Martin Larsen, director, Rich Plastics provides insight into five critical steps in ensuring your medical injection moulded product gets to market on time.
Developing plastic injection moulds for the medical industry requires following certain steps (below) and understanding the purpose of each. Short cuts in the process are often made because of the pressures to release medical tooling quickly.
Imagine this process scenario: you produce a good tool, you run tooling trials and after a few modifications you have dimensionally acceptable parts leaving you to think you can get the tool into production. If this sounds familiar, you may have overlooked the critical steps of process analysis. Completing these steps will enable you to understand your processing window and will guarantee reliable and quality moulding production as you release this into mass production.
Vallard;Steve
Figure 1: Flow chart from design approval to ‘OK to ship’.
There are five main areas that come into play once the customer gives the ‘OK to cut’ order (Figure 1):
- steel measurements
- electrode measurements
- part critique list
- moulding process analysis and documents
- mould inspection.
‘OK to cut’ is a term used to signify that all the design reviews and analysis are complete, the customer and tooling supplier have frozen the design, steel can be ordered and component production can start on the steel.
Step 1: steel measurements
Once the machining of the steel has been completed it is essential to determine that the tooling has been cut to the correct shrinkage, which is dependent on the resin material used. The steel dimensions need to be checked and verified on the coordinate measuring machine (CMM). This must be done before any tool moulding takes place, because to mould the tool with the incorrect shrinkage would be pointless.
Step 2: electrode measurements
To alleviate any recutting of the core, cavity, slides and lifters due to incorrect machining, all electrodes manufactured by the supplier must be measured (to ensure the part is within tolerance) and the intended spark gap.
Step 3: part critique list
Vallard;Steve
Figure 2: An example of a tooling approval document.
A document called a “tool approval tracking sheet” (Figure 2) will capture all issues with the tooling during the first trial and will serve as the checklist of actions to complete before a tool can be released. It will document all the open issues (errors found during the trials) discovered on the mould tool itself, as well as the part dimensional issues. It will also include the issue’s location on the tool/part and a small description of its cause.
A countermeasure will then be added to the document at the meeting following the tool trial. The customer should be involved in the countermeasure discussion as part changes may be requested. Assigned responsibility for the countermeasure will be added along with the proposed completion date. Each tool trial will consider the issues; any others found during each trial will be added. Only when all issues are corrected will they be closed and the tool considered ready for shipping.
Step 4: process document
This document is used to optimise the process needed to develop stable moulding and produce balanced and directionally acceptable parts that are free of cosmetic defects — such as short shot, sink, drag, burn, pin push — at the best achievable cycle time.
The process document considers each of the following:
Mould information: a quick summary sheet of the tool and the runner type/resin used.
Mould water flow data: used to confirm the water flow through all water circuits on the tool to help avoid the appearance of hot spots on the tool, which cause process issues.
Vallard;Steve
Figure 3: An example of process set-up documentation.
Process set-up sheet: although injection moulding machines vary in size and brand, the programable logic control systems (PLCs) all have the same functions and the data you program into them will be the same. The process set-up sheet provides the process data that will be entered into the PLC each time you set up a moulding press to run production (Figure 3). The results will be the same regardless of whether you are running the tool in different moulding presses within your factory or in a different country. The document contains all the necessary information to prepare the tool in readiness for production runs. It also provides the information needed to get the resin ready for use (including drying times and temperatures), the press that the process was set up on (including barrel capacity and screw diameter), the runner system being used and the temperature of each zone on the tool, and the press settings.
Short shot and cavity balance study: this analysis is performed to understand the cavity balance at full shot weight and to study the resin flow front (the phase-volume averaged velocity of the liquid resin). The study is done at 25, 50 and 75% and at cut off normally 95–98%. A good balance keeps the internal pressures in the tool even and stops over packing of the parts. This is achieved by runner and gate size changes.
Vallard;Steve
Figure 4: An example of gate freeze analysis.
Pressure loss study: needed to show there are no high-pressure areas inside the nozzle of the moulding machine, the hot runner system or through the gates into the part. Nozzle, sprue, runner, and part measurements are taken using the peak press hydraulic injection pressure, also referred to as pressure loss. The location of the high pressure will determine whether modifications to the nozzle, runners or gate size are required to reduce this pressure to an acceptable level. If the gate sizes are changed, they must all be opened to the same amount; the amount depends on the gate style, each of which has a different maximum size depending on the material used to keep the cavity balance.
Gate freeze study: used to determine the minimum hold time required to cause th
Vallard;Steve
Figure 5: An example of a rheology curve study.
e solidification of the gate (Figure 4). With the gate frozen, the part weight will stay the same even if the hold time is increased. If this happens before the part is full and packed out, then sink and short shot will appear on the part.
Rheology curve study: used to plot polymer viscosity versus the shear rate to determine the optimum fill time (Figure 5). Start with the maximum fill velocity the press will allow. Once the viscosity curve calculation is complete, the fill speed can be selected on the flat side of the curve to ensure that any viscosity changes in the material will not affect the process.
Mould temperature study: this secondary check of the tool temperature will test whether the water circuits are performing as planned. Heat mapping of the tool will show the different temperatures on the tool surface; the more uniform this is, the better the process window. Typical hot spots tend to occur on the nozzle areas.
Step 5: mould inspection
Figure 6: An example of how a finished tool will look.
Mould inspection typically happens after the first tool trial to find any areas in the tool that need adjusting or fixing (Figure 6). When the tool is given the ‘ok to ship’ it will be totally stripped down and each part inspected to ensure no damage occurred during the trial, and cleaned before rebuilding the tool so it arrives at the designated moulding area ready for mass production.
Summary
Following the above processes provides a clear understanding of how a mould will perform with the materials provided by the resin supplier. Once the verification is complete you will have developed the process parameters and established a performance range to ensure your parts fall within the specified quality limits (tolerances). The tool can now be transferred into mass moulding production with confidence — you have developed the widest possible process window to produce the most economic and efficient injection moulded plastic parts for complex medical applications.