Know Your Part Well Before Sizing Machine & Tooling
Originally published in Plastics Techology magazine, April 2010.
Industrial blow molding evolved from art to science with the marriage of PC controls and proportional hydraulic valves.
Today’s machines are highly efficient and predictable, and can generally be relied on to produce sophisticated parts from the first shot.
Newer parts are now being produced with 50-, 75-, and even 100-lb shot sizes.
Regardless of how technology advances, it’s still wise to brush up on some basic guidelines to help you get started, especially if you’re making a particular part of the first time.
First, determine the specifications of the actual parts you’ll be running. Will it require flash only on the top and bottom of the part? Or maybe the only way to make good parts will be to flash all the way around?
Project the actual finished weight of the part, and then estimate the shot size, taking the flash into account.
Flash only on the top and bottom of the part will mean a complete shot weight about 25% to 40% more than the final trimmed part weight. If the flash is all the way around, this could increase the shot weight by upwards of 60%. I have seen that reach 100%–double the final part weight—for certain parts, so be careful with your estimate.
In most cases, flash can be recycled back into the parts.
Shot size and cycle time will also influence the output requirement for the extruder. To make sure the machine is sized right for the job, a rule of thumb is to project the needed output capacity of the extruder at 80% of its maximum screw speed.
The extruder must be working well to ensure a low melt temperature. The hotter the material, the longer the cycle time.
The finished wall thickness of the part also plays an important role in cycle time.
If the part wall thickness is 0.060 in. or less, the cycle time will be in the area of 40 to 50 sec. A wall thickness of 0.080 to 0.100 in. will result in a cycle of around 60 to 70 sec. Very thick parts with walls of 0.120 to 0.180 in. could result in cycles from 90 to 180 sec.
Be careful, as these are only estimated guidelines. Depending upon tack-offs, for example, handles and very thick areas of the part will influence the overall cycle.
Another matter to be carefully considered is the size of the accumulator head that your part and process will require. Here are some guidelines you might find handy:
- Determine necessary head tooling size. You might require larger head capacity than output needs alone would indicate, in order to get proper head tooling size for the part.
- Find out the parison layflat needed based upon top/bottom or all around part flash. (Layflat = tooling diam. x 3.14 ÷ 2) This equation does not consider die swell or parison preblow inflation.
- Factor in parison die swell. Normally, larger tooling diameters have smaller parison swell. Small tooling produces a larger percentage of swell.
- Remember that competitive heads may produce different parison sizes for a given tooling size.
- Determine if there is a limit on how much regrind can be put back into the finished part. Some flash requirements might be more that the weight of the finished part.
- Based upon maximum shot size, determine head capacity needed. Consider what other parts might be run in this machine and to what specifications.
- Select tooling size based on head size. Determine if converging or diverging head tooling is needed. If using dual heads, determine the required head center distance.
- Next, you’ll need to identify the actual size of the mold, including any outriggers, cylinders for split molds, water connections, and blow pins. This is extremely important in the event you’ll be running dual heads with side-by-side molds on a fixed-head center line. The platens must be sized to fit the molds on the head centerline.
Now you are ready to determine the clamp tonnage required to mold your part. I use this as a guide:
- HDPE: 500 to 600 lb of force per linear in.
- HMW-HDPE: 600 to 700 lbf/in.
- PP homopolymer: 500 to 600 lbf/in.
- PP copolymer: 600 to 700 lbf/in.
Next, calculate the pinch clamp force needed to seal the parison:
Length of pinch x lbf/linear in. ÷ 2000
Find the blowing clamp force (to keep molds closed during air blow):
Projected area of blown section x blowing pressure
(around 100 psi) ÷ 2000
Remember that once you clamp up, the pinch clamp force ends and the blowing clamp force takes over. Don’t add these two values together.