When you think about injection molding, you probably don’t envision all of the options to injection molding. The most common type you see in manufacturing is cold runner, or just cold injection molding. It consists of two mold halves that molten plastic is forced into to form a part or multiple parts. The cold reference in this type of injection molding is the cooling runners with water or coolant flowing in them to help cool the part after the injection molding has been completed.
The second type of injection molding, known as hot injection molding, is similar in process but does not use a cooling method in the same respect. Hot injection molding was invented in the early 2000s in Japan due to the stringent surface-quality requirements in their demanding electronic consumer goods. Many of the consumer products required very thin walls to reduce weight, yet the parts were very large and had to ensure zero surface defects. The process was developed in collaboration of multiple companies, with Ono Sangyo Co. patenting the Rapid Heat Cycle Molding (RHCM) process in 2002. Later in 2009 Matsui Manufacturing Co. joined the RCHM collaboration and brought to market a new controller for the hot injection molding process. Since 2009 this technology has become the standard process among Asian consumer electronics manufacturers for large thin-walled products and uses steam as the cavity-heating medium.
The best-known product that uses the Rapid Heat Cycle Molding (RHCM) process is a flat-screen television bezel. Prior to RHCM being developed, TV bezels were injection molded, then painted and clear coated. The paint would hide any molding defects and add color before the clear coat would add a high gloss finish. The RHCM process now allows for a defect free part with a high-gloss color finish without the extra secondary processes to achieve similar results. These eliminated steps enabled significant cost reductions of painting operations, VOC emissions, and any extra handling required.
These benefits were proven well in the Asian electronics markets, but it took time for the concept to move to other areas of the world. European companies brought to market a similar technology of hot/cold injection molding called Variotherm. That term is a registered trademark of Hofmann Innovation Group in Germany. Options for hot or cold injection molding has gained steady acceptance for automotive interior applications and many other consumer-goods applications requiring high surface aesthetics with minimal defects.
What Heating Options Are Available with Hot Injection Molding?
One of the greatest challenges of conventional molding is the immediate cooling of the molten plastic as it enters the mold. The molecular structure of the polymer immediately changes due to the rapid cooling of touching a colder surface, and it tends to lose some gloss on the surface too. The cooling causes the polymer to additional gain viscosity as it flows, which makes it increasingly difficult to fill a thin-walled mold effectively. The benefit to hold/cold injection molding is to keep the viscosity constant and prevent the surface gloss from changing due to a cold surface. Cooling the part after complete molding is still done with chilled water or coolant, but the medium to heat the mold surface has four main options: hot water, hot oil, steam, and induction heating.
Option 1 – Hot Water
One of the more popular options is pressurized hot water. It typically requires the least capital investment and equipment changes, plus doesn’t require changes to tooling or tool design. The cooling channels in the tool are used to circulate hot water instead of chilled water, and that keeps the mold a consist hot temperature. The low cost is an advantage, but the water can only achieve a maximum temperature around 320F (160 C) without extended cycle times. Hot water as a heating medium is a good choice for semi-crystalline materials but may not be a good option higher-temperature amorphous materials or for molding applications with short cycle times or that involve foaming.
Option 2 – Hot Oil
When hot water doesn’t achieve the high temperature needed, the next step is to look at a hot oil as a medium. Hot oil has a very high heat-transfer characteristic and can be heated to approximately 608 F (320 C). That is almost double the temperature of hot water, and it makes it a perfect match for polymers such as PEEK and those with high-carbon content. One of the downsides to using hot oil, at least in a production environment, is that the injection mold should be designed for hot oil from the beginning. A pre-production or prototype mold can be retrofitted for hot oil as a medium, but on a larger scale it won’t be cost effective due to the long heating and cooling times. If you’re unsure if hot oil is the best heating medium for your project, be sure to reach out to us.
Option 3 – Steam
Steam, as mentioned previously at the beginning of the technology, is the longest used medium for heating hot/cold injection molds. It provides a very economic option for all levels of production. One of the major economic benefits of steam is the heat energy it carries. It has more than six times the heat energy of hot water and eighteen times that of hot oil. That difference can speed up cycle times and decrease the cost of every part produced. The biggest downsides to steam is the temperature is only slightly higher than hot water, and it does require a separate controller and boiler to make the steam.
Option 4 – Induction Heating
Electromagnetic induction is the latest technology that has entered into the hot/cold injection molding process. This technology enables the molder to hold precise temperature anywhere on the mold, including varying the temperature at multiple points. This can enable rapid temperature cycling and high maximum temperatures of 400 F (over 200 C). There are multiple ways induction heating is used for injection molding, from induction coils built into the mold to holding coils directly in front of the cavity side of the mold before the mold cycle starts. The induction technology can be added to existing tools for an economical price and it won’t affect productivity significantly.
The Best Option for Your Project
Determining which of the mentioned technologies currently available is going to be a challenge. Cold injection mold has been in use for decades, and has shown economic benefits for both small and large molding production. Hot/cold, or just hot for short, has less of a track record, but is currently used across the globe for difficult molding conditions. The first decision for your new project needs to be which molding option is best based on your resin use, your production volume, and a handful of other criteria to achieve your required results. If you have the idea, but are stuck from that point on, we’re here to help sort through all the options for success. Our elite team produces world-class prototypes and products, so call us at (425) 339-0288 or email us at info@sealectplastics.mystagingwebsite.com. We look forward to offering support and advice on your next project.
Matthias Poischbeg was born and raised in Hamburg, Germany. Matt moved to Everett, Wash., after finishing his bachelor’s degree in business in 1995 to work for Sea-Dog Corporation, a manufacturer, and distributor of marine and rigging hardware established in 1923.
In 1999, Matt took over the reins at Sea-Lect Plastics Corporation, a sister company of Sea-Dog and a manufacturer of plastic injection molded products with an in-house tool & die shop. Matthias Poischbeg is also a contributor to Grit Daily.