GB2515348A

GB2515348A - A method of 3D printing a flexible or compressible structure

Info

Publication number: GB2515348A
Application number: GB1311142.2A
Authority: GB
Inventors: Tim Denholm
Original assignee: Individual
Current assignee: Individual
Priority date: 2013-06-23
Filing date: 2013-06-23
Publication date: 2014-12-24
Also published as: GB201311142D0

Abstract

A method of printing a flexible and/or compressible three dimensional polymer object, and an associated system per se, includes providing a printer with a freezer component to cool at least part a print volume and whereby the polymer is additively melted so that when the printed object is warmed its rigidity changes to allow it to flex or be compressed. The polymer, such as polypropylene, may have a glass transition temperature between 5C and -35C, preferably between 0C and -20C.

Description

A method of 3D printing a flexible or compressible structure The present invention relates to methods for producing flexible or soft objects by additive manufacturing where the desired object has a size, structure and flexibility such that it can be expected to deform under gravity whilst being printed -termed self-deforming objects.

3D printing is a rapidly progressing field, however a key limitation is the ability to print such self-deforming objects. Common objects which it would be useful to be able to print, such as soft toys for children, realistic prosthetics, soft skinned robots and animatronic toys, soft furnishings such as cushion, and clothing such as shoes and hats.

As an example, one current commercial 3D printer has a print volume sufficient that it has been demonstrated printing a full size model of a basketball. However, to ensure the structure does not flop over' during construction, it is clear that it must be made either with a comparatively inflexible material or with an increased wall thickness, and is therefore unsuited for use as a real basketball.

The range of materials available and suitable for printing flexible objects continues to grow, and currently includes polypropylene or simulated polypropylene (which is generally cheaper), polycarbonate, polyethylene, rubber-like plastics, nylon and ABS and many others. To achieve greater flexibility than a particular material allows, it is possible to print a structure which has greater porosity than a normal 3D printed object has (3D printers generally are designed to minimize bulk porosity unless specifically required to do otherwise). Examples of porous structures include open cell structures, closed cell structures, parallel capillaries, cross-linking frameworks of rods (or membranes), including frameworks having a partially fractal nature, and these frameworks can be regular, irregular or arbitrary. Different porous structures offer different bulk flexibility or bulk compressibility, and some can be designed to give preferential flexibility in one or two directions compared to other directions. Such structures can be devised very easily by the man skilled in the art, and are not discussed in more detail for brevity.

The combination of commercially available polymers that are comparatively flexible in use, combined with the option to print a porous bulk structure, enable the user to control the bulk compressibility to vary across a wide range. However in the past, a key design restriction has been that any part of the object being printed, cannot have such compressibility or flexibility such that the weight of material to be printed on top of that part will cause it to deform in such a manner that the top of the object will move during printing to the extend that this will interrupt the correct operation of the printer.

One route to overcome this difficulty is to print the object in several parts, and indeed it is known to automate the identification of how an object could best be split into several parts with keying press-fit interfaces so that each part can fit into the print volume of a particular 3D printer. However this approach, especially if requiring a keying interface generally works best for objects that are rigid. For a soft object it would be necessary to glue the parts together, perhaps using flexible glue, and this would detract from the convenience and value of 3D printing.

It is therefore desired to 3D print objects that retain their shape during the printing process, despite the structure, the orientation of the object on the print platform, and material(s) used being such that the structure would be expected to substantially deform during the printing process to such an extend or in such a manner that would interfere with the 3D printer's ability to print the rest of the object in the desired shape.

First aspect of the invention: A method of 3D printing a flexible and/or compressible object having the steps of: Providing a 3D printer having a print volume, and a freezer arranged to cool at least the print volume to a reduced temperature, Providing a variable-rigidity polymer which is comparatively rigid at the reduced temperature and comparatively flexible at room temperature, Selectively and additively melting and depositing the variable-rigidity polymer using the 3D printer according by computer control, to print at least part of a print object within the print volume, and Warming the printed object to room temperature and allowing the variable-rigidity polymer to consequently change from being comparatively rigid to being comparatively flexible, and for the at least part of the object to flex and/or compress under the weight of the object.

Optionally the freezer is a dual use freezer sold or purchased separately from the 3D printer and may be a conventional freezer marketed for domestic storage of food, in which case the 3D printer is placed inside the freezer.

Second aspect of the invention: A 3D printer system for printing flexible and/or compressible objects, the 3D printer system comprising a 3D printer and a freezer, the freezer being arranged to reduce the temperature of a print volume of the 3D printer to a reduced temperature, the 3D printer being adapted to selectively and additively melt and deposit a variable-rigidity polymer that is comparatively rigid at the reduced temperature and comparatively flexible at room temperature, to print at least part of a print object with the print volume.

Optionally a housing of the 3D printer is arranged within a housing of the freezer. Alternatively a housing of the freezer is arranged within a housing of the 3D printer and preferably the freezer housing is open at the top. Alternatively, the freezer housing has a flexible, sliding or concertinaing cover to permit the print head to extend downwardly into the freezer and to move laterally around the print volume.

Third aspect of the invention: There is provided a polymer for 3D printing of flexible and/or compressible objects, for deposition in a print volume maintained below OC, the polymer exhibiting a glass transition temperature lower than OC and higher than -20C The polymer is preferably in the form of a strand rolled into a reel, for dispensation to a print head of a 3D printer. The polymer optionally includes polypropylene. The polymer includes an additive to control the glass transition temperature. The glass transition temperature is preferably between -5C and -15C.

Fourth aspect of the invention: A flexible and/or compressible 3D printed object formed at least partially of a variable-rigidity polymer which has a glass transition temperature between SC and -35C, the object having a structure and composition such that it deforms under its own weight at room temperature but substantially does not deform under its own weight immediately below the glass transition temperature of the variable-rigidity polymer, the object being of a laminar construction, the laminar structure being substantially free of distortions deriving from deformation of the object under its own weight during a printing process.

Preferably the reduced temperature is between bC and -35C, more preferably between 5 C and -20C, most preferably between OC and -20C. These ranges are increasingly beneficial, firstly because temperatures well below room temperature have the benefit that the material will have a consistent hardness both at room temperature and outdoors in typical temperate climates, and secondly because temperatures above -35 enable use of affordable and readily available freezer technologies.

Preferably the polymer has a glass transition temperature below room temperature and above the reduced temperature. Preferably the glass transition temperature is below bC, and ideally is between -20C and SC most preferably between -1SC and OC. Preferably the glass transition temperature is at least 5 degrees C higher than the reduced temperature as this enables the user to have confidence that all parts of the material will cross the glass transition temperature.

While typical rigid printable polymers may have a Shore A hardness of about 83, polypropylene-like materials may have a Shore A hardness of about 76 while proprietary rubber-like materials such as Fullcure93O TM are advertised with a Shore A hardness of about 27.

Examples of suitable pure, unmodified materials include Polyvinylidene fluoride (PVDF) which has a glass transition temperature of -35C, Polypropylene (atactic) which has a glass transition temperature of -20C, Polyvinyl fluoride (PVF) which has a glass transition temperature of -20C, Polypropylene (isotactic) which has a glass transition temperature of OC, and Poly-3-hydroxybutyrate (PHB) which has a glass transition temperature of 15C.

It is common general knowledge in the art of polymer design, that the glass transition temperature can be modified, and this art can be beneficially employed for use with the present invention. Widely known methods include 1) adding plasticizers such as phthalates, 2) making chemical modifications such as adding non-reactive side-groups 3) mixing of different polymers to form a copolymer. It will be within the ability of the person skilled in the art of polymer design to select, modify and/or design a polymer with a suitable glass transition temperature. One example of a suitable polymer is Isostatic polypropylene, which is suitable without a reduced glass transition temperature, but is more suitable if its glass transition temperature is lowered from OC to around between -SC and -1SC by the addition of a plasticizer or more preferably by copolymerization with another polymer such as atactic polypropylene, in one example being in a 50:50 ratio but more generally being between 99:1 and 1:99 ratio or between 95:1 and 1:95, or between 80:20 and 20:80.

Optionally the 3D printer, including its housing sits within a freezer (or fridge). A reel of printable polymer may be arranged inside or outside of the freezer. Alternatively an open top chest freezer is arranged on the print platform, and a print head of the 3D printer is arranged to protrude into the freezer to be able to print onto the floor of the freezer. Further embodiments are set out in the claims.

Definitions: The term freezer encompasses a fridge, and in some embodiments the freezer may be a multipurpose freezer suitable for being used for storing food, which may be sold separately from the 3D printer. The term 3D printer predominantly relates to printers that deposit polymers additively, typically thermoplastics, and it includes printers that can perform other manufacturing techniques or steps too. The term reduced temperature means substantially below room temperature. The print object may be any object that a user selects to print and a computer controls the printer to deposit including composite objects with moving parts. The print volume is the region that a print head can be moved to to deposit material. The term polymer includes copolymers, plastics and rubbers. The rigid and flexible states are both solid states which may or may not be either side of a glass transition temperature of the polymer. Room temperature means around 15-25C. Variable-rigidity polymer means that the polymer has a substantial change in rigidity, flexibility and or hardness between the reduced temperature and room temperature.

Selectively means that the computer controls locations where the polymer is deposited so as to construct the print object according to a desired shape. Allowing includes any of turning off the freezer, removing the object from the freezer, or actively heating the object using a flow of room temperature or above-room-temperature air or water. Flexible or compressible includes flexible and compressible.

Description of the preferred embodiment.

A 3D printer is arranged to accept a strand of thermoplastic from a reel and to melt it and deposit it selectively on a print platform to build up an object according to a computer program as is known in the art. The 3D printer is arranged wholly within a freezer that cools the print volume to around -20C. Ideally the computer or hardware compensates for the thermal contraction of the guidance wormgears at the reduced temperature, by printing the object fractionally larger. Similarly an electric heater in the print head may have to drive a larger current to ensure adequate melting in the cooler environment.

The object to be printed is large and soft, or otherwise contains structures that will be expected to deform during the printing process, as more weight is added on top of them. Among countless examples are large soft toys for children and fake household plants. The polymer used, as well as the colour, may vary according to computer control. However in at least one region a comparatively soft polymer and or a compressible porous structure is needed which should be flexible in use but needs to be rigid during the printing process, otherwise the object will distort during printing, which will impede the ability of the print head to build up the desired shape. In that region particularly, the computer controls the printer to deposit a thermoplastic with a glass transition temperature below room temperature but above the temperature of the freezer. Soon after being deposited each droplet, line or layer of deposited plastic solidifies and then passes below its glass transition temperature, rendering it rigid and enabling the object to be printed without excessive distortion In this specific and non-limiting example, the polymer used is a copolymer of isostatic polypropylene and atactic polypropylene having a glass transition temperature between -SC and -15C, and where modest flexibility is needed in a large/heavy object, the comparatively flexible nature of polypropylene is relied upon, whereas where greater flexibility is needed (whether in a large/heavy object or not), increased flexibility is achieved by depositing the polymer in an open cell structure. The porous structure is typically filled with air, or if flexibility without compressibility is desired the porous structure can be filled with another material which is flexible at both the reduced temperature and at room temperature, such as a soft rubber-like polymer.

Claims

Claims: 1. A method of 3D printing a flexible and/or compressible object having the steps of: Providing a 3D printer having a print volume, and a freezer arranged to cool at least the print volume to a reduced temperature, Providing a variable-rigidity polymer which is comparatively rigid at the reduced temperature and comparatively flexible at room temperature, Selectively and additively melting and depositing the variable-rigidity polymer using the 3D printer according by computer control, to print at least part of a print object within the print volume, and Warming the printed object to room temperature and allowing the variable-rigidity polymer to consequently change from being comparatively rigid to being comparatively flexible, and for the at least part of the object to flex and/or compress under the weight of the object.
2. The method of claim 1 where the freezer is suitable provided with removable food storage compartments or locating and supporting means therefor.
3. A 3D printer system for printing flexible and/or compressible objects, the 3D printer system comprising a 3D printer and a freezer, the freezer being arranged to reduce the temperature of a print volume of the 3D printer to a reduced temperature, the 3D printer being adapted to selectively and additively melt and deposit a variable-rigidity polymer that is comparatively rigid at the reduced temperature and comparatively flexible at room temperature to print at least part of a print object with the print volume.
4. The 3D printer system of claim 3 where a housing of the 3D printer is arranged within a housing of the freezer.
5. The 3D printer system of claim 3 where a housing the freezer is arranged within a housing of the 3D printer.
6. A polymer for 3D printing of flexible and/or compressible objects, for deposition in a print volume maintained below OC, the polymer exhibiting a glass transition temperature lower than OC and higher than -20C.
7. The polymer of claim 6, in the form of a strand rolled into a reel, for dispensation to a print head of a 3D printer.
8. The polymer of claim 6 including polypropylene.
9. The polymer of claim 6 including an additive to control the glass transition temperature.
10. The polymer of claim 6, where the glass transition temperature is between -SC and -15C.
11. A flexible and/or compressible 3D printed object formed at least partially of a variable-rigidity polymer which has a glass transition temperature between SC and -35C, the object having a structure and composition such that it deforms under its own weight at room temperature but substantially does not deform under its own weight immediately below the glass transition temperature of the variable-rigidity polymer, the object being of a laminar construction, the laminar structure being substantially free of distortions deriving from deformation of the object under its own weight during a printing process.