Plastic Injection Moulding

Proto Labs provides quick-turn thermoplastic injection moulding often used for prototyping, bridge tooling and low volume manufacturing up to 10,000+ parts. We use advanced aluminium alloy moulds that allow us to eliminate the costly and time-consuming custom engineering that normally goes into the development of tooling. Proto Labs offer hundreds of engineering-grade resins that produce moulded parts that are strong and can have excellent finishes.

Quick-turn injection moulding uses the industry standard production process for parts, so it’s an excellent predictor of manufacturability when you move to large-scale production. At Proto Labs, we support an ever-increasing maximum part size and degree of complexity, and continue to introduce new materials that help designers and engineers make better products.
Get 25-10,000+
parts in 1-15 days.
Process
Design Guidelines
Fundamentals of Moulding

Design Guidelines

Size

The maximum part outline that can be moulded is approximately 480mm by 770mm with a maximum part volume of approximately 967cc. Depth up to 101mm from the parting line with 3 degrees of draft, or up to 202mm total if the parting line can pass through the middle of the part, inside and outside. Deeper parts are limited to a smaller outline.
Size
Protomold size

Draft

A taper applied to the faces of the part that prevent them from being parallel to the motion of the mould opening is called draft. This keeps the part from being damaged due to scraping as the part is ejected out of the mould. Recommended draft:
  • 0.5 degrees on all vertical faces is strongly advised.
  • 2 degrees works very well in most situations.
  • 3 degrees is minimum for a shutoff (metal sliding on metal).
  • 3 degrees is required for light texture (PM-T1).
  • 5 or more degrees is required for heavy texture (PM-T2).
Draft
Protomold undrafted
Undrafted
Protomold drafted
Drafted
Protomold undrafted
Undrafted
Protomold drafted
Drafted

Tolerances

Typically, injection moulding tolerances of +/- 0.08 mm + 0.005 mm/mm are expected and achieved.

Wall Thickness

With injection-moulded parts, observing proper (and uniform) wall thickness helps parts avoid potential issues such as sink marks and warpage. Recommended thicknesses vary by material:
Resin
Recommended Wall Thickness (mm)
ABS
1.1 - 3.5
Acetal
0.8 - 3
Acrylic
0.6 - 3.8
Liquid crystal polymer
0.8 - 3
Long-fiber reinforced plastics
1.9 - 25.4
Nylon
0.8 - 2.9
Polycarbonate
1 - 3.1
Polyester
0.6 - 3.1
Polyethylene
0.8 - 5
Polyphenylene sulfide
0.5 - 4.5
Polypropylene
0.6 - 3.8
Polystyrene
0.9 - 3.8
Polyurethane
2 - 19
The table* shows wall thicknesses Proto Labs recommends according to resin. Please note that thin walls only work on small parts and thicker walls are required where the resin has a long way to flow (Proto Labs injection moulding makes parts with dimensions of about 6mm to 770mm or more).

* Adapted from www.manufacturingcenter.co
Wall Thickness
Protomold wall thickness

Surface finish

A number of standard finishes are available for injection-moulded parts at Proto Labs. Our current finishes include non-cosmetic, low-cosmetic and EDM finishes; bead-blast texturing; and high-end mould polishing.

Standard finishes (from least to most costly):
PM-F0:
Non-cosmetic: finish to Protomold discretion
PM-F1:
Low-cosmetic: most toolmarks removed
SPI-C1:
600 grit stone, 10-12 Ra
PM-T1:
Protomold texture, SPI-C1 followed by light bead blast
PM-T2:
Protomold texture, SPI-C1 followed by medium bead blast
SPI-B1:
600 grit paper, 2-3 Ra
SPI-A2:
Grade #2 Diamond Buff, 1-2 Ra
*"PM" in the table signifies a surface finish adjusted to fit the rapid injection moulding process, where SPI (The Society of the Plastics Industry) denotes an industry-standard finish.
Surface finish
Protomold PM-F0 finish
PM-F0
Protomold SPI-A2 finish
SPI-A2
Protomold T2 finish
PM-T2
Protomold PM-F0 finish
PM-F0
Protomold SPI-A2 finish
SPI-A2
Protomold T2 finish
PM-T2

Radii

Because Proto Labs employs an automated CNC milling process to make the mould for your parts, some part corners will end up with a radius rather than a sharp edge.

This does not usually require a change to the model, but the designer is made aware of these radii before the mould is made.
Radii
Protomold radii

Ejector Pins

The injection-moulding process uses ejector pins of various sizes to push the plastic part out of the mould after it has solidified. The sizes and arrangement of these pins are selected to minimize the impact on your part design. Ejector pins range in size from 1.18mm to 9.53mm, with sizes larger than 1.58mm preferred.
Ejector Pins
Protomold ejector pins

Undercuts

Our moulding process supports simple undercuts in your part geometry. Maximum side core dimensions:
Width
Height
Pull
≤ 8.419 inches
≤ 2.377 inches
≤ 2.900 inches
≤ 213.84mm
≤60.38mm
≤73.66mm
Undercuts
Protomold undercuts

Materials

Protomold stocks hundreds of thermoplastic resins that offer a variety of benefits for many different applications and industries. See full list of materials.
  • ABS
  • ABS/PC
  • Acetal Copolymer
  • Acetal Homopolymer
  • HDPE
  • LDPE
  • Nylon 6
  • Nylon 66
  • PBT
  • PEI
  • Polycabonate
  • Polypropylene
  • PPS
  • PS
  • TPE
  • TPU
  • TPV