The most common problems with epoxy and urethane applications.
Blemishes due to shrinkage
Probable Cause |
Possible Fix |
| Significantly different wall thicknesses | Good part design allows for reasonably uniform wall thickness in the encapsulant. During the curing process the thicker sections solidify first and the shrinking material will replenish itself from those areas that have not been cured to the same degree. Blemishes and cosmetic defects will occur in areas where the material cures last (thinner section). Provide uniform wall thickness or cool the thick sections so that all areas gel at the same time. |
| Incorrect heat profile | Most often evidenced with heat cure in metallic moulds. This requires experimentation to determine the correct heat profile. Ideally, the material should gel half way between the insert and the inner surface of the mould. This will require determining the correct difference between the temperature of the insert and the temperature of the mould. |
| Insufficient reservoir | Epoxies and Polyurethanes shrink during cure. Surface defects and a weakened structure will result if the shrinking material is not able to replenish from a reservoir of cooler material. |
| Incorrect cure / gel temperature | Some systems require the application of heat to start the reaction. The gellation process will not commence or will not complete without heat. Warm the mix and/or warm the components and/or place the potted assembly into an oven if necessary. |
Cavities in the castings
Probable Cause |
Possible Fix |
| Excessive localized shrinkage | Localized shrinkage usually occurs over cooler areas of the curing mass. Cavities form if the top layer of the compound, being next to the hot mould, skins over and cures while the area below remains cooler allowing adjacent material to draw from it to replenish shrinkage. Adjust the heat profile to make all areas more uniform in temperature. |
| Air entrapment | It is essential to remove the air that is inevitably introduced while mixing the resin and hardener together. The trapped air will attempt to rise to the surface as it is heated during the reaction. The distance the bubbles will travel depends on the viscosity of the material and the speed of gellation. Bubbles which are unable to reach the surface before gellation will be trapped in the structure. Trapped air reduces the structural integrity and the electrical properties of the material. Depending on the circumstances, some unacceptable surface blemishes could result. The easiest fix is to de-air the mix under 29" vacuum prior to use. Alternately, warming the mix will aid in releasing trapped air. |
| Trapped volatile ingredients | Bubbles and blemishes can result from using a resin or a hardener that is inappropriate for the application at hand. Some systems have been designed for use in thin film applications and could contain volatile components that will flash off during cure. These volatiles may be trapped in the structure if the system is used in casting applications with heavier wall thicknesses. The only solution is to select a different resin/ hardener combination. |
Cracking
Probable Cause |
Possible Fix |
| Improper Mix | Make certain that the fillers (if any) are dispersed thoroughly throughout the material. Ensure that there are no pockets of unmixed hardener present in the mix. Unevenly dispersed fillers or hardener will cause changes in the shrinkage profile causing the product to cure with "built in stresses". |
| Mix off ratio
Click below for details: |
The mix ratio is based on product chemistry. If it is incorrect there will be a change in the crosslink density. Any change in the crosslink density will cause changes in tensile strength and the elongation properties of the cured material. The combination of tensile strength and elongation is key in preventing cracking. |
| Improper / insufficient cure | Most, if not all, compounds are brittle just after gellation, before they are fully cured. All products must be cured at the specified temperatures for the specified length of time to develop full strength. In some cases, where a step cure is specified, full cure is not achieved until after all the stages are complete. Curing at temperatures other than those specified will result in less than optimal properties. Cracking can occur if the cast component is subjected to mechanical or thermal stresses prior to full cure. |
| Residual built in stresses due to improper heat profile during cure | Whether the system is cured at room temperature or with the application of heat, it is important to have a uniform heat profile so that curing takes place at a uniform rate throughout the part. For example; zoning in a curing oven will cause the material to cure at different rates in different areas of the part. The area that cures first will want to replenish its shrinkage for adjacent cooler material. This will result in built in stresses within the cured mass.
- Almost all products will cure in a more "relaxed state" if the final cure temperature is above their Tg (glass transition temperature). |
| Inadvertent thermal cycling prior to full cure | Almost all materials are brittle after gellation. Care must be exercised not to subject components to undue mechanical or thermal stresses while in this state. For example; gelling a product in an oven and then storing the parts at room temperature prior to final post cure would in fact be thermal cycling. |
| Weakened areas due to embedded sharp edges | Internal components, having sharp corners or sharp pointed edges, will cause the formation of internal micro cracks as the material shrinks during gellation. These internal cracks will propagate further during post cure and will represent seriously weakened areas in the encapsulant. These cracks will eventually propagate to the surface with the application of mechanical stresses or thermal cycling. It is best to eliminate the undesirable contours or at least cushion these surfaces prior to encapsulation if possible. |
| Improper adhesion to internal components | Non-uniform adhesion to internal components can result in built in stresses. The curing material can pull away from contaminated areas as it shrinks during gellation. This will result in varied strength within the structure. The ultimate bond strength of the particular layer will be governed by the weakest link (i.e. the bond strength of the contaminant to the substrate vs. the bond strength of the compound to the contaminant). Typically areas with oily contaminants are the worst offenders but residual mould release, grease and various chemicals will also cause problems. |
| Weakening due to excessive shrinkage during cure | The areas that gel last will act as reservoir to replenish the shrinking material in adjacent areas. This could leave the area that gels last in a weakened state, with built in stresses, just looking for an excuse to crack. More uniform wall thickness and evenly heated mass should fix this problem.
- In some cases, choosing another material or a different hardener may be necessary to reduce the amount of shrinkage during gellation and cure. Filled materials and slower hardeners tend to shrink less. |
| Large differences in the coefficient of thermal expansion between the embedded components and the encapsulant | Large differences in thermal expansion between the embedded component and the encapsulant, if not compensated for, either through the formulation or through mechanical means, could result in cracking. Employing different types of fillers and/ or hardeners can solve this problem. Alternately, mechanical cushioning around embedded components may be the only solution. In some cases, utilizing a system with higher filler content or a slower reacting hardener may solve the problem . Compounds with higher elongation characteristics may also be sufficient. It is best to contact the material supplier for a recommendation |
| Insufficient tensile strength | Most often results in cracking during thermal cycling since the material "breaks" usually at the higher temperature. Part design has a major effect on the amount of tensile strength that is required for the application. Part design that includes smooth, rounded corners with heavier encapsulant wall thickness usually fare better. Alternately, an encapsulant with a more favorable combination of tensile strength and elongation may be required. |
| Insufficient elongation | See above |
| Insufficient filler content (due to filler settling or layering during cure) |
Evenly dispersed fillers are critical to avoid cracking. Care is required to make sure that the fillers remain in suspension throughout the encapsulation process. This includes taking appropriate precautions to prevent the fillers from layering, through settling in the mould, prior to gellation. Moulds that are heated from the bottom are prone to this. Thoroughly dispersed fillers and even heat throughout the mould are the answer to this problem. Settled fillers will cause drastically different shrinkage and stresses throughout the mass, not to mention the differences in the resulting tensile and elongation properties within the casting. |
| Insufficient heat dissipation around embedded heat generating components |
The strength of the cured material gradually drops as heat is applied. The most drastic change occurs around its Tg (glass transition temperature). If this occurs, in combination with applied mechanical stresses, cracking could result. The most effective solution is to provide a good metallic heat sink that is not encapsulated with the material. Epoxies and polyurethanes are insulating materials both from the electrical and thermal standpoint. Thermal conductivity can be improved somewhat through formulating techniques but usually not enough to accommodate severe requirements. |
Inconsistent Colour
Probable Cause |
Possible Fix |
| Separation caused either during transit or storage | Stir thoroughly to disperse pigment(s). De-air mix after stirring. |
No gel or will not cure
Probable Cause |
Possible Fix |
| Incorrect mix ratio | Mix individual components separately to disperse fillers. De-air after mixing. Confirm that the correct weight of resin and hardener are being mixed. |
| Resin and hardener not mixed well enough | Mix thoroughly scraping the sides of the container. |
| Incorrect resin or hardener used | Not all hardeners react with all resins. Check and confirm that the correct components are being mixed together. |
| Missing resin or hardener component | Check for blocked lines or inoperative valves on dispense equipment. Confirm that all the required components are present in the mix. |
| Insufficient heat applied or the pot life has not expired | Some systems require the application of heat to start the reaction. In these cases the reaction is extremely slow or does not start without the application of heat. Apply heat as stated on the technical data sheet. |
Click here for steps in troubleshooting: Curing and Gellation Problems
Pot Life is shorter than expected or is shorter than stated on the Technical Data Sheet
Probable Cause |
Possible Fix |
| The mix is off ratio
Click for details: Mix Ratio Related Problems |
Confirm that the mix is not resin rich or short on hardener. Make sure that the fillers are properly dispersed. Make certain that all supply lines are clear from blockage and material is not leaking past the seals on dispense equipment. |
| The processing temperature is too low | Cool ambient temperatures allow the heat generated by the reaction to be transferred to the surrounding air or a cold bench top. Removing some of the heat generated by the reaction will lengthen pot life and slow the curing process. Insulate the part from the cold or choose a more aggressive hardener. |
| The amount of material being mixed at one time is too small | Smaller masses generate less heat and slow the reaction. Heat the mix or the components being processed or choose a more aggressive hardener. |
Soft Gel or Cure
Probable Cause |
Possible Fix |
| Not cured long enough | Insufficient time or temperature. Smaller masses or assemblies that contain components, such as metallic inserts which can sink away heat from the curing material will cause the gel time to lengthen. Apply moderate heat or allow for a longer gel time. |
| Incorrect mix ratio (could be due to settled fillers). Click below for details: | By far the most common cause of this problem. Make certain that the fillers are dispersed within the mix, the resin and hardener are present in the correct amounts. In some cases it may be necessary to select a different hardener. Dispense machines with heated feed lines are prone to filler settling if the heated resin or hardener is left stagnant for longer periods of time. This could result in an off ratio mix. |
| Insufficient mixing | Check that the fillers are properly dispersed and that the hardener is thoroughly mixed into the resin. Concentrated pockets of resin and/or hardener will not react with anything and remain soft. Insufficient number of elements in static mix heads or improperly sized feed lines will also result in uneven mixing |
| Incorrect cure / gel temperature | Some systems require the application of heat to start the reaction. The gellation process will not commence or will not complete without heat. Warm the mix and/or warm the components and/or place the potted assembly into an oven if necessary. |
Click below for detailed troubleshooting: Epoxy / Urethane Curing and Gellation Problems
Soft Spots
Probable Cause |
Possible Fix |
|
Incomplete Mixing. Click below for step by step confirmation: |
Common symptom with malfunctioning dispense equipment or improperly mixed hand batches. Increase the mix time or trouble shoot the automated dispense equipment. |
| Lead/lag problems with dispense equipment. (incorrect amounts of resin or hardener present intermittently throughout the dispense cycle. Most frequently evident at the beginning or toward the end of the cycle-less often during.) |
The most common causes are incorrect back pressures that develop during the dispense cycle. Improperly sized or partially blocked feed lines, an incorrectly sized mix head, defective check valves or leaky seals will cause this problem. Confirm that the material reacts as specified when mixed by hand in a cup. If the product is fine when mixed by hand, it is time to overhaul the equipment. |
| Varying or too much back pressure in dispense equipment |
The soft spots appear randomly throughout the dispense cycle. Check for leaky seals, partially blocked lines and defective check valves. |
| Insufficient number of elements in static mix head |
Depending on the viscosities of the components, there are different number of mixing elements that are required to obtain a thorough mix. Most static mix heads are designed to allow sections to be added or removed to achieve the correct mix. Consult the supplier to obtain the correct part.
|
| Contaminant preventing complete cure in certain spots |
Certain chemical residues, coating materials etc. can interfere with uniform gellation and cure especially if these materials are not properly removed or fully cured and are able to mix with the epoxy. Make certain that all these materials have been processed according to supplier instructions and any substrates are thoroughly cleaned. |
| Incomplete cure | Material covering components that have the ability to sink the heat generated by the curing epoxy or polyurethane away from the mix will cure last. Surrounding areas, where this condition does not exist, will cure first and it will be necessary to extend the time to achieve uniform hardness throughout the encapsulant. If this time is insufficient, the areas over the heat sinking devices may remain soft leaving the appearance of soft spots. |
Surface blemishes and cosmetic defects
Probable Cause |
Possible Fix |
| Bubbles on surface | If the component being manufactured has a complex or intricate shape with a number of corners and curvatures, there is a good chance that the air will not be able to rise and escape fast enough from the mould cavity prior to gellation. This will result in small surface cavities that are cosmetically unacceptable. Increasing the number of strategically placed vent openings will go a long way to help eliminate this problem. Filling the tool or potting container more slowly will also have a major impact (keep in mind the pot life). Pouring the compound into one corner only and letting it rise slowly will also help to eliminate the problem. In severe cases, the only solution may be to apply vacuum after the part is poured but be careful not to strip the volatile components from the formulation as these vapours can also be the cause of surface blemishes. If the product has a long pot life it is possible to warm the mix to reduce the viscosity thereby making the release of air easier. |
| Surface blemishes caused by shrinkage | There are 3 key factors to look at. 1) The mix temperature should not be too high since the gellation process will be too fast resulting in increased shrinkage. 2) The mould and insert temperatures should relate to each other so that the material gels toward the center. 3) There should be enough of a reservoir containing cooler material to replenish the curing product as it shrinks. |
| Skinned over cavities near the surface | Chances are the tool or the container is too hot. The material gels extremely fast adjacent to the surface while the cooler material below acts as a reservoir. Cooling the mould or container would be the first step. Cooling the mix could also help. |
| Irregular patterns on the surface | The probable cause is too much or un-evenly applied mould release on the surface of the tool. If the mould release is silicone suspended in a solvent carrier, the cause could be that the silicone is not properly dispersed in the carrier. Applying the mould release in an evenly sprayed fine mist is best. |