Acrylic is a popular hard plastic, generally applicable for hearing aids with gain up to about 55dB. It is easy to insert and remove, and is usually also available in a non-allergic formula as well. An acrylic canal is rigid and may lead to sound leakage during chewing or other facial movement if applied to a high gain aid on a patient with a "flexible face." On the other end of the spectrum, acrylic earmolds are ideal for many open-fit applications.
The addition of a soft canal to an acrylic mold reduces unwanted feedback due to facial movement while retaining the advantage of the rigid earmold body.
Semi-soft molds made of a fairly rigid vinyl material which softens noticeably at body temperature are used for additional comfort while maintaining ease of insertion, or in an Acrylic/semi-soft canal arrangement where minor facial flexing problems are present.
Soft vinyl -- a very popular soft material where allergies are not involved. Excellent for active children in any fitting, as a hard mold could cause injury if the ear were to be struck while playing. Vinyl will shrink with body contact and age, and can harden and discolor as well. Tubing is often difficult to remove for replacement, and new tubing installation normally requires a highly toxic solvent if cementing is desired. Special locking devices can sometimes be used to eliminate the cement, provided they do not distort the shape of the earmold. In open-fit and RIC applications, soft vinyl earmolds are often desirable not only from the comfort aspect, but also as an excellent retainer of the receiver tubing from the hearing instrument.
Regular silicone solves most allergy cases, and is comfortably flexible. A tubing lock is required as cement does not adhere to silicone. This is the purest silicone available.
For the toughest power aid applications, use soft silicone material in a canal or canal-shell style. An accurate impression beyond the second bend is required, but the power/comfort relationship is usually maximized with this fitting. Because of its unsurpassed comfort, this material is replacing the Vinyls in many applications, even where high power hearing aids are not being used. Additionally, soft silicone is an excellent choice for many open-fit and RIC applications.
Polyethylene is a hard material, appropriate for severe allergy cases. Not always a thing of beauty, it may be effective in cases of allergies to silicones.
Consider too the potential need for additional modification once the mold is delivered to the patient. Acrylic, semi-soft and polyethylene are the easiest materials to grind and polish; the silicones are very difficult to change, except for straight cuts to remove excess bulk, without high-speed tools. Vinyl molds can be ground down with some level of success, meaning that they must not be re-inserted in the ear for extended wear without at least a 24-hour wait.
Although major in-office modifications can be minimized most easily by proper impression technique and intelligent style selection procedures, they can never be eliminated, and the fitter should retain as much flexibility and control as possible when choosing a material. Just as everyone can't wear Acrylic molds, not everyone should have silicone molds, either.