shrinkage of rubber compounds
TRANSCRIPT
SHRINKAGE OF RUBBER
COMPOUNDS
Luis Tormento
July/2017
Introduction
• One of the biggest problems encountered in the manufacturing of rubber goods is its shrinkage during the molding process or post molding.
• This phenomenon is commonly known as shrinkage and to obtain parts within the desired dimensions, it is necessary to increase the size of the mold cavity to compensate for such dimensional variations and to obtain an artifact within the required tolerances.
• A number of factors act to obtain a part within the required dimensional parameters.
Polymers
• For example, different types of polymers have their own shrinkage characteristics, but this is not the only thing that affects the shrinkage of rubber artifacts; we will discuss this below.
• To begin, we present below a generic table where we highlight the typical shrinkage of a given polymer; other factors have an influence on this shrinkage, and all of them are summative for the final shrinkage of the rubber artifact
Polymers
ASTM Designation Shrinkage Rate, %
ACM 2,0-4,0
AEM 2,0-4,0
AU/EU 1,6-3,3
CR 1,0-3,0
CSM 1,8-3,0
ECO 2,0-3,0
EPDM 1,9-3,5
FEPM 3,0-4,5
FKM 2,0-4,5
FVMQ 2,8-4,7
HNBR 2,0-3,0
IIR/CIIR 1,0-2,5
NBR 1,5-3,5
NR/IR 2,0-3,5
PVMQ/VMQ 2,0-5,0
SBR 2,0-3,0
Typical shrinkage of some polymers
Expansion duringcuring/vulcanization
• An important point is that during the heating and pressing steps to transform the uncured material into a finished product, there will always be some degree of linear expansion of the rubber compound. The problem is that the dies or molds required to withstand process temperatures and pressures are usually made of aluminum or steel - these metals have a significantly lower linear expansion rate than the rubber contained in the mold. While this difference in expansion helps to impart additional pressure within the mold, with consequent improvement in the curing conditions, it can also fracture weak molds.
Expansion duringcuring/vulcanization
• It also means that when limited by the mold during curing, with a size smaller than the rubber would have reached if heated in a relaxed state, when the piece is "removed from the mold", cooling of this restricted size means that the highest degree of linear expansion ratio of the rubber in relation to steel or aluminum, resulting in the shrinkage of the part, proportionally larger than the molding tool, at the end substantially less than the corresponding mold cavity.
Filler Characteristics
• The filler proportion inside a rubber compoundalso have a significant effect, mainly because its coefficient of linear expansion is smaller than that of steel and rubber.
• The fillers are used to increase the hardness and physical properties of the rubber compounds, so that more loaded a polymer is to increase its hardness, the smaller the shrinkage will be.
Filler Characteristics
• This is a common fact, where customer specifications for the same artifact requires different hardnesses with the same dimensional characteristics, i.e made from the same mold.
• If a part has tight tolerance, depending on its size and shape, large differences in size can be observed after complete cooling of it;
• In extreme cases this may require individual molds as different shrinkage rates may exist on parts of the same size but different hardnesses.
Crosslink Characteristics
• The curing process is necessary to formmolecular crosslinks, that impart to the rubber its "memory"; the way these crosslinks are formed for a given polymer can also affect shrinkage, and this is another factor that can influence the final size of any part.
• The contraction is influenced by the cure system, too:– Peroxide cure systems: have contraction between
0,8-1.0%– Sulfur-based cure systems may range from 0.1-0.3%
Crosslink Characteristics
• But, in the case of particular blends of polymers, variations may occur outside these orientations.
• However, in thin tolerance parts, even if it is something seemingly minor such as changing the curing system, it can influence the final size of the part.
Volatiles Loss
• The loss of volatiles is another aspect of the curing (and post-cure) process: polymers have plasticizers or curatives that volatilize from the material during the curing process - once extruded, reduce the volume of the material and result in shrinkage of the part.
• The quantification of the decrease of the part due to this effect can be controlled to some extent: since some plasticizers are more volatile than others, it may be possible to conceive a change in the final dimensions of a part without modifying the cavity of the mold; this can be achieved simply by changing the formulation of the rubber compound.
Textile Insertion
• When fabrics or staple fibers are introduced into a rubber compound to reinforce it, there will be:– Evident smaller shrinkage along the orientation of the
fibers, where they are aligned, and;– Evident greater shrinkage throughout the alignment,
where the shrinkage in the polymer pulls the closest parallel fibers.
• This may be a difficult effect to predict and control, due to polymer and fiber type combinations, and the weave / weave of any tissue structure, have a significant influence on the result, and laboratory tests are often the only way to predict the end result.
Textile Insertion
• This effect is also a problem when solid inserts are attached in a rubber component; The bond strength will restrict the rubber to the side of the substrate to which it is attached. The coefficient of linear expansion of the rubber compared to the substrate will cause contraction of rubber zones, not limited by a substrate.
Geometry
• The shape of a part can influence the degree of shrinkage of the rubber; long and thin pieces tend to have levels of shrinkage much higher than would be expected from more solid forms; Initial trials may be required to establish the particular characteristic of the product. Once this is done, for certain parts of the same polymer with similar dimensional requirements, the mold designs can be run with confidence.
Process
• The method of manufacturing molded parts can also be a problem. In compression molded parts, different shrinkage between the length and width of the material may be observed.
• If necessary, this effect can be attenuated with the use of transfer and injection molding; However, for some large or long artifacts, traditional compression molding is still the most suitable way of manufacturing these molded parts.
Conclusion
• There are many factors that influence the final results of shrinkage in the manufacturing processes of rubber parts.
• Even with years of experience and technical knowledge, it can be difficult to anticipate the results from any combination of tools or new material in particular.
• There are formulas and guidelines generated from various research projects carried out over the years, but it remains a gray area; The best way to achieve consistently reliable parts is the prior experience of process conditions and tooling design.
Contact
LT Químicos
Av. Pedro Severino Jr., 366 Cjto 35
04310-060 – São Paulo – SP – Brasil
Luis Tormento
NPD Director
Tel: +55 (11) 5581-0708