Rotational Molding – Growing Global Applications
Rotational Molding is both an industry and a process that is based in the use of thermoplastic resins. Chem-Trend has developed a long-standing reputation as a leader in the support of rotational molding around the globe through our development of high quality and high performance release agents. The rotational molding process (rotomolding) enables relatively small volume part production for the following types of applications;
Bulk fluid storage
- Water, oil, gasoline and other bulk storage tanks and header tanks
Industrial and domestic cleaning equipment
- Covers, guards for cleaning machines, janitor’s carts and trash carts/cans
- Point-of-purchase display structures
- Dip tanks, pesticide dispensers, feed troughs, vehicle mud-guards
- Water-slides, cars, play houses and accessories
- Storage bins, gasoline tanks, mud wings and fenders
A large percentage (90% plus) of the polymeric resin consumed in rotational molding is polyethylene in various densities. However, as rotational molding gains popularity around the globe, a wider variety of thermoplastic molding resins is becoming available. Polyethylene will likely remain the dominant resin in rotational molding, since total consumption is relatively small compared to injection molding of thermoplastic materials, and because there are only a few dedicated suppliers to support demand for rotational molding grades of the plastic. From an economic perspective, there is little incentive to engage in the development of other polymers due to current low levels of demand.
The majority of rotational molded products are hollow. Rotomolding is a technique suitable for products that need a good cosmetic external appearance but do not require the inside surface to be as well finished or aesthetically pleasing. A certain degree of variation in the wall thickness of products can also be tolerated, even though rotomolded parts are becoming more and more dimensionally controlled.
Release Agents in Rotomolding
The release agent plays a critical part in the correct formation of rotomolded parts. It needs to perform several tasks, some of which can appear to be conflicting:
- Provide a protective barrier for the metal surface of the mold.
- Allow the molding resin to become attached to the mold wall (of course, the release agent always needs to be present between the mold wall and the plastic resin) during the initial heating phase of the molding cycle.
- Ensure that the fully molten skin of the formed part remains in close contact with the mold wall throughout the heating cycle and the initial part of the cooling cycle.
- Ensure that the formed part stays in close contact with the mold wall until the part has sufficient cohesion within its structure to resist collapse when the release process begins.
- Allows the molded part to release cleanly, quickly and evenly, once the release process actually begins.
- Allows the molded part to slide out of the mold without any surface abrasion damage.
Chem-Trend has been in the business of developing and manufacturing release agents for some of the most challenging industrial applications for over fifty years. Our line of semi-permanent mold release agents for rotomolding are specially designed and formulated to perform all of the above tasks. As well, our release agents for rotational molding come in both solvent and water-based forms to meet the specific needs of our customers.
Challenges for Rotational Molding Release Agents
Producing a high quality, dimensionally-controlled rotomolded part would be easy if all parts were the same shape and size, but they are not. For example, a gas tank for a marine application usually needs to be molded in a shape that will allow it to fit into a very irregular and tight space envelope. A water storage-tank, on the other hand, is necessarily large and quite regular in shape. The demands placed on the release agent for these two applications are very different. The marine gas tank will usually require a release agent that provides maximum assistance in releasing the part from the mold, whereas the large water storage-tank requires a release agent with very different characteristics. Based upon our many years of shop level experience at our rotational molding customers’ facilities, Chem-Trend understands the problems you experience and develops solutions for those problems from among a wide ranging portfolio of customizable products.
Among other things, our products help you address production problems such as:
- Poor release around flanges and vent-tubes
- Double skin parts
- Parts exhibiting “swirls”
- Poor gloss level
This can be a particular concern when molding large regular-shaped parts, such as water storage-tanks. The polyethylene wants to release from the mold wall much earlier in the cooling cycle than is desirable. When this occurs, it leaves large areas with an undulating or even distorted appearance. The plastic is not stiff enough to support itself at this stage. Chem-Trend has developed specific products that promote the plastic staying in contact with the mold wall for a longer period during the cooling cycle. This ensures that large areas of the molding retain their correct shape. Being able to choose a release agent to do this job puts the molder in control of the process.
Poor release around flanges and vent-tubes
Flanges and vent-tubes are areas of the mold where super-easy release is required. When the mold is opened, the flanges should separate cleanly. Similarly, the vent tube should easily slide out when required. If plastic is trapped on the flange, above normal effort will likely be required to release the flange. Chem-Trend has developed specially formulated release agents for the flange and vent-tube. Ongoing use of these products from the day the mold is initially commissioned will minimize the chances of the mold flange distorting due to stuck-on plastic. Treating all thread-forming inserts with the same products not only allows easy extraction of the insert, but ensures that the plastic forms perfectly around the threads, providing an extremely smooth finish. Cut-out areas in molds are often places where plastic is encouraged to form a thin skin. Use of these products on these areas of a mold can ensure that this skin is easily removable.
The molded part wants to pull away from the “female” part of the mold, but the plastic shrinks very tightly on the core – or male part – of the mold. Chem-Trend has developed highly-lubricious conventional release agents for these circumstances to ensure that the core of the mold can easily be removed. This type of product is also useful in molding pallets, which have many inserts designed to form “kiss-points” within the pallet’s structure. These inserts can be notoriously difficult to remove, but release easily with these specially formulated Chem-Trend products.
Parts exhibiting “swirls”
Parts molded with dry-blended pigmented resins often exhibit high and low-pigmented molded walls that manifest as “swirls” when viewed with back-lighting. The phenomenon is readily evident when different-colored pigments are used to provide something like a speckled effect on the surface of the molding. The “speckles” often migrate to form unsightly concentrated areas. The phenomenon is caused by static electrical charge build-up during the initial stages of the heating cycle when the molding resin is still in powder form and “tumbles” over the mold walls. Separation of electrical charge occurs within the pigment and cannot discharge anywhere (this situation can be exacerbated by the mold release agent). As a consequence, some pigment particles try to form agglomerates due to concentrations of unlike charges. Chem-Trend has developed a special coating that can be applied to the mold before every molding to avoid this problem. The coating we developed dissipates the electrical charge, thereby eliminating pigment swirls.
Poor gloss level
Certain parts require high gloss levels, but often, post molding, the gloss level proves to be incorrect. Chem-Trend has developed a special primer product that is easily applied to a clean mold surface before the release agent is applied. The extremely high gloss, which is fixed upon the mold surface is then replicated onto part after molded part. Regular re-applications of a corresponding specialized Chem-Trend mold release agent will ensure that the gloss level remains intact for a long period of time. Using this special primer reduces or even totally eliminates the need for flame-polishing of molded parts.
A Lot More than Release Agents
Our history in the rotational molding industry, combined with our many years of experience on shop production floors, means that we can help you optimize your mold release application to improve your bottom line. When you purchase Chem-Trend products for the rotomolding applications you get a lot more than just products, you gain access to a worldwide network of industry professionals committed to developing the right solutions and providing excellent support. It’s all part of our commitment to providing our customers with greater value, performance and dependability.
To find out more about how our products and expertise can help you improve your rotomolding operations contact us.