From a tutorial @ audiology online:
Acrylic/Lucite
Many professionals are likely familiar with three standard earmold materials: acrylic or lucite, vinyl, and silicon (Figure 12). Acrylic might be considered the old standby. This hard material has been around for a long time, and while a large majority of patients with more significant degrees of hearing loss were moving to softer materials for a while, more people are actually going back to acrylic these days. One main reason for this is the advent of better digital hearing instrument signal processing technology and feedback managers. Acrylic, generally speaking, remains the most durable earmold material. It does not shrink, break down over time, or harden with time (since it is already hard). Additionally, it is the easiest of the three materials to modify or re-tube and glue. From a patient standpoint, it is easy to insert and remove, and it comes in every earmold style and color available. Acrylic material is very easy to clean with most mild cleaning agents, and is resilient no matter how many times the surface has been cleaned.
Acrylic is best suited for patients with a softer ear texture. When considering ease of insertion, if the ear is very soft and the skin has lost some elasticity, acrylic material is the easiest for that person to maneuver. The only exception is the soft, delicate ears of infants and children. It would only be in a rare case that an acrylic or hard mold would ever be recommended for a child. The greatest disadvantage of a hard earmold for a child is the potential for injury. A 12 year old who is very active in sports from baseball to soccer would have a potential risk that a ball or other player would collide with his ear, and that hard earmold would injure the delicate ear structure. Generally, soft molds are more comfortable to children with growing ears, as well. There is no magic age where a child becomes able to wear an acrylic mold;however, infants and children learning to walk and navigate should never be fitted with a hard earmold.
A general disadvantage of acrylic material is mainly that it is not flexible. Although it is generally the easiest material to insert, since the mold will not compress it can be difficult to insert into very narrow canals. A flex-canal material can address that problem to some degree, however. Secondly, acrylic earmolds are more prone to acoustic leakage when the ear canal changes shape because they don’t move with the jaw. If someone with a significant loss has excessive jaw movement or the earmold does not stay tight during chewing, that user may experience feedback issues despite a good feedback manager in the hearing instrument.
Vinyl
Vinyl, also known as polyvinyl chloride (PVC), is a material that falls in between acrylic/lucite, which is the hardest earmold material, and the softest materials at ear labs. Vinyl is a softer material for soft or flaccid ears and works well for older individuals or children. It inserts fairly easily, although some argue not as easily as acrylic, and has the advantage of providing a better acoustic seal for high-gain instruments. It is easily tubed, but does require a glue to hold it in place. Some may argue that vinyl is not easily modified;however, a medium-coarse grinding stone can be used to trim and smooth edges quite nicely in the convenience of your office. The disadvantages of vinyl are that it does shrink, harden, and discolor over time. Very old vinyl earmolds that have hardened, may in fact, look and sound like an acrylic earmold as you tap it on your desk. Vinyl earmolds tend to turn a yellowish-brown with time due to individual body chemistry. Sunlight also speeds discoloration. Because of these inherent property problems, vinyl earmolds need to be replaced more often than acrylic or silicon. Most earmold labs cannot make vinyl earmolds in all of the bright multicolors or glitters, either.
Furthermore, vinyl earmolds are not generally recommended for use with patients who have allergy concerns. Many earmold labs offer a boil and saline option, which means the earmold is literally boiled, making the earmold a little more user friendly with people who may report allergies. Now, for the person seeing an allergist for airborne allergies, food allergies, and the like, they might be able to use a boiled vinyl earmold, but usually they require something more neutral such as silicon or polyethylene.
Polyethylene
Polyethylene is a semi-hard waxy material, with an appearance much like candle wax. The color resembles an older milky-colored ear hook on a hearing aid. This is the earmold material of last resort for allergic cases. Generally, we encourage you to try everything else before using polyethylene because it is not cosmetically appealing, and it is difficult to modify. It is a very safe mold for extreme allergies, but there are also no color options, and style options are somewhat limited. Keeping a tube in place has also proven to be very difficult with this material, so you may have frequent tubing problems or remakes back to the lab. But, all things considered, if you have a patient who is absolutely unable to wear anything else, consider the polyethylene earmold.
Silicone
Silicone has many advantages. It is very flexible and comfortable, tight fitting for high-gain instruments, and durable, especially when compared to acrylic. Very little change in shape or size occurs over time and it is an ideal material for allergy cases (when ordered in a neutral color or clear with no glazing or high-shine). The disadvantages of silicone include difficulty of insertion, especially when it is brand new. Because it is quite flexible, it almost has a rubbery feel when pressed to the skin and can grab the skin upon insertion. One way to address this is by ordering a matte finish instead of a glazed finish, which will slide into the ear much easier. A second way to address this is by applying a water-based lubricant to the canal portion of the earmold before insertion. Silicone is not appropriate for patients who have very soft or flaccid ears because it grabs the loose skin and pushes back, making it difficult to get the earmold in with a good fit.
While silicone is very soft and flexible, these characteristics make it difficult to modify and glue tubing. Making in-office fit changes to this earmold is more difficult than to earmolds made of acrylic or vinyl material. It takes a grinding motor with high RPM speeds (20,000 RPM or higher), which many offices do not have, to modify silicone earmolds. It is also difficult to glue tubing into the sound bore of a silicone earmold. On the original make of the earmold, the tubing is actually not glued. Most earmold laboratories use a plastic or metal/brass retention device of some sort on the tube. This type of tubing is referred to as a TRS (tubing retention system) tube or tube lock (Figure 13). There is also friction tubing, in which the tubing is tight in the sound bore where tugging does not release the tubing easily.