In the world of consumer goods, we rely on engineers and designers to select the plastics molding process that will deliver a functional, robust product that looks attractive at a reasonable price. For example, no consumer goes into a super market and says “Can I get my shampoo in an injection molded container?” We trust the experts.
However, in the B2B world we often have options. We are able to satisfy a need by purchasing a product made from a variety of plastics processes. And with a little insight, you can make more informed decisions about which plastics molding process is best suited for your food processing applications.
Cross-referencing factors such as size, strength and quantity against the four most common molding methods will provide some simple “rules of thumb” that can guide you towards better decisions.
The plastic processes we will concern ourselves with are: injection molding, blow molding, thermoforming and rotational molding.
Injection Molding
As the most widely known process, injection molding requires very expensive molds carved from steel. Two blocks of steel are machined so that a portion of one block fits inside a cavity cut into the other block. In some areas there is a gap created, and this gap is in the shape of the desired finished product. Melted plastic is then “injected” through a small opening until it fills the gap. The mold is held in a large clamp and remains closed as cooling fluid is circulated through the blocks of steel to bring the temperature of the plastic back down to the point that it becomes solid again. Finally, the finished part is removed.
Advantages of Injection Molding
- Fast production time, therefore unit pricing tends to be relatively low.
- Parts are reasonably precise because the mold defines every surface.
- Offers the broadest range of materials with differing physical properties.
Limitations of Injection Molding
- Tooling is very expensive and can’t be justified unless there is demand for very large quantities.
- Equipment is expensive.
- Parts are more brittle because the plastic is forced into the mold under high pressure, producing a part with internal stress molded in.
- Large minimum orders due to long machine setup times and long production runs.
- Color changes are time consuming and expensive.
- Hollow parts are not possible.
- Stiffness is achieved by adding ribbing to parts.
Blow Molding
Demand for plastic bottles for soda, detergents, milk, shampoo etc. have driven the process of blow molding to be a significant consumer of plastic materials. Two blocks of metal are carved so that when they come together they create a void that is the outside shape of the desired part. The two blocks are separated and a tube of molten plastic is suspended between them. They are then closed together and a small pin penetrates the molten tube and air is pumped in. Pressure from the air forces the molten tube to expand and fill the cavity. Cooling the blocks then solidifies the material so the blocks can be opened and the finished part is removed.
Advantages of Blow Molding
- Fast production time, therefore unit pricing tends to be relatively low.
- Tooling costs less than injection molding (though still expensive).
- Hollow parts are possible.
- Broad range of materials with differing physical properties are available.
Limitations of Blow Molding
- Tooling is still relatively expensive.
- Machinery is expensive.
- Large minimum orders due to long machine setup times and long production runs.
- Walls thin out as the material stretches to corners.
- Color changes are time consuming and expensive.
- Not as precise as injection molding.
Thermoforming
Packaging is also the primary driver of thermoforming. A very simple process, thermoforming calls for an open cavity carved out of aluminum. A sheet of plastic heated to its softening point is positioned over the cavity and then a vacuum is drawn which sucks the material down into the cavity, giving the sheet of plastic the desired shape. The mold is cooled to solidify the plastic, and then the molded part is removed.
Advantages of Thermoforming
- Tooling is inexpensive.
- Production runs can be relatively short.
- Color changes are easily accommodated.
- Can employ a variety of materials, with varying properties.
- Machinery is inexpensive.
Limitations of Thermoforming
- Slower processing time than that of injection or blow molding, creating higher costs.
- Labor intensive.
- Hollow shapes are not possible.
- Thin corners are created as the sheet stretches to the far reaches of the mold.
- Expensive to accommodate small quantity orders.
- Difficult to mold precision parts, especially as size increases.
Rotational Molding
Toys were at the forefront of the development of rotational molding, but industrial applications really took off when materials improved to the point where molders could produce large chemical/agricultural tanks in the early 1970’s. The basic process starts by putting the desired amount of plastic powder into a hollow mold. That mold is then tumbled slowly in an oven until the plastic melts and sticks to the walls. The mold is cooled, forcing the plastic to solidify in the desired shape, and the part is removed.
Advantages of Rotational Molding
- Capable of a large range of sizes.
- Inexpensive tooling.
- Can create thick-walled, robust units.
- Changing wall thickness does not require a change to the tooling.
- Corners and edges tend to be the thickest area of the unit for great durability.
- Small production runs are feasible.
- Machinery is inexpensive.
Limitations of Rotational Molding
- Slower production time than other methods.
- Labor intensive, creating higher costs.
- Limited materials are available.
- Precision tolerances can be difficult to hold.
- Huge quantities are not feasible.
Which Container Type is Right for Your Food Processing Facility?
Each food processing facility will benefit from these molding processes differently.
The need for injection molded products in the food processing industry is created when extremely large demand is combined with the need for low cost (driven by disposability) and precision. Examples include caps for every liquid consumer product that comes in a bottle, and single-trip, light-duty pallets in applications where getting the pallet returned is not economically feasible.
Blow molding shines in large quantity applications that require a hollow part—the classic definition of a bottle. Products like soda, water and condiments are the prime end markets for blow molding because this process can produce large volumes of thin-walled units in very short periods of time.
Thermoforming contributes substantially to the world of packaging in high volume applications that do not demand precision. Yogurt cups and the shallow trays used for meat and poultry are products that maximize the efficiency of this process.
Rotational molding finds its food processing sweet spot within the factory, creating heavy duty, reusable products that generate extremely high levels of ROI. The robust construction of rotationally molded bins and totes allow them to survive years of continuous service in applications requiring ease of cleaning. Rotationally molded products are waste fighters that punch in both directions; they can replace stainless steel products at a fraction of the cost, or they can replace disposable corrugated options with payback periods calculated in months, not years.
The chart below summarizes a few of the key points to consider when deciding which type of plastic bulk container is the best fit for your needs. Comparing your long-term food processing needs with the capabilities of each plastic molding type can lead to overall savings and fewer headaches for your business.
About the author
Peter Connors is the President of Remcon Plastics, Inc., a plastics manufacturer specializing in material handling solutions for the food processing and pharmaceutical industries. Remcon’s rotational molding capabilities include CAD design, product prototyping, custom mold development, manufacturing, and more.
