When overmolding liquid silicone to thermoplastics, one must consider the glass transition temperature of the plastic substrate. The higher the melting point of the plastic, the more options available to overmold liquid silicone materials onto.
Some of the thermoplastics that liquid silicone has been sucessfully overmolded to are PC (polycarbonate), PBT (Polybutylene Terephtalate), PPO alloys (Polyphenylene Ether/ Oxite PEEK, (Polyetheretherketone) and polyphenylsulfone.
Methods of bonding silicone to thermoplastics
The Use of Primers: Primers are typically applied some time prior to the application of the liquid silicone, the amount of time and application technique will be dictated by the primer to be used. Most liquid silicone suppliers offer platinum (and other catalyst) primers to support their line of liquid silicones. The problem with primers is that the application process is difficult to perform repeatedly. Also, in medical silicone applications the primer could affect the bio-capability.
Self Bonding Silicones: Many liquid silicone manufactures offer self bonding silicones, also called primerless liquid silicone. The adhesion promoters (bonding agents) are contained in the liquid silicone material itself, these adhesion promoters are typically included with a specific bond application in mind, in other words, the same silicone supplier may offer a different self bonding material for adhesion to polycarbonate than nylon. But with liquid silicone overmolding still being in its early stages of development and tremendously case specific, it is advisable to try different combinations and test for the best results. An issue with self bonding liquid silicone is their tendency to stick to everything, which makes releasing the finished part from the mold considerably more difficult. Also in medical silicone application the adhesion promoters could affect the bio capability.
Plasma and Corona Treatment: Pretreatment of the thermoplastic part with a plasma or corona treatment is a third alternative. This method of overmolding liquid silicone is new and is gaining acceptance. This is largely due to the fact that no expensive platinum based primers, or self bonding silicone is required. Additionally, the bond between plasma treated thermoplastic and liquid silicone is typically stronger than the bond between primed or self bonded materials. Plasma and corona treatment are also often viewed to be the more repeatable options, however they still suffer the problem of high temperatures, while exposure time is typically limited to fractions of a second, and exposure distance enough to maintain structural integrity of the part, there is still a risk of causing burn damage to the substrate. Plasma and corona treatment for bonding liquid silicone to substrates is very favorable for medical silicone application since there are no added chemicals.
While the three methods of liquid silicone bonding to thermal plastics each take a unique approach, they all suffer some shortcomings and it should be noted, that there is no reason why the methods could not be combined to result in a stronger bond than any one of the three is capable of. Albright Technologies will frequently apply all three methods, when bond strength is critical.
Other overmolding pitfalls to steer clear of are additives in thermoplastic resins. Many thermoplastics will have a letter “R” in their compound name, ie 123R, the “R” signifies that a release agent has been added to the plastic chemical compound to promote flow through the barrel of the injection molding machine. Resin distributors should also carry 123 (without the “R”), this is the same plastic compound but without the additional release agent. A certificate should be obtained from the resin supplier stating there are no releases in the plastic resin. While this resin may not run as smoothly and as quickly as resin with release built in, that release agent makes it impossible to overmold liquid silicone onto. It should also be noted that the addition of release agents to plastic resins has become so common place, that not everyone at the resin distributor that you are dealing with will know what the “R” stands for and that there is a difference between the material with and without the “R”.
Stand alone, spray on mold releases of any kind should be avoided (if at all possible) in both the plastic molding and liquid silicone overmolding steps in the process, as they can weaken the adhesion strength if it is permitted to contact the bonding interface.
Tests have indicated that bond strength of the liquid silicone to the overmolded part increases over time, ie the level of a liquid silicone adhesion measured when the part is removed from the injection molding machine will be inferior to the adhesion measured the following day.
There are many advantages to liquid silicone overmolding, including reduction of assembly costs of devices and the opportunity to design devices that otherwise could not be created. Silicone also acts as a protective cover against dust, water, impact, heat and electrical shock, as well as a biofriendly implantable barrier, between device and patient.