So what's the difference?
Comparison | Desktop | Benchtop | Industrial |
Tonnage Range | 3-5 Tons US | 5-9 Tons US | 7-10 Tons |
Average Footprint | 10” x 34” | 13” x 36” | 24” x 60” |
Shot Capacity | 15-20cc | 7 cc–30cc | 30-50cc |
Throughput (at 5cc) | 60 cycles/hour | 240 cycles/hour | 240 cycles/hour |
Decoupled Injection | No | Yes | Yes |
Injection Max Pressure | Unknown | 5000-15000 psi | 20000+ psi |
Process Controls | Limited | Advanced | Advanced |
Initial Cost | <$10K | $13K-$50K | $80K-$150K |
Maintenance Cost/year | <$100 | <$100 | $500-$1500 |
Amortized Cost | Very High | Low | Low |
Automatic Mode | No | Yes | Yes |
Tie Bar Spacing | 5” x 5” | 6” x 6” | 6.5” x 6.5” |
Portability | By hand | Push on wheels | Set by equipment |
Weight | <100lbs | 200-300lbs | 700lbs+ |
Different parts require different performance needs. Your part size and design will determine your needs. Let’s compare specifications and see what needs fit what type of machine you would need.
When going through the sets of workflow during the part and tool design step you come across your parts projected area this is the cross section of the tool when the a half and b half of you tool meet when clamping to create the sealed negative space of your part you are trying to create a sold of. This cross sectional area times the pressure applied (plastic injection pressure as described in the section of workflow from process building) will help determine the calculation of clamp tonnage (the mold closing force needed to contain the plastic being filled in the molds cavitated space). Smaller section of parts need less tonnage and larger need more. For example if the area of the part you trying to form has a projected area of 2 square inches and the force needed to fill your part in the mold based on material type, temperature and its thickness is 6000 psi then 2 square inches X 6000 pounds per square inch equals 12000 pounds of force being applied in the middle of the two halves of the mold. To counter act this force a minimum clamping force of 6 tons (2000 pounds equaling 1 ton) is needed an additional force of 15-20% is recommended equaling a total need of 7 tons. In this example your part would require a Benchtop or Industrial size molding machine.
Anytime you are working with machines they will take up a physical space. Now some of us are fortunate to have an abundance of available space to work with be it owned or rented but other either are limited to or prefer working in a limited space sometimes for the location or the up and down stream steps of the final product assembly don’t require it. So if you only have a few square feet less than 2 square feet to place your equipment and you also need the dynamic layout of your setup to move things around then a desktop or benchtop molding machine will meet your space and portability requirement needs.
How big is your part… Our should the question be how small? Since different materials have different densities it’s more common to describe the size of your in units of how much space it takes up also described as volume. Units of Cubic Centimeters aka CC’s is commonly used as most plastic resins have a very close weight of 1 gram for every cc of volume it takes up. As described in the process development section of the Workflow series, the ratio of the size of the circle pushing the melted plastic into the mold to the distance it pushes it to fill it called the stroke effects the precision of control when manufacturing your part. A molding machines with a large capacity or volume can make larger parts, but it’s not optimal for smaller parts because its more difficult to move and stop a larger circle a short distance quickly. A good rule of thumb is to use a machine that is in the 30-80% range of its volume. Example if your part takes up 3cc of space then a desktop molder with a capacity of 15cc would only use 20% (3cc/15cc * 100) of its stroke, using a short section of its injection which is difficult to maintain control. Using a benchtop molder that has a smaller diameter longer stroke injection barrel totaling 7cc would use 43% of its capacity increasing the ability to properly control the dose of material being put into the mold. Industrial machines that haven’t be customized for an additional cost have even larger shot capacities and would not be ideal for making smaller parts.
Does your part fit with or connect with another part? If your part interfaces with another part then expect it to have dimensions and those dimensions to have tolerances, a range of how close to the exact measurable dimension you are trying to achieve. When heating, melting, moving and cooling thermoplastics the factors used to accomplish this vary moment to moment from things like the environment of the composition of the resin material supplied to the level of moisture in the air. More factors are described in the Workflow series of polymer processing. These factors effect the way thermoplastics shrink under cooling and pressure changing the parts final dimensions. The more controls the molding machine has to counteract the environmental changes the better chance you have at maintaining the dimensional tolerance you are trying to achieve from part after part. Desktop molding machines tend to have limited controls and are not ideal for manufacturing plastics parts that have tighter tolerances. Benchtop and industrial molding machines have sufficient amounts of controls to manipulate the maximum range of thermoplastics shrinking condition.
What cost makes sense for my parts. It’s easy to think well the lower the cost the better, but there’s more to the cost of making plastics parts than the price of the raw resin, machine and the tool. One of the biggest hidden costs is waste. Waste of material and waste of time. Example you want to make 10,000 plastics parts to last one year of sales or use volume and these parts have dimensional control needs, have a projected area of 1.5 square inches and a volume of 4 grams. The thought is a desktop molder specs meet the needs of my part with an upfront savings vs a benchtop molder and even more so from an industrial molder. To produce 10,000 parts on a desktop molder would take about 160 hours or 20 eight-hour shifts and using these numbers you could estimate the cost of each part to produce but what is often forgotten is scrap. Because the unit has limited controls the parts being made may not meet its dimensions every shot meaning not all the parts can be used, which means you would have to highly monitor the parts being made and scrap plastic because of lack of control requiring a person’s time throughout the entire run. By choosing to use a benchtop molding machine with controls you can now produce parts confidently from the robust process developed described in the Workflow series and complete the entire volume in just 5 shifts and with minimal operator interface time saving thousands of dollars on scrap material and time. If you have the additional floor space, power and budget to produce this part with an industrial molding machine the only advantage would be to produce multiple parts at once cutting down the time to make all the parts needed into 2 or 3 shifts. This multicavity option would likely cost 3-4 times the cost of a benchtop unit at which point it would be more cost effective to run two benchtop units simultaneously if time was an issue.
So what molding machine is ideal for when? The ideal time to select a desktop molding machine for upfront cost savings is if your part is small but not too small between 5-15cc, the amount of parts you need is very low between 5-100, and it doesn’t need to be very dimensionally accurate. An example for a part that could be ideal for a desktop molder would be a custom toy action figure that doesn’t have any attachment options. On the other end the ideal time to select an industrial molding machine would be if your part is larger in size 15cc or greater or you need to produce a very high volume of parts in a short amount of time like 1,000 parts per hour or greater. For all parts in between a benchtop molding machine is the idea solution.