Compression molded tooling is made by machining and EDM (Electric Discharge Machining) large blocks of steel. However, there are machining limits which must be considered in the finalized design. This page will provide design rules which, when followed, will allow the silicone keypad to be properly tooled up and function correctly.
There are three major constraints to tooling compression molded silicone parts:
1. Delicate features that are too close together have a high risk of being damage during machining or production.
2. Most shapes in the tool are created using EDM. This process requires corners to have a radius.
3. Any openings in the silicone part must be hand torn - if the openings are too small, hand tearing is not possible.
Tooling is made from steel. If part geometries are very close together, then the tooling of these geometries becomes very thin and there is a risk of thin tool portion breaking during machining or even during molding. To prevent this, all features (webs, openings, edges, etc.) should have a spacing of at least 1mm from other features.
See how to design the web to calculate distance between the web and features
Under Key Features: For features at the bottom of the key, more spacing is needed so that the delicate web structure is not affected. Abatek suggests leaving 1.5mm between features.
See also: Designing with Conductive Pills (pill clearance section).
Features such as notches are molded by using very thin steel in the tool. These features should be at minimum 0.3mm wide and their depth not more than twice their width (0.6mm minimum).
Silicone Keypad molds are typically created using an EDM process. The advantage is that the EDM can achieve significantly more custom and delicate designs in steel compared to standard machine tooling processes. One limit of EDM is that very sharp corners are not possible. For this reasons, Abatek recommends a radius of 0.5mm as a minimum for all corners.
Ideally the web structure would have the largest possible radiuses to prevent sticking. A radius of 0.75mm minimum should be used for non-circular keys. The radius key top rim should be larger than 0.25mm (ideally more than 1mm).
The flash on the perimeter of the keypad, as well as on each opening must be removed by a manual, hand tearing process (see flash). Very small opening or tight corners will result in poor hand tearing and potential residual flash. If possible, the number of openings should be minimized. Abatek suggests a minimum of Ø1mm for openings and a minimum radius of 2mm for cutouts/recesses.
When a silicone key is pressed against a flat PCB, a vacuum can form preventing the key from returning back up (see sticking). Air channels will help prevent vacuums from forming.
Air channels should be 0.3mm deep and 1.5mm wide. For small keys (less than 6mm) one or two air channels should be sufficient to evacuate air. For larger keys, three or more may be needed.
Unless you don't have a choice, air channels leading to the perimeter of the keypad should only be used if humidity or other environmental exposure is not a factor.
Discreet LED lighting is used, for example, to illuminate indicator lights or for backlighting. To reduce cost and improve quality Abatk does not suggest using openings/holes for the LEDs.
Instead of openings, use the following Abatek solutions:
1. pockets with thick windows for backlighting LEDs - light bleeding is acceptable so silicone can be clear
2. pockets with thin windows for indicator LEDs - light bleeding is not allowed so dark silicone must mask LED
Additional Design Tips for LED Pockets:
The keybase should be at least 1mm thick to support the window. It is also suggested to leave 1-2mm of space between the LED and the silicone to help prevent overheating of the LED.
In both cases, the pocket's window should be supported by 1mm walls. The window can be polished to maximize light transmission.
Thick Windows for Backlighting LEDs: Backlighting LEDs are used to illuminate keys - with this design light bleeding is typically not an issue. For this reason, the pockets (and the rest of the keypad) can be made from clear silicone. The pocket's window thickness should be made 0.5mm or more to prevent tearing during molding
Thin Windows for Indicator LEDS: When colored silicone is thin enough, it becomes almost transparent. This characteristic can be leveraged to design a window for indicator LEDs where non-transparent silicone can block light leakage. Abatek suggest using 0.08% black pigment mixed with the clear silicone. See also Transparent & Tinted Silicone
Vertical walls that are 5mm or taller on a silicone keypad must have some angle (draft) to help with part removal after molding.
For features that are 15mm tall - a draft of 2º is needed.
For features that are 5mm tall - a draft of 1º is needed
Because compression molding tools have long cycle times (see molding), the molds are built with many cavities.
A cavity marking is used to identify the mold cavity used to make the part. The cavity marking is typically placed on the underside of the keypad. Cavity markings are made from raised material and extend about 0.2mm - the extra material will not interfere with the keypad's assembly fit or function.
Additional Markings If given enough space, additional identification markings, such as the part number, can be added to the keypad.
However, Abatek cautions against adding any identifiers that may potentially change during the life of the tool, such as a revision number. Unlink plastics tools, silicone compression molds cannot accommodate inserts - instead a (costly) tool modification will be needed to change the marking.
If it is absolutely necessary to add a revision number marking on the part, then Abatek suggest using dots which can easily be machined as needed.
Abatek recommends using a base thickness of 1mm to 4mm. If the base thickness is too thin, it will not be able to stabilize the key-webs during key presses. If the base thickness is too thick (4+ mm), the molding time will be unnecessarily increased.
Abatek also recommends using base thickness as the only Cpk/Critical reference dimension.
