INJECTION MOLDING: How to Specify an Injection Molding Machine
With the market percolating, more molders are looking at new presses. This series will discuss what you should consider before writing a check, beginning with the clamping unit.
#scientificmolding #processingtips #bestpractices
The economic upturn is bringing the injection molding manufacturing market along for the ride. Demand for molded parts has risen, and in response many molders are up-grading or expanding their plants. This has increased the demand for new presses.
NPE2015 provided evidence, as several reports indicated more machines were sold from the show floor than ever before. An even stronger indication is that lead times for new machines are extending beyond six months, so it might take a year to get delivery of a higher-tonnage press. Make sure you buy what you need to support production, and not something that will add to your problems.
With all of these dynamics as a backdrop, I figured it was a good time to start a series of columns devoted to the issues involved in buying an injection machine. The list is long, due to the multitude of details necessary to get right. Buying the right machine for your needs is complex and mistakes are often made, causing lost production and increased costs to upgrade or retrofit. Let’s get it right the first time.
If your decision to buy is spur-of-the-moment, you’re already behind the eight ball. Do the following scenarios sound familiar? Somebody looks at plant utilization, sees a stream of new orders in the pipeline, and pushes the panic button to buy a new machine. Or an older machine’s controller dies, and a replacement is either not available or is as expensive as a new machine. Again, the panic button is pushed.
I would call these scenarios typical. The result is a rush purchase, which often means taking whatever is available from a machine builder’s stock inventory—and that often is limited. It is worse in times like today, when inventories are at a minimum and lead times are long. The result is buying whatever is available and trying to make it fit the application. Square peg, round hole. This often leads to disaster on the shop floor, with manufacturing personnel wondering why they cannot make production on a brand-new machine.
A better start is to plan for machine retirement. Even if the money is not available, the process of planning allows you to define your list of needs. It also saves time, even if a new order comes in. Planning on machine retirement will provide time to get the multitude of details established to properly specify the machine you need.
Specifying is different than settling for what’s available. Each machine is built with stock components, so make sure you get the ones that work for you and the special components required for your jobs. Further, specify it so that it is as flexible as possible to accommodate as many different molds as possible. Do not specify a machine to run only one job, unless you are 95% positive a single type of mold will run for its life span.
You can’t predict the future, so you must be able to adapt to change. How many times have you ordered a machine and auxiliary equipment for a big new project only to see the project die in three to six months? Is that capital working for you now? Think of how much easier scheduling would be if the above rule about flexibility were followed from the beginning. Yes, I can hear the screams from the offices of your financial people about how you wasted money on options that you will never use. But they are wrong—scheduling flexibility improves profits! It also avoids the explatives when you realize a mold will run in only one machine, and it is occupied with another job.
So how to you detail what is needed? To specify a machine, break it down into its component parts. Every mold is its own peculiar beast, and so are the major components of the machine. They are the clamp, controller, and injection unit. Each has its own function and needs to be specified to your requirements.
Try your best to keep your purchasing agent out of this process, as he or she will make decisions based solely on getting a better price. Let those involved with processing, molds and mold setup, and scheduling make these decisions. Follow the Deming principles: The people that use the equipment specify the equipment.
We’ll start with the clamp. The following is partial list of the criteria to review (with thanks to Kip Doyle and Bill Hartwick):
1. Toggle, fully hydraulic, or hydro-mechanical (two-platen) style?
2. Horizontal or vertical?
3. Core-pull requirements—electric, hydraulic, or hybrid? (New servo core pulls are available.)
4. Tiebar spacing required? Will it accept your molds or are wide platens needed?
5. Do molds occupy 70% of distance between tiebars? They should in order to minimize platen wrap.
6. Is clamp tonnage adjustable from the controller?
7. Is clamp tonnage is easily set in minimum time?
8. Is clamp tonnage sufficient? Thin walls or certain resins can change the rules. Have more than necessary; you can dial down. (Do not run full tonnage unless necessary.)
9. Clamp self-zeros, and reads actual position on controller?
10. Clamp is consistent in open stop position? (Can be a problem in some machines.)
11. Are clamp and platens of proper thickness and strength?
12. Do the platens provide cooling (temperature control) for hot molds? Actually insulating the mold from the clamping plates and cooling the clamping plate is better and cheaper.
13. Is clamp movement fast enough for cycle demands?
14. Is the auto lube system easy to monitor? Will it provide an error message if low on grease or the line is plugged?
15. Does the lube system use grease or oil?
16. Is the clamp force constant as platen changes temperature?
17. Is the clamp force measured, or calculated by position?
18. Do the ejector and bolt-hole patterns fit your needs?
19. Want a T-slot in the platens to be safe from stripping?
20. Is the drop bar functional and auto-adjusting?
21. Is the nozzle contact force adequate and adjustable? I like to see at least 3-7 tons of contact force available (<3 is questionable). This does not need to change with the size of the machine.
22. What are the correct minimum and maximum mold daylight—not only to clamp the mold but to get the part off the core? Note: magnetic platens reduce die height.
23. Can clamp movement be accelerated and decelerated without jolting the press?
24. Is ejection possible as the moving platen opens?
25. Are ejection velocity and pressure adjustable?
26. Is mold protection easy to set? Does it work properly?
27. Is clamp speed fast enough for cycle demands?
28. Is it important to be able to pull a tiebar?
29. Need coining capability, now or in future?
30. Is part-drop area open for part removal?
31. Will the mold need support? Check machine specs for maximum mold weight.
I’m sure I missed some. Just remember: Do not buy what is available; buy what you expect to need. And if possible, build in flexibility. Next month we’ll focus on the injection unit.
The polymers we work with follow the same principles as the body: the hotter the environment becomes, the less performance we can expect.
To properly understand the differences in performance between PET and PBT we need to compare apples to apples—the semi-crystalline forms of each polymer.
Molders should realize how significantly process conditions can influence the final properties of the part.