Does your production require the use of tools such as plastic injection molds? Here are some tips on how to plan for using the right tooling and for inspecting their proper use.
1. Getting the Specification Right
When planning your injection mold project, it is important for you to understand the key aspects of the process and try and include these within your Mold Tool Specification or Tooling Agreement, that way, the toolmaker should provide you with evidence of your requirements as part of the tooling package.
Example of a few key aspects for a tool would be:
- 1.1 Tool material selection
Proof of material being used (this is normally provided with a chemical composition analysis report showing the percentages of the chemicals included in the material). - Hardness of the material (it is important to ensure the correct hardness is achieved and this should be provided in a hardness test report).
1.2 Standard parts being used
- The use of standard parts is an important aspect when constructing a mold tool because any parts that need replacing can be quickly swapped out without the need for special manufacturing. Parts would include cooling system connectors, ejector system and pins, heating system, pneumatic system, air valves, locking components, guides and bushes etc…
1.3 Use of sliders within the tool
- If there is a need to have sliders in the tool (these are parts of the tool that move in and out in order to create undercuts or lips or slots in the finished molded part), ensure that non-like materials are used and that grease grooves are included for lubrication. It is important that non-like materials are used because of the wear issues. If like to like materials are used it causes what is known as galling (this is where the two surfaces generate friction, heat and ultimately the surfaces will break down and small balls of metal will start to form), this results in damage to the tool and consequently issues with molding the part.
1.4 Shrink rate of your polymer
- When the tool is being designed, the tool designer will take your 3D CAD model and use that as the data to make the mold. Another aspect the designer will take into consideration is the polymer you have specified and they will calculate the shrink rate of that polymer and include this value into the dimensions of the tool.
- Final dimensions of your part will be produced by processing the polymer through the injection mold cycle. Any change of polymer AFTER the tool has been finished could result in your part coming out with different overall dimensions due to the different shrink rate of the alternative polymer.
- Polycarbonate (PC) for example has a shrink rate of around 0.5% ~ 0.7% whereas High Density Polyethylene (HDPE) has a shrink rate of around 2.0% ~ 6.0%, so as you can see, changing from PC to HDPE will have a large effect on the dimensions of the final product (these percentage shrink rates are a guide only).
2. Inspection of Tooling
2.1 The Process
Once your tool material is specified, you have signed off the tool drawings and paid your initial down payment, the manufacturing process starts. The tool maker should provide you with regular updates and photos of work in progress. A key milestone in any tool manufacture is T1, this is the first trial of the tool (which is where the term T1 comes from, ‘Trial 1’) where the tool is tested and run for the first time. At this point there may be some finishing work required by the toolmaker in order for the tool to function as expected in production conditions.
2.2 What to Check for
Either you or a consultant you appoint should ensure that the tool functions as expected while still on the bench, listen out for knocks and bangs (these are not good). If everything is smooth, run the tool in the intended size injection mold machine and check how the tool is working, how the parts are being ejected out of the tool – they should be ejected out nice and clean with no sticking in the tool.
This check process should follow a checklist so that all aspects of the tool are covered as well as providing a mechanism to record finding against each of the checklist items. An example of some top level elements that should be checked is shown below.
Top Level Headings of Check Points (examples)
- Visual tool checks – good condition, clear, correct lifting fittings, labelling etc…
- Physical tool checks – cross check against tool drawing, all inlets and outlets are clearly marked, correct gate design etc…
- Dynamic bench check – ejector plate movement, pin fit in holes is not too loose, correct length of ejector pins etc…
- Pre-running check – water ways are connected correctly, locating ring and sprue bushing are in-place, limit switches have been set and checked etc…
- Checking the tool in the mold machine – screw parameters set, injection pressure, does the tool open and close smoothly, etc…
- Tool Trial – Correctly filled parts, sink marks, flash, cycle times, etc…
- Checking molded part – check the part weight with and without runners, clean separation of gate, dimension inspection, etc…
- The person doing this inspection should have a comprehensive checklist for this inspection work.
2.3 Tool Sign off
Once you are happy that the tool has been manufactured from the correct material, you have all the certification that shows the hardness is correct, you have ensured the tool is functioning and has been built to specification and the final molded parts are acceptable in every way, only then should you sign off the tool fit for production.
There could be occasions where the tool is manufactured in one location and the production molding is carried out in a different location. In these scenarios we highly recommend that the tool sign off process is completed at the toolmakers factory and once the tool has been transferred to the molding factory a secondary inspection is to be carried out. This secondary inspection ensures the tool has not been damaged during transit and the transfer of tooling is signed off and accepted by the molding factory as they are now responsible for the tool and the parts being molded from it.
As a note; we would recommend some run time before final sign off. This is standard in many industries. For example, twenty hours of production run time to determine overall capability. This is just an example of a way to determine good molds.
