Why it pays to take your tools to the repair shop
21 Apr 2021
Updating or repairing an injection mould tool can be complex yet helps extend lifespan and quality advises Broanmain’s Kamil Stec, who provides some pointers for how to do it...
The cost of creating an injection moulding tool can run into thousands of pounds. One of the key considerations for product engineers has to be the operational lifespan of the tooling.
The process usually begins with a drawing. However, proficient toolmakers can eyeball a tool and present viable modification options and cost breakdowns.
It’s always easier to cut rather than add material in a tool. Removal is often done in small increments. Metal can be added. But this isn’t usually advisable for high-speed moulding.
Making a moulded polymer part thicker is relatively straightforward. By making the initially geometries in the tooling thin, the thickness can be expanded by making cuts into the tool. It’s advisable to make the outside diameter and shape of the part with the tool as small as possible. This then enables the tool to be altered instead of replacing it.
Until the features of the part are fully determined, leave them out. In some instances it is possible to replace the core of the tool. This is more commonplace in today’s modern moulding tools, which rather than being a solid hardened steel tool comprises 200+ individual components assembled together.
Factor different iterations that might potentially be needed into the development. The most common adaptation is changing hole sizes because these are typically formed using easily replaceable core pins.
Moulding tools are typically designed for processing a particular plastic. Occasionally it’s possible to mould multiple plastics in the same mould. However, there’s always a size difference in the part – troublesome if the component forms part of an assembly where pieces are slotted together with great precision.
Changing the material can also cause other quality issues such as flash and burns. Heat transfer also needs to be considered. Using copper alloys for certain sections of the tool can help to defuse the heat a bit better. Generally, a tool can be built using 30+ different materials, tool steels and alloys. Mixing different grades and hardness of tool steels is advisable, especially when there are sliding parts and complex shut offs within the mould cavity.
For corrosive or abrasive plastics, like glass filled nylon, consider wear and tear. Broanmain offersdifferent treatments and coatings, such as nitriding and hard chroming to further improve wear resistance.
As well as increasing the number of rejected parts and potentially affecting the finish of the component, not having a draft angle can also damage the mould tool. Rather than having squared walls in the injection moulded part, they need to be slanted at an angle to the ejection and opening of the tool. The general rule of thumb is 1 degree of draft per 1 inch of cavity depth.
An undercut is a feature that is formed perpendicular to the mould tool standard opening direction and prevents easy demoulding. Yet for an open and shut moulding tool, filling the perpendicular opening can be complex and costly.
The more detailed information a toolmaker has at the outset, including original drawings, the simpler and quicker moulding tools are to fix.
Kamil Stec is tool manager at Broanmain Plastics
