling, or dimensional instability. While these defects may still occur even when working with less demanding plastics and even with the best expertise, the level of precaution that needs to be taken to prevent them is higher when working with plastics that are inherently challenging under injection molding processing conditions.
Manufacturers seeking to improve product quality, reduce production costs, and minimize scrap rates must have a good understanding of how the processing affects the material and product properties and how to optimize the production conditions for best result. This involves selecting the best suited material as well as optimizing mold design, using the right machine operating parameters, carrying out drying and other pretreatment procedures, and implementing efficient cooling systems.
The Table below compares the properties of the plastics selected
| Polymer | Typical Melt Viscosity (Pa·s)* | Processing Temperature (°C) | Injection Pressure (MPa) | Typical Shear Rate (s⁻¹) |
|---|---|---|---|---|
| PEEK | 150–600 | 360–400 | 80–180 | 1,000–10,000 |
| PC | 500–3,000 | 280–320 | 70–150 | 500–10,000 |
| PPS | 80–500 | 300–340 | 60–150 | 1,000–20,000 |
| Nylon (PA6/PA66) | 200–1,000 | 240–290 | 60–140 | 500–10,000 |
| Liquid Crystalline Polymer (LCP) | 20–200 | 300–360 | 40–120 | 5,000–100,000 |
Note
- Melt viscosity values are representative apparent viscosities measured at typical processing temperatures and shear rates of approximately 10³–10⁴ s⁻¹. Actual values depend on resin grade, molecular weight, filler content, and processing conditions.
- The listed processing temperatures represent typical melt temperatures used during injection molding.
- Injection pressure and shear rate ranges are representative industrial values and may vary with mold geometry, part thickness, and machine capability.
1. Polyether Ether Ketone (PEEK)
PEEK is a high performance engineering thermoplastic whose wide ranging application extends from medical implants to aerospace….
It is indispensable in applications where parts need to maintain their high performance under high temperatures. These same properties that make it indispensable also make it challenging for injection molding.
Why PEEK Is Challenging to Injection Mold
PEEK is exceptionally strong, chemically resistant, wear resistant, and thermally stable. It maintains its properties even at temperatures exceeding 250°C, making it suitable for demanding applications. However, these outstanding characteristics come at a cost during processing.
PEEK requires extremely high melt temperatures, often between 340°C and 400°C. Mold temperatures must also remain significantly higher than those used for commodities plastics like PP and PP for example that are typically processed between 160°C to 200°C.
Typical melt viscosity of PEEK varies between 100 to 800 Pa.s when processing in the range of 340 to 400°C. It also shows shear thinning behaviour. While increasing the temperature reduces the melt viscosity, higher temperature increases the likelihood of degradation and associated defects.
Maintaining such elevated temperatures puts a lot of burden on the mold and requires specialized molding equipment with advanced heating systems and wear-resistant components.
Common Processing Problems
- Incomplete mold filling
- Surface blemishes and imperfections
*Voids - Internal stresses
- Warpage
- Dimensional inaccuracies
- Thermal degradation
PEEK only allows a relatively short processing window as small variations in processing conditions can significantly impact crystallinity and this directly affects the mechanical performance of the finished part.
Best Practices to Avoid Processing Issues and Defects
While PEEK does pose the aforementioned processing challenges, experienced processors still manage to achieve well formed parts by sticking to high processing standards. Precautions they take include:
- Preheating the plastic and the mold to minimize thermal shock
- Implement temperature control systems with high precision
- Use mold materials that can withstand high processing temperatures and pressures
- Adequate drying stage in pre processing
- Optimize cooling channels
- Carefully balanced injection pressure and speed to ensure good flow and mold filling
- Annealing of the formed part in post processing
Following these precautions allow manufacturers to overcome the challenges of injection molding PEEK, manage costs and achieve molded parts that have all the exceptional performance of PEEK.
2. Polycarbonate (PC)
Polycarbonate is widely recognized for its outstanding impact resistance, transparency, and dimensional stability. Over the years it has come to replace glass in many applications for these properties without the brittleness and heavy weight of glass. Applications of PCs include personal safety equipment, automotive lighting, medical devices, and housing for electronic equipment.
Like the other plastics on this list, for all its attractive properties, polycarbonate can be surprisingly difficult to mold. Particularly to successfully mold the part without losing the desirable properties of PC that would allow for good performance.
Why PC is Challenging to Injection Mold
One of the biggest challenges with a PC is its moisture sensitivity. This polymer has a tendency to absorb moisture from the surrounding environment. When it absorbs even very low levels of moisture, this can result in hydrolysis which leads to degradation of the polymer structure into shorter chains. Polymer chain length is directly linked to properties such as tensile strength, crystallinity and optical properties.
PCs also require relatively high processing temperatures. While not as high as that of PEEK, the processing temperature of polycarbonate, typically ranging between 280°C and 320°C is higher than the commodity plastics such as PP or HDPE. PC also allows a short processing window as too high temperatures can lead to discoloration and loss of transparency while too low temperatures will result in flow challenges and improperly filled molds.
PCs are also quite prone to internal stress. Rapid cooling often results in generation of residual internal stress. These stresses may not be detected during quality checks and remain undetected until the component experiences mechanical loads or chemical exposure, leading to premature cracking causing part failure or poor performance.
Common Processing Problems and Defects with PC
- Brittleness
- Reduced impact strength
- Surface defects
- Silver streaks
- Poor optical clarity
Best Practices to Avoid Processing Issues and Defects
- Thoroughly dry the resin
- Maintain consistent barrel temperatures
- Avoid excessive shear
- Use gradual cooling
- Optimize gate design to minimize stress concentration
With careful process control, transparent, high impact strength and durable PC parts can be achieved through injection molding
3.Polyphenylene Sulfide PPS
PPS is a specialty, high performance, engineering plastic with competitive performance. It finds applications in electronics, automotive and medical industries. It is favoured for its exceptional chemical resistance and heat resistance as well as its dimensional stability. It is often used as an alternative to metals in applications where heavy duty performance properties of metals are required but with higher chemical resistance, since metals are prone to corrosion. In medical application for example, a medical instrument can be produced using PPS as an alternative to metal since it can withstand high temperatures required for sterilization while having higher chemical and corrosion resistance than most metals.
What Makes PPS Challenging to Injection Mold?
PPS is highly crystalline with rather unpleasant flow properties as far as injection molding is concerned. This poses particular challenges for injection molding parts with thin walls and complex geometries.
With processing temperatures that can reach 320oC PPS also requires relatively high energy input for processing. The temperature also needs to be tightly controlled as slight shifts may result in dimensional instability and brittleness and poor heat resistance of the molded part.
In addition to this PPS tends to release corrosive gases, which although in low amounts, over time takes a toll on the equipment parts and mold and calls for more frequent maintenance and replacement, hence higher costs.
Fillers such as glass fibers are often used in order to improve the material properties of PPS such as stiffness and flame resistance. While this offers improved performance and versatility, these fillers tend to be abrasive and cause wear on the parts and mold. Compounding with fillers and fibers also pose the additional challenge of controlling fiber orientation during injection and mold filling to avoid loss of mechanical properties and dimensional accuracy.
Common Problems Manufactures Encounter with PPS
*Flash because PPS can be rather “leaky” at optimal processing conditions
- Tool wearing
- Degradation
- Corrosion
- Brittleness
- Warping
Best Practices for Successful Injection Molding with PPS
*Tight mold temperature control to ensure complete crystalization
- Maintain relatively high mold temperature between 120 to 160°C
- Optimize injection speed while balancing flow and shearing
- Maintain moderate pressures to allow for mold filling while avoiding flash and fibre breakage or loss of fiber orientation
- When molding complex parts additional care should be taken in gate placement to avoid warpage or weld lines
As a specialty, high performance engineering plastic, PPS usually comes at a premium price compared to other more common plastics like PP and PET. so manufacturers would want to minimize waste in the form of scrap or rejected parts.
4. Nylon (Polyamide)
Having a wildly popular start as the famous material used in women’s stockings in the 1940s, nylon over the years has come to be one of the most versatile engineering plastics. It offers excellent strength, wear resistance, and fatigue performance.
These days its more common applications include gears, bearings, fasteners, automotive components, electrical connectors, and industrial machinery.
Despite its popular early and current applications and its advantages, nylon does introduce some challenges in injection molding.
What Makes Nylon Challenging to Injection Mold?
Moisture absorption is one of the most well known problems with processing of nylon. Nylon readily absorbs atmospheric moisture. This makes injection molding of nylon prone to moisture associated problems if not properly controlled. It is important to note that even after molding, finished parts continue absorbing moisture. This results in time related dimensional instability which must be taken into account when designing parts with nylon.
The semi-crystalline nature of nylon results in anisotropic shrinkage as the material cools. This poses challenges with maintaining precision with complex geometries. This requires well controlled even cooling.
Fiber-reinforced grades are often used to address some of these challenges and improve strength and stiffness. However the use of these reinforcements introduce additional molding challenges of abrasion. This leads to increased wear on equipment parts and molds resulting in more frequent replacement and maintenance.
Introduction of fibers also calls for better control over fiber orientation thus adding to complexity of the process.
Common Problems Encountered with injection molding of Nylon
- Surface splay
- Reduced strength
- Bubbles
- Poor surface finish
- Dimensional instability
- Anisotropic shrinkage
- Warpage
- Twisting
- Distortion
- Dimensional instability
Best Practices for Successful Injection Molding with Nylon
- Thoroughly drying resin
- Maintaining uniform mold temperatures
- Optimizing cooling channels
- Designing balanced gating systems
- Monitoring moisture content continuously
Nylon had a popular start for its inherent exceptional properties. Injection molding of nylon proves particularly challenging due to its tendency to absorb moisture and non uniform shrinkage. Through implementing best practices, manufacturers are able to still achieve parts for demanding applications using injection molding.
5. Liquid Crystalline Polymers
Here we have added a whole class of polymers to the list since LCPs have certain properties in common that all together make their injection molding challenging for similar reasons.
LCPs are high performance engineering thermoplastics that include plastics like kevlar a polyamide, [vectra](VECTRA® MT® LChttps://www.celanese.com/products/medical-vectra-lcp-mt-zeniteP | Medical Grade Liquid Crystal Polymer | Celanese) an aromatic copolyester based polymer, xydar also an aromatic polyester and Zectran another aromatic polyester. Their applications include fiber optic cables, electrical connectors, chip carriers, sterilizable trays, dental tools, surgical instruments and printer components amongst others.
While most polymers become highly disordered in their liquid state, LCPs are characterized by their highly ordered molecular structure in their liquid state. Although the level of orderliness is less than that of solid state orderliness (some level or disorder is required to achieve liquid state), this ability to maintain such relatively high levels of orderliness in the liquid state gives them the exceptional properties they are known for; outstanding mechanical strength even at high temperatures, highly inert, exceptional chemical resistance, wear and abrasion resistance as well as good flame retardant properties.
LCP has the advantage of near zero shrinkage due to the inherent orderliness of the liquid state and the good flowability.
What Makes LCP Challenging to Injection Mold
LCP actually does not pose flow issues like a polymer like PPS as discussed above. Their exceptional tensile strength does depend on the
Common Problems Manufacturers encounter Injection Molding LCP
- Delamination due to excessive shearing or wrong mold and melt temperature
- Flash is less likely than other plastics prone to flash but can occur if mold is not well clamped of injection pressure is too high
- Warping may occur if gate is not properly placed in alignment with flow orientation or there is uneven cooling
Best Practices to Successfully Injection Mold LCPs
- A drying stage before feeding into the machine
- Minimize regrind where possible
- Mold temperatures between 70 to 150oC for good surface finish
- Use relatively high injection speed
- Moderate to high injection pressure
- Position gate in alignment with flow orientation
Balancing Designing for Performance with Designing for Manufacturability
The material selection process should not only consider the suitability of the inherent material properties for the performance in specific applications. It should also consider the processibility of the material and whether the material properties survive the manufacturing process, in this case injection molding.
Engineers must balance functional requirements with production efficiency. For example, while PEEK may offer unmatched temperature resistance and strength that means it should be able to withstand the heat sterilization process and offer good mechanical support for consideration for use in a medical implant, the complex shape of a particular implant might make the mold design and the injection molding process challenging. In such cases the design and production team will have to weigh the option of using another material or stick to PEEK and take on the additional cost and complexity of injection molding the part using PEEK.
Injection Molding Grades
As the plastic industry has greatly expanded over the years, many companies like Celanese have been able to focus of production of polymer grades that are specially formulated for injection molding. Therefore for most of the plastics listed herein, you can always find injection molding grades. While this does not mean throwing away all the measures required for handling these challenging plastics simply because you have a special injection molding grade, the challenges will still exist, however having a particular grade that’s been specially formulated for injection molding will minimize the risk of defects and processing issues during injection molding.
It is also important to note that certain applications might not allow the use of injection molding grades of some polymers. This could be due to, for example, certain additives or fillers used in the injection molding grade that are not allowed by regulatory standards. This is often the case in plastic products used in food or medical applications.
Conclusion
Although injection molding remains one of the most versatile and widely used plastic processing technologies, its process generally exerts a significant level of pressure and shearing on the material and equipment parts. The specific operating conditions and ease of processing highly depends on the material properties. With some materials being more challenging than others. PEEK, PC, PPS, nylon and LCPs are known to be particularly challenging in injection molding. Yet their exceptional properties make them indispensable in certain applications. Therefore over the years as injection molding advances, plastic professionals have developed techniques for getting around these challenges.