Mold vs. Machine: Evaluation Considerations
By: Jason Travitz, Sr. Instructor, Processing
Much can be written about Mold vs. Machine criteria and selecting the correct machine for a given mold. This evaluation can become stressful for a molder because if one of the criteria is missed or incorrect, the mold may not run or will run poorly in the selected machine.
The Mold vs. Machine evaluation process may begin as soon as the mold concept is complete, or it may begin when the planner asks if the mold “can run in Machine 5 because it is open” and they urgently need to ship parts. The Molder’s objective may be to determine if a mold can run in a machine, or may be to select the optimum machine to run the mold.
Evaluation criteria can vary from determining if the mold will physically fit in the machine to an evaluation of the shot utilization, injection pressure, and clamp tonnage. This article focuses on the categories of criteria a molder may need to consider when evaluating a machine to run a given mold.
The Mold vs. Machine criteria fall into two categories – Go / No Go or Risk Level. The Go / No Go items prevent the mold from being run in the machine while the Risk Level items typically have ranges, guidelines, or rules of thumb, though they may be debatable.
Here are some considerations when evaluating a machine under each category of criteria:
Go/No Go Items
These items clearly have to “fit” to allow you to run the mold in a given machine.
• Tie Bar Spacing – does the mold fit between the tie bars in at least one direction?
• Mold Open Stroke – does the clamp open far enough to eject the parts?
• Min / Max Die Height – will the mold clamp up in the molding machine?
• Ejector Pattern – does the machine ejector pattern match the mold?
• Ejector Stroke – does the machine have adequate ejector stroke to eject the part?
• Platen Layout – does the bolt hole pattern allow for clamping the mold in the machine?
• If possible add the mold, platen, tie bars, and clamps in a 3D model for new projects to verify this before steel is cut
• Injection Pressure – is the pressure sufficient to fill the part(s)?
• Barrel Temperatures – does the machine have sufficient temperature capability for the given material?
Risk Items have ranges associated with them. Some of those ranges are likely industry or company Rules of Thumb. They may be considered optimum, but don’t necessarily indicate fixed limits that will produce good parts within the range and bad parts outside the range.
Take for example Shot Utilization. If the recommended range is 25%-65%, does that mean that 66% produces bad parts? Typically, this is not the case. What it does indicate is that as a process operates further outside the optimum range, the risk of producing bad parts increases.
I recommend giving these criteria a ranking – this could be a simple Red (no), Yellow (caution), Green (optimum), or a 1-10 ranking (1 least desirable vs. 10 most desirable) and present them to the team in terms of risk. A yellow or low-ranking item may create a risk – for example, a small shot utilization creates risks of degradation, shot-to-shot variation, etc. From there, the team can evaluate the risk items as a whole to determine whether or not a machine may be an ideal fit, or at least be capable of producing parts within spec.
Regardless of what criteria are being evaluated, a molder should have a comprehensive understanding of how the four pillars – material, mold, process, and part design – impact a given Risk Item. For example, consider the process with a Shot Utilization of 66% again. Is the material amorphous or semi-crystalline? What is the molding machine’s screw geometry? How fast is the cycle time? Does the material require drying? Understanding all of these items, and any others factors that arise during the evaluation, will help the molder assess the Risk Criteria and make a well-informed decision when evaluating their Mold vs. Machine.