There’s a lot of room for expensive errors when it comes to injection molding. Quality issues in injection-molded products can range from minor surface defects to more serious problems that can affect the safety, performance and function of the product. They can be caused by problems related to the molding process, material use, tooling design or a combination of all three.
Some molding defects may be difficult or costly to address. Others can be prevented by adjusting the molding process, without the need to redesign the mold tooling or replace other production equipment. You can typically avoid these defects relatively easily simply by adjusting the flow rate, temperature or pressure of your mold.
- Flow lines
Flow lines appear as a wavy pattern often of a slightly different color than the surrounding area and generally on narrower sections of the molded component. They may also appear as ring-shaped bands on a product’s surface near the entry points of the mold, or “gates”, which the molten material flows through. Flow marks won’t typically impact the integrity of the component. But they can be unsightly and may be unacceptable if found in certain consumer products, such as high-end sunglasses.
Causes and remedies for flow lines
Flow lines are most often the result of variations in the cooling speed of the material as it flows in different directions throughout the mold. Differences in wall thickness can also cause the material to cool at different rates, leaving behind flow lines. flow lines on a plastic injection mold For example, molten plastic, cools very quickly during the injection process and flow marks are evident when the injection speed is too slow. The plastic becomes partially solid and gummy while still filling the mold, causing the wave pattern to appear.
Here are some common remedies for flow lines in injection-molded products:
Increase the injection speed, pressure and material temperature to ensure the material fills the mold before cooling
Round the corners of the mold where wall thickness increases to help keep flow rate consistent and prevent flow lines
Relocate mold gates to create more distance between them and the mold coolant to help prevent the material from cooling too early during flow
Increase the nozzle diameter to raise flow speed and prevent early cooling
- Burn marks
Burn marks typically appear as black or rust-colored discoloration on an edge or surface of a molded plastic part. Plastic injection molded part with burn marks Burn marks generally don’t affect part integrity, unless the plastic is burned to the extent of degradation.
Causes and prevention of burn marks
The usual cause for burn marks in injection-molded parts is trapped air, or the resin itself, overheating in the mold cavity during injection. Excessive injection speeds or heating of the material often lead to overheating that causes burns. Consider the following preventative measures to avoid burn marks in molded components:
Lower the melt and mold temperature to prevent overheating
Reduce the injection speed to limit the risk of trapping air inside the mold
Enlarge gas vents and gates to allow trapped air to escape the mold
Shorten the mold cycle time so that any trapped air and resin don’t have a chance to overheat
Warping is deformation that can occur in injection molded products when different parts of a component shrink unevenly. Just as wood can warp when it dries unevenly, plastic and other materials can warp during the cooling process when uneven shrinkage puts undue stress on different areas of the molded part. This undue stress results in bending or twisting of the finished part as it cools. This is evident in a part that’s meant to lie flat but leaves a gap when laid on a flat surface.
Causes and prevention of warping in molded parts
One of the main causes for warping in injection-molded plastic and similar materials is that cooling happens too quickly. A warped injection molded part Often excessive temperature or low thermal conductivity of the molten material can worsen the problem. Other times mold design can contribute to warping when the walls of the mold are not of uniform thickness—shrinkage increases with wall thickness. Here are some common ways to prevent warping in your molded parts:
Ensure the cooling process is gradual and long enough to prevent uneven stresses on the material
Lower the temperature of the material or mold
Try switching to a material that shrinks less during cooling (e.g. particle-filled thermoplastics shrink much less than semi-crystalline materials or unfilled grades)
Redesign the mold with uniform wall thickness and part symmetry to ensure greater stability in the part during cooling
- Vacuum voids / air pockets
Vacuum voids, or air pockets, are trapped air bubbles that appear in a finished molded component. Quality control professionals typically consider voids to be a “minor” defect (related: 3 Types of Quality Defects in Different Products). Air pockets on a plastic injection mold But larger or more numerous voids can weaken the molded part in some cases, as there’s air below the surface of the part where there should be molded material.
Common causes of and steps to prevent vacuum voids in molded parts
One of the chief causes of voids is inadequate molding pressure to force trapped air out of the mold cavity. Other times, the material closest to the mold wall cools too quickly, causing the material to harden and pull the material toward the outside, creating an abscess. The material itself may be especially vulnerable to voids if its density changes significantly from the molten to hardened state. Voids are more difficult to avoid in molded parts which are thicker than 6 mm. Common ways to prevent voids include:
Raise the injection pressure to force out trapped air pockets
Choose a grade of material with lower viscosity to limit the risk of air bubbles forming
Place gates close to the thickest parts of the mold to prevent premature cooling where the material is most vulnerable to voids
- Sink marks
Sink marks are small recesses or depressions in an otherwise flat and consistent surface of a molded part. These can occur when the inner part of a molded component shrinks, pulling material from the outside inward.
Causes and prevention of sink marks
Sink marks are similar to vacuum voids but are reversed in cause and effect. sink marks on a plastic injection molded part Rather than the material cooling too rapidly near the exterior of the part, the material cools too slowly. The resulting shrinkage pulls the outside material inward before it’s had a chance to adequately cool, leading to a depression. As with voids, sink marks are more likely to occur in thicker parts of a component. Here are some steps you can take to prevent this defect:
Increase holding pressure and time to allow the material near the part’s surface to cool
Increase cooling time to limit shrinkage
Design your mold with thinner component walls to allow for faster cooling near the surface
- Weld lines
Weld lines can appear on the surface of a molded part where the molten material has converged after splitting off into two or more directions in a mold. The hair-like weld line is the result of weak material bonding, which lowers the strength of the part.
Common causes of weld lines and how to prevent them
Two or more fronts of polymer or other molten material need to maintain a certain temperature when colliding. Weld lines on a plastic injection mold Otherwise, they become partially solidified and won’t sufficiently bond where they meet, resulting in weld lines. Common remedies for weld lines in molded parts include:
Increase material temperature to prevent partial solidification
Raise injection speed and pressure to limit cooling before the material has filled the mold
Redesign the mold to eliminate partitions
Switch to a material with a lower melting temperature or viscosity to allow faster flow and prevent early cooling
Jetting refers to a kind of deformation in a molded component that can occur when there’s an initial “jet” of molten material injected into the mold cavity that starts to solidify before the cavity is filled. Jetting often appears as a squiggly line in the surface of the finished component, typically leading from the initial gate of injection. This visible flow pattern can result in part weakness.
Causes and remedies of jetting in molded parts
The chief cause for jetting is excessive injection pressure. Jetting marks on a plastic part When molten polymer or other material is injected through a small gate at high pressure it often squirts rapidly through the gate, rather than filling the mold cavity gradually. As this initial line of material cools against the mold walls and starts to harden, the remaining mold material pushes it, leaving impressions in the surface of the finished part. Avoid jetting in molded parts by:
Reducing injection pressure to prevent rapid squirting of the material into the mold cavity
Increasing material and mold temperature to keep the initial jet of material from solidifying early
Designing the mold with the injection gate located such that the material is directed across the mold, rather than lengthwise
Injection molding defects can often originate from the material itself or how the manufacturer stores and handles the material prior to the production process. These defects can range from minor aesthetic issues to compromised strength of the finished component. Serious safety concerns may also result, depending on the intended application of the product affected.
Discoloration, or “color streaking”, occurs when a molded part is a different color than intended. Often the discoloration is limited to a localized area or a few streaks of abnormal color on a molded part. This defect typically affects the appearance of the part without reducing its strength Causes of discoloration in molded products and how to prevent it
A common cause of discoloration is leftover pellets in the hopper or residual resin in the nozzle or mold from a previous production run. discolored injection molding defect Poor thermal stability of the coloring agent or improper mixing of the masterbatch are other potential causes. Take the following precautions to limit the risk of discoloration in your injection-molded products:
Ensure that workers properly clean the hopper, nozzle and mold between production runs to eliminate any residual pellets or base material
Consider using a purging compound to remove excess color from the machine
Ensure you or your supplier is using a color agent with adequate thermal stability
Ensure that the masterbatch is evenly mixed for consistent color output
If you find thin layers on the surface of a molded part are easily separating or peeling off the underlying material, you’re seeing a molding defect called delamination. Delamination is a defect characterized by a flaking surface layer, similar to what you’d commonly find on flake mica. This is generally regarded as a relatively serious defect because it reduces the strength of the component.
Causes and prevention of delamination in injection molding
The most common cause of delamination is contamination of the resin pellets or other base material with a foreign material. Flaky separation results when the two materials cannot properly bond to each other. For example, you might combine a common base plastic like acrylonitrile butadiene styrene (ABS) with an incompatible plastic, such as polypropylene (PP). The resulting loss of strength of the material would be very dangerous if your part is intended for a safety-critical use. delamination injection molding defect
Aside from material fed into the hopper, the contaminant could also be any excess release agents coating the mold for easier component separation. Excess moisture on the material, due to improper drying prior to use, can also cause delamination. Consider the following corrective actions to prevent recurrence if you discover delamination affecting your molded parts:
Increase the mold temperature or pre-dry the material properly if excess moisture is an issue
Ensure workers are properly storing and handling the resin pellets or base material to prevent contamination
Consider redesigning the mold with a focus on the injection nozzle to limit your dependence on release agents
Defects can be introduced into molded products by issues with the mold tooling itself. Certain defects are likely to occur when the mold is poorly maintained or designed. Especially in the latter case, these defects can be difficult or costly to address in future production runs when it’s necessary to completely overhaul the mold.
- Short shot
A short shot occurs when the flow of molten material doesn’t completely fill the cavities in a mold. The result is that the molded component is incomplete after cooling. Short shot might appear as incomplete compartments in plastic shelves of a display or missing prongs on a plastic fork, for example. Short shots are typically classified as a major defect that can inhibit the function or appearance of the molded part. short shot injection molding defect
Causes and remedies of short shot in molded products
The most common cause of short shots is flow restriction resulting from narrow or blocked gates. Sometimes the material is too viscous or the mold is too cold to allow the molten material to completely fill the mold before cooling. And other times trapped air pockets may be hindering proper flow or injection pressure may be inadequate. Consider the following steps to prevent short shot:
Redesign the mold with wider channels or gates for better flow
Increase injection speed or pressure or choose a thinner base material to improve flow
Increase mold temperature to prevent material from cooling too rapidly
Add additional air vents or enlarge existing vents in the mold to allow trapped air to escape
Flash, also called “spew” or “burrs”, is an excess of molding material that appears as a thin lip or protrusion at the edge of a component. Flash appears because material has flowed outside of the intended flow channels and into the space between the tooling plates or at the injector pin. flash injection molding defect Flash is usually subtle but might be considered a major defect if particularly obvious on a product. The process for reworking a molded product with flash often includes trimming the excess material.
Common causes of flash and prevention in molded products
A poorly-designed or worn and degraded mold is the most common contributor to flash. Excessively high mold temperature or injection pressure can also cause flash. Material flowing through the mold cavities can force its way between plates when plate clamping force is inadequate.
Common methods for addressing flash in molded products include:
Retool or redesign the mold if plates don’t fit together properly or allow material to flow outside the channel
Increase plate clamping force to confine material flow to the channel
Adjust mold temperature, injection pressure and ventilation to improve material flow
Manufacturing using injection molding typically requires significant upfront investment in tooling. That’s why it’s especially important to design your mold right the first time, rather than having to start again after finding serious defects. Defects related to the molding process or material tend to be easier and less costly to solve. But no matter the cause, defects in molded products can hurt your bottom line tremendously.
Plastic injection molding is so widely used, for good reason. The process offers high production at reasonable costs, with a broad set of material selection options offering different material properties to meet the needs of the application. With the right material and design, plastic injection molding can even offer material strength and hardness rivaling that of metal materials, at a fraction of the weight.
When taking advantage of these benefits, however, it is important to avoid plastic injection molding defects, which can be all too common in cases of poor mold design or engineering — as well as inexperienced operators who may not be used to certain material types or mold characteristics.
Below, we will explore injection molding defects and solutions that can avoid them.
Plastic Injection Molding Defects — And How to Avoid Them
Read on for a list of common injection molding defects, why they occur and how to troubleshoot them:
Brittleness: The mold does not have suitable strength, and will easily crack or break. Brittleness may be caused by overly dry conditions, by the overuse of recycled materials, or by mold runners that are too narrow. Remedies include decreasing the use of recycled materials, increasing runner and gate sizes, and reducing the temperature of the material closer to the melt point.
Delamination: The finished part has visible surface layers that can be peeled. This may be caused by a mixture of incompatible materials, inconsistent material temperature, or poor mold design with right angles in the material flow path. To remedy, verify that materials are compatible and contaminant-free, increase mold temperature, and chamfer or remove right angles from the material flow path.
Jetting: A deformation in the finished piece that can cause structural weakness as well as aesthetic problems. Jetting occurs when some of the molten plastic material sprays into the mold cavity before the normal flow. This initial spray cools and hardens, and prevents material flow as intended through the cavity. Jetting can be remedied by reducing injection pressure.
Flash: Occurs when some material remains on the outside of the mold, creating a flap or excess material that must be trimmed. Most common in worn-out or poorly designed molds. Though flash can also occur due to excessive material temperature or pressure. To remedy flash, you may need to refurbish the mold or create a new one.
Sinks: Depressions in the surface of the piece. Sink occurs when the mold is not properly filled, and can be remedied by increasing injection pressure, and extending holding and cooling time. Sink may also occur due to mold design that inhibits proper cavity filling, so you may need to review part and mold design.
Voids: Air bubbles beneath the surface of the finished part, which can threaten the strength and structural integrity of the part and cause it to fail. Remedy voids by increasing injection pressure or choosing a material with lower viscosity for improved flow.
Splay: A cosmetic defect where moisture in the material creates streaks on the surface of the part. Rectify splay issues by guaranteeing that the mold and materials are properly dried, and that excess moisture is not present in the ambient environment.
Bubbles/blisters: These form within the part when air cannot escape the mold cavity as the material is injected. Bubbles can affect the strength of the part and may create cosmetic defects. Eliminate bubbles and blisters by adding a vent to the mold, making part wall thicknesses more uniform, increasing mold temperature, or increasing injection pressure.
Warping: A deformation where part surfaces or walls twist or bend as the part cools. It is caused by inconsistent cooling of the material, poor part design, or material that is especially likely to shrink as it cools. Remedy warpage by ensuring uniform wall thickness in part design, by adjusting material selection, and by gradually reducing mold temperature to create a smoother cooling process.
Flow lines: Visible streaks or waves in the surface of the part, caused by inconsistent cooling of the material. This defect is especially critical when the appearance of the part is important. Solve flow line issues by increasing injection speed, as well as pressure and temperature, to fill the entire cavity before the material cools. Variations in wall thickness can also cause flow lines, so review part design as well.
As you can see, many injection molding defects causes come from similar roots — part design, mold design, material temperature, injection pressure and cooling time. we are experts in design for manufacturability — providing advice and suggestions to greatly reduce or eliminate the occurrence of these defects and working with you to remedy them when they occur. Visit our previous post for even more common defects, and contact us to learn more about how we can help you.
Most plastic parts are small and look quite simple in their finished form. In truth, a lot goes into designing and manufacturing these parts to ensure they are just right. In most cases, plastic injection molding is the process used for the fabrication of such components. It’s a process with its fair share of potential pitfalls and defects, all of which can delay projects, ruin entire production batches and set manufacturers back thousands of dollars.
There’s an upside, though: experts have had decades of molding to use for understanding the causes of these problems and what to do. In this article, we’ll look at some of the most common injection molding defects and the solutions that can help avoid the
Flow lines from injection molding are streaks, lines or random wavy patterns that show on plastic molded components. They usually appear as a slightly different color than the rest of the part. Flow lines occur because of the cooling that takes place in the mold. Sometimes, different parts of the material in a mold cool at different rates. The material that cools earliest often leaves behind flow marks.
Several solutions can address flow marks in injection molding. Increasing the speed of injection when filling a mold is the simplest option. Plastic starts cooling the moment it enters the mold, so if there is a slow fill rate, some of the material will already begin cooling before the entire mold cavity fills. Another option is to focus on the wall thickness of the mold, which can avoid consistency issues with cooling.
During the cooling process, the plastic in a mold shrinks and contracts slightly. Sometimes, this process occurs unevenly, which can cause the part to warp from the stress of internal forces. A warped plastic component will be bent or twisted in ways that could potentially render it unusable. Uneven cooling can even introduce additional plastic defects, such as cracks and fractures, which could compromise the part’s integrity under stress.
Inconsistent cooling often occurs in molds where the cooling process occurs too rapidly. Rapid cooling increases the risk of uneven shrinkage, putting undue strain on different design parts and causing warping.
Controlling the speed and consistency of the cooling process is vital to avoid this problem. You can do so by designing your plastic molds to have more consistent wall thickness or by altering the temperature of the plastic material or the mold itself. You may also consider using a different plastic compound if your original material is particularly prone to shrinkage
Burn marks on plastic-molded parts manifest as sections of black or discolored material on your plastic part. In most cases, the burns only affect the surface of the plastic, which means they look worse than they are. You’ll see much discoloration on the part’s surface, but the plastic beneath the burn mark will be fine.
In some rare cases, the plastic may burn badly enough to compromise the integrity of the plastic, in which case you’ll have to discard the part.
This molding defect appears when trapped air in the mold or the plastic resin itself becomes overheated. The two most common causes are 1) excessive heating of the plastic resin before injection and 2) injection speeds that are too fast, which can trap hot air in the mold.
The solutions, therefore, are to reduce both the resin temperature and injection speed. Other options include ventilation in the mold itself (to give trapped air a way to escape) and shortened cycle times for the molding process.
Sink marks are injection molding defects that resemble small dents or craters on the part’s surface. They are the result of uneven mold walls. Specifically, thick wall sections that don’t allow the part to sufficiently cool in areas of larger mass will cause sinking.
When this happens, the thickest sections of the part can’t cool as quickly as the thinner walls; thus, these thicker sections continue to shrink, causing material from the surface to sink into itself. This process leaves behind small but noticeable depressions in the part’s surface.
Usually, the cause of this plastic defect is simple: the exterior parts of the molded component cool too slowly. Thus, if shrinkage occurs in the part’s interior, the material will naturally pull from the surface to fill that space. The surface layer of plastic has not yet cooled enough to be fully stable.
Solutions include reducing the wall thickness of the mold to allow for faster surface cooling or increasing the holding pressure in the mold to compensate for shrinkage
In the world of molding defects, vacuum voids are the opposite of sink marks. Where a vacuum causes sink marks in the middle of the part, pulling material down from the surface, vacuum voids are air pockets or bubbles that appear on the surface of a molded part.
In most cases, these bubbles occur due to insufficient holding pressure. High enough holding pressure levels will essentially force all air out of the mold, leaving no opportunity for air pockets to form. Lower holding pressure allows air to stay in that environment, sometimes resulting in vacuum voids. These defects are usually on the minor end of the spectrum but can affect a part’s strength and structural integrity.
Increasing the pressure at which you inject material into your mold can help force out air pockets and avoid vacuum voids. Lower-viscosity plastic resins are also worth trying out if you have been struggling with vacuum voids, as they are less likely to cause the air bubbles that create an injection molding defect in the first place.
Setting Up for Successful, Quality Production
These types of plastic injection molding defects are not the only ones that a plastic manufacturer may face, nor are the proposed injection molding solutions the only options you will have to solve each problem. Ultimately, you will likely need to do some trial and error to find the right balance between the materials you use, the temperatures of the resin and the mold, the thickness of the mold walls, the speed of injection, and other factors. There are many variables with plastic injection molding, and as you can see, those variables can sometimes lead to defects in your finished parts.