Design tips ... on a plate!

Martin Larsen, director, Rich Plastics, offers OEM part design engineers advice on design issues relating to well plates and microplates.

COVID-19 has turned the medical industry upside down in many ways. First, it has affected development times: where previously tooling development projects might have taken 6 months, the pandemic has demanded urgency when time, among other commodities, was in short supply. Second, it has affected output: the world needed hundreds of millions of test plates, immediately. Every day, manufacturers were faced with having to increase production 10-fold or 100-fold. 

Despite the seemingly insurmountable hurdles, the industry has come up trumps, bringing many advancements in injection mould tooling designs for medical products such as microplates, pipettes and tubes. Examples include improvements in designs to control warp/deformation of the plastics during processing and to eliminate bubbles in the plastic. Alongside such advancements is the industry’s increased demand for maximising output, specifically through shorter cycle times, increased number of cavities and extended tooling life.

Design for manufacturability

OEM medical plastics part design engineers for the most part care about the function of the part, as it relates to the equipment with which it is intended to work. All dimensions must be achieved within the tolerances specified by the engineer. Once the injection mould supplier receives the part geometry (data) from the design engineer, the supplier will perform a Design for Manufacturability (DfM) analysis to understand whether the geometry is manufacturable. Results of this analysis will inform the tooling supplier whether to make recommendations to change the part geometry. The results will also highlight whether the manufacturer needs to add draft for demoulding, address thin steel conditions, and identify areas for potential sink marks and deformation, part ejection, cooling. In addition, the supplier will advise how best to construct the tooling, thus providing a preliminary design concept. The DfM will also specify the exact resin/polymer to be used and its shrinkage rate. 

The DfM review will go back and forth several times based on sharing experience and knowledge of similar programmes. Once the DfM has been agreed, the supplier performs a complete mould flow analysis to simulate the moulding conditions, providing valuable information on, for example, the cycle time, warp, deflections, gas traps, fill speed and balance fill. After a final review of the mould flow analysis with customer, the design will be accepted and approved. The final approval will initiate the production process, beginning with ordering steel and other components to be used in the production. On the acceptance date the quoted production build schedule will officially start. 

The following design considerations will help the tooling supplier to choose — and recommend to the customer — the choice of hot runner system: 

• The spacing between each of the hot runner drops. This is relevant to how the material flows into the tool, which in turn results in the best fill locations and even fill.  
• The volume of resin needed for each shot.
• The selected resin material and its melt flow rate.
• The stack height of the tool.

Another important design consideration is the gate type and size, bearing in mind that occasionally two types of gates can be used. Positioning of the gates is also important as it determines the balance of feed in the part; cycle times; minimum internal pressures; and part surface finish. 

Mould flow analysis 

Mould flow is a powerful simulation tool used to analyse the chosen gate positions. The mould flow will highlight the weld lines of the material flow, gas traps that will be formed and warpage of the part once it has cooled, thus allowing the tooling supplier to change gate position, as well as

the gate size and/or injection speed. 

A proficient design team will perform several (six or eight are common) mould flow analyses with different melt and tool temperatures to find the optimum design. Mould flow analysis is not 100% accurate, but when combined with experience of similarly designed products it gives the supplier a clear steer on hot runner and gate choice.

Cooling system

Cooling is used to keep the tool at the desired temperature during moulding production to optimise production output — the faster and more efficiently you can cool the part, the faster you can open the tool to complete the shot. Reducing cycle times by even a few seconds can increase productivity by thousands of parts per day/week/month … and, therefore, improve profit. 

Cooling lines are needed to remove heat from hot areas within the tool. These cooling lines ensure the flow of the material is even throughout the part. Ideally, the two halves (the core side and cavity side) of the tool need to cool at the same speed to ensure quick ejection.  

The most common method of cooling is by drilling holes into the core and cavity steel and running water 

through them. You add as many cooling lines as needed to cool the part evenly. A newer method for optimising cooling is to add heat transfer pipes (into the design) to extract heat from particularly small areas. An innovation in cooling technology is using 3D printing for complicated designs that could not normally be machined. Figure 1a and 1b shows two new options for cooling.

Venting

The injection mould venting expels air from the plastic

injection mould cavity during the injection process of the molten plastic material. Without venting, internal pressure will build up and damage the longevity of the tool — gas traps appear on the tool and eat into the steel.  

A good designer will not vent just at the end of the fill, they will also vent the entire outside of the tool and use the ejector pins and vent inserts in the tool to expel the air. Mould flow can be used to optimise and reduce weld lines (Figure 2). 

Summary 

In today’s fast passed and quickly changing medical tooling industry, we are challenged with rapid tooling production schedules and high demands on the tooling to produce millions of parts at the shortest cycle times to achieve profitability targets, as well as to protect patients. 

When selecting tooling design and manufacturing partners, look for companies with extensive experience in the product you need to manufacture, including their ability to perform complete engineering analyses. As different resins require different tool designs and tooling steel, a supplier who has full knowledge of the resin materials you want to use is also highly recommended.