Silicone Elastomers US Summit: Comparing Molding Techniques

At Extreme Molding, we offer both injection molding and transfer molding as part of our full-service range. The draw of efficiency coupled with high-volume manufacturing leads many of our partners to select injection molding for their products, but there are those who choose transfer molding for their products. During Lynn’s presentation she discussed the various properties and advantages of materials, as well as the differences between injection and transfer molding.

There are two primary materials involved in injection molding, compression molding, and transfer molding:

• heat cured rubber (HCR or HCE)
• liquid silicone rubber (LSR or LIM)

HCR has a high density (and therefore viscosity) so it feels a little like playdough to touch. It also is easily customizable and is fairly cheap ($2-4 per lb.).

LSR feels more like peanut butter, and ships from any supplier in 5 or 55 gallon amounts ready to use. There’s not a lot of choice for customization once your molder has it, mostly just color, and it’s typically $4-6 per lb.

When it comes to these materials there are a few ways you can use them to make parts:

• Compression Molding
• Transfer Molding
• Injection Molding

We offer all of these molding processes in our silicone manufacturing facilities in Albany, N.Y. because each one of them has their own place, and advantages, when it comes to production.

Compression Molding is the cheapest way to make a silicone molded part by far. We often recommend this for prototyping because it’s cheap and you can use either one of HCR or LSR materials. Some silicone manufacturers will tell you this method is labor intensive, and it is — a manufacturer must hand-load silicone into a two-plate mold before applying heat and pressure. While it is not suited to high-volumes, compression molding can be ideal for small volumes where precision is not the focus i.e. prototypes. The parts rarely look pretty, so we do not recommend this method for precision molding.

Transfer molding using HCR materials, and so milling is required before molding. Milling is a manual process and requires several end-over-end passes. It is used to freshen material, eliminating crepe hardening/structuring.

While Transfer molding is faster than compression molding, its cycle times are slower than Injection molding. Using HCR during transfer molding, most molds will not produce flash free parts, so this process also requires demolding and deflashing. Peroxide cured parts also require post-baking

Injection molding is by far the most efficient process and is geared towards high volume silicone manufacturing. It has a low labor content — the process is selectively automated to ensure efficiency — and so it’s by far the most popular choice of our partners. While this method is the most cost effective it’s worth noting initial set up is the costliest, which is why this makes the most sense for larger volumes. The parts injection molding produce are pristine and precise, which is a strong draw for businesses making pet, infant, or healthcare related products.

We often find people approach us to transfer mold a part because the molds are cheaper, making initial cost lower.  Taking a look at the graph below, we can see the attraction in terms of cost when it comes to transfer molding and compression molding.  While transfer molding is suited to low to mid volumes, when it comes to higher volumes of parts Injection molding takes the lead. The mold cost is more than the others but the cost per part is remarkably low.

When it comes to transfer molding, this process also shines with large parts— there comes a point when it’s hard to get big injection molded parts as you need a massive injection molding machine to do this. Transfer molding can also ease overmolding and is ideally suited for composite seals — so this process is popular in the hydraulic and electrical industries.

At Extreme Molding we have over 50 combined years of experience in business and industry. Our focus is excellent customer service coupled with the highest quality products. Partnering with the right silicone manufacturer can make all the difference when it comes to selecting the right materials and processes. If you’re looking for a rubber manufacturer able to best advise you on which molding you should undertake with your new product, contact us today.